Geomorphological Impacts on Benthic Macroinvertebrates in a Headwater Tributary of the South

Prong West Branch of the Rocky River

David Rogers

Abstract

This paper will discuss the impacts of gully erosion on stream health by specifically looking at benthic macroinvertebrate populations downstream of a gully knickpoint. It will cover the background information, methods, and results of the study conducted through the course of a 6.5-month span in the South Prong West Branch of the Rocky River, a gully stream reach located in Davidson, North Carolina. I used the Leica Flexline TS02plus to conduct three surveys of the gully reach through the course of the study that allowed me to create high-resolution digital maps of the gully. To assess benthic macroinvertebrate populations downstream of the gully knickpoint, I conducted kick-net sampling at monthly intervals in a three-month span. At the lab I sorted and identified kick-net samples and used the Hilsenhoff Biotic Index (HBI) to determine stream tolerance level at each site. My results showed no significant relationship between distance from the gully knickpoint and benthic macroinvertebrate populations, however, the overall HBI of the stream was very high at an average of 7.07 indicating poor stream health. Additionally, gully migration rate was quantified at ~26 m/y and soil lost rate at ~192 m3/y or ~430 ton/y, indicating rapid gully erosion taking place.

Introduction

This study examined how headcut gully migration is affecting benthic macroinvertebrate

communities by assessing invertebrate species abundance downstream of the migrating headcut. A

headcut, also commonly referred to as a knickpoint, is a point where there is an abrupt change in base

level. Gullies are important aspects of the environment to study because they are often caused by land use

change and can significantly alter the environment around them. While the two topics of gully erosion

and benthic macroinvertebrates have been researched extensively, they have not been studied in

conjunction with each other and the result is a poorer understanding of impacts on benthic

macroinvertebrates by gully formation. I hypothesize that invertebrate community health will be least

abundant close to the knickpoint and healthier downstream from the knickpoint as the area appears to

have stabilized.

Background

What are gullies and gully erosion?

Gullies are incised channels with steep walls and an eroding headcut. While the primary cause of

erosion is vertical incision and headward erosion, often incision leads to slumping or bank wall collapse

that also significantly contributes to erosion (Istanbulluoglu, Bras, Flores-Cervantes, & Tucker, 2005).

There are three different types of gullies that are typically studied: ephemeral gullies, permanent gullies

and bank gullies. Ephemeral gullies occur episodically throughout the year in response to runoff events

and can be manually filled in or evened out by farmers (Poesen, Nachtergaele, Verstraeten, & Valentin,

2003). Bank gullies are those that form when excess hydraulic flow occurs over a bank, causing headward

migration and erosion (Bennett, Alonso, Prasad, & Römkens, 2000; Poesen et al., 2003; Wells & Alonso,

2009). Permanent gullies are very static in place with little to no headward migration occurring. They are

typically more deeply incised and larger than bank and ephemeral gullies, having the potential to be as

deep as 30 meters (Soil Science Society of America, 2001).

The South Prong West Branch of the Rocky River is interesting in its characteristics because it is

a bank gully with an ephemeral stream. The gully likely responds differently throughout the year due to

the seasonal influences on water flow, but it is a bank gully because despite the absent water flow it still

actively migrates up the channel. The migration occurring when there is no consistent water flow at the

headcut is caused by high intensity flows as a result of intense rainfall events or spates.

Causes of Gully Erosion

There are multiple factors that play a role in gully formation, but the two largest contributors are

land use change and discharge increases associated with rainfall intensity (e.g. Ireland et al., 1939, Poesen

et al., 2003, Valentin et al., 2005). In many areas, cultivation has led to overgrazing and deforestation that

has increased surface runoff and caused gully formation (e.g., Grissinger & Murphey, 1989; Ooswoud

Wijdenes, Poesen, Vandekerchkhove, & Ghesquire, 2000; Poesen, Vandaele, and Van Wesemael, 1996).

Land use change is significant, regardless of type because many different alterations to land have led to

gully activity (Ireland et al., 1939, Poesen et al., 1996). Inappropriate farming techniques, cultivation and

irrigation as well as overgrazing, construction, logging tracks, and urbanization have all shown to trigger

gully formation (Valentin et al., 2005). As lands are altered for cultivation, overgrazing, or any of the

above techniques, runoff increases and therefore gully erosion is intensified (Oostwoud Wijdenes et al.,

2000).

In the Southern Piedmont, this is very relevant. Gullies in the Piedmont have shown to migrate

headwardly at a rapid and devastating rate (Ireland et al., 1939). As examined by Ireland et al. (1939)

many causes of gullying include “improper methods of farming, construction of roads and railroads,

digging ditches, and throwing up of poorly planned terraces.” Due to these usages of land that are

different than the past, many gullies have been formed. Abandoned crop and farmland specifically is a

larger factor in causing gullies to form than terracing or cultivation alone. Unlike actively tilled land

where farmers can even out small channels, on abandoned land those channels get larger and deeper

through time until gullies are formed (Ireland et al., 1939).

Land use change and resulting increases in peak discharge alone are a major factor in gully

formation, but rainfall events trigger the process as well. In fact, a study done by Valentin et al. (2005)

showed that through the historical recreation of gully formation in Europe, spates or heavy rainfall events,

were just as much a factor in gully formation as deforestation or land overuse. Forested land that used to

catch run off is no longer there and this increase in runoff can lead to gully activity as mentioned above.

In streams of ephemeral nature like the South Prong West Branch of the Rocky River, spates are an

integral part of how fast the gully erodes. The more intense the spate is, the higher and faster the run off

and stream discharge, thus resulting in mass bank and headward erosion (Istanbulluoglu et al., 2005).

Gully Erosion Concerns

Gully erosion is of high concern because it can significantly transport sediment and alter

geomorphological landscapes. Studies have shown that bank gullies can erode 0.2-.5 ton ha-1year-1, while

ephemeral gullies can erode a whole magnitude higher, ranging from 2.3-5.8 ton ha-1year-1 (Poesen et al.,

2003). In some areas, over 90% of total sediment lost is due to gully erosion (Rydgren, 1990). The

sediment that is eroded is then deposited further downstream, many times creating alluvial fans due to

excess sediment. Deposited sediment can disrupt water flow and bury invertebrate species. Additionally,

actively migrating gullies expanding headward can cause severe deterioration to surrounding land. In the

South Prong West Branch of the Rocky River, for example, gully erosion has caused numerous trees,

banks, and even a bridge to collapse and continued erosion could effect the adjacent Spring Street,

possibly even destroying it.

Challenges With Modeling Gully Headcut Erosion

Modeling headcut erosion is an extremely difficult process due to the unpredictable nature of

gullies in conjunction with weather. The amount of gully erosion is largely dependent on increased

discharge due to spates, therefore making it hard to accurately predict. Many attempts have been done to

effectively model headcut erosion, but have been strictly limited to controlled, manipulated studies in a

flume (Bennett et al., 2000; Bennett & Alonso, 2003; Wells et al., 2009). These studies have shown that

gully erosion can be very dependent on the soil of the gully and that higher discharge leads to higher

erosion rates (Bennett et al., 2000; Bennett & Alonso, 2003; Wells et al., 2009). However, there is still no

effective way to model headcut erosion in a natural setting. Therefore, through my gully assessment I

hope to provide an alternative and more effective way of modeling and predicting gully headcut erosion.

Benthic Macroinvertebrates as Indicators of Stream Health

Benthic macroinvertebrates are good indicators of how gully erosion is affecting stream health

because they are very sensitive to physical change such as pollution or sediment deposition (e.g.,

Cheimonopoulou, Bobori, Theocharopoulos, & Lazaridou, 2009; Masese, Muchiri, & Raburu, 2009).

Certain macroinvertebrate taxa are much more tolerant than others thus their presence can indicate

excellent or very poor water quality. (Hilsenhoff, 1987). Tolerance levels vary on a 0-10 scale, the higher

the number, the higher the tolerance level of the species (Hilsenhoff, 1987). For example, taxa in the

Plecoptera family have a tolerance range of 0-2, meaning their presence indicates excellent water quality

while the Odonata Coenagrionidae have a tolerance level of 9, therefore only it’s presence in a stream

suggests very high amounts of organic pollution (Hilsenhoff 1987). By measuring benthic

macroinvertebrate abundance, diversity, and HBI, I can get a good grasp at how disturbance is affecting

stream health.

From numerous studies assessing benthic macroinvertebrates populations in streams, it is evident

that urbanization negatively effects stream health (e.g. Gage, Spivak, & Paradise, 2004; Lenat, 1988;

Oberlin, Shannon, & Blinn, 1999; Violin et al., 2011). Urbanization can increase stream flashiness and

pollution runoff thus negatively affecting stream health and macroinvertebrates. Intense rainfall events

and disturbances have also shown to effect macroinvertebrate populations in streams, dropping their

populations significantly for a temporary period (Boulton, Peterson, Grimm, & Fisher, 1992; Fisher &

Gray, 1982)

However, gully migration impact on benthic macroinvertebrates has not been as extensively

researched. In the few studies that have been conducted there are significant findings that indicate benthic

macroinvertebrate populations are higher upstream of the knickpoint than downstream (Wantzen, 2006;

Muehlbauer & Doyle, 2012). Continued research on this subject is addressed through this study.

In addition to benthic macroinvertebrates there are other species that can be used as biological

indicators such as salamanders. Like benthic macroinvertebrate, salamanders are good indicators of

stream water quality because they are easily affected by organic pollution (Willson & Dorcas, 2003). One

study done by Southerland et al. (2004) found significant correlation with stream salamanders and water

quality in Maryland while other studies show similar results (Welsh & Oliver, 1998; Willson & Dorcas,

2003).

Setting

The South Prong West Branch of the Rocky River is located between Davidson Elementary

School and The Pines At Davidson, a local retirement community (Figure 1). Both Davidson Elementary

School and The Pines At Davidson have experienced construction in the past 50 years (History, 2015;

Town History Timeline; 2015). Due to this, land use has changed and urbanization has helped contribute

to the gully formation. The headcut of this stream is migrating rapidly upstream, especially after times of

heavy rain. My project aimed to quantify the erosion occurring through surveys and to see if it is affecting

stream health by sampling for benthic macroinvertebrates at three sites 100, 200, and 300 meters

downstream.

Figure 1. Map of the stream labeled with property ownership. Created by David Kroening, the lead project manager for Charlotte-Mecklenburg Storm Water Services.

Methods

Measuring Gully Erosion

To assess erosion in the gully during the course of my study, I created three high-resolution,

digital topographic maps of the stream reach through the course of a 6.5-month span. The first survey was

taken in the spring on May 4, 2015 while the following two were taken in the fall on September 15, 2015,

and November 16, 2015. I used the Leica FlexLine TS02plus to create these high-resolution, digital maps

to understand the eroding behavior of the stream as well as to quantify migration rate. The Leica FlexLine

TS02plus works by using a laser to measure the distance between the surveying equipment and a

reflector. At each location the reflector is located, the Flexline measures the distance to the nearest

millimeter and converts the data to X, Y, and Z coordinates. Therefore, points are taken along the banks

of the gully, in the floodplain, and in the gully itself. Higher concentrations of points were taken near the

knickpoint and in areas that were under direct influence of erosion. The more concentrated the sampling,

the higher resolution the map is. The map was then created by inputting the data collected by the Leica

FlexLine TS02plus into the program Surfer.

Another method used for assessing gully erosion was inserting six erosion pins in the bank walls.

Erosion pins are 20 cm pieces of rebar that were hammered flush with the bank wall at three different

locations. There were two erosion pins assigned to each location about a meter apart with the exception of

location two, where one erosion pin was a few feet below the other. The locations were chosen based on

their characteristics as drainage spill offs or cut banks. As the banks of the stream erode, the erosion pins

stick out and allow for quantification of erosion rates. This allowed me to measure how much bank wall

has eroded in the 6.5-month span of the study.

Benthic Macroinvertebrates Sample Collection

At each of the three sites, two samples were collected approximately 10-20 meters apart, resulting

in a total of 18 samples through the course of three months. The samples were collected on August 29,

2015, September 28, 2015, and October 20, 2015.

The characteristics of each site were recorded using a stream evaluation sheet customized by Dr.

Chris Paradise. This assessment took note of various physical characteristics that could affect benthic

macroinvertebrate abundance such as flow rate, water characteristics like riffles or color, stream habitat,

and percent vegetation cover. In addition to this, pH, DO, conductivity, alkalinity, and temperature were

also recorded to help determine water quality. Flow rate was measured by using a Global Flow Probe.

The kick-net sampling method was used to collect samples. Kick-net sampling works by placing

a kick-net in the water and disturbing a 0.5m2 area upstream for 30 seconds. Samples were primarily

collected along stream banks or in areas with small riffles and leaf litter. After sampling, the kick-nets

were emptied into plastic jars and preserved in 75% ethanol. Once in the laboratory, the samples were

emptied into trays and benthic macroinvertebrates, surface aquatic bugs and salamanders were collected

and recorded.

Data Analysis

Yearly gully migration rate was calculated by subtracting the previous distance of the stream

reach from the current distance, dividing that number by time between the gully surveys and multiplying

that monthly rate by 12 to find the yearly rate (Equation 1).

(1) GMR=(Current length of stream reach−Previous length of stream reach )

Timebetween gully surveys∈months* 12

The length of the stream was determined by running a string along the scaled topographic maps

that were created with the Leica Flexline TS02plus. The string was run along the middle of the channel up

to the knickpoint and then straightened out to find the total distance of the stream reach in meters.

To calculate the total soil lost of the gully throughout my project, cross sections at every meter

were created using the topographic map. At each one meter mark, the contours were plotted and the cross

sectional area was calculated. At the end, the sum of the cross sectional areas was calculated, resulting in

total soil lost. The yearly rate of soil lost was calculated in the same manner of gully migration rate where

the difference between the current soil lost and previous total soil lost is divided by time in between the

surveys in months and then multiplied by 12.

For benthic macroinvertebrates, the independent variable of the statistical tests analysis was

sample site location. The dependent variables were total benthic macroinvertebrates, total biological

indicators, specific macroinvertebrate taxa, Hilsenhoff Biotic Index (HBI), and other stream health

indicators such as pH, alkalinity, and stream health.

The HBI was calculated using Equation 2, where n is the number of invertebrates in the taxa, a is

the predetermined value of given taxa, and N is the total number of taxa. By calculating the HBI, we get a

good grasp of the overall tolerance level of the community (Hilsenhoff, 1987).

To

test for benthic macroinvertebrate abundance in conjunction with site location, I performed a Kruskal-

HBI=∑ ni × ai

N(2)

Wallis statistical test. Due to the low numbers of data, a more powerful, parametric ANOVA test could

not be used. After the Kruskal-Wallis test, I conducted a linear regression to test for results that showed a

linear relationship.

Results

Gully Erosion Quantification

The high-resolution digital maps created by the Leica Flexline TS02 show a major change in the

gully channel through the span of 6.5 months. Through the time span of this study, the gully channel has

both migrated upstream rapidly and expanded outwardly.

The surface comparison and wire point maps show a 3D visual of the gully reach at the time of

the surveys throughout the course of the 6.5 month study. The results are shown in chronological order

(Figure 2). The quantified total soil lost is ~104 m3, which is equivalent to ~190 m3/y or ~430 ton/y.

Additionally, the gully erosion rate was much higher in the fall than it was in the summer. In the summer,

rate of soil lost was ~110 m3/y or ~260 ton/y while in the fall it was more than three times that ~370 m3/y

or ~830 ton/y.

The topographic maps of the gullies show the elevation of the gully reach. The results are shown

in chronological order (Figure 3). It is evident that the gully reach is significantly increasing in incision

upstream and along the banks. The calculations show that headward migration rate is ~26 m/y.

Additionally, there is a significant difference in migration rate seasonally. In the summer, migration rate

Figure 2. Surface comparison and wire point maps of the gully through time. From top to bottom, the surveys were taken on 5/4/2015, 9/15/2015, and 11/16/2015

was calculated at ~17 m/y while in the fall it was calculated at 2.5 times that of the summer at ~44 m/y

(Figure 4).

Figure 3. Topographic map of the gully through time. From top to bottom, the surveys were taken on 5/4/2015, 9/15/2015, and 11/16/2015

Figure 4. Gully migration rate in summer versus fall. The gully headcut in the fall migrates at a much more rapid pace.

Of the original six erosion pins placed flush with the bank gully, three were washed out

completely during a spate, one showed erosion of 13 cm, while two showed no bank retreat. The three

pins that were washed out completely were placed under flood plain spill off areas showing that banks

where spill off areas were present eroded at least 20 cm. The pin that showed bank retreat of 13 cm was

located a foot next to a spill off area, thus showing areas that non spill off areas were still eroding, just not

as fast. The two pins that were unaffected were located about 150 meters downstream, indicating that

downstream banks have stabilized.

Benthic Macroinvertebrate Assessment

Mean total benthic macroinvertebrates showed a gradual increase with site, however the

difference amongst site was not significant (Chi-squared=1.1429, df=2, p=0.5647).

1 2 30

2

4

6

8

10

12

14

16

Mean Total Benthic Macroinvertebrates by Site

SiteMea

n To

tal B

enth

ic M

ancr

oinv

erte

brat

es

The mean Total Biological Indicators showed very little variation among the sites, with the total

range being from 20.5-22. Therefore, there was no significant relationship between distance from gully

and biological indicator presence (Chi-squared=0.0735, df=2, p=0.9639).

Figure 5. Kruskal-Wallis Test. Chi-squared=1.1429, df=2, p=0.5647. There is no significant variation of benthic macroinvertebrate populations among sites.

1 2 319.5

20

20.5

21

21.5

22

22.5

Mean Total Biological Indicators by Site

Site

Mea

n To

tal B

iolo

gica

l Ind

icat

ors

Salamander populations showed a significant, negative correlation with distance from the gully

knickpoint (R2=0.689, p=0.0415).

Figure 6. Kruskal-Wallis Test. Chi-squared=0.0735, df=2, p=0.9639. There is no significant variation of biological indicators among sites.

50 100 150 200 250 300 3500

2

4

6

8

10

12

14

Salamander Populations by Distance from Knickpoint

Distance from Gully Knickpoint

Tota

l Sal

aman

der P

opul

ation

s

Figure 7. Linear Regression. R2=0.689, p=0.0415. Salamanders show a significant decrease in population with increase in distance from the gully knickpoint

Mean HBI remained relatively constant throughout the three sites, ranging from 6.93-7.27.

Therefore, there was no significant difference among the sites’ HBI (Chi-squared=0.85714, df=2, p-

value-0.6514).

1 2 36.8

6.856.9

6.957

7.057.1

7.157.2

7.257.3

Mean HBI by Site

Site

Mea

n HB

I

Figure 8. Kruskal-Wallis Test, chi-squared=0.85714, df=2, p=0.6514. There is no significant variation of HBI among the three sites.

Mean pH increased with distance from the gully knickpoint. The difference among the

sites was significant (Chi-squared=6.4889, df=2, p-value=0.03899).

1 2 37

7.1

7.2

7.3

7.4

7.5

7.6

7.7

7.8

pH by Site

Site

pH

Discussion

Gully Erosion Assessment

My study was intended to see if gully erosion significantly affects stream health by looking at

benthic macroinvertebrates in the gully stream reach of the South Prong West Branch of the Rocky River

(SPWB). In order to accurately assess this question, I aimed to first assess the gully erosion as a whole.

By doing so, it would give me an accurate picture of how the gully reach of study compares to other

gullies in the Piedmont and worldwide.

The quantification of gully migration rate and total soil lost initially shows that this gully is

behaving in an extreme manner especially relative to gullies assessed in other studies. The yearly

migration rate of ~26 m/y is well beyond the typical gully retreat rate found in studies done throughout

the world. On average, gully headward retreat rate ranges from 0.5–1.5 m/y (e.g. Oostwoud Wijidenes et

Figure 9. Kruskal-Wallis Test. Chi-squared = 6.4889, df = 2, p= 0.03899. There is a significant increase of pH among the three sites

al., 2000; Radoane M., Ichim, & Radoane N., 1995; Poesen 2003; Rieke-Zapp & Nichols, 2011;

Vanmaercke & Poesen ,2015). However, although average gully retreat rate may be relatively small, it

can get to rates much higher than even what the SPWB gully is showing. Gully rates have shown to be as

high as 70 m/y (Vanmaercke & Poesen, 2015). However, rates that high are much less frequent. Of the

900 active gullies assessed in a study conducted by Vanmaercke and Poesen (2015), the median retreat

rate was 0.82 m/y. Therefore, it is safe to conclude that the headward migration rate is a very rapid one. In

the fall especially, the SPWB retreats at a rate that is incredibly fast at ~44 m/y.

Total soil lost is also a large indicator of how extreme the gully erosion is. In comparison to other

studies it also is much larger than the average. The yearly soil lost rate of the SPWB is ~190 m3/y or ~430

ton/y. Like the migration rate, the fall and summer behave much differently. The summer rate of soil lost

was ~110 m3/y or ~260 ton/y while in the fall it was more than three times that at ~370 m3/y or ~830

ton/y. These numbers convert to a yearly rate of ~35,000 ton ha/y, a summer rate of ~22,000 ton ha/y and

a fall rate of ~69,000 ton ha/y. Relative to other studies, these numbers are magnitudes higher. Gullies

assessed in other studies, have averages of about 0.2-.5 ton ha/y for bank gullies and 2.3-5.8 ton ha/y for

ephemeral gullies (Poesen, 1989; Poesen, 1993). The reason for such a large difference in rate of total soil

lost is likely due to the nature of the gullies in other studies. A majority of the gullies being assessed are

much larger and stagnant than the SPWB. In fact, many of the gullies are 50-100 meters wide and up to

30 meters deep (Poesen et al. 2003; Soil Science Society of America, 2001). These gullies erode about a

meter a year across a much larger scale while the SPWB erodes ~26 meters a year at a much more

concentrated scale.

Another interesting factor of the gully erosion assessment is the large difference between summer

and fall rates. Headward migration rate in the fall (~44 m/y) is more than 2.5 times higher than the

migration rate in the summer (~17 m/y) and total soil lost rate is more then 3 times higher in the fall than

in the summer at ~370 m3/y and ~110 m3/y respectively. The differences between the fall and summer

rates is likely due to the ephemeral nature of the stream in conjunction with the weather. In the summer,

there is ample vegetation around the gully and little rain. As a result, the stream is quickly dried out

upstream. When there are times of rain and water fills the upstream channel, it is quickly absorbed by the

surrounding vegetation. Additionally, the vegetation likely holds the soil in place. In the fall, more

specifically late fall, the nature of the gully is much different, a large portion of the vegetation dies and

there is more rain. As a result, there is much more rapid erosion. The continual presence of water

upstream is one reason for that, but fall storms likely contribute to the erosion more so. As discussed

earlier, spates significantly contribute to gully erosion (Boulton et al., 1992; Fisher & Gray, 1982). This is

prevalent in the SPWB where intense rainfall occurred periodically through the course of the past two

months. Due to the presence of water in the stream, the dieback of vegetation, and spates, it is

understandable why fall rates are so much higher than the summer.

However, the differences in fall and summer erosion rates are probably much more drastic than

usual due to the very dry summer and very wet fall of this year. This summer (May-August) there was

8.56 inches of rain while in the past five summers in Mecklenburg County the average amount of rain is

19.17 inches (USGS, 2015). That is more than double what was precipitated this past summer. The fall

rains show the exact opposite, where this fall was nearly double the average the past five falls. This fall

(September-November) it rained ~16.57 inches while in the past five falls it rained an average of 8.72

(USGS, 2015). It is evident that the unusually dry summer and wet fall had a major influence on the

erosion rates in this study, an influence that is surely not to be as drastic in previous years. In fact, based

on averages, summer may in fact have higher erosion rates than the fall in previous years. For example

take 2010 where the summer received 22.39 inches of rain and the fall only received 7.3 inches. The rate

of erosion is surely to have been much higher in the summer that year than the fall. However, in the

summer there is still likely not a huge difference due to the other factors present that could slow down

erosion rate such as vegetation and the drying up of upstream. Erosion was likely as extreme this year as

it has been in the past several years because the heavy rainfall occurred in the fall with continual water

flow and dieback of vegetation.

Benthic Macroinvertebrates Assessment

My assessment of benthic macroinvertebrates was designed to determine how gully erosion is

affecting stream health. Benthic macroinvertebrates have been studied frequently to help determine

stream quality, however they are not often studied in conjunction with gully erosion. Therefore, through

my investigation, I tested to see if gully erosion was having an impact on stream health downstream the

knickpoint. It is important to note that only one of the statistical tests had a significant result. Therefore,

the following claims are observations and assumptions from my assessment of the data.

The stream reach appeared to be much more stabilized further downstream, therefore, one would

expect benthic macroinvertebrate communities to have higher populations in Site 3 at 300 meters

downstream the knickpoint than at Site 1, which is only 100 meters from the knickpoint. However, the

statistical tests did not show a true relationship between distance and population. Although population

appeared to increase gradually with site, the Kruskal-Wallis test showed no significant difference among

the sites. The likely reason for this is that there was high variation in the sampling. For example, the sub-

sites in Site 2 had a large difference in number that caused a high standard deviation. Site 2A had more

than three times the amount of benthic macroinvertebrates present than site 2B. Additionally due to the

overall low numbers of benthic macroinvertebrate present, it is difficult to get any result with

significance. By pooling the sites together, the chances for a significant result are increased, however,

even pooled the numbers are still really low. A typical study looking at benthic macroinvertebrate

populations would have thousands of individual taxa and around 45 different families (Gage et al., 2004).

This study had 65 individual taxa and only six different families, a much smaller number.

The same reasons can be said about the total number of biological indicators in the stream. This

group includes all the benthic macroinvertebrate along with stream salamanders and Collembola

Isotomidae, which also can indicate stream health (Southerland et al., 2004; Welsh & Oliver, 1998;

Willson & Dorcas, 2003). I thought it would be useful to use biological indicators in data analysis in

addition to benthic macroinvertebrates because the initial number of invertebrates was so low. Not only

does this increase my sample size but it does so while remaining true to the main question of whether or

not gully erosion is affecting stream health. However, even on first glance there appeared to be no

relationship between total biological indicators and distance from knickpoint, for all three totals fell

within a range of 20-22.

Of all the different taxa, only one showed to have a significant relationship with distance from the

gully knickpoint. However, the relationship was inverse of what I originally imagined and instead of there

being a positive correlation between distance from knickpoint and salamander populations, there was a

negative one. As distance from the knickpoint increased, salamander populations decreased significantly.

The reason for such a result could be because salamanders are very sensitive to changes in water thus

changes in water chemistry could severely effect their presence (Willson & Dorcas, 2003). From

assessing simple water chemistry, there is a significant difference between Site and pH where distance

positively correlates with pH. As distance from the stream increases, the water becomes significantly

more basic, which could contribute to the decline in salamanders present.

However, the more likely reason salamander populations are significantly higher closer to the

knickpoint at Site 1 than at Site 3 is because the ideal salamander habit is found nearer the knickpoint.

Salamanders are typically found under leaf litter or logs, both of which are very present near Site 1 and

not nearly as present in Site 2 and Site 3 (Conant & Collins, 1998). With the intense erosion that is

occurring in the SPWB gully, a lot of vegetation and trees have fallen into the streambed, creating the

perfect habitat for salamanders and making this a very plausible explanation for the interesting results

present.

HBI was calculated to assess the overall tolerance level of the benthic macroinvertebrates in the

stream. By understanding what the average tolerance level of the sites is, I was able to see if there were

less tolerant taxa located in sites closer to the knickpoint. The results again show no significant variation

among the sites and HBI remained relatively constant, ranging from 6.97-7.27. However, although there

is no significant variation among the sites, the HBI is incredibly important to look at. As discussed earlier,

HBI ranges from 0-10 with 0 being a very intolerant species and 10 being a very tolerant species.

Therefore, an HBI of 7 is indicative of poor stream health. In fact, according to Hilsenhoff (1988), there is

likely to be very substantial organic pollution present. Accompany that with the presence of not one

Ephemeroptera, Plecoptera, or Trichoptera taxa along with the very low number of other benthic

macroinvertebrates and it is safe to assume that this stream is in poor health.

The reason for such poor stream health could be as much due to the ephemeral nature of the

stream as it is the gully erosion, for both largely affect downstream activity. The stream being ephemeral

causes there to be no water upstream of the knickpoint. As a result, there is very low amount of water

downstream. The water level is so low that recording flow was impossible because not only was there too

little water to stick the whole probe in but also the water was moving at a rate so slow that it wouldn’t

even trigger a flow rate. The reason for water being present downstream while being absent upstream is

likely the result of the channel incision being below the water table. The water present is not the result of

a stream source, but rather simply ground water. Additionally, because there was no top source for the

stream, in times of heat with very little rain, the water table would lower causing drying out downstream

as well. In fact, on September 7, 2015, 21 days prior to sampling, Site 3 was completely dry. The

following sampling at Site 3 on September 28 showed a total benthic macroinvertebrate collection of six,

which is nearly half of the average total benthic macroinvertebrates from the other two dates at that site.

The stream being ephemeral also causes poor conditions for benthic macroinvertebrate

communities because when it rains, discharge increases by magnitudes and soil erodes rapidly.

Macroinvertebrates have been shown to decrease in population in response to both sediment deposition

and intense rainfall (e.g., Gage, Spivak, & Paradise, 2004; Lenat, 1988; Oberlin, Shannon, & Blinn, 1999;

Violin et al., 2011). Consequently, that likely has a large effect on the benthic macroinvertebrate

communities downstream. As discussed earlier, the SPWB gullly is rapidly eroding, especially in the fall

when spates are more likely to occur. This is likely very negatively affecting benthic macroinvertebrate

populations.

Conclusion

After quantifying gully erosion and migration rate as well as comparing site benthic

macroinvertebrate populations, biological indicator presence, and HBI, I conclude that gully erosion is

significantly affecting stream health. Although I fail to conclude that benthic macroinvertebrate

populations will increase with distance from the gully knickpoint, it appears based on the data that overall

stream health is being affected by gully erosion. The high HBI average among the sites in addition to the

low benthic macroinvertebrate populations indicates poor stream quality. Additionally, the rapid

migration rate and total soil lost rate shows that the gully is eroding at a relative fast pace and depositing

sediment downstream. However, it is difficult to conclude anything with the number of other variables

that could contribute to the poor results we see and the short time scale of this project.

In future research, I would like to see this study carried out long term, especially in the late fall,

winter, and spring when the stream will be flowing. That will allow for the sampling of benthic

macroinvertebrates upstream, immediately above the knickpoint, and immediately below the knickpoint

in addition to the three downstream sites. For this investigation, I was originally planning on collecting

my samples in that manner, however, was unable to due to the stream’s ephemerality. Additionally, more

sampling would be beneficial in order to increase sample size. I would also like to see the continuation of

the gully surveying. That would allow me to understand the nature of the gully erosion year round and see

how it responds to the winter and spring conditions. It would also allow me to survey through another

summer and fall when the rain received is more reflective of the averages. It would be very interesting to

see how erosion rate changes with normal seasonal amounts of rain.

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