Riffle-Pool spacing in relation to channel variables along a meandering course of the River Lin

By James Runacres

Dissertation presented for the honours degree of BSc Geography

Department of Geography

University of Leicester

2015

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Acknowledgements

I would like to thank Simon, Angela and Molly Runacres for their assistance in obtaining field

data. Additional thanks goes to my supervisor Dr. Andrew Carr for his continuing support

and advice during the completion of this dissertation as well as Declan Goodwin for proof-

reading. Their combined co-operation is very much appreciated.

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Table of Contents

Acknowledgements. .......................................................................................................... ii

List of Figures and Tables .................................................................................................. vi

List of Plates ................................................................................................................... viii

Abstract ........................................................................................................................... ix

1. Introduction .................................................................................................................. 1

1.1 Riffle-Pool Characteristics ................................................................................................ 1

1.2 Categorisation of Pools .................................................................................................... 3

1.3 Integration with Restoration Schemes .......................................................................... 4

1.4 State of Current Research ................................................................................................ 5

2. Literature Review .......................................................................................................... 6

2.1. Free Formed Riffle-Pool Formation ............................................................................... 6

2.2. Forced-Pool Formation ................................................................................................... 9

2.3. Riffle-Pool Spacing ......................................................................................................... 10

2.4. Dominant Discharge ...................................................................................................... 12

2.6. Riffle-Pool Morphometry .............................................................................................. 15

2.7 Review of the Literature ................................................................................................ 15

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3. Methods ...................................................................................................................... 17

3.1. Study Site ........................................................................................................................ 17

3.2. Sampling .......................................................................................................................... 19

3.3. Riffle-Pool Criteria .......................................................................................................... 20

3.4. Data Analysis................................................................................................................... 21

4. Results ........................................................................................................................ 22

4.1. Riffle-Pool Spacing and Width Analysis ...................................................................... 22

4.1.1. Channel Width ............................................................................................... 22

4.1.2. Bankfull Width................................................................................................ 22

4.1.3. Forced Pools ................................................................................................... 23

4.2. Frequency Distribution Graphs .................................................................................... 24

4.2.1. Riffles and Free-Formed Pools ..................................................................... 25

4.2.2. Forced-Pools ................................................................................................... 25

4.3. Riffle-Pool Spacing to Other Channel Variables......................................................... 28

4.3.1. Length .............................................................................................................. 28

4.3.2. Velocity............................................................................................................ 28

4.3.3. Cross-Sectional Area ..................................................................................... 29

4.4. Riffle-Pool Length Analysis ........................................................................................... 31

4.4.1. Channel and Bankfull Width to Length ....................................................... 31

4.4.2. Average Depth to Length .............................................................................. 32

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4.5. Omission of Data Points ................................................................................................ 34

5. Discussion ................................................................................................................... 35

5.1. The Significance of Channel and Bankfull Width ....................................................... 35

5.2. An Explanation of Riffle-Pool Spacing ......................................................................... 38

5.3. An Assessment of Dominant Discharge ...................................................................... 39

5.4. Forced-Pool Spacing ...................................................................................................... 40

5.5. Effects of Human Intervention ..................................................................................... 45

5.6. Limitations of the Methodology and Data ................................................................. 46

6. Conclusion ............................................................................................................................ 48

Bibliography .................................................................................................................. 511

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List of Figures and Tables

Figures

Figure 1: The location of riffles and pools through a meandering channel reach. Points

A and D represent riffles on inflections and points B and C show pools on the

outside of meander bends (Mount, J. F., 1995).

Figure 2: The formation of riffle-pool sequences from irregular erosion initiated in

Stage 1. Each stage represents further development of both the meander and

riffle-pool morphology (Keller, E. A., 1972).

Figure 3: A typical meander sequence with the location and thalweg profiles of riffles,

pools and point bars shown. This is a simplified schematic but is used to

demonstrate basic riffle-pool concepts (Keller and Melhorn, 1978).

Figure 4: Dominant discharge in relation to sediment transport rates and frequency of

occurrence in (a) fine and (b) coarse grained channels (Heritage, G. L., Milan,

D. J. 2004).

Figure 5: The study reach of the River Lin encircled in red. Notice the differing land use

and urbanisation surrounding the channel. The river flows from north to

south (Google Earth, 2011).

Figure 6: A detailed geological map of the land surrounding the study site in the centre.

Areas in red indicate resistant igneous rock and pink shows areas of softer

mudstone. Adapted from BGS (2002).

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Figure 7. Relationships between RP spacing and (a) channel width (b) bankfull width as

well as forced-pool spacing with (c) channel width and (d) bankfull width. The

circled point in Figure 7a indicates the location of two near-identical values

that may not be visible on the graph.

Figure 8. Frequency distribution of (a) RP channel width spacing, (b) RP bankfull width

spacing, (c) forced pool channel width spacing and (d) forced pool bankfull

width spacing to one decimal place. These are included to illustrate the range

and spread of the data. Standard deviation values (SD) are also included to

quantitatively show variation.

Figure 9. Relationships between RP spacing and (a) length, (b) velocity and (c) cross-

sectional area with r2 values.

Figure 10. Relationship of riffle-pool (RP) length with (a) channel width, (b) bankfull

width, (c) average depth and (d) maximum depth with accompanying

correlation coefficients.

Tables

Table 1. Variances among mean riffle-pool characteristics for the entire data set with

accompanying t-test results and associated p-values.

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List of Plates

Plate 1: An example of a channel constriction caused by a vegetated outcrop

producing a forced-pool. A recirculating eddy is present, although not evident

in the plate, located at the far side of the channel and encircled in red.

Plate 2: An example of LWD which did not produce a forced-pool. Such obstructions

were present throughout the study reach. The plate shows evidence towards

bank erosion caused by the LWD.

Plate 3: A permanent and well-vegetated bar bisecting the channel into two sub-

channels within which a forced-pool is formed. Notice the location of an

upstream and downstream riffle immediately before and after the channel

bar.

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Abstract

Riffle-pool spacing is known to approximate 5-7 bankfull channel widths however its

application to channels with a high density of coarse alluvial sediment and the presence of

forced-pools is limited. In addition, the use of bankfull width to explain riffle-pool

morphology is debated in the literature and consequently so too is the type of dominant

discharge. The study focused on the River Lin, a minor channel located in NW Leicestershire,

which is heavily influenced by resistant granite surroundings. A total of 13 riffles and 13

pools (including six forced-pools) were included in the 520m study reach with channel width,

bankfull width, length, near-bed velocity, cross-sectional area and depth measurements

taken at each unit. Results showed a stronger correlation between spacing and channel

widths associated with observed low flow (Q = 0.13m³s¯¹) than with bankfull width

(r2=0.219 and 0.191 respectively). Despite rhythmic spacing, similar weak correlations and

magnitudes between the two widths implies that the dominant discharge for the River Lin is

above that of bankfull and approaching flood stage where the large stream power associated

with this is able to overcome the resistant channel bed and banks and mobilise sediment.

Forced-pools in the study showed little or no correlation between spacing and

channel/bankfull width however this was attributed to the relatively low density of pool

forming obstructions. Whilst certain channel variables had a more significant influence on

riffle-pool morphology than others (most notably velocity), they were all ultimately

influenced by discharge and was therefore identified as the key variable. The study

illustrates the need for more research into the impacts on riffle-pool spacing especially in

relation to forced-pools and the effects of increased pressures from human interaction.

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1. Introduction

1.1. Riffle-Pool Characteristics

Riffle-pool units are meso-scale features found predominantly in bedrock and coarse-grained

rivers on slopes less than 2%. Pools can be considered as small, asymmetric depressions in

the channel bed formed as a result of channel scour and are theoretically found on the

outside of meander bends often accompanied by a point bar on the opposing bank

(Figure 1). Under regime flow (approximating mean annual flow), pools exhibit calm and

near-horizontal water surface slopes with low surface and near-bed velocities and represent

the deepest sections of a rivers cross-section. Riffles are topographic high points on the river

bed and characteristically produce more symmetrical cross sections than pools. Riffles form

and develop from the deposition of sediment along the inflection points of meander bends

(Keller and Melhorn, 1978) and are typically constructed of tightly packed, coarse-grained

alluvium. These shallower channel sections have faster surface and near-bed velocities and

subsequently produce turbulent surface slopes that are steeper than those of pools under

regime flow. Riffles and pools are orientated in an alternate sequence as shown in Figure 1

and are separated by a connecting run or straight that marks the changing point between a

riffle and a pool in terms of bed elevation, near-bed velocity, water surface slope and

sediment characteristics.

A key aspect of the system is riffle-pool maintenance which refers to the constant balance

between depletion and replenishment of sediment forming a quasi-equilibrium state

(Dolling, 1968). The concept of maintenance helps to explain how riffles can be sustained

through deposition and pools by scour despite greater velocities (and hence sheer stress)

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over riffles during low flow. The velocity reversal hypothesis, first observed by Gilbert (1914)

and part-developed by Leopold et al. (1964), was combined into a theoretical framework by

Keller (1971) to describe how channel velocities could produce the observed sediment

sorting in riffles and pools. The key findings of the paper describe how at low flow, riffles

scour fine grained sediment and deposit in pools, but at high discharges sediment is scoured

from pools and deposited at riffles. In addition, the larger sediment in riffles, which is

immobile at small discharges, is also transported through the sequence and as discharge

returns to regime flow, large sediment becomes trapped in riffles. This mechanism explains

why riffles are typically comprised of larger sized material and why pools contain an

abundance of fine sediment mostly constituting sand and silt.

Figure 1: The location of riffles and pools through a meandering channel reach. Points A and D represent riffles on inflections and points B and C show pools on the outside of meander bends (Mount, J. F., 1995).

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1.2. Categorisation of Pools

Pools can be grouped into two categories: free-formed and forced-pools. The former

concerns those that form as a direct result of flow and sediment interactions whilst forced

pools, first coined by Montgomery et al. (1995), refers to pools formed by irregular channel

obstructions and constrictions including bedrock outcrops (Arrington and James, 2013), large

woody debris (LWD) (Hassan and Woodsmith, 2004) and boulder obstructions (Dolan et al.,

1978; Lisle, 1986; Harrison and Keller, 2007). Forced-pool sequences often develop in steep

sloping, mountaneous environments where erratic bedrock outcrops and large boulders are

common. They can also form in non-bedrock dominated rivers where there is sufficient

riparian vegetation and LWD to provide channel obstructions (Montgomery et al., 1995;

Jackson and Sturm, 2002). Forced pools and accompanying riffles can therefore form in

forest channels where there may be more restriction on channel migration compared to

free-formed units in reaches surrounded by a floodplain.

Together, riffles and pools can be considered as a single morphological unit connected by

sediment and flow dynamics (Dietrich et al., 1979) which create downstream undulations in

bed topography. The cross-sectional dimensions of pools and riffles therefore reflect the

balance between the erosive forces linked to discharge and the resistive forces provided by

bank cohesion, vegetation, sediment size and structure. Despite riffles and pools being

dominant along coarse-grained channels a small number of studies have reported their

existence in rivers dominated by finer grained sediment (Hudson, 2002) however it is

necessary for a supply of coarse sediment to form riffles and hence maintain the sequence.

Riffles and pools represent channel self-adjustment and drive processes such as meander

migration, channel development and modifications to the rivers thalweg (the points of

lowest bed elevation representing areas of lowest friction and hence fastest velocities).

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1.3. Integration with Restoration Schemes

The riffle-pool sequence is a fundamental element of developed gravel-bed rivers and is

shown to be a permanent and stable channel feature unlike micro-scale ripples and dunes

found in ephemeral rivers. Research including Dury’s (1970) study of a riffle-pool sequence

in New South Wales, Australia found no morphological or geographical change in their

dimensions or location respectively over a 100yr period. Riffles and pools are evidence of

deposition and erosion of sediment respectively and are therefore representative of the

behaviour of the channel. This coupled with the persistence of riffles and pools and their

influence on channel morphodynamics has led to them being incorporated into river

management and restoration schemes as well as implementation into river quality policies.

Forced-pools are of particular interest as they can be easily recreated and used to restore

riffle-pool sequences in damaged river systems. The re-introduction of such sequences is of

particular interest to ecologists as pools provide key habitats for fish and other aquatic

creatures which spawn in the deep and calm water (Hanrahan., 2007; Pasternack et al.,

2008). The importance of these features highlights the need for a better understanding of

the complex flow and sediment fluxes involved in forming and maintaining riffles and pools.

Increased pressure from land use and land cover change (Goode and Wohl, 2007) including

the expansion of agriculture and increased urbanisation alongside lowland reaches of rivers,

can drastically alter sediment and water inputs to the river, altering the morphology of free-

formed and forced-pools as well as compromising their ecological and hydrological

importance to the environment.

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1.4. State of Current Research

For restoration projects to be successful, fundamental riffle-pool relationships that occur in

natural channels such as their rhythmic spacing must be well-understood. Despite the

significant role that riffles and pools play in the development and ecology of alluvial

channels, the large literature base has been unsuccessful in producing a unified theory which

explains the nature of these channel features. Although general patterns in riffle-pool

morphology have been well documented, the specific variables responsible and their relative

significance have produced confusion in the literature with many contradictive findings. This

is often attributed to how different studies distinguish between riffles, pools and runs as

well as the large number of possible variables involved which increases the complexity of the

riffle-pool system. Recent trends in studies have now begun to cite a number of variables,

operating together in feedback mechanisms to explain riffle-pool dynamics. This increasingly

common approach disregards the concept of a single overriding variable being responsible

for development and modification, and has instead looked at coupling models as a more

realistic explanation which, as stated previously, reflect the balance between the erosive

forces linked to discharge and the resistive forces provided by bank cohesion, vegetation,

sediment size and structure.

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2. Literature Review

To effectively interpret the data produced in this study the processes involved in the

formation of riffles and pools will be discussed as well as those concerning forced-pools. The

relationship of riffle-pool spacing to a multitude of channel variables will be debated with

reference to the large, but undecided literature base with an emphasis on highlighting the

key conflicts. The concept of dominant discharge is considered as well as the established

relationships between riffle-pool morphology. A brief appraisal of the literature will be

provided to evaluate the key issues relating to the aims of the study which are outlined at

the end of literature review.

2.1. Free Formed Riffle-Pool Formation

The formation of riffles and pools is not fully understood despite their abundance in alluvial

rivers. An understanding of how, and under which specific conditions riffles and pools form

is key to explaining observed morphology which has a wider use in restoration schemes.

What is clear is that riffles and pools develop a symbiotic relationship with the channel as

the behaviour of one ultimately affects the other – whilst riffles and pools are essentially

part of the channel, they are both able to adjust independently of one another. Although

incomplete, Keller’s “Five Stage Model” (Keller, 1972) stands as the most efficient method

for explaining channel pattern development and the apparently spontaneous formation of

riffle-pool sequences (Figure 2).

The model consists of five stages which depict the transition of a river from a straight to a

meandering course into which Keller (1972) claims all natural rivers can be categorised.

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Stage one is considered as the embryonic stage of a river consisting of a straight channel

with no riffles or pools; most rivers enter stage two very quickly and stage one can be

considered partly hypothetical as rivers with these criteria are rare. Keller identified riffles

and pools as bedforms which develop in later stages of his model. Earlier stages are

dominated by asymmetric or skew shoals first coined by Quraishy (1944). These first-order

bedforms slope towards the banks in an alternate pattern and create concentrations of

faster flowing water which wander from one bank to the other in the form of a thalweg. This

consequently creates variations in erosion along the channel. As demonstrated in Figure 2,

the high velocity jet of water erodes into the bank as it is deflected around the shoals and

then disperses between shoals. Flow convergence due to the asymmetric shoals leads to the

development of meander bends with erosion occurring on the outside bank due to inertia.

In stages two and three, convergence of flow in bends and divergence of flow between

bends cause vertical scour and deposition respectively and leads to the formation of the first

riffles (symmetric shoals) and pools. At this stage riffles and pools are undeveloped and are

still dominated by the asymmetric shoals in terms of their ability to alter flow dynamics. As

the river continues to migrate, riffles and pools enlarge at the expense of asymmetric shoals

which transform into point bars on the inside of the meander bend. These point bars

represent areas of deposition due to low velocities and hence explain their location; it is

important to note that although asymmetric shoals are transformed to point bars, relic

formations may survive and can alter the inflection points of the meander (Keller, 1972).

Stages four and five represent a decrease in radius of curvature and increasing channel

length due to the migrating meander and produce well developed riffles and pools that are

deeper, longer and are spaced 5-7 channel widths apart.

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Theoretically, the wandering thalweg creates pools at bends and riffles at inflections

(Figure 2). This idealised model is rarely seen in natural streams as more than one pool can

be found on a bend as well as along inflections. This is in part due to sequences of straight

and meandering sections commonly found in mature rivers in stages four and five. One of

Keller’s main insights in his 1972 paper was that riffle-pool spacing remains constant during

the transition between straight and sinuous channels despite longer channel lengths

associated with meander migration. To simplify, a straight reach with four pools will

transform into a sinuous reach with four pools. If channel length increases sufficiently, riffle-

pool spacing can exceed 5-7 channel widths; this range appears to be a constant value found

in nearly all rivers although there is no theory which can currently explain this phenomenon.

Lofthouse and Robert’s (2008) study in southern Ontario, Canada found that when riffle-pool

length exceeded a critical threshold for a given sediment size, the transport capacity of the

sequence was disrupted. The disruption in the movement of sediment between riffles and

pools caused a new riffle to form upstream in order to shorten the sequence and maintain

constant spacing. This process is particularly apparent in actively meandering channels.

Figure 2: The formation of the riffle-pool sequence initiated by irregular erosion in Stage 1. Each stage represents further development of both the meander and riffle-pool morphology (Keller, 1972).

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2.2. Forced-Pool Formation

Studies concerning the formation of forced-pools are limited in comparison to free-formed

units and are therefore also not fully understood. The main body of literature consider large

boulders (Harrison and Keller, 2007; Thompson, 2012), log jams (Magilligan et al., 2008;

McBroom et al., 2014) and bedrock outcrops (Keller and Melhorn, 1978; Wohl and Legleiter,

2003) to be the main source of forced-pools with studies from Montgomery et al. (1995) and

Lisle (1986) linking 82% and 92% of these obstructions to the location of forced-pools

respectively. These obstacles, present in the channel, disrupt the natural flow of discharge

and create backward flowing currents and steeper water slopes immediately downstream of

the obstruction. This rerouting of water can generate jet flows, higher velocities and form re-

circulating eddies at the constriction which promotes channel scour and hence the

formation of pools (Kieffer, 1985; Schmidt et al., 1993; Thompson, 2001). The ability for an

obstruction to create a forced-pool is reflected in its relative size to the channel. In the case

of LWD, research from Magilligan et al. (2008) found that whilst their frequency was high

(15-50 pieces per km) they rarely extended across a significant portion of the channel and

therefore did not produce a forced-pool due to their inability to alter flow and sediment

fluxes.

Five to seven bankfull channel width spacing is commonly attributed to free-formed pools

but little attention is paid to spacing in coarse-grained and constriction-dominated rivers.

Whilst it is well-known that the concentration of channel obstructions, most notably LWD,

reduces riffle-pool spacing (Bilby and Ward, 1991), issues arise when the notion of rhythmic

spacing of forced-pools is introduced. The most pressing concern is how seemingly random

distributed obstructions can produce a rhythmic spacing pattern. Thompson’s (2001) study

focused on this issue and used a simulation model for riffle-pool formation in a river

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dominated by channel constrictions including LWD, bedrock outcrops and large boulders.

The author found that the random positioning of channel obstructions could still produce a

constant spacing of riffles and pools; his model indicated an average of between four and

eight bankfull channel widths but with significant deviation due to the large variation in the

characteristics of pool-forming obstacles.

These studies show that the presence of LWD, bedrock outcrops and large boulders can be

directly correlated with the location of forced-pools and reported to produce rhythmic

spacing but with greater variation than sequences in non-constriction dominated reaches.

The limited literature means that the precise mechanisms influencing forced-pools and the

notion of rhythmic spacing itself is highly contested and does not provide a definitive model

to explain observed patterns.

2.3. Riffle-Pool Spacing

A key aspect of riffles and pools is their individual spacing and in particular, the relationship

they exhibit with channel variables. Leopold et al. (1964) established that riffle-pool

sequences average 5-7 bankfull channel widths as a result of the formative processes

explained above. This rhythmic spacing is widely reported in the literature and appears to be

a constant value in alluvial, bedrock and supra-glacial rivers containing meandering reaches

– this implies that spacing is independent of sediment calibre (Keller and Melhorn, 1978).

There is currently no theory as to why rivers tend towards this spacing however some

studies have linked it to the geometry of meanders. Early work form Leopold and Wolman

(1960) showed that meander wavelength was a function of channel width (W) and

quantified this at 10.9W. Later theoretical research by Richards (1976) put meander chord

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length at 10-14W, averaging 12W. By assuming two riffle-pool sequences per meander

wavelength (based on an idealised approach that pools form on the outside of meander

bends and riffles on inflections), Richards (1978) put riffle-pool spacing at 6W. These insights

are demonstrated in Figure 3 which shows a simplistic meander course and associated riffle-

pool wavelengths. Whilst riffles and pools rarely incline to this model, it illustrates the

fundamental concepts behind riffle-pool spacing and its integration with channel width.

Supplementary to meander wavelength and width, other channel variables have been

correlated to riffle-pool spacing to assess their relative significance in controlling key

characteristics. Stream power, a product of discharge and channel gradient, has been

considered as providing the necessary mechanisms for riffle-pool maintenance and hence

spacing. Wohl et al. (1993) studied the effects of channel gradient on spacing, depth and

length and found that pool depth and distance between pools both increased with

Figure 3: A typical meander sequence with the location and thalweg profiles of riffles, pools and point bars shown. This is a simplified schematic but is used to demonstrate basic riffle-pool concepts (Keller and Melhorn, 1978).

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decreasing channel gradient. They concluded that at areas of high channel gradient,

discharge and therefore stream power are relatively low, this coupled with more resistant

channel boundaries means that energy is expended in overcoming channel resistance rather

than channel scour. In lower gradient reaches, discharge is significantly greater so provides

higher rates of stream power and shear stress. In conjunction with finer grained bank and

bed material found in lower reaches, more energy is used in channel scour in both the

vertical and horizontal planes. It is known that rivers consistently try to minimise their

energy expenditure (Cherkauer, 1973) along its course and Wohl’s study indicated that

energy expenditure varies downstream due, in part, to changes in bank material.

Whilst significant correlations are found between riffle-pool spacing and variables such as

gradient (Wohl et al., 1993; Thompson, 2002), sediment flow (Lisle and Hilton, 1992; Milan,

2013), and valley width (White et al., 2010) they are all linked to discharge which is shown to

be the underlying determinant which dictates spacing and overall adjustments to riffle-pool

morphology.

2.4. Dominant Discharge

The idea of a dominant discharge in this study refers to a specific flow regime that maintains

and controls channel bedforms including riffles and pools. This specific discharge is reflected

in Figure 4 which links sediment transport with return period for (a) fine and (b) coarse

sediment. As riffles and pools are controlled by sediment movement, it can be assumed that

the dominant discharge for such bedforms is the product of these two variables. Bankfull

discharge is often recognised as the dominant discharge in maintaining riffles and pools and

subsequently bankfull width is used to quantify riffle-pool spacing. Shear stress reversal and

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sediment routing hypotheses argue that high velocity jets initiated by near-bankfull

discharges are responsible for channel scour and conclude that any major modifications to

the channel can only be performed by high shear stresses associated with bankfull flow and

above (Robert, 1997; Wilkinson, 2004).

Harvey (1975) correlated median riffle spacing and channel width with six types of discharge

ranging from mean annual discharge to flood discharge with a recurrence interval of 1.1

years. Harvey’s results showed that the strongest correlations were with channel widths

associated with discharges ranging from mean annual flow and discharges exceeded 20% of

the time. The study argued that whilst overall adjustments were caused by rarer discharges,

minor alterations to the riffle-pool sequence were related to smaller, more frequent

discharges well below bankfull flow. Harvey concluded that although high discharge events

exerted the largest shear stress values, they occurred far too infrequently to explain the

constant reworking of riffles and pools and may be better explained by smaller, more regular

discharges approximating mean annual flow.

The simplistic model shown in Figure 4 describes the mechanisms involved with dominant

discharge but does not provide numerical values for its magnitude in fine and coarse-grained

channels. The disagreement in the literature in relation to dominant discharge is related, in

part, to difficulties in accurately measuring channel variables such as near-bed velocity

especially at discharges equal to and above bankfull (Almeida and Rodriguez, 2011).

Consequently, measuring more attainable variables which may not be directly linked to

riffle-pool maintenance or morphology is preferred at such large discharges. In the case of

channel velocity, studies such as Wohl (2007) measured average cross-sectional surface

velocities rather than near-bed velocities which have a marked difference in both their

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magnitude and influence over the riffle-pool sequence. Keller (1971) specifically measured

bed velocities as he identified this flow to be responsible for exerting tractive forces on the

channel bed and was therefore accountable for shear stress, scour and overall sediment

transport through the riffle-pool sequence.

The perspective from Keller (1971) is adopted in this study with near-bed velocities being

measured over surface velocities as the channel discharge during fieldwork will be

sufficiently low for accurate data to be taken. In addition, as the study concerns riffle-pool

morphology which is strongly linked to sediment transport and downstream sorting, it is

only the flow in direct contact with the bed that can entrain sediment and is therefore the

flow that should be measured when investigating riffles and pool.

Figure 4: Dominant discharge in relation to sediment transport rates and frequency of occurrence in (a) fine and (b) coarse grained channels (Heritage, G. L., Milan, D. J. 2004).

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2.6. Riffle-Pool Morphometry

Aside from riffle-pool spacing, studies have investigated the correlation between channel

variables and the associated widths, depths and lengths of riffle-pool units to establish the

relationships that exist. Key insights from Carling and Orr (2000) concerning the River

Severn, found that riffle height and pool depth are both positive functions of riffle length

and pool length respectively, but length was found to increase quicker than bed amplitude

so long riffles and pools are relatively flat in form. High levels of variation were recorded

between individual units in the study which indicated a complex relationship between flow

and sediment controls. Similar research from Richards (1976) established that channel width

increased by 15% at riffles as flow is diverted around medial bars and subsequently cuts into

the bank. Active erosion infers excess energy at riffles which conflicts with the minimum

variance hypothesis that predicts excess energy at pools (Langbein, 1964). This adds to the

debate of whether riffle-pool sequences are initiated and maintained by bed scour at pools

or bank erosion at riffles.

2.7. Review of the Literature

A review of the literature indicates a lack of agreement amongst authors on many aspects of

riffles and pools. One key, unresolved issue is the spacing of riffles and pools in rivers

containing a high density of channel obstructions. There is limited data concerning how

forced-pools can maintain a constant spacing of 5-7 bankfull channel widths if the

obstructions that cause them are randomly distributed along the river. There is also

contradiction concerning the significance of channel or bankfull width in explaining riffle-

pool morphology. Although most studies consider bankfull width to be of most importance,

a small number maintain that channel widths associated with smaller discharges have a

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stronger influence on riffle-pool morphology – most notably Harvey (1975). It seems evident

that achieving a unified theory of riffle-pool morphology is difficult due to differences in how

riffles and pools are located and measured in the literature, as well as the large number of

channel variables which creates complicated feedback mechanisms.

2.8. Aims of the Study

This study aims to correlate riffle-pool spacing to both channel width and bankfull width in

order to determine which has the greatest influence in a channel containing high densities of

resistant granite sediment. These widths can then be linked to their associated discharge in

order to provide an estimation of the dominant discharge for the River Lin. The distribution

of spacing measured in channel (CW) and bankfull (BW) width units will also be analysed to

test if 5-7 CW/BW is applicable to the River Lin. As the channel reach contains a mixture of

both free-formed and forced-pools, an evaluation of forced-pool spacing will be undertaken

to determine if the random location of pool-forming obstructions can initiate rhythmic

spacing as suggested by Thompson (2001). In addition, the influences of other channel

variables (average near-bed velocity, cross-sectional area and average depth) over riffle-pool

morphology are scrutinised to assess if the sequences in the River Lin are similar to those

examined in the literature and also to test their relative significance to spacing.

17

3. Methods

3.1. Study Site

This study focuses on the River Lin, a small river (Q=0.13m³s¯¹ at time of fieldwork),

occupying stages four and five of Keller’s model (1972), located in Newtown Linford, NW

Leicestershire (Figure 5). The channel itself is sourced from a network of minor tributaries in

surrounding farmland to the NW of Figure 5 before meandering through Bradgate Park to

the east and into Cropston Reservoir.

The River Lin is situated within a geology setting split between igneous and mudstone rock.

To the immediate east is the extinct Bradgate Park volcano which has subsequently littered

Figure 5: The study reach of the River Lin encircled in red. Notice the differing land use and urbanisation surrounding the channel. The river flows from north to south (Google Earth, 2011).

N

18

the surrounding area with resistant Pre-Cambrian/Cambrian granite and diorite composites.

This is illustrated in Figure 6 which shows a dominance of mudstone sediment in the local

area with intrusive igneous outcrops concentrated around the study site. As a result, the

sediment sourced by the River Lin is a mixture of resistant and relatively soft material. The

igneous sediment is mostly represented by large boulders present in the banks and bed of

the channel with a potentially smaller proportion entering the channel from the surrounding

hillslopes. The inclusion of both resistant and erodible alluvium of varying calibre will

therefore allow riffle-pool sequences to form in the River Lin due to its meandering course.

The chosen channel reach of the River Lin is a 520m long section comprising of 26 riffles and

pools. The site is located within arable farmland and contains several meanders of varying

degrees of curvature with a slope approximating 1%. Of the 13 pools, seven are free-formed

with six forced-pools as a result of LWD, boulders and island bars present in the channel.

From 370m-445m along the reach no measurements were taken due to a localised natural

dam which caused dramatically deeper upstream cross-sections and therefore masked any

evidence of riffles and pools. Records of riffle-pool morphology recommenced when the

rivers discharge had returned to a level similar to that at the beginning of the reach – this

happened to be 75m after the temporary blockage.

19

3.2. Sampling

This dissertation aims to correlate riffle-pool spacing, widths and depths with other stream

variables along a section of the River Lin. These other stream variables are channel width,

bankfull width, riffle-pool length, average depth, average near-bed velocity and cross-

sectional area. Channel and bankfull width were measured at the midpoint of each riffle or

pool as this usually represented the widest section of the channel and therefore best

reflected existing channel-bank interactions. Cross-sectional area was measured at the same

point as channel and bankfull widths; it was calculated by splitting the channel into ten

vertical subsections of equal width and recording the depth and subsequent area of each

respective subsection to best estimate the cross-sectional area (Lu et al., 2006). Due to the

small channel reach used in the study, channel width was not expected to vary significantly

so the same number of vertical subsections was used at both riffles and pools throughout

the reach. Velocity readings were recorded using a current meter at five points along the

Figure 6: A detailed geological map of the land surrounding the study site in the centre. Areas in red indicate resistant igneous rock and pink shows areas of softer mudstone. Adapted from BGS (2002).

20

same channel transect as width and cross-section measurements. As near-bed velocities are

responsible for the maintenance of riffles and pools, they were measured instead of surface

velocities. Riffle-pool spacing values were taken by measuring the straight-line distance

between the midpoints of two successive riffles or pools. Discharge values during fieldwork

were assumed to represent mean annual flow as no significant rainfall had occurred prior to

data collection. Basic riffle-pool characteristics which are established in the literature

including velocity, depth, length and cross-sectional area profiles are investigated which

ultimately allows an evaluation of the appropriateness and suitability of the methodology to

the aims of the study.

3.3. Riffle-Pool Criteria

Identifying pools and riffles was a major concern in this study especially when considering

the relative size of the river which inevitably produced small scale morphology. When

attempting to locate pools, a minimum width criterion was used whereby pools were

distinguished from smaller scour holes when they exceeded at least one-half the channel

width (Smith and Buffington, 1993). In addition to this, deep, calm sections of flow verified

their location (Leopold et al., 1964). Riffles were identified by shallow, turbulent sections of

the channel and were used to measure riffle length. Although similar studies have used

complex criteria for riffle-pool identification including ‘zero-crossing’ and ‘spectral analysis’

(Yang, 1971; Richards, 1976), the limitations of the study required simple differentiation

methods weighted towards surface turbulence.

Forced-pools were identified as being those formed by the presence of LWD, channel

boulders, bedrock outcrops and or channel constrictions that significantly influenced flow

21

dynamics. Whilst the presence of potential pool-forming obstructions was high in the study

reach, they had to produce a significant difference in depth to be identified as a forced-pool.

3.4. Data Analysis

Recorded values for riffle-pool spacing as well as their associated widths, depths and lengths

were analysed using a scatter graph, with associated r2 values, against several channel

variables to show the type and strength of correlation. Riffle-pool spacing, in units of

channel and bankfull widths, were incorporated into frequency distributions graphs to

assess the variation in the data and its tendency to generate rhythmic spacing in the order of

5-7 widths – relative variation was tested with standard deviation (SD) values assuming a

normal distribution. Separate figures were used to illustrate both the correlation and

variation of forced-pool spacing to test hypotheses made by Thompson (2001).

All channel variables (RP spacing (m), RP spacing (CW), RP spacing (BW), channel width,

bankfull width, length, average velocity, cross-sectional area and average depth) for riffles

and pools were subject to a two-tailed t-test. In all cases the null hypothesis was that there

was no difference in the mean values (h0=0) between riffles and pools. The results were

interpreted such that no significance difference was found when p>0.05 and a significant

difference when p<0.05.

22

4. Results

4.1. Riffle-Pool Spacing and Width Analysis

4.1.1. Channel Width

Data concerning riffle-riffle and pool-pool spacing shows a distinct positive correlation in

relation to channel width (Figure 7a). Subsequent analysis produces an r2 value of 0.219

indicating a weak but significant relationship that channel width increases with riffle-pool

spacing. Figure 7a shows evidence that riffles tend toward a stronger correlation than pools

which display more variation in spacing. This variation is mostly due to a few points which

strayed from the line of regression and may indicate anomalous values as they significantly

reduce the correlation coefficient. There is a high density of both riffles and pools located

around 3m in channel width which represents a large proportion of the data set. This cluster

contains the smallest channel widths with larger values found to be more sparsely

distributed. Despite variances in the distribution of riffles and pools, t-test analysis shows no

significant difference in their mean average values. It is worth noting that in one instance the

spacing and channel width for one riffle and one pool were near-identical and are therefore

indistinguishable on Figure 7a; as a result these data points has been encircled on the figure.

4.1.2. Bankfull Width

Bankfull width (Figure 7b) ranges from approximately 3-6m and displays similar spacing

variation in to Figure 7a – this is confirmed by t-test analysis which confirms that the bankfull

width at riffles and pools is statistically identical. These similarities have produced a similar

but slightly less well correlated data set (r2 = 0.191) for bankfull width. These regression

23

analyses show that channel width at low flow has a marginally stronger influence over riffle-

pool spacing than bankfull width however neither demonstrate a dominant control over

observed morphology. Differences in the correlation of riffles and pools seen in Figure 7a are

less evident in Figure 7b as riffles exhibit a more scattered distribution. Also absent from

Figure 7b is evidence of clustering as both riffles and pools show an even distribution in both

spacing and bankfull width. The presence of the two aforementioned pools iare less

noticeable as anomalies in Figure 7b but do have a prominent impact on the strength of

correlation.

4.1.3. Forced Pools

The presence of six forced pools in the study site is shown to produce no obvious correlation

between their spacing and channel width (Figure 7c) which explains just 5% of the data. The

spacing of forced-pools as a result of bankfull width shows a stronger relationship (r2=0.260)

than channel width (r2=0.053) although the small data set significantly increases the

sensitivity of the correlation coefficient. This is particularly prominent in Figure 7d where the

smallest bankfull width value is largely responsible for the observed correlation. Despite a

poor correlation, forced-pools were found to produce similar spacings (10-30m) and

channel/bankfull widths (2-5m and 3-6m respectively) to those of free-formed units but with

significant distribution.

24

R² = 0.053

0

10

20

30

0 1 2 3 4 5Forc

ed

Po

ol S

pac

ing

(m)

Channel Width (m)

(c)

R² = 0.260

0

10

20

30

0 1 2 3 4 5 6Forc

ed

Po

ol S

pac

ing

(m)

Bankfull Width (m)

(d)

Pools Riffles

Figure 7. Relationships between riffle-pool spacing and (a) channel width (b) bankfull width as well as forced-pool spacing with (c) channel width and (d) bankfull width. The circled point in Figure 7a indicates the location of two near-identical values that may not be visible on the graph.

R² = 0.219

0

10

20

30

0 1 2 3 4 5 6R

P S

pac

ing

(m)

Channel Width (m)

(a)

R² = 0.191

0

10

20

30

0 1 2 3 4 5 6 7

RP

Sp

acin

g (m

)

Bankfull Width (m)

(b)

25

4.2. Frequency Distribution Graphs

4.2.1. Riffles and Free-Formed Pools

Frequency distribution graphs were produced to show the spread of spacing data so an

assessment of whether riffle-pool spacing approximated 5-7 channel or bankfull widths

could be made. Figures 8a and 8b are both dominated by channel and bankfull width

spacings typically in the range of 4-7 and 3-6 widths respectively. The range of values is

greater for channel width (SD = 1.54) than bankfull width (SD = 1.13) however both were

found to contain low frequencies of particularly larger spacings. As shown in Table 1, riffles

average 5.7CW and 4.8BW whilst pools approximate 6.3CW and 4.9BW but with

considerable variation. In relation to channel width, the variation appears to be random in

nature as the data does not produce a normal distribution but is instead weighted towards

smaller width spacings. Bankfull width does however generate a more normally distributed

data set peaking in frequency at 5.0-5.9m but is still shown to be dominated by smaller

values.

4.2.2. Forced-Pools

Forced-pool spacing was analysed to see if they developed rhythmic spacing of similar

magnitude to free-formed riffles and pools despite their random distribution along the study

reach. As with Figure 7c and 7d, the small number of forced-pools in the study heavily

influences the relationships of Figures 8c and 8d however some patterns have emerged.

Figure 8c displays an extremely large range of values (SD = 2.42) in the order of several

channel widths within which the remaining forced-pools are sporadically distributed. Whilst

4.0-4.9 and 8.0-8.9 represent the modal values for channel width, there is no indication that

26

the data is conforming to a normal distribution. In addition, although forced-pools average

6CW, Figure 8c shows that this does not represent the true nature of the data. Bankfull

width spacing (Figure 8d) shows a more normally distributed data set averaging between

4.0-4.9 BW (SD = 1.49) but as with Figures 7c and 7d, it is highly sensitive due to the small

data set. Similar to Figure 8c, bankfull width spacing displays a large, albeit smaller, range of

data from 2-7 BW. The normal distribution focuses on a spacing of 4.0-4.9 and is found to

average 4.5BW which is smaller than that of free-formed pools (5.3BW).

27

0123456789

3.0 - 3.9 4.0 - 4.9 5.0 -5.9 6.0 - 6.9 7.0 - 7.9

Fre

qu

en

cy

RP Bankfull Width Spacing

0

1

2

3

4

5

6

7

4.0 - 4.9 5.0 - 5.9 6.0 - 6.9 7.0 - 7.9 8.0 - 8.9 9.0 - 9.9Fr

eq

ue

ncy

RP Channel Width Spacing

0

1

2

3

2.0 - 2.9 3.0 - 3.9 4.0 - 4.9 5.0 - 5.9 6.0 - 6.9 7.0 - 7.9 8.0 - 8.9

Fre

qu

en

cy

Forced Pool Channel Width Spacing

0

1

2

3

2.0 - 2.9 3.0 - 3.9 4.0 - 4.9 5.0 - 5.9 6.0 - 6.9

Fre

qu

en

cy

Forced Pool Bankfull Width Spacing

Figure 8. Frequency distribution of (a) RP channel width spacing, (b) RP bankfull width spacing, (c) forced pool channel width spacing and (d) forced pool bankfull width spacing to one decimal place. These are included to illustrate the range and spread of the data. Standard deviation values (SD) are also included to quantitatively show variation.

(a)

(b)

(c)

(d)

SD = 1.54

SD = 1.13

SD = 2.42

SD = 1.49

28

4.3. Riffle-Pool Spacing to Other Channel Variables

Other channel variables have been correlated to riffle-pool spacing to assess if they

produced relationships which conform to the general understanding of riffle-pool

characteristics. Such relationships can be used to validate the adopted methodology. They

were also included to test the strength and significance of their influence over observed

riffle-pool morphology.

4.3.1. Length

Riffle-pool length is shown to develop a moderately strong relationship with spacing

(r2=0.332) whereby spacing increases with unit length (Figure 9a). Whilst the range of length

and spacing values for riffles and pools is similar, the data shows that riffles concentrate at

relatively small lengths, typically 4-7m with the exception of three riffles which contain

significantly larger values. In comparison, pools show a more scattered variation in lengths

and do not tend to cluster; they instead represent both the maximum and minimum values

in the data set at 4.2 and 10.4m respectively. Although Figure 9a infers that pools produce

longer units than riffles, a t-test analysis found no significance in the variation of mean

values and therefore shows that riffles and pools are of the same average length.

4.3.2. Velocity

The relationship between spacing and average near-bed velocity (Figure 9b) is unique

throughout all figures in that it shows a negative correlation with a significantly strong

coefficient (r2=0.360). The data indicates that riffles typically contain faster average

velocities in the range of 0.3-0.6ms¯¹ compared to pools which vary predominantly from

0.1-0.3ms¯¹ – a t-test found a significance in the mean average values and confirmed the

29

distinction between riffles and pools. As seen with areas of clustering in Figure 9a, the

potential separation of riffles and pools in terms of average near-bed velocity is also

accompanied with exceptional values. In the case of velocity there are three pools and one

riffle which stand out as having significantly faster and slower velocities respectively. These

three pools form part of a small cluster of values located at the highest values on the

velocity scale along with four riffles and signify a disconnection from the main body of data.

There is also another clear exceptional point (0.15ms¯¹, 15m). This unit was a forced-pool

found prior to a dense log jam which had a localised damming effect producing a maximum

depth of 0.22m which was amongst the deepest pools in the study. As a result of the jam, re-

circulating eddies were present and were responsible for the low velocity readings.

4.3.3. Cross-Sectional Area

Cross-sectional area (Figure 9c) was found to produce little correlation with riffle-pool

spacing (r2=0.114) as illustrated by the large degree of scattering. Despite a minor riffle-pool

convergence zone located at approximately 0.4m2, pools appear to be confined to larger

cross-sectional areas than riffles averaging 0.48m2 and 0.33m2 respectively (Table 1.).

Distinctions between riffles and pools are expressed in the t-test analysis as being significant

regardless of the few exceptional points. This validates the interpretations of Figure 9c that

pools exhibit a larger cross-sectional area than riffles in the study although with large

deviation and little evidence of correlation to spacing.

30

R² = 0.332

0

10

20

30

0 2 4 6 8 10 12

RP

Sp

acin

g (m

)

RP Length (m)

(a)

Figure 9. Relationships between RP spacing and (a) length, (b) average velocity and (c) cross-sectional area with r2 values.

R² = 0.114

0

10

20

30

0 0.2 0.4 0.6 0.8

RP

Sp

acin

g (m

)

RP Cross-Sectional Area (m²)

Pools Riffles

(c)

R² = 0.360

0

10

20

30

0.0 0.1 0.2 0.3 0.4 0.5 0.6

RP

Sp

acin

g (m

)

Average Velocity (m/s)

(b)

31

4.4. Riffle-Pool Length Analysis

Due to the high density of resistant sediment in and around the channel, analysis was

undertaken into the relationships between cross-sectional morphology and riffle-pool

length. By correlating depth and channel/bankfull width to unit length, the consequential

influence of the channel bed and bank on these dimensions can be analysed.

4.4.1. Channel and Bankfull Width to Length

Channel width is found to have a weak correlation to riffle-pool length (Figure 10a) with an

r2 value of 0.135. Riffles and pools display large distribution in length and both are found to

occupy the minimum and maximum values (4.2-10.6m respectively). Individually, neither

riffles nor pools show a clear tendency towards forming a clear relationship and appears to

be the result of a small cluster of points at 4m in length. The majority of riffle lengths are

found towards the lower end of the scale typically between 4m and 6m whilst pools depict

an even distribution of length values.

Riffle-pool length was found to correlate strongest with bankfull width (Figure 10b) which

explained 35% of the observed variation. Whilst the small range of bankfull widths, similar to

those in Figure 10a, is still present, riffle-pool lengths display significantly less variation from

the line of regression than channel width. As mentioned in relation to Figure 10a, a region of

clustering representing the shortest length values in also apparent in Figure 10b. These

values vary in bankfull width but maintain near identical lengths and may signify a

disconnection or partial separation from the main data set.

32

4.4.2. Average Depth to Length

The average depth of riffles and pools in the study show no significant correlation to length

(Figures 10c) as illustrated by an r2 value of 0.007. The regression line shows no evidence of

a positive or negative correlation and appears to show several minor clusters which form

part of a larger central cluster of values encompassing all riffles and pools. Whilst no

correlation is found, there is a distinct discrepancy with pools typically found to be deeper

when compared to riffles – the boundary separating the two is well illustrated by the

regression line. This conclusion is supported by the subsequent t-test which shows a

definitive difference between the mean depth values of riffles and pools (p-value=0.0001).

Table 1. Variances among mean riffle-pool characteristics for the entire data set with accompanying t-test results and associated p-values.

Variable Riffle Mean Pool Mean Result of t-test

Riffle-Pool Spacing (m) 20.2 19.7 Not significant at p=0.815

Riffle-Pool Spacing (CW) 5.70 6.30 Not significant at p=0.382

Riffle-Pool Spacing (BW) 4.80 4.90 Not significant at p=0.740

Channel Width (m) 3.70 3.30 Not significant at p=0.277

Bankfull Width (m) 4.40 4.10 Not significant at p=0.439

Length (m) 6.60 7.40 Not significant at p=0.405

Average Velocity (ms¯¹) 0.42 0.28 Significant at p=0.006

Cross-Sectional Area (m²) 0.33 0.48 Significant at p=0.006

Average Depth (m) 0.11 0.18 Significant at p=0.0001

33

Pools Riffles

Figure 10. Relationship of riffle-pool (RP) length with (a) channel width, (b) bankfull width and (c) average depth with accompanying correlation coefficients.

R² = 0.135

0

2

4

6

0 2 4 6 8 10 12

RP

Ch

ann

el W

idth

(m

)

RP Length (m)

(a)

R² = 0.007

0.0

0.1

0.2

0.3

0 2 4 6 8 10 12RP

Ave

rage

De

pth

(m

)

RP Length (m)

(c)

R² = 0.352

0

2

4

6

8

0 2 4 6 8 10 12

RP

Ban

kfu

ll W

idth

(m

)

RP Length (m)

(b)

34

4.5. Omission of Data Points

Certain data points were excluded from specific figures as they represented anomalous

values which in some instances significantly influenced the correlation coefficients and were

seen to be potentially masking the true relationship of the data. One such data point was

removed from Figures 7a and 7b as it contained an extremely small channel and bankfull

width measurement (CW=1.5m, BW=2.0m). This value was a pool located at a permanent

channel bar which bisected the channel flow and significantly reduced the channel and

bankfull width. This pool was later categorised as a forced-pool and was therefore included

in all the relevant figures that exclusively concerned forced-pools.

Also omitted form specific figures was a pool containing a large unit length (14.7m) that was

considerably larger than any values within the data set. This pool was not found to be

induced by a channel obstruction and its large value may be due to difficulties in locating

riffles and pools in this area of the study reach. The pool itself contained channel variables

which were comparable in magnitude to the rest of the data so were included in other

figures.

Whilst the majority of the study site was subject to natural flow and sediment interactions

there were two small areas of channel management present. These managed sections

represented two riffles and one pool and consisted of reinforced banks on the outside of a

meander bend and at the location of a bridge. Due to their restrictions on channel migration

and spacing, these points were excluded from the figures however their implications for

future river management and restoration schemes are considered.

35

5. Discussion

The results of riffle-pool spacing in relation to channel variables are interpreted to assess the

magnitude and respective influences of the physical processes behind observed

relationships. From this, a range of dominant discharges are analysed for their suitability in

explaining riffle-pool spacing in the River Lin. In addition, the mechanisms behind the

occurrence of forced-pools are analysed as well as the examination of criteria for pool-

forming obstructions and channel constrictions. As minor channel restoration schemes were

active in the study reach, their interaction with the riffle-pool sequence is investigated with

possible implications for further river management. The limitations of the methodology are

also discussed.

5.1. The Significance of Channel and Bankfull Width

Although riffle-pool spacing is repeatedly shown to be the result of widths associated with

bankfull discharge, regression analysis in this study shows a stronger correlation to channel

width associated with low flow and supports findings by Harvey (1975). The large variation

(as reflected by the high SD value) in the data is represented by relatively weak correlations

(CW=0.219, BW=0.191) but shows no discrimination between riffles and pools with a p-value

of 0.382 for channel width and 0.740 for bankfull width. Although extreme anomalous data

points were removed from all figures except those specifically relating to forced-pools, the

presence of further anomalies, less evident in the figures, may be masking the true

relationships of spacing to channel and bankfull width - most notably in Figures 7a and 7b.

The removal of certain points reveals a significantly stronger correlation to spacing however

36

there was no rationales for this as such units were not found to contain distinguished

features like those that were omitted from the results (Section 4.5.)

The similarity in the variation and correlation of channel and bankfull width (Figures 7a and

7b) can be attributed to their comparable magnitudes. With the inclusion of all data points,

channel and bankfull width averaged 3.7m and 4.4m and ranged from 1.5-5.4m and 2.0-

6.0m respectively. Bankfull width was found to be only 23% larger than observed channel

width and therefore generated data that was difficult to differentiate between. The

analogous statistics is most likely the result of cohesive channel beds and banks which have,

to a certain extent, limited cross-sectional morphology (Andrews, 1984; Millar and Quick,

1993). Riparian vegetation ranging from grasses and brambles to substantial trees was

present throughout the entire study reach. In many instances, tree roots were visible in the

banks and were also seen to occupy the neck of meander bends. It is well known that the

presence of vegetation around channels provides a bond for bank sediment. Research from

Hey and Thorne (1986) estimated that densely vegetated banks limited the lateral expansion

of alluvial rivers by up to a half. Further research by Millar and Quick (1993) found values of

0.6-1.4 times that of sparsely vegetated banks using the same data. This demonstrates that

although the exact influence of vegetation on bank stability is not fully understood, it is

reasonable to assume a limiting effect is produced.

In the case of the River Lin, the comparable data between channel and bankfull width

suggests that the surrounding geology may have an influence. Whilst the majority of the

banks were comprised of soft, albeit compact sediment, apparently random distributions of

larger granite sediment were also present throughout the reach. In conjunction with

vegetation, it was evident that the banks were particularly resistant especially in relation to

37

the low stream power and shear stress generated by the River Lin. Also evident, most

notably at riffles, were well-cemented channel beds comprised of larger sized sediment. As

such sediment would become mobile only at discharges equal to or greater than bankfull,

nearly all available energy goes into overcoming the resistant channel bed and banks. Any

excess energy will be spent in lateral and vertical erosion, and as available energy is severely

restricted in the River Lin, the rate of channel widening and deepening will be low at the

discharges measured during fieldwork – at low to moderate flow, any channel incision is

likely to occur in pools where very fine sand and silt was present. The data in this study

shows that both average channel and bankfull width is greater at riffles (3.7 and 4.4m

respectively) than pools (3.3 and 4.1m respectively). As reported by Richards (1976), this

implies that excess energy is available at riffles and is the result of flow diversion around

shallow central bars which were apparent in the River Lin. Although this violates the

minimum variance hypothesis, proposed by Langbein (1964), rates of channel widening and

deepening were not measured so it is unclear as to whether the sequences in the River Lin

are maintained by erosion at riffles or pools.

As a result of riparian vegetation and resistant channel beds and banks, the River Lin is

unable to continuously alter its morphology in relation to discharge patterns and therefore

produces relatively narrow cross-sections with similarly small channel and bankfull widths.

The seemingly random concentrations of large granite sediment in the banks may produce

riffle-pool morphology that respond to location specific channel and bank characteristics

(Milan et al., 2001; Thompson and Wohl, 2009). Although unclear in the results, the idea that

riffles and pools operate individually (Thompson, 2001) may explain the large variation seen

in channel and bankfull width and extending to their relationship to spacing. It is worth

noting that although the density of granite rocks varied along the study reach, such sediment

38

was present at all riffles and pools and was of sufficient density to have some influence over

channel morphology.

5.2. An Explanation of Riffle-Pool Spacing

Riffle-pool spacing was shown to produce values in the range of 5-7 channel widths (5.7CW

for pools and 6.3CW for riffles) with bankfull width found to be marginally below this,

averaging 4.8BW for riffles and 4.9BW for pools. The incidence of particularly large spacings,

although cited as potential anomalies, is also found in other studies where spacings as high

as 20 widths are observed (Milne, 1982). Similar research from Lofthouse and Robert (2008)

suggest that this may be an indicator that the riffle-pool sequence is in a state of adjustment

(representing a quasi-equilibrium state) whereby the actively migrating channel increases

riffle-pool spacing beyond a threshold value – this value is dependent upon sediment calibre

(Foster, 1998) but is typically expected to occur beyond seven widths. In the case of the

River Lin, an additional riffle may be forming upstream of the sequence with a particularly

large spacing value in order to retain a spacing value of 5-7 widths – if such a proto-riffle was

present, its morphology would have been undistinguishable and would have gone

unrecognised during fieldwork.

Although there was no direct evidence for this in the study, the presence of high curvature

meanders, particularly in the upstream reach, would have had an effect on sequence length

(Milne, 1982; Hooke and Harvey, 1983) and the rate of sediment transport (Lofthouse and

Robert, 2008). The modification of the sequence in actively migrating sections could explain

the observed location of riffles and pools. Although pools should form on meander bends

and riffles on inflections (Keller and Melhorn, 1978), the River Lin rarely generated this

39

simplistic plan-form. Meander curvature varied significantly throughout the study reach and

consequently, both riffles and pools were found to occupy bends and inflections

respectively. This provides strong evidence that the formation and adjustment of riffle-pool

sequences in the River Lin is in part, the result of meander migration (Hudson, 2002).

5.3. An Assessment of Dominant Discharge

Such low spacing-width correlations in Figures 7a and 7b may represent a minor causal link

but more importantly implies that the discharges associated with these widths do not have

an overriding influence over riffle-pool spacing and are therefore not the dominant

discharges. Although a large body of the literature assumes bankfull discharge (Harrison and

Keller, 2007) it can be assumed that the dominant discharge for the River Lin lies somewhere

in the range of:

(a) discharges between those associated with channel width at low flow and bankfull width

(b) discharges equal to and above flood stage

If option (a) is true it would suggest that despite the large stream power and erosive

potential associated with high discharges, their long return period is such that it is not

sufficient to explain observed riffle-pool morphology. In comparison, it is apparent that low

discharges, similar in magnitude to those measured in this study, occur frequently but do not

obtain the required stream power to continuously alter the channel. Although beyond the

scope of the study, the dominant discharge may be of similar magnitude to that of Harvey’s

(1975) study where those exceeded 40% of the time was found to produce widths with the

strongest correlation to spacing.

40

The presence of resistant granite sediment in the channel sourced from within the

catchment provides support for option (b) and the need for a discharge considerably larger

than that during fieldwork to be able to maintain the riffle-pool system. The discussion of

dominant discharge and the agreement on bankfull width relates strongly to studies based

on rivers of more erodible boundaries. As the River Lin operates within more resistant

settings, it can be determined that only through very large discharges, approaching flood

stage (Emmett and Wolman, 2001), can the river obtain the required stream power to

readily erode and transport sediment through the sequence (Chapuis et al., 2015) and

therefore produce stronger correlations to riffle-pool spacing. This deduction is supported by

stronger correlations reported between riffle-pool length and bankfull width (r2=0.352)

rather than channel width (r2=0.135). This increases the likelihood that riffle-pool

morphology is the response of especially large discharge patterns.

5.4. Forced-Pool Spacing

A large proportion of channel obstructions in the study produced a forced-pool however

results in Figures 7c and 7d show little sign of rhythmic spacing relating to either the direct

distance between riffle-pool units or in relation to channel and bankfull width. These

findings contradict those of Thompson (2001) where evidence for rhythmic spacing of forced

pools was found. Although the variation of spacing in terms of bankfull width is less

noticeable (Figure 7d) as represented by a smaller SD value, it is still unclear from the study

as to how channel obstructions can produce rhythmic spacing if they are unsystematically

distributed. In Thompson’s (2001) paper, the author states that the spacing of a series of

riffle-pool sequences is the collective result of individual units responding to random channel

obstructions rather than reach-scale conditions. Research suggests that at each of these

41

obstructions where re-circulating eddies are present, a positive feedback mechanism formed

whereby pool length was adjusted in accordance with discharge (Schmidt et al., 1993;

Thompson et al., 1999). This concept implies a minimum distance between pools which,

coupled with riffle length, is supposedly able to create rhythmic spacing in the order of 5-7

bankfull channel widths. These findings were taken from rivers dominated by channel

obstructions; as forced pools represented only 46% of total pools in the River Lin it seems

plausible that channels must contain a minimum density of pool forming obstacles (which

the River Lin did not achieve) to initiate rhythmic spacing (Thompson, 2012).

Despite the limited number of forced-pools, the mechanisms involved in their formation and

subsequent maintenance are identical to those reported in the literature. Plate 1 illustrates

the effect that a channel constriction, present in the River Lin, has on re-routing flow. In this

instance, the constriction concentrates flow which subsequently diverges immediately after.

As a result, recirculating eddies form (encircled in Plate 1) which cause vertical scour into the

channel bed and hence form a forced-pool. The six forced-pools in this study were formed

from a variety of channel obstructions including LWD and channel boulders as well as the

constriction shown in Plate 1. Whilst the majority of pool-forming features did produce a

forced-pool, there were several instances of LWD which did not alter channel morphology

sufficiently to be categorised as a forced-pool; this is demonstrated in Plate 2 where the

relative size of the LWD to the channel does not result in a forced-pool.

Although unnoticed during fieldwork, upon further examination of Plate 2 there is evidence

that the LWD has caused bank erosion – this can be seen by the sudden widening of the

channel immediately after the obstruction. The differential rates of vertical and lateral

erosion relates strongly to the characteristics of both the channel bed and banks. Whilst the

42

banks of the River Lin contained a high density of resistant granite pebbles and vegetation,

the majority consisted of easily erodible sediment. With the exception of pools, the channel

bed throughout the reach contained highly compacted, coarse sediment and consequently is

significantly more resistant than the banks (Karim and Holly, 1986; Willetts et al., 1987;

Akbari et al., 1997). The high width:depth ratio for the River Lin is indicative of the channels

preferential tendency to erode laterally into the banks rather than vertically into the bed and

explains the active meander migration which was evident in the field.

The ability for LWD to create a forced-pool is known to be related to its orientation, size

relative to the channel and height above the channel (Bilby and Ward, 1989; Richmond,

1994). In addition, observations made in this study suggest that channel and bank

sedimentology has a significant role in both the formation and morphology of forced-pools.

It follows that the specific dimensions of the forced-pool is based on individual

circumstances rather than reach scale conditions. Although unclear in this study, the size of

the obstruction relative to the channel may be of less importance if the surrounding bed and

bank is particularly erodible. Such interpretations would however be more suited to similar

channels which contained a greater number of forced-pools and pool-forming obstructions.

Plate 1: An example of a channel constriction caused by a vegetated outcrop producing a forced-pool. A recirculating eddy is present, although not evident in the plate, at the far side of the channel encircled in red.

43

One key aspect that is evidently missing from the literature is a definitive model that can

accurately predict the lifespan of channel obstructions. It is clear that in the instance of

bedrock outcrops, their persistence will last a significant period of time, however it is less

obvious for the presence of boulders and log jams. McBroom et al. (2014) estimated a 12-14

year timespan for LWD, however this concerned a much larger river than the River Lin and

therefore cannot be directly compared. The ability for a river to remove large obstacles from

the channel is reflected in its discharge and associated stream power and shear stress

patterns. Certain boulders or LWD that are capable of creating forced-pools may be quickly

removed by particularly large discharges depending upon the characteristics of the obstacle

(Buffington et al., 2002) and the discharge patterns of the river. As pools gradually alter their

morphology it follows that forced-pools can only form rhythmic spacing if they can persist

for a reasonable length of time. Whilst the River Lin contains relatively small cross-sections,

Plate 2: An example of LWD which did not produce a forced-pool. Such obstructions were present throughout the study reach. The plate shows evidence towards bank erosion caused by the LWD.

44

the observed boulders and LWD were also small in nature (typically less than 1m in length

for LWD and 0.5m for boulders). It can therefore be assumed that these would be

transported downstream (albeit not far) during the next bankfull event or steadily removed

over time. If the occurrence and persistence of channel obstructions are temporally periodic

they would be unable to develop rhythmic spacing and would therefore explain the lack of

correlation seen in this study.

One particular forced-pool stood out in the data set as it was unrelated to the forms of

channel obstructions previously mentioned. Plate 3 shows the channel bar which bisected

the flow into two separate channels resulting in the formation of a forced-pool in the right

sub-channel as seen in Plate 3. Although unrecorded in the data, a pool is likely to have

formed, or is in the process of forming in the other sub-channel however its morphology,

most notably its depth, did not meet the required pool criteria used in this study. Although

this bar does not represent a braided system, observations from (Leopold and Wolman,

1957) suggest that the presence of channel islands may reduce the spacing of riffles and

pools as the channel width is significantly reduced. This theory did not relate to the channel

bar in Plate 3 as spacing was of similar magnitude to free-formed units as well as the

associated length, depth and velocity measurements. It can be hypothesised that the

reduction in width concentrates the flow into a high-velocity jet which subsequently erodes

vertically into the bed at high discharges and forms a pool (Schmidt et al., 1993; Thompson

et al., 1996). This was the only channel bar in the reach and therefore more features would

be needed to elaborate on this hypothesis. In the current scenario, it is unclear if the

constriction had actually created a forced-pool or if it represented a free-formed pool but

with a significantly reduced width – the morphological influence of permanent and semi-

permanent channel bars in small rivers is notably lacking in the literature.

45

5.5. Effects of Human Intervention

Although largely unaffected, the River Lin contained two locations of minor channel

management consisting of bank reinforcement at a meander and a small bridge on an

inflection. Riffles and pools occupied these sections of the study reach and recorded

measurements showed that these man-made features had a significant influence over their

morphology. The two riffles and one pool affected by human intervention produced erratic

spacings (14.9m, 29.9m; 11.7m respectively) which contained both the minimum and

maximum values in the data set. Whilst the riffle located at the bridge had a marginally

reduced width, all other channel and flow variables were not found to deviate from the main

body of data. These can be attributed to the fact that the reinforced banks are similar to the

natural banks in terms of their resistance and therefore produce similar width, length,

Plate 3: A permanent and well-vegetated bar bisecting the channel into two sub-channels within which a forced-pool is formed. Notice the location of an upstream and downstream riffle immediately before and after the channel bar.

46

velocity, cross-sectional area and average depth values at low flow. In relation to the riffle

with an extremely large spacing, this may be reflected in the loss of sediment supply due to

bank protection provided by the bridge and bank reinforcement directly upstream. Together

with the disruption to natural flow patterns, a cumulative effect may be produced whereby a

lack of large sediment and an inability to transport it downstream may result in a large

spacing to the next riffle. The subsequent inter-riffle will form when sufficient coarse

sediment is provided from scour in upstream pools or from local bank collapse (Hassan and

Woodsmith, 2004) however the specific effects of river management and sediment mobility

in general is beyond the scope of the study. It is clear however that the presence of

boulders and LWD frequently generate forced-pools and therefore has some value in

restoration schemes (Gurnell et al., 1995).

5.6. Limitations of the Methodology and Data

Despite the rigorous methodology used in this study, there are several areas where recorded

data may not represent the true nature of the fluvial processes in operation in the River Lin.

The measurement of channel width at each riffle and pool was, as previously stated, chosen

to be the midpoint of each unit. This approach was used as it is assumed to be the point of

maximum width and therefore signifies aspects of channel erodibility and flow dynamics.

However, it is not clear if this method has justification in all forms of rivers especially those

where channel obstructions are present. It was clear during the field study that some

recorded channel widths did not represent the widest section of that particular riffle or pool;

in some units maximum width was present at both the head and tail. Although purely

theoretical, issues related to channel width may be responsible for producing data with no

47

obvious correlation (see all Figures) however this view has no backing without further

investigation.

As referred to previously, identifying the exact start and end point of each riffle and pool is

challenging and it has been shown that many similar studies poorly define their criteria

(Carling and Orr, 2000). This study located units by observing surface velocity and turbulence

and further identification from bed sediment characteristics was used. The limitations of this

study induce a small percentage error based on inaccurate measurements of unit length and

hence midpoint channel widths which have an incalculable effect on the results and

regression. The results and conclusions taken from this study do however show support that

the methods used in taking measurements were sufficiently accurate to display riffle-pool

relationships.

48

6. Conclusion

This study aimed to analyse the spacing of riffles and pools to channel and bankfull width as

well as several other channel variables, from which a dominant discharge could be

estimated. The data shows that riffle-pool spacing is marginally more correlated to channel

width than bankfull width with r2 values of 0.219 and 0.191 respectively. Whilst channel

width explains more of the observed data, the relatively poor correlation and similarities to

bankfull width implies that neither acquires a dominant influence over the observed

morphology. The River Lin represented a small channel (Q=0.13 - assumed to approximate

mean annual flow) and was found to contain a high density of riparian vegetation and large

resistant sediment (mostly granite and diorite composites) in the banks and bed. As a result,

this study concludes that only at discharges approximating flood stage can the channel

obtain the required shear stress to readily erode and transport sediment and hence maintain

the riffle-pool sequence. Although the dense literature refer to bankfull discharge as being

dominant (Sear, 1996), and regime flow to a lesser extent (Harvey, 1975), there is limited

data concerning riffle-pool spacing in channels containing both low shear stress values and

high concentrations of resistant sediment. This study therefore contributes to the field and

illustrates the need for further research especially towards low-order streams as there

appears to be a bias towards large and well-developed rivers.

Rhythmic spacing in the order of 5-7 widths, first observed by Leopold et al. (1964) and

widely accepted in the literature, is found to be present in the River Lin but with significant

variation: 4-10CW and 3-8BW. This variation is hypothesised to reflect the resistant channel

and an inability to readily erode the bed and banks. As proposed by Thompson (2001), the

data suggests that riffles and pools may be operating on an individual scale rather than

49

collective reach-scale inputs. There is also evidence that additional riffles and pools are

developing in response to a lengthening of the sequence due to meander migration. This

was however based on the rare instances of significantly large spacing values rather than

direct observation and cannot therefore be fully justified for the River Lin.

A further aim of this study was to assess the morphology of six forced-pools in relation to

free-formed units. Whilst no significant correlation was found, this was attributed to the

small data set and therefore represents a possible extension of this study in order to fully

assess the relationship that exists in the River Lin. A key paper by Thompson (2001)

suggested that the random orientation of pool-forming obstructions in the channel could

produce rhythmic spacing in forced-pools however this study fails to observer similar

patterns. The small number of such obstructions and constrictions is recognised as the

limiting factor and explains the large and sparse variation in spacing – ranging from 2-9CW

and 2-7BW. This study proposes a minimum density of pool-forming obstructions which are

able to persist for a prolonged time period before rhythmic spacing can be initiated. This

highlights a noticeable gap in the literature as there is no model to predict the lifespan of a

channel obstruction and therefore makes the examination of forced-pool morphology more

difficult.

The inclusion of other channel variables was to assess the basic characteristics of riffles and

pools and the appropriateness of the methodology to the River Lin. Key findings included

faster near-bed velocities at riffles and larger cross-sections and average depths at pools all

of which were found to produce significant p-values from t-tests. The similarity between

variables observed in this study and those reported in the literature confirm that the

50

adopted method for direct measurements and riffle-pool criteria is suitable for the study

reach.

The occurrence of river restoration projects at two specific areas of the reach, containing

two riffles and one pool, were found to have a dramatic effect on their morphology. Whilst

many channel variables including near-bed velocity and width were unchanged, spacing

values represented the full range of data. Although unclear in this study, it is apparent that

the introduction of man-made features has a significant effect on sediment fluxes which

extends to riffle-pool morphology and illustrates the need for restoration schemes to

consider all possible impacts to the channel. Of particular importance is the link between the

location of channel obstructions and forced-pool sequences (Lisle, 1986). As riffles and pools

influence channel migration and provide key ecological habitats, their involvement in future

river management schemes is highly recommended.

51

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