interpreting avulsion process from ancient alluvial ... et al, 2000.pdf · interpreting avulsion...

1787

GSA Bulletin; December 2000; v. 112; no. 12; p. 1787–1803; 15 figures; 3 tables.

Interpreting avulsion process from ancient alluvial sequences:Guadalope-Matarranya system (northern Spain) and Wasatch

Formation (western Colorado)

David Mohrig*Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA

Paul L. HellerDepartment of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA

Chris PaolaDepartment of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA

William J. Lyons†

Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA

ABSTRACT

Alluvial deposits of the Guadalope-Ma-tarranya system (Oligocene, Ebro basin,Spain) and the Wasatch Formation (Eo-cene, western Colorado), provide time-in-tegrated records of the process of river-channel avulsion. These sequences consistof isolated channel-belt sandstones incisedinto, and abruptly overlain by, flood-plainsiltstones, indicating deposition by avulsiveriver systems. The geometry and distribu-tion of channel incisions suggest that avul-sion was not controlled by tectonics, cli-mate, or base-level changes, but formed byautocyclic processes.

Measurements from 221 channel fills inthe Guadalope-Matarranya system and 38from the Wasatch Formation allow us tostatistically characterize channel geome-tries we infer to be associated with estab-lishment and abandonment of individualriver avulsions. Paleoflow depths in bothsystems average 1.4 to 1.6 m. Aggradationheight (superelevation) of channel marginlevees are, on average, 0.6 and 1.1 times pa-leoflow depth in the Guadalope-Matarran-

*Present address: ExxonMobil Upstream Re-search Company, P.O. Box 2189, Houston, Texas77252, USA; e-mail: [email protected].

†Present address: Department of Earth, Atmo-spheric, and Planetary Sciences, Massachusetts In-stitute of Technology, Cambridge, Massachusetts02139, USA.

ya and Wasatch systems, respectively.These results are consistent with valuesfrom recently avulsed modern rivers andsuggest that (1) flow depth is the appropri-ate parameter against which to scale thecritical superelevation necessary for chan-nel avulsion; and (2) the increase in poten-tial energy due to channel perching drivesthe lateral instability that is needed foravulsion to be successful.

Numerous stacked channel fills indicaterepeated reoccupation of the same site byavulsing channels. These reoccupationchannels indicate that inherited flood-plaintopography, here abandoned channelforms, was an important control on the ar-rival site of newly avulsed channels.

Comparison of our results to others sug-gests two end-member types of avulsion cantake place. Incisional avulsion, seen here, ischaracterized by an early incision phasefollowed by infilling by migrating barforms. Aggradational avulsion begins withaggradation followed in time by stream in-tegration into a single downcutting channel.We suggest that the type of avulsion isstrongly influenced by whether or not theadjacent flood plain is well or poorlydrained. In both cases subsequent aggra-dation and channel perching increase thechances that some triggering event will leadto avulsion.

Keywords: alluvial deposits, avulsion, flu-

vial sediments, rivers, sedimentary depos-its, stratigraphy.

INTRODUCTION

River avulsion is the relatively rapid trans-fer of river flow out of an established sectionof channel belt and into a new flow pathwayelsewhere on the flood plain. In river systemswhere avulsions occur, the process typicallydominates the long-term dispersal of sedimentand water across their alluvial surfaces. As aresult, understanding and characterizing thecontrols on avulsion are important to the geo-morphologist studying evolution of deposi-tional landscapes, to the sedimentologist re-constructing the history of ancient riversystems, and to the civil engineer interested incontrolling the position of waterways.

Recent numerical models that simulate con-struction of the depositional architecture(channel stacking pattern) of fluvial depositshave emphasized the role of avulsions as acontrol on the distribution of ancient riverchannels in space and time (Leeder, 1978; Al-len, 1978; Bridge and Leeder, 1979; Mackeyand Bridge, 1995; Heller and Paola, 1996).Key assumptions of these models concern (1)when a channel will avulse, and (2) the posi-tion of the new channel belt following avul-sion. In the first assumption, it is essential toknow if there is some single aspect of thechannel system that primarily, and predict-ably, creates conditions favorable for avulsion.If so, this parameter may serve as a standard

1788 Geological Society of America Bulletin, December 2000

MOHRIG et al.

Figure 1. Location map of (A) the Guadalope-Matarranya alluvial system (dark shading)in northeast Spain (modified from Anadon et al., 1989); and (B) the Wasatch Formationexposed in the study area (dark shading) in western Colorado. Bold white arrows indicategeneral paleoflow directions in the units studied.

among systems so that different-sized riverscan be compared. The second assumptionevaluates whether preexisting topography ex-erts a role on site selection of the newlyformed stream. Some workers have suggestedthat former channel positions create alluvialridges along the flood plain that repel newchannels (Allen, 1978; Bridge and Leeder,1979; Mackey and Bridge, 1995). In contrast,studies of modern avulsions show that newlyavulsed rivers are strongly attracted towardpreexisting channels, the beds of which arepreserved as lows on the flood plain (Aslanand Blum, 2000; Morozova and Smith, 2000).Stratigraphic evidence for this attraction waspresented by Maizels (1990).

Whereas study of the avulsion process canbest be done by direct observation, avulsionsare infrequent events; characteristic recurrenceintervals in natural rivers are tens to thousandsof years (e.g., Rannie et al., 1989; Tornqvist,1994; van Gelder et al., 1994), although thisinterval may depend upon sedimentation rate(Bryant et al., 1995). The stochastic compo-nent of the avulsion process precludes makinggeneral models of channel switching from thesmall number of realizations associated withany single modern system. Scaled physicalmodeling in the laboratory provides an alter-native approach that allows many realizationsby compressing time and has the advantage ofunambiguously defining all of the input vari-ables (Hooke and Rohrer, 1979; Bryant et al.,1995). However, laboratory scale models donot include some aspects of the fluvial envi-ronment (e.g., vegetated levees and floodplains) that may play a key role in avulsiondynamics.

In contrast, studying the deposits of a well-exposed ancient river system has the distinctadvantage of displaying many avulsion eventsthat occurred over a long time period at theexpense of detailed knowledge of timing andflow processes in each case. This record is atfull natural scale, and has all relevant processesembedded in it; however, it is impossible totrace the history of a specific avulsion, or toknow the precise timing of the events or thehydraulics of the associated flows. Nonetheless,careful analysis of the deposits of ancient geo-morphic systems can provide quantitative in-formation about sedimentary processes that acton long time scales and, therefore, cannot bedirectly measured in modern systems.

In this study we examine well-exposedchannel deposits of the Guadalope-Matarranyaalluvial system of Oligocene age in northeastSpain and the Wasatch Formation of late Eo-cene age in western Colorado (Fig. 1). In theexample from Spain, broad, low-relief dissec-

tion of these deposits has etched out hundredsto thousands of ancient channel fills in such away as to permit observation of channel formsin both cross-sectional and plan view (Fig.2A). Although the Wasatch Formation doesnot have such plan-view exposure, the channelfills are very well exposed in continuous crosssection along the Colorado River valley nearParachute, Colorado (Fig. 2B). These obser-vations provide a statistically meaningful se-ries of measures of channel geometry relatedto avulsion.

RIVER AVULSION SETUP

The process of river avulsion can be con-sidered as having two fundamental require-

ments: a setup, where the river aggrades overtens to thousands of years and becomes poisedfor avulsion, and a trigger, which is a short-term event that causes the slow or abruptabandonment of the channel (typically floods;Mosley, 1975; Slingerland and Smith, 1998;Brizga and Finlayson, 1990). River avulsionsetup is generally recognized to be a conse-quence of the tendency for deposition ratesnear river channels to be greater than those onthe adjacent flood plain. The coupled deposi-tion of sediment on the bed of a channel andon the levees flanking the channel marginsleads to the channel becoming progressivelyperched above its flood plain while maintain-ing a cross-sectional shape that is best suitedfor the throughput of water and sediment.

Geological Society of America Bulletin, December 2000 1789

INTERPRETING AVULSION PROCESS FROM ANCIENT ALLUVIAL SEQUENCES

Figure 2. (A) Aerial photo of exhumed channel threads in the Guadalope-Matarranyasystem, 22 km north-northwest of Alcaniz. The field of view is 4.5 3 6.0 km. Sinuouschannel forms stand as dark, vegetated ridges scattered among light colored orchards.Paleoflow here was generally toward the northwest (upper left corner of photo). Althoughdips of beds in photo are near horizontal, stratigraphic level of exhumed channels variesslightly throughout the area. (B) View of the Shire Member of the Wasatch Formation(lower half of cliff face) along the Roan Cliffs, west of Parachute, Colorado. Lenticularchannel-belt sandstone bodies (typically dark bodies) become less common upsection.Light colored overlying unit is the Green River Formation.

Avulsion Setup by Levee Slope versusSuperelevation

Two different measures have been proposedto characterize the setup of a channel for avul-sion. The first compares the lateral slope onchannel levees (levee slope in Fig. 3) to theslope of the long profile of the system (Hookeand Rohrer, 1979; Mackey and Bridge, 1995),and the second, superelevation (Fig. 3), cal-culates the relief between the water-surface el-evation in a channel at bankfull discharge (i.e.,levee height) to the minimum elevation of theadjacent flood plain (Heller and Paola, 1996).It is possible that these two measures are cor-related if levee slope becomes steeper as thetotal amount of superelevation increases, suchas in the model of Slingerland and Smith(1998).

One approach to determining whether leveeslope or superelevation exerts the primarycontrol on avulsion setup is to observe whichvariable has the least scatter among modernrivers that have avulsed, or would have if hu-mans had not intervened. Before such a com-parison can be made, the slope or height val-ues must be normalized to allow comparisonof rivers of vastly different size (e.g., Slinger-land and Smith, 1998). To normalize slopedata from different rivers we calculate the ra-tio of levee slope to downstream slope. Tonormalize the elevation data we use the ratioof superelevation height to flow depth. Nor-malized values of these two parameters forfive modern rivers taken at, or near, the siteof avulsion are reported in Table 1. Althoughthe database is small, we are struck that theratios of superelevation height to flow depthare very similar despite widely different dis-charges for these rivers, while the levee todownstream slope ratios cover two orders ofmagnitude.

Alternative arguments for superelevationwere made by Bryant et al. (1995), Heller andPaola (1996), and Imran et al. (1998). The lat-ter study reasons that there must be somethreshold amount of potential energy for flowsto cut through fully developed levee material.Imran et al. (1998) pointed out that, for marineturbidite systems, the excess density drivingturbidity currents downslope is small com-pared to the excess density driving river flow.As a result, they predicted that submarine le-vees along turbidite channels are about 50times taller than river levees, a ratio matchingthe excess density differences between the twoenvironments (Imran et al., 1998). This dif-ference in the height of levees of submarineand terrestrial channels is documented by re-cent seafloor bathymetric data (Pirmez and


MOHRIG et al.

Figure 3. Definition sketch of preserved channel-belt geometry showing key characteristicsconsidered and measured in this study. Channel core is shaded. Channel wings thin awayfrom the core and where they abut the core are interpreted as channel levee deposits.

Figure 4. Topographic map (upper) and cross sections (lower) of perched channels of theAssiniboine River and fluvial fan, Manitoba, Canada (modified from Rannie, 1990). Con-tour interval is 3 m. For clarity, the fan feature is shaded; darker shades fill the contourintervals on the upper part of the fan. Note that the active channel is also the highestchannel form along the cross sections.

Figure 5. Histogram of superelevation, nor-malized by local flow depth, for the activechannel and the abandoned channels alongthe Assiniboine River. Data are measuredfrom cross sections of Rannie (1990).

Flood, 1995). Thus, the comparative relief oflevees in these two environments suggests thatrelative potential energy (i.e., local differencebetween the perched channel and the distalflood plain), not slope, is the dominant controlon levee development and associated channelavulsion.

Control by superelevation makes physicalsense to us because height of the water col-umn sets both the force driving the water lat-erally and the extent of the flow depth avail-able for extraction. Although not an argument,the superelevation approach has the uniqueadvantage over the slope model in that it canbe measured in ancient rock sequences.

Superelevation in the Modern

An example of an avulsing system, and onethat serves as a modern analog for the systemsstudied here, is the Assiniboine River, Mani-toba, Canada (Fig. 4; Rannie, 1990). As seenin cross sections (Fig. 4), levees confine chan-nels, allowing them to become perched abovethe surrounding flood plain. A comparison ofthe amount of superelevation, normalized bylocal bankfull flow depth, along publishedcross sections (Rannie, 1990) shows that, onaverage, 65% of the bankfull channel is abovethe elevation of the nearby flood plain (Fig.5). Nearly all of the channels had avulsed be-

fore the base of the perched channel becameequal in elevation to the flood plain (i.e., su-perelevation/flow depth 5 1).

A survey of modern rivers (e.g., Fisk, 1952;Smith, 1983; Brizga and Finlayson, 1990;Rannie, 1990; van Gelder et al., 1994) showsa limit to which a channel can be superele-vated in this way. Rarely do natural channelsbecome superelevated to the point where theriver bed reaches the average elevation of theflood plain. This observation suggests that itis difficult for the bed of an active channel beltto aggrade above the adjacent flood plain be-fore avulsion takes place.

GUADALOPE-MATARRANYA STUDYAREA

Deposits of the Guadalope-Matarranya al-luvial system (Figs. 1 and 6) crop out in thesoutheastern corner of the Ebro basin, theyoungest and largest subbasin associated withthe southern Pyrenean foreland basin (Puig-defabregas et al., 1986). The Guadalope-Ma-tarranya system is the largest of several Pa-leogene alluvial systems that flowed into thiscorner of the basin from the south and fromthe east (Colombo, 1986; Anadon et al.,1989). The source for the Guadalope-Matar-ranya system was the linking zone (Fig. 1A),a highland produced during the Paleogene bynorth-directed thrusts of limited displacementand northwest-southeast strike-slip basementfaults (Guimera, 1984; Anadon et al., 1989).Tectonic subsidence during deposition wassufficient to trap along the margin of the basinmost of the coarse clastic sediment suppliedby the Guadalope-Matarranya and the otherPaleogene alluvial systems, producing an in-terior lake that was filled mainly with carbon-ates (Cabrera, 1983; Cabrera et al., 1985).



TABLE 1. MODERN RIVER DATA

River Name Mean Levee Downstream Ratio of Levee Channel Ratio to Referencesdischarge slope slope levee slope height depth levee height

(m3/s) to downstream (m) (m) to channelslope depth

Yellow River delta 1332 0.00081 0.00010 8.1 1.32 2.56 0.52 van Gelder et al. (1994)Thomson River 104 0.0057 0.0011 5.2 3.3 5.3 0.62 Brizga and Finlayson (1990)Assiniboine River 64 0.0035 0.0013 2.3 2.28 5.71 0.4 Rannie (1990)Saskatchewan River 484 0.0079 0.00012 65.0 4.29 7.37 0.582 Smith (1983) (lengths); Smith

et al. (1989) (discharge)Mississippi River in vicinity of

Baton Rouge, LA9542 0.00186 0.000037 50.3 6.85 35.7 0.192 Mississippi River Commission

(1947) (lengths); USGS Sta-tion 07374000 (discharge)

Notes: Relevant geometric data are from sections of rivers at or near the sites of avulsion. LA—Louisiana; USGS—U.S. Geological Survey.

Figure 6. Map showing sample locations (triangles), measured paleoflow directions (linesegments and arrows), and the general grain-size trend of channel fills for the Guadalope-Matarranya system. Line segments denote locations where orientations of the channel axeswere used as indicators of the paleoflow directions, and arrows mark locations where dunecosets were used. Squares mark the locations of the towns of Alcaniz, Caspe, and Gandesa.

Rodent fauna (Anadon et al., 1992) andmagnetic polarity reversals in associated lakebeds (Barbera et al., 1994) indicate that theGuadalope-Matarranya alluvial system wasdeposited within a 3 6 1 m.y. interval duringlate Oligocene (Chattian) time. The paleocli-mate during deposition, as interpreted fromevaporitic lithofacies and fossil pollen assem-blages (Cabrera et al., 1985), was arid to semi-arid. The Guadalope-Matarranya system mayexceed 1 km in total thickness (Cabrera et al.,1985); however, locally exposed stratigraphicthickness never exceeded 350 m in the studyarea. Strata in the study area are primarily as-signed to the Caspe Formation (J. Cuevas,1994, oral commun.). The deposits coverabout 5000 km2 between the southern sourcearea in the linking zone and the terminal lakebeds (Los Monegros Formation) exposed to

the north in the central Ebro basin (Fig. 1A).The trends of exposed channel fills, as well asinternal sedimentary structures (48 channelaxes, 24 dune cosets) and grain-size trends(based upon 116 grain-size measurements),confirm that paleoflow in the system was gen-erally to the north, and its pattern was broadlyfan like (Fig. 6). Postdepositional deformationof the Guadalope-Matarranya system has beenminor. Most of the strata in the basin are closeto horizontal (,18 regionally) but locallysteepen near the thrusted southern margin. Be-cause of the absence of clearly correlatablehorizons within the Guadalope-Matarranyasystem, we were not able to precisely deter-mine the stratigraphic position of sample siteswithin the unit.

Spectacular exposure has made the Gua-dalope-Matarranya one of the most studied

fluvial systems in the world (Riba et al., 1967;Friend et al., 1979; Allen et al., 1983; Cabreraet al., 1985; Anadon et al., 1989). Three typesof sandstone bodies have been described inthis system (Cabrera et al., 1985). Laterallyaccreted channel bodies, as thick as 3 m, in-terpreted to have formed by meanderingstreams (Puigdefabregas, 1973; Friend et al.,1979), become increasingly common north-ward toward the margin of the Los Monegrospaleolake system. Thin sheet-like sandstonebodies, as thick as a few decimeters, are typ-ically burrowed and massive but display rip-ple- to dune-scale cross-bedding in places.Most of these sheet bodies have been inter-preted to represent crevasse-splay deposits,particularly where associated with ribbonchannel sand bodies (Cabrera et al., 1985).

The third sandstone type are ribbon channelbodies, as thick as 6 m (Cabrera et al., 1985),and are the channel fills of interest in thisstudy (Fig. 7). These sandstone bodies arenestled within thin-bedded to massive mud-stones that make up the bulk of the Guada-lope-Matarranya deposits. Mudstones aremostly composed of siltstone and minor fine-grained sandstone. For the most part the unitis reddish to yellow-tan, in places mottled, of-ten with moderate to strongly developed clay-rich horizons suggesting formation as paleo-sols. Variable paleosol development and closeassociation of shallow lacustrine and evapo-ritic environments suggest that flood plainsranged from well to poorly drained (Cabreraet al., 1985). Burrowing, plant debris, and thethin sheet-like sandstones are common. To-ward the margin of the Los Monegros lacus-trine system there is an increase in (1) thegray-green color within the mudstone; (2) thecarbonate and, to a lesser degree, gypsum con-tent, both as thin beds and nodules; and (3)locally the coal content (Cabrera et al., 1985;Cabrera and Saez, 1987). The mudstone lith-ofacies is widely interpreted as overbank de-posits (e.g., Williams, 1975; Allen et al.,1983) that grade basinward into lake fringe


MOHRIG et al.

Figure 7. Examples of channel sand bodies in cross section in the Guadalope-Matarranyasystem with interpreted line drawings (cf. Fig. 3). Labeled line drawings (A9, B9) are shownbelow each photo. The approximate measures of superelevation (S) and maximum incision(I) are shown. (A) Two-story channel west of Caspe. Note convex-up rollover on tracedbar forms, suggesting preservation of the entire bar height. Maximum thickness of channelcore is 4 m. (B) Channel margin exposed northeast of Alcaniz. The channel wing containswell-defined thin beds that conformably overlie finer grained flood-plain deposits. In con-trast, the channel core is more massively bedded and steeply incises into adjacent flood-plain deposits. Rock hammer (circled) is for scale.

and deltaic settings (Cabrera, 1983; Cabrera etal., 1985).

Ribbon Channel Bodies

Thousands of spectacularly exhumed chan-nels and channel complexes stand out in crosssection as well as plan view; individual chan-nel segment lengths are often in excess of 1km (Fig. 2A). Of this large number of expo-sures, several percent are complete enough toclearly show channel margin geometry and itsrelationship with adjacent overbank mud-stones (Fig. 7).

Planform exposure of the ribbon channelbodies (Fig. 2A) allows direct measurement ofsinuosity, a property often inferred but seldomobservable for paleochannels. From aerialphotographs, Williams (1975) calculated the

sinuosities for 400 preserved Guadalope-Ma-tarranya channels segments ranging from afew hundreds of meters to 5 km in length. Thereconstructed widths of these paleochannelsare between 10 and 30 m, so all segments areat least 10 channel widths in length, and thusprovide a reasonable measure of sinuosity.Williams found that 66% of the segments havea sinuosity between 1 and 1.1, whereas 97%have sinuosities ,2. With such low values forsinuosity, fully developed meandering chan-nels, their point bars situated against the innerbank of the tightest channel bends, are the ex-ceptions. Field and photo examination revealsvery little lateral migration through time. Thisobservation is consistent with the laboratoryexperiments of Whiting and Dietrich (1993),who found in similarly low sinuosity channelsthat bars are not permanently trapped in bends

as point bars, but rather migrate freely downthe channels. We observed stratification at thetops of many exhumed channel fillings (e.g.,Fig. 7, A and B) that we interpret as a seriesof alternate bars. Bars of this style, migratingdownstream in single-thread channels, seem tohave been typical in the Guadalope-Matarran-ya system.

Ribbon sandstone bodies are primarilycomposed of sandstone; gravel macroforms(bars) are common only within 20 km of thelinking zone and entirely conglomeratic fillsare limited to within 5 km of the thrustedsouthern basin margin (Figs. 6 and 8A). Themedian grain size for the preserved gravelbars is in the range of very fine to fine peb-bles. The average caliber of channel sand-stones is medium sand, the upper mediumsand giving way to upper fine sand over 60km in the downstream direction (Fig. 8A).

The cores of these sandstone bodies arechannel form in cross section (Fig. 7A) andcontain accretion sets formed by migratingbars. Groups of accretion sets are typicallybounded by low-angle erosion surfaces thatcan be traced across the width of the sand-stone body dividing the channel fill into sto-ries (Fig. 7A). Most preserved sandstone bod-ies contain single, continuous basal scoursurfaces filled with one to three stories ofchannel-filling deposits (Fig. 9A). Single-sto-ry channel fills become more common towardthe north (Allen et al., 1983). Sandstone coresare the thickest parts of the channel body(Figs. 3 and 7A), and represent the filled axisof the paleochannel.

Channel wings (Fig. 3; Friend et al., 1979)emanate from the upper part of many channelcores. A wing is a wedge of thin-bedded sand-stone that thins and fines away from the chan-nel core (Fig. 7B) and grades laterally intoadjacent overbank fines. Wings are dominant-ly composed of fine-grained sandstone, typi-cally bioturbated but otherwise composed ofripple- to dune-scale cross-bedding and planarlamination. Thin mudstone partings separatebeds within the wings (Fig. 7B) and carbo-naceous plant roots were noticed in places.The bases of the wing deposits sharply overliesubjacent flood-plain deposits with minorscour. In general, moving away from the chan-nel core there is a decrease in grain size, bedthickness, total thickness, bedform height, andscour depth, and an increase in mudstone con-tent. These channel wings have been inter-preted as channel levees (Friend et al, 1979;Allen et al., 1983), where sediment-ladenflows spilled laterally onto overbank areas.

Various types of channel fills can be seenin the Guadalope-Matarranya system (Fig. 9;



Figure 8. Downstream trends in (A) median grain size, and (B) paleoflow depth of channelfills of the Guadalope-Matarranya system. Distance is measured from point 0, 0 in Figure6, and data are from Table 2.

Figure 9. Diagrammatic channel configu-rations seen in the Guadalope-Matarranyasystem. Sandstones are shown in white.Overbank mudstones are shown in black.Sigmoidal lines within sandstone bodiesrepresent bar accretion bedding. Wavy sur-face at base of channels represents scouredbase. ‘‘Wings’’ at upper edges of channelrepresent levee deposits building out uponthe attendant flood plain. (A) Simple in-cised channel with a single-story fill. (B) Su-perposed, single-story, channel sandstonebodies separated by a small amount ofoverbank mudstones that are locally erodedthrough. (C) Sandstone body with step-likemargins. In many cases the steps can betraced within the channel body as a verythin, discontinuous horizon of mudstone,suggestive of multiple superposition ofchannels.

Friend et al., 1979). Abandoned channels withmud plugs are relatively common, as are othersimple sandstone-filled channels (Fig. 9A).The absence of obvious mud plugs in the latterchannels may result from relatively slow avul-sions where the channel continued to transportwater and sediment as it was slowly aban-doned. A surprisingly frequent occurrence isthe superposition of one channel belt sand-stone body above another (Fig. 10). Theseamalgamated channel belts are often separatedby a thin erosional remnant of overbank mud-stones and/or paleosols preserved directly be-neath the overlying channel belt succession(Figs. 9B and 10). These amalgamated chan-nel belts are distinctive in that while the chan-nel-belt deposits are superimposed, generallythere is evidence, sometimes subtle, that thesite had been abandoned and later reoccupiedby an avulsed channel. Typically this evidenceincludes (1) a thin, sometimes discontinuous,intervening interval of overbank deposits thatcan be traced laterally into adjacent mudstones(e.g., Fig. 10), suggesting a period of flood-plain aggradation; or (2) sometimes this ho-rizon shows evidence of oxidation, burrowing,and/or root traces, suggesting a period ofabandonment and subaerial exposure.

Margins with distinct step-like forms (Figs.9C and 10C) were also interpreted as formingthrough multiple reoccupations rather than bya punctuated filling of a single deep, arroyo-like incision (Fig. 9A). In some cases otherevidence of reoccupation is present. In other

cases, channel reoccupation is suggested by anincrease in grain size and/or paleosol thick-ness in the adjacent mudstones that is sym-metric and centered about the channel positionthat closely coincides with the level of themargin step (Fig. 10C). These deposits sug-gest that levees grew adjacent to the channelearly on, and after a short period of abandon-ment and flood-plain aggradation the site wasreoccupied and scoured into by a newlyavulsed channel. In addition, the margin stepsare continuous with story breaks within thechannel fill (Fig. 10C) and are overlapped bybarforms that can be traced along the storybreak. This indicates that this level is identicalto, and continuous with, the scoured base of achannel belt through which complete barsfreely migrated. Similarity and continuity ofdeposits above the step with the rest of thechannel fill demonstrates that the steps are notpreserved terrace or strath remnants of an ear-lier channel. They indicate the base of a newlyavulsed channel belt.

We found 11 excellent examples of paleosoldevelopment between channels, 24 excellentexamples of stepped channel development,and two channel belts that clearly showedboth. Of all the measured channel belts, 24%appeared to result from reoccupation. In gen-eral, the channel belt (sandstone) to floodplain (mudstone) ratio is ..100/1, so thereis a surprisingly high number of reoccupiedchannels if the process is driven by randomprobability.

WASATCH STUDY AREA

To provide a comparison to the Guadalope-Matarranya system, we also collected identicaldata from the Shire Member of the WasatchFormation, exposed along the Colorado Rivervalley near Parachute, western Colorado (Fig.1B). The Shire Member, of alluvial origin, isthe uppermost unit in the Wasatch Formationin this area and immediately underlies lacus-trine beds of the Green River Formation (Fig.2B). Ample dating by microfossils, pollen,and invertebrate and vertebrate assemblageswithin, and immediately overlying, the ShireMember brackets the age as late Eocene(Wood, 1962; Donnell, 1969; Franczyk,1992). Paleocurrent indicators (dune foresetsalong accretion surfaces) indicate river flowdominantly to the southwest, suggesting der-ivation from the nearby White River uplift ofLaramide age (Lyons, 1998). While the unit isexposed nearly continuously over more than30 km along the valley wall, the alluvial basin


MOHRIG et al.

Figure 10. Examples of stacked channel fills from the Guadalope-Matarranya system in-terpreted as the products of channel reoccupations. Labeled line drawings (A9, B9, C9) areshown below each photo. (A) Three channel-fill sequences, each separated from the nextby a thin interval of mudstone to fine-grained sandstone (interpreted to represent over-bank deposits). (B) Two stacked channel-fill sequences separated by an overbank mud-stone interval. Rock hammer (circled) is for scale. (C) Two-story channel fill with step-like margins found west of Gandesa. The step is at the level of the story-bounding surfacewithin the fill and is overlain by complete barforms as seen elsewhere in the story. Thissecond, wider story follows a more resistant paleosol horizon. Evidence for subaerial over-bank origin of fine-grained deposits includes root traces and burrows, oxidized mudstone,and paleosol horizons.

Figure 11. (A) Front of a midchannel barin the North Loup River, Taylor, Nebraska.Bar top is nearly at the surface of the flow.Dots mark locations where bar heightswere measured. (B) Histogram of values forthe ratio of local bar height to reach-aver-aged flow depth. These measurements werecollected from four downstream-migratingbars in the North Loup River. Data weretaken after a long period of constant riverdischarge to ensure that the bar forms werein equilibrium with the river flow.

was much larger and our sample size wassmall relative to the Guadalope-Matarranyasystem. As a result we were not able to assessdownstream changes in channel geometry.

Sandstone bodies in the Shire Member aresimilar to the three types seen in the Guada-lope-Matarranya system; ribbon channel bod-ies predominate. These channel bodies are

simple, composed of one to a few stories iso-lated within mudstones. The mudstones showstrong reddish, yellowish, and grayish bandingand mottling, suggesting paleosol develop-ment. Channel body cores are primarily com-posed of medium-grained sandstone and con-tain well-developed accretion surfacesrepresenting bars that migrated oblique to pa-leoflow. Channel wings are of similar lithol-ogy as those seen in the Guadalope-Matarran-ya system.

STUDY METHOD

Geometric measures of individual channelforms must be reconstructed from the pre-served sedimentary deposits (Fig. 3). The taskwas simplest in sedimentary bodies created byfilling of a single channel (single story). How-ever, some sandstone bodies consist of com-plexes containing more or less amalgamateddeposits of a succession of temporally distinctchannels (multistory). These complexes firsthad to be separated into their individual chan-nel components (stories) before individualchannel geometries could be reconstructed(Figs. 7 and 10). Separation was done by as-suming that the most prominent bounding sur-faces, those traceable in the cross-stream and



streamwise directions over the distance of asingle outcrop, represented the base of indi-vidual channels.

We measured 221 ancient channel fills inthe Guadalope-Matarranya system and 34channel fills in the Wasatch Formation. Thesemeasurements were taken from vertical cutsthrough the channel fills that extended pri-marily across the paleoflow direction. To cor-roborate our interpretations, prominent surfac-es were traced as far in the upstream anddownstream directions as exposure permitted(always ,40 m). The dimensional data col-lected at each site, therefore, are best thoughtof as capturing the local form of a paleochan-nel fill, equivalent to dimensional data onemight measure in a modern river at any givencross section. For example, the reconstructedvalues for depth of channel flow, incisiondepth, and channel superelevation discussed inthe following represent local values ratherthan the reach-averaged values for that partic-ular channel. When interpreting the data set itshould be recognized that the scatter amongpoints includes not only the variability amongchannels, but the variability within a singlechannel as well.

Measurements were not corrected for com-paction of the sediment bodies during accu-mulation and burial. Recent studies argueagainst differential compaction of sandy chan-nel fills versus muddy overbank deposits asplaying a significant role in modifying theoriginal geometry of individual alluvial chan-nel bodies (Willis, 1993b; Nadon and Issler,1997). Early sediment dewatering and paleo-sol development and the low abundance ofclay deposits in fluvial systems limit differ-ential postdepositional compaction. We didnot notice any offsets of deposits between themain channel bodies and the channel wings,which suggests that differential compactionand postdepositional modification of the sand-stone bodies were insignificant.

At each channel fill three key measurementswere taken that we believe capture the essen-tial geometry of the channel immediately be-fore and soon after an avulsion took place.These measurements are taken as proxies for(1) flow depth, which provides a measure ofthe characteristic depth of an individual chan-nel during bankfull flow, and is the lengthscale by which flows of different size can becompared; (2) maximum incision depth,which represents the scouring into preexistingflood plain during, or soon after (i.e., beforethe final channel fill was deposited), channelavulsion; and (3) superelevation, which rep-resents the maximum height of the bankfullfree surface (levee height) above the adjacent

flood plain, which provides local potential en-ergy. These channel belt geomorphic elementsare shown in Figure 3 and described in thefollowing.

Flow Depth Proxy

We used the thicknesses of preserved bardeposits as a measure of paleoflow depth (Fig.3), an approximation that has been used nu-merous times in interpreting fluvial deposits(Allen, 1965a, 1965b; Lorenz et al., 1985;Miall, 1993; Willis, 1993a). Bar height is asurrogate for flow depth because, unlike rip-ples and dunes, bars in rivers are able to growin height nearly to the free surface. There aretwo potential problems in applying this con-cept to the stratigraphic record. The first is thepossibility that much of the preserved bar hasbeen removed by subsequent erosion. If so,we could greatly underestimate paleoflowdepths. This problem is circumvented by de-liberately searching for bar accretion units thatare completely preserved, as indicated by fullysigmoidal shapes (including rollover at the bartops, e.g., Fig. 7A) and/or the presence of finebartop deposits.

The second concern is that even if com-pletely preserved, bar forms might not be rep-resentative of true channel-forming flowdepths. In order to assess this problem, wedocumented the relationship between local barheight and average flow depth of the modernNorth Loup River, at Taylor, Nebraska (Fig.11). Bar forms in this modern river freely mi-grate downstream (Fig. 11A) and, as describedhere, are therefore appropriate analogs for thebar accretion sets observed in the Guadalope-Matarranya system. The measured bar heightsrange from 0.1 to 3 times the measured av-erage depth of flow in the channel; the averageand median values are 1.03 and 1.23 times theaverage flow depth, respectively (Fig. 11B).This magnitude and distribution of bar heightsis consistent with the well-established rela-tionships that maximum channel depths maybe as much as three or five times the meanchannel flow depth in meandering (Bridge andMackey, 1993) and braided rivers (Best andAshworth, 1997), respectively. For this rea-son, any single untruncated bar set probablyprovides an overestimate of average flowdepth. Erosional truncation, however slight,would tend to reduce this overestimation. Sta-tistical studies suggest that in braided riversthe average ratio of preserved bar height tomean depth is about 0.7 (Paola and Borgman,1991), which we regard as acceptably close tounity. Nonetheless, we attempted to minimizethis truncation effect by measuring the heights

of several completely preserved, or nearly so,bar surfaces in a paleochannel and averagingthese values. It is important to note that theheights of individual bar forms are measuredand not the heights of cumulative bar sets, be-cause the latter can include an aggradationalcomponent as bars climb at low angles duringmigration (oblique climbing bar forms).Where possible, we also measured the heightsof mud plugs that represent the channel formleft behind as the stream abandoned the site,and averaged all of these values.

Incision Depth Proxy

Measures estimating the depth to which anysingle paleochannel had cut into its alluvialsurface were collected only at sampling loca-tions with complete exposure of at least onechannel margin and the flood-plain deposit di-rectly adjacent to it. The scour depth is takenas the shortest vertical distance between thebase of the channel-filling deposit and thebase of the aggradational (levee) deposit (Figs.3, 7, and 10C). It is straightforward to mea-sure this length in a single-story channel body,but for those bodies of sediment representingthe stacked deposits of multiple channels, theincision associated with each individual paleo-channel was carefully broken out throughanalysis of the composite lateral margins.

Superelevation Proxy

Superelevation is a measure of the relief be-tween the river’s free surface and some pointon the adjacent flood plain that represents thelocal potential energy minimum. While thefree surface is reflected by maximum leveeheight, and so is relatively easy to determine,it is less obvious how to uniquely identify thelow point on the nearby flood plain fromwhich relief should be measured. On an ide-alized flat flood plain, the lowest adjacentpoint is, or would soon become, the placewhere aggradation rate is minimal. In an un-confined flood plain, overbank sedimentationasymptotically approaches zero far from thechannel margin (Pizzuto, 1987). As a result,without evidence to the contrary, we assumethat there is a point normal to the channel atwhich sedimentation is nil, in which case thetotal aggradation height of the levee marginapproximates local superelevation (Fig. 3).Any preexisting topography on the flood plaincould either increase or decrease local super-elevation to some extent, but cannot be as-sessed and so is ignored.

A potential problem is that the preservedlevee sands could represent a form of artificial


MOHRIG et al.

TABLE 2. DATA FROM THE GUADALOPE-MATARRANYA SYSTEM

Distance Flow Incision Superelevation I/D S/D(km) depth depth (S)

(D) (I) (m)(m) (m)

8.9 2.109.1 1.25 1.95 1.569.2 2.509.9 1.4210.0 1.2010.0 1.2010.0 1.7010.0 1.5010.3 1.0010.4 1.30 1.60 1.05 1.23 0.8110.4 1.6010.4 1.2010.4 1.3010.6 1.20 2.40 2.0010.6 1.3010.6 1.1011.0 1.0913.2 2.08 2.81 1.3513.4 1.25 1.45 0.90 1.16 0.7214.1 1.2015.6 1.49 1.25 0.94 0.84 0.6315.6 1.7515.6 0.7015.7 1.3215.9 2.3515.9 1.7515.9 2.2016.2 1.4016.2 0.9616.2 1.3016.2 1.9016.8 1.54 1.46 0.80 0.95 0.5216.8 1.1016.8 0.6017.8 0.9017.8 0.8017.8 0.8818.1 0.95 2.05 2.1618.1 1.0018.2 1.35 1.71 1.2718.2 1.1518.6 1.0418.6 1.7518.7 1.3018.7 1.20 0.87 0.90 0.73 0.7518.7 1.2519.0 1.5519.0 1.5019.0 1.9019.2 1.20 0.75 0.6319.2 1.00 1.80 0.55 1.80 0.5519.2 1.6519.2 0.7019.2 2.00 2.84 2.04 1.42 1.0219.2 1.58 1.80 0.60 1.14 0.3819.3 1.78 2.05 0.72 1.15 0.4119.3 1.85 1.40 0.78 0.76 0.4219.7 1.4220.1 1.50 1.50 0.60 1.00 0.4020.1 1.5320.1 1.00 2.15 0.30 2.15 0.3020.2 1.89 2.00 1.85 1.06 0.9820.3 1.4520.3 1.9020.3 1.6521.4 1.03 1.65 0.70 1.60 0.6821.4 1.95 0.77 1.58 0.40 0.8121.5 1.0021.6 1.10 1.90 1.7321.6 2.0021.6 1.1021.7 1.9021.7 1.80 1.20 0.6721.7 0.7421.7 0.79

TABLE 2. (Continued)


(D) (I) (m)(m) (m)

21.7 0.9222.0 3.2222.0 3.0122.0 1.5722.6 1.93 2.35 1.38 1.22 0.7223.6 1.2823.9 1.70 3.09 1.20 1.82 0.7123.9 1.2524.2 2.85 3.51 1.20 1.23 0.4224.2 1.9024.2 1.7024.2 0.9324.7 1.6324.7 1.2024.7 1.0224.7 1.0028.1 1.05 1.45 1.3828.7 2.6028.7 1.6029.0 1.9629.3 1.00 1.00 0.65 1.00 0.6529.8 1.40 1.60 1.1429.8 1.3030.1 1.3030.1 1.9030.1 1.2030.5 1.3031.0 1.09 2.15 0.83 1.97 0.7632.6 1.10 1.45 0.77 1.32 0.7032.6 1.2032.9 1.68 2.70 0.78 1.61 0.4633.4 1.46 1.88 1.2934.0 1.65 2.39 0.80 1.45 0.4935.6 1.30 1.85 1.4235.6 1.2535.6 1.4436.2 2.04 1.02 0.5037.2 1.0537.2 1.3538.1 1.22 0.57 0.4738.1 1.5038.2 1.04 2.25 0.55 2.16 0.5338.5 0.7040.1 2.0040.1 2.1640.1 2.7640.6 1.4540.6 1.3940.6 2.40 1.35 0.5640.6 1.50 1.95 1.3040.6 1.2142.2 1.40 1.50 0.50 1.07 0.3642.3 1.75 2.00 0.75 1.14 0.4342.3 1.5542.8 1.0043.3 1.35 1.10 0.8243.8 1.40 0.60 0.4343.8 1.80 2.50 0.80 1.39 0.4443.8 0.6843.8 1.4543.9 1.60 2.13 0.93 1.33 0.5843.9 1.5544.3 1.00 0.90 0.68 0.90 0.6844.7 0.5245.6 0.60 0.75 1.2545.8 1.5946.1 0.7646.6 1.50 2.24 0.83 1.49 0.5546.7 1.6046.8 2.10 0.73 0.3547.1 1.2047.1 1.0747.1 0.7247.1 2.5047.1 1.3047.9 0.68

TABLE 2. (Continued)


(D) (I) (m)(m) (m)

48.9 2.0849.0 1.45 1.99 1.06 1.37 0.7349.0 0.80 0.47 0.5949.0 1.3549.5 1.18 2.60 2.2049.5 1.2350.2 1.0450.2 1.0050.2 1.0050.2 1.25 1.35 1.0850.2 1.2250.9 2.00 1.29 0.6550.9 1.4551.5 0.92 0.91 0.9951.6 1.0651.6 0.8953.2 1.20 0.40 0.3353.4 1.68 0.18 1.50 0.11 0.8953.6 1.3054.0 1.6054.0 1.1054.0 1.3055.2 1.41 2.10 0.62 1.49 0.4455.2 1.50 1.65 1.1056.8 0.90 0.60 0.6757.1 1.2057.3 1.90 1.20 0.59 0.63 0.3157.3 0.8757.5 0.6657.7 2.10 1.52 1.62 0.72 0.7757.8 0.6557.9 0.5859.1 0.6059.1 1.1259.1 0.6359.3 1.0659.3 0.9160.4 0.92 1.50 1.20 1.63 1.3060.6 1.7061.3 0.7062.0 1.6562.1 0.7662.4 2.30 2.30 1.0063.3 1.1063.3 1.2063.3 1.9063.3 0.67 0.35 0.5263.7 1.2063.9 1.20

Notes: Distance is measured from the ‘‘LinkingZone’’ reentrant. I/D—normalized incision depth; S/D—Normalized superelevation.

stratigraphic relief if the flood plain continuedto accumulate fine-grained material that inter-fingered with levee sands. This can only betrue in confined valleys where flood-plain sed-imentation pervades the valley width. How-ever, the Guadalope-Matarranya and Wasatchsystems do not appear to have occupied majorpaleovalleys. In addition, other field observa-tions in these units suggest that the total ag-gradational thickness of the levees approachesthe original topographic relief: (1) in placesthe uppermost sandy bed of the levee depositprogrades laterally out onto the flood-plainmudstones at a level only slightly above thelowest levee deposit next to the channel (Fig.



Figure 12. Histograms of (A) paleoflowdepth, (B) normalized incision depth, and(C) normalized superelevation measured inthe Guadalope-Matarranya system. Popu-lation means are shown by heavy verticallines. Data are from Table 2.

10C); (2) typically the levees contain beds thatapproximately parallel the upper boundingsurface of the levee and in no case are seento cross it and downlap against the subjacentmudstones (Fig. 7B), suggesting that they pri-marily merge downward into underlying over-bank deposits; and (3) the upper surface of thelevee is a sharp, continuous bounding surfacebetween the levee sandstones and the over-bank mudstones (Fig. 7).

RESULTS

Channel Measurements

All of the measurements made in the Gua-dalope-Matarranya system are presented in

Table 2. Flow depth measurements for the sys-tem vary by less than an order of magnitude;the mean value is 1.38 m (Fig. 12A). Al-though there may be a very slight decrease inaverage proxy flow depths along the length ofthe system that coincides with the gradual de-crease in mean grain size (Fig. 8), we assumethat flow depths represent a single populationfor the entire system. The population histo-gram for the entire system (Fig. 12A) is welldescribed by a two-parameter gamma distri-bution (mean 5 1.38 m, coefficient of varia-tion [standard deviation/mean] 5 0.35, n 5200). This compares well with depth distri-butions from the Niger River (Paola and Borg-man, 1991) and bar-height data from the NorthLoup River (Fig. 11B), both of which are alsogamma distributed with low coefficients ofvariation (0.51 and 0.55, respectively). Suchconsistency among all of these data sets sug-gests that our sampling adequately describesthe natural variability of flow depth throughthe Guadalope-Matarranya system.

The near constancy in measured paleoflowdepths and the narrow range of widths sug-gests that the channels active on the alluvialsurface during any given interval of time didnot systematically merge or diverge with dis-tance downstream. We interpret the many ex-humed channel-filling sediment bodies of theGuadalope-Matarranya system to represent thedeposition by some small number of frequent-ly avulsing channels that flowed across abroad alluvial plain, similar to the modern As-siniboine River (Fig. 4).

Incision depths range from 0.18 to 3.51 m,and superelevations range from 0.40 to 2.04m. The scatter in this raw data is great andoverwhelms any possible trends that might ex-ist along the length of the system between themountain front and lakeshore.

Flow depth, incision depth, and superele-vation measurements for the Wasatch Forma-tion are shown in Figure 13 and Table 3. Pa-leoflow depths, estimated from the maximumthickness of mud plugs and the maximum re-lief of nearly complete accretion surfaces (barheights), range from 0.65 to 3.4 m (average 51.58 m). Incision depths range from 0.62 to4.85 m (average 5 2.11 m) and supereleva-tions, measured at channel edges, range from0.55 to 4.40 m (average 5 1.76 m).

DISCUSSION

Interpreting Normalized Data

In our analysis, we normalize incision depthand superelevation by local flow depth so thatdata from both large and small flows can be

easily compared. We have done this by divid-ing maximum incision depth (I) by flow depth(D) at each site to yield normalized incisiondepth (I/D). Superelevation (S) is likewise di-vided by D to calculate normalized superele-vation (S/D).

Normalized Incision Depth (I/D)

The distribution normalized incision depth(I/D) from the Guadalope-Matarranya system(Fig. 12B) is fairly narrow; all measured in-cisions are ,2.4 times their associated flowdepths, and the average is 1.3 times flowdepth. In the Wasatch Formation, values of I/D cluster around 1.4, similar to the Guada-lope-Matarranya system. These average valuesfor I/D indicate that, as channels avulsed to anew position on the flood plain, or soon there-after, they typically incised into the overbankmaterial by some small amount greater thantheir characteristic flow depth.

It is unlikely that channel cutting, and sub-sequent infilling, are controlled by climate orbase-level change (cf. Anadon et al., 1989).Whereas allocyclic controls are capable ofproducing cut and fill cycles, such effects tendto concentrate near, and decay away from, up-stream or downstream boundaries of a fluvialsystem (e.g., Leopold and Bull, 1979; Wyroll,1988; Humphrey and Heller, 1995). No suchtrend exists along the length of the Guada-lope-Matarranya system. In addition, in boththe Guadalope-Matarranya and Wasatch de-posits there is a relatively narrow range ofnormalized incision depths, none more than afew times the paleoriver flow depth. Whilethis range of incision is consistent with auto-cyclic variations in modern rivers (Best andAshworth, 1997), serendipitous base-level (orother allocyclic) changes would be required toalways result in incisions that are closelyscaled by river flow depth. In these systems,base-level changes would always have to be afew meters at most and to have affected theentire length of the alluvial system subequally.Thus we believe autocyclic incision, associ-ated with avulsion, is the most likely expla-nation for the incision acting in this systemduring the time interval under study.

Normalized Superelevation (S/D)

The maximum thickness of bounding le-vees, or superelevation, normalized to the lo-cal flow depth (S/D) is on average 0.61 (Fig.12C). As with the I/D value, there is no sys-tematic change in S/D along the length of theGuadalope-Matarranya system. The S/D mea-surements from the Guadalope-Matarranya


MOHRIG et al.

Figure 13. Histograms of (A) paleoflow depth, (B) incision depth/flow depth, and (C) su-perelevation/flow depth measured in the Shire Member of the Wasatch Formation. Pop-ulation means are shown by heavy vertical lines. Data are from Table 3.

TABLE 3. DATA FROM THE SHIRE MEMBER OFTHE WASATCH FORMATION

Total Flow Incision Super- I/D S/Dsandstone depth depth elevation

body (D) (I) (S)thickness (m) (m) (m)

(m)

5.90 2.45 3.15 2.75 1.29 1.126.30 1.90 3.30 3.00 1.74 1.582.65 2.70 1.12 1.53 0.41 0.572.33 1.05 0.83 1.50 0.79 1.432.75 1.10 1.30 1.45 1.18 1.322.22 1.70 0.62 1.60 0.36 0.944.10 1.70 2.15 1.95 1.26 1.154.93 2.37 1.33 3.60 0.56 1.524.90 1.80 3.00 1.90 1.67 1.065.00 2.70 2.30 0.855.15 1.40 3.66 1.49 2.61 1.063.60 0.70 1.70 1.90 2.43 2.719.15 3.40 4.75 4.40 1.40 1.294.70 0.65 2.32 2.38 3.57 3.664.64 1.90 2.84 1.80 1.49 0.955.10 1.60 3.08 2.02 1.93 1.262.24 0.95 1.34 0.90 1.41 0.952.95 2.05 0.92 2.03 0.45 0.993.10 1.20 2.16 0.94 1.80 0.781.35 0.74 0.80 0.55 1.08 0.744.95 1.60 2.85 2.10 1.78 1.315.65 1.30 3.85 1.80 2.96 1.381.70 0.65 0.85 0.85 1.31 1.312.90 1.75 1.70 1.20 0.97 0.691.95 1.50 0.65 1.30 0.43 0.877.00 2.70 4.85 2.15 1.80 0.802.90 1.50 1.30 1.60 0.87 1.072.15 1.15 1.45 0.70 1.26 0.612.70 0.65 2.15 0.55 3.31 0.852.10 0.70 1.45 0.65 2.07 0.93

Note: I/D—normalized incision depth; S/D—normal-ized superelevation.

and Wasatch systems (Figs. 12C and 13C) of-fer strong support for the idea that superele-vation is limited by local flow depth. In theGuadalope-Matarranya system, where wehave the largest data set, only 4% of measuredsuperelevation values are greater than their as-sociated flow depths, and then only slightlyso, whereas ,1% of the superelevations areless than one-quarter of their associated mea-sured flow depths. The data suggest that theability of the streams to aggrade above theflood plain was limited, and only in rare casesin the Guadalope-Matarranya system did thebase of the active channel become perchedabove the flood plain.

In the Wasatch Formation, a mean value of1.1 for S/D (Fig. 13C) suggests that, on av-erage, channel floors became only slightlyperched above the adjacent flood plain beforeavulsion took place. Conversely, the absenceof values ,0.5 suggest that as long as the baseof the channel was the lowest spot along ornear the flow path, it maintained the flow,

even when levees were breached duringfloods.

The distributions of the S/D data from thesesystems suggest that in order for avulsion tobe successful, the channel floor of the activeriver must approach or be above the flood-plain elevation. If the channel is too deep,avulsion will likely not occur, regardless ofhow far above the flood plain the free surfaceof the channel flow is found.

We interpret the mean value for S/D as aminimum estimate for the typical value asso-ciated with avulsion. A fundamental problemwith directly using local values of levee heightas a measure of critical superelevation (i.e.,the superelevation just prior to avulsion) isthat we do not know how far away a particularcross section is from the actual avulsion point.When avulsion takes place, some reach of thedownstream channel is abandoned. Therefore,for some distance downstream, channel formsthat become preserved in the rock record maynot be truly representative of the channel ge-

ometry at the avulsion site, and thus the mea-surements underestimate the requisite avulsionsetup conditions. However, we do not thinkthat our levee measurements significantly un-derestimate the critical superelevation of thesechannels because we found no systematicdownstream variation in superelevation todepth ratio in the Guadalope-Matarranya sys-tem, suggesting that the relative rate of chan-nel and flood-plain deposition does not varystrongly downstream. Thus, like the Assini-boine River (Fig. 4), it appears that channelaggradation was relatively constant over longreaches of the channel belt and that many sitesalong a channel approached a critical super-elevation at nearly the same time.

The well-defined gamma distribution of thesuperelevation data from both systems (Figs.12C and 13C) indicates that while there wasa minimum height that generally was achievedbefore avulsion, there was also a stochasticprocess involved in determining when anygiven channel would avulse. This distributionlikely reflects the variations in local conditionsthat were superimposed upon the more generalrequirement of a minimum superelevation.Such triggering processes likely includedflood hydrograph shape and magnitude;



changes in flux and caliber of sedimentaryparticles transported and deposited; and cata-strophic bank collapse. Differences in meanvalues between the Guadalope-Matarranyaand the Wasatch Formation may reflect thesesecondary controls, which varied between thetwo study areas.

Significance of Variances

In both the Guadalope-Matarranya and Wa-satch systems, there is more variance in thenormalized incision depth data than in the nor-malized superelevation data (0.19 vs. 0.04 forthe Guadalope-Matarranya and 0.59 vs. 0.16for the Wasatch Formation; Figs. 12 and 13).These differences most likely reflect importantdifferences in both processes and time scalesof incision versus aggradation. In order to de-velop an incised channel form, either basalscour must occur as water is introduced to anew position on the flood plain, or the scoursurface develops some time soon thereafter, byrepeated passage of the deepest scour pits.Therefore channel incision depth records, atmost, a few unusually deep erosion events thatpass through any given channel sector. It maytake time before maximum incision isreached; however, incision precedes final fill-ing of the channel. In contrast, levee aggra-dation is an incremental process requiringmany individual flood events. Because aggra-dational processes tend to subdue landscapesby filling topography, slow growth of leveesacts to suppress development of extremeforms. As a result of differences betweenthose processes that tend to average out overtime (S/D) versus those that result from a fewlarge events (I/D), the distribution of normal-ized superelevations is much tighter than thedistribution of normalized incision depths.

Evaluation of Uncertainties

Uncertainties in measurements come fromseveral sources; the most important is incom-plete preservation of original thickness whereerosion has taken place. We have attempted tominimize these problems by choosing themost complete sections where lateral conti-nuity is also available. In addition, there is noway of knowing if data sites are located nearwhere avulsions took place or from points far-ther away from the avulsion sites. In the lattercase, a channel becomes abandoned, eventhough locally it was not ready to avulse onits own. Therefore, we deemphasize the spe-cific values measured at any given channeland instead emphasize the clustering of thedata and the well-developed probability dis-

tribution as being meaningful and useful inunderstanding river behavior.

To some degree, the importance of theseuncertainties is reduced by using normalizedsuperelevation in the analysis. By using a ratio(S/D) in cases where flow depth exceeds su-perelevation height (nearly all the sites), un-certainties are reduced. Conversely, in caseswhere normalized superelevation is .1, moretypical in the Wasatch example, there is atrend toward increasing error.

Channel Reoccupation

The superimposed channel belts within themultistory sandstone bodies (Fig. 10) in theGuadalope-Matarranya system show clear ev-idence of reoccupation of a fixed thoroughfareby a succession of channels. The relative fre-quency of superimposed channel belts sug-gests that, at least part of the time, abandonedchannels remained as relative lows on theflood plain and acted as attractors to avulsingriver channels. Such reoccupation of recentlyabandoned channels or secondary flood-plainchannels is a common characteristic of manymodern river avulsions. Examples include theLower Old River connecting the Mississippiand Atchafalaya Rivers (Fisk, 1952), Missis-sippi River flood-plain channels (Kesel et al.,1974), Rainbow Creek (Brizga and Finlayson,1990), the Assiniboine River flood-plain chan-nels (Rainne, 1990), and the River Rapti, inIndia (Richards et al., 1993). Some reasonsreoccupation occurs are: (1) as long as theyare not filled in, preexisting channels provideready-made conduits; and (2) even if they arefilled, sandy channel fills are often easier toscour out than surrounding flood-plain mate-rial (Smith et al., 1998). Reoccupation of to-pographic lows should produce stacked chan-nel sandstone bodies in the stratigraphicrecord.

Reoccupation of abandoned channels thatare low points on the flood plain is in markedcontrast to the occurrence of abandoned chan-nels as alluvial ridges. Ridges act as topo-graphic barriers that repel newly formed chan-nels until the ridge is buried by flood-plainsedimentation. Such repulsion is a built-incomponent of the early alluvial architecturemodels (Allen, 1978; Bridge and Leeder,1979; Mackey and Bridge, 1995). It may be,however, that abandoned channel sites that arenot completely filled act as semicontinuousthoroughfares that present preferential pathsfor avulsions. As such, channel position in theflood plain may not be as random as is oftenassumed in architecture models.

Model for River Avulsion

Comparing our observations with those ofSmith et al. (1989) leads us to generalize atwo-end-member model of river avulsion.These members are primarily distinguished bywhether flood-plain scouring occurs early orlate in the avulsion process. The avulsion styleseen in this study, which we believe is com-mon in the rock record, we call incisionalavulsion, and it includes the following key el-ements (Fig. 14).

Some trigger event, such as bank collapse,major flood event, or biologic disturbance, al-lows the channel to empty out onto the floodplain. Avulsion may take place either rapidly,such as during a single flood, or more gradu-ally (e.g., Tornqvist, 1994) over a prolongedperiod during which multiple, anastomosedchannels are active.

During avulsion a river moves to a relative-ly low spot on the flood plain. Connection oftopographic lows, in some cases facilitated bythe reoccupation of abandoned channels, pro-vides a thoroughfare for the new river reach(Fig. 14A).

Early in its development, the avulsed chan-nel cuts into the underlying overbank materialby as much as a few flow depths (Fig. 14B).Over time aggradation along the channel belttends to perch the active channel to the pointabove the flood plain. By the time the channelbase approaches the average elevation of theadjacent flood plain, there is sufficient poten-tial energy for avulsion to take place. Oncethis setup has developed, avulsion will againoccur due to some trigger event.

Incision at the base of the newly formedchannel may develop gradually over time bythe passage of many bars, the largest of whichhave attendantly deep scour pits that cut theunderlying flood-plain materials. The amal-gamation of many of these largest local scoursurfaces can create the continuous scour sur-face that forms the boundary of the incisedchannel. Alternatively, the incisional base ofthe nascent channel may be caused by head-ward erosion of a knickpoint that first formswhere the newly formed rejoins the activechannel or a tributary downstream (Fig. 14A).Any knickpoint formed where the river flowsfrom the flood plain back down into the pre-existing channel will migrate up the newlyformed channel through time (Fig. 14B). Theability of an avulsion event to succeed likelyreflects such factors as duration and peaked-ness of the triggering flood, ratio of sedimentflux to water flux, geometry of the opening atthe avulsion node, and preexisting topographyof the flood plain.


MOHRIG et al.

Figure 15. Aggradational avulsion model. (A) Early stage where avulsion has abandoneda river reach and flows out across the flood plain. Rapid aggradation during this phaseforms a prograding wedge of braided stream deposits filling in the topographic lows ofthe flood plain. We have shown the case where avulsion reattachment occurs along atributary channel that returns flow to the main channel. (B) Later, flow strands integrateinto a single flow that locally erodes the early aggradational deposits, and avulsion iscomplete.

This model of avulsion contrasts with theaggradational avulsion model (our term) de-scribed by Smith et al. (1989). Their obser-vations of the Saskatchewan River led themto suggest that an early phase of rapid aggra-dation takes place as sediment-laden water ex-its the main channel and flows across the floodplain. Many small channels thread across theavulsion zone and eventually feed back intothe main channel, possibly along a preexistingtributary, at some point downstream (Fig.15A). Eventually the multiple channel threadsare succeeded by a single large channel of suf-ficient flow depth to locally scour throughthese earlier aggradational deposits (Fig. 15B).In essence the incisional avulsion model gen-erates a ‘‘cut then fill’’ sequence and the ag-gradational avulsion model predicts a ‘‘fillthen cut’’ sequence.

The key to identifying aggradational avul-sion in ancient strata is to find the early phaseaggradational deposits immediately underly-ing the channel fill. Sediment transport bymany small channels requires a larger netchannel width than a single, deeper channelunder conditions of constant discharge. There-fore, early phase deposits should be left be-hind in the rock record even after the avulsionreverts back into a single channel. Such de-posits have been identified in the WillwoodFormation of northern Wyoming (Kraus,1996). However, absence of early phase de-posits may result from subsequent erosion dueto channel enlargement or migration. We notethat these models are end members in thesense that incisional avulsion may have manyimbedded episodes of aggradation and viceversa. The key difference is that in the formermodel incision dominates the early phase ofavulsion, and the opposite is true for the ag-gradational avulsion model.

Role of Flood-Plain Drainage

To some extent, whether incisional or ag-gradational avulsion occurs depends upon thepreexisting morphology of the flood plain. Ifthe water and sediment flow directly onto apreexisting flood-plain channel, modificationof the channel form may be minor if the chan-nel geometry is adequate to handle the newdischarge. Even if the channel must expand toaccommodate the increase in discharge, mod-ification of reoccupied channels is rapidly ac-complished by sedimentation and erosion asthe new flow is focused along the channel.

However, if local relief of the flood plain issignificantly less than the depth of the inun-dating flow, then the exiting flow is not con-fined. The subsequent evolution of the outflowdepends on whether the first event is erosionalor depositional (Smith et al., 1989; van Gelderet al., 1994; Schumann, 1989). This evolutioncan depend upon a number of factors relatedto the geometry of the flood plain, the sedi-ment and water flux in the main channel, andoverbank flows and grain size.

Ultimately, however, we suggest thatwhether incisional or aggradational avulsiondevelops is mostly a function of the state ofthe flood-plain surface, including its gradient,water-table elevation, sediment type (perme-ability), and vegetation type (acting as a baf-fle). These characteristics control how well theflood plains are drained.

Channel construction on a relatively lowgradient, well-vegetated, poorly drained floodplain was thoroughly documented by Smith etal. (1989). Water spreading over a poorly

drained flood plain decelerates everywhere asit runs into standing water bodies and vege-tation, and deposits sediment. The result isthat the initial period following avulsion ischaracterized by unconfined flow in rapidlyshifting, shallow channels, possibly braided(Fig. 15A). As the fan-shaped body aggrades,the local stream gradient is increased. Even-tually flow focuses into a single channel thatdeepens by cutting into, and possibly through,its earlier avulsion deposit (Fig. 15B). It ismost likely that the avulsion is local and even-tually reoccupies the preexisting main channelin some fashion, either taking advantage of atributary or local low along the preexistingchannel levee.

The well-drained case differs in that the ini-tial unconfined avulsive flow is not forced toimmediately deposit a great deal of its sedi-ment load across the flood plain due to a largedeceleration by running into standing water.While these flows may locally deposit sedi-ment due to the effects of flow expansion,



Figure 14. Incisional avulsion model. (A) Early stage where avulsion has abandoned onereach of the river and returned to the main flow downstream. We have shown the casewhere the avulsive flow has reoccupied a previously abandoned channel and where incisionat the base of the avulsed channel is primarily a function of knickpoint retreat, whichinitiated at the reattachment point. However, return to the main channel may be facilitatedby intersecting a tributary stream, as shown in Figure 15, and basal scour may developby migrating scour pits in front of bar forms. (B) Later stage where the avulsed channelis fully incised into the flood plain.

overall they drain across the flood plain backinto the main channel or a tributary, some dis-tance downstream of the avulsion site. Anylocal drop in elevation, such as along the bankwhere the flow reenters the main channel, canpromote knickpoint retreat that propagates up-stream (Fig. 14A). An example of this processis seen at Red Creek, Wyoming (Schumann,1989). As the new channel elongates (Fig.14B), it eventually captures the flow from theoriginal channel (McCarthy et al., 1992). Inour example, we suggest that the avulsive wa-ter path reoccupies, in part, the topographiclow left behind from an earlier channel path(Fig. 14A). However, reoccupation is not a re-quirement of the model.

The observation that incisions seem tobound the fluvial sandstone bodies, with noevidence of sheet-like early avulsive deposits,suggests that avulsions in the Guadalope-Ma-tarranya system and Wasatch Formationformed in well-drained flood plains. This in-

terpretation is consistent with the developmentof well-oxidized paleosols in silty overbankdeposits in both of these systems. However,detailed paleosol interpretations and paleocli-mate reconstructions have yet to be done.

CONCLUSIONS

Fluvial deposits of the Guadalope-Matar-ranya alluvial system in Spain and the ShireMember of the Wasatch Formation in Colo-rado record deposition by avulsive river sys-tems. A large number of well-exposed channelfills in these systems allows us to collect astatistically significant data base that providesinsight into the channel setup required foravulsion to be successful. Conclusions includethe following.

1. Average maximum incision depths ofbasal channel scours, as normalized by localflow depth, are about 1.3 to 1.4 times thebankfull flow depths, and average superele-

vation of levee crests above attendant floodplain are 0.6 to 1.1 times channel flow depthfor the Guadalope-Matarranya and Wasatchsystems, respectively. These data are consis-tent with autocyclic processes in modern riv-ers and indicate that: (1) channel incision intothe flood plains following avulsion and sub-sequent filling by channel deposits are closelyscaled with flow depth, and (2) the channelsaggraded and were perched such that the chan-nel floor was either not far below, or even alittle above, the elevation of the adjacent floodplain before there was sufficient lateral insta-bility for avulsion to occur. Perching of thewater surface above the flood plain is not suf-ficient to drive avulsion. The magnitude ofboth the channel incision and filling is scaledby flow depth so that larger rivers have deeperincisions and higher superelevations than doshallower channels. The limited range of cut-ting and filling suggests that allocyclic con-trols may be more easily identified wherethese values are greater than a few times flowdepth.

2. In a surprisingly large number, up to24%, of channels preserved in the Guadalope-Matarranya system we found that rivers hadreoccupied previously abandoned channels. Insuch cases channels acted as long-term thor-oughfares to which streams would tend to re-turn during a later avulsion. In this way pre-vious channel belts can act as attractors tosubsequent avulsions.

3. These results and comparison to pub-lished avulsion studies suggest a modelwhereby avulsion is first associated with cut-ting by channels into the flood plain and sub-sequent filling by channel deposits. This in-cisional (cut then fill) model differs from theaggradational (fill then cut) model of Smith etal. (1987). The latter model requires an earlyphase of aggradation by multiple channels,followed by reversion of flow into a singlechannel, which enlarges and cuts through itsearly phase deposits. We propose that flood-plain drainage characteristics exert major con-trol between the incisional and aggradationalavulsion modes. Poorly drained flood plainscan support standing water bodies that wouldtend to slow flood waters, forcing deposition.In contrast, well-drained flood plains promotethrough flowing channel systems and associ-ated early erosion by a variety of mechanisms.

4. Studying ancient fluvial sequences inwhich hundreds or thousands of iterations ofchannel avulsion can be observed may pro-vide a valuable database to augment modernprocess studies. This inverse actualism ap-proach provides a useful component in theanalysis of this, and other, geologic problems.


MOHRIG et al.

ACKNOWLEDGMENTS

Funding was provided by a grant from the Na-tional Science Foundation (EAR-9217171). Wehave benefited from thoughtful reviews from PeterDeCelles, Mary Kraus, Beth McMillan, and NormSmith.

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