GEOMORPHOLOGY AND NATIVE FISH HABITATS IN THE UPPER COLORADO RIVER BASIN Principal Investigator: John Pitlick Contact: pitlick@spot.colorado.edu Geography Department, Box 260 University of Colorado Boulder, CO 80309-0260 phone: 303-492-5906 Sponsors: US Bureau of Reclamation and US Fish and Wildlife Service Upper Colorado River Endangered Fish Recovery Program

•   2006 CHANNEL MONITORING REPORT

•   2000 FOOD WEB REPORT

•   1999 GEOMORPHOLOGY REPORT
  

Problem Statment: Several species of native fish that were once common in the upper Colorado River basin are now endangered and threatened with extinction. Changes in flow hydrology caused by reservoir operations and water diversions have altered important habitats, and restricted the fishes' range and access to former habitats.  Added pressure from non-native species, and changes in food web dynamics, have also contributed to the decline. To fully understand the scope of this problem and better define stategies for recovering the endangered fish, we have been working with biologists from the US Fish and Wildlife Service to determine how habitats used by native fishes are affected by streamflows, sediment transport and channel geomorphology. Approach: Our field studies have focused on alluvial reaches of the Colorado River in western Colorado and eastern Utah.  In these reaches the Colorado River undergoes significant changes in geomorphology, grading from relatively shallow but steep gravel-bed reaches upstream to deeper, more incised sand- and gravel-bed reaches downstream. To evaluate the potential ecological significance of these trends, we made detailed measurements of channel chararacteristics along a quasi-contiguous 300-km reach of the Colorado River, extending from areas above Grand Junction, Colorado, to areas downstream of Moab, Utah (Fig. 1).  The data set includes: 149 measurements of bankfull channel width and depth determined from surveys taken every 1.6 km; quasi-continuous measurements of channel gradient using a mapping grade GPS; and approximately 100 samples of the channel bed material, including both surface and subsurface sediment samples.

Colorado Pikeminnow        Illustration by Joseph R. Tomelleri Razorback sucker                Illustration by Joseph R. Tomelleri

Figure 1. Location map of upper Colorado River study area.

Figure 2. Gravel bar in Ruby-Horsethief Canyon.

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Field Observations: The bankfull width and depth of the Colorado River increase systematically downstream, as one might expect; however, unlike typical rivers, the bankfull depth increases much more rapidly than the bankfull width.  These trends are illustrated in Figure 3, which shows a series of cross sections arrayed from upstream to downstream.  Note that the bankfull depth increases from about 2.5 m at RKM 345 to more than 6 m at RKM 106, whereas over the same distance, the bankfull width increases from about 110 m to 150 m.  The Colorado River appears to have developed these characteristics in response to regional-scale processes which supply water and sediment in disproportionate amounts: Over the length of the study area, the average annual discharge of the Colorado River increases by a factor of only two, whereas the average annual suspended sediment load increases by a factor of almost four [Pitlick and Cress, 2002].  The relations for hydraulic geometry (Fig. 4) show that downstream increases in discharge, Q*, produce a modest increase in bankfull width, B*, a stronger increase in bankfull depth, H*, and strong decrease in slope, S.  The change in depth offsets the change in slope and grain size, resulting in a roughly constant value of the bankfull dimensionless shear stress, t*(0.049).  This result suggests that the bankfull channel geometry of the Colorado River is adjusted to transport bed load at shear stresses roughly 50% higher than the threshold for motion.

Figure 3. Channel cross sections of the Colorado River.

Figure 4. Downstream hydraulic geometry relations.

Implications: The biological implications of our results can be summarized as follows: Mobile gravel substrates exist throughout the study area, as do off-channel habitats such as backwaters.  However, these features vary in their relative abundance and potential suitability as habitat, depending on location within the study area.  Alluvial reaches above Westwater Canyon are characterized by lower flow depths and somewhat coarser substrates than the reaches downstream.  However, because of the influence of channel slope, substrate mobility is roughly the same in all reaches; field observations and modeling results indicate that framework gravels begin moving at flows equal to about half the bankfull discharge.  These flows are an important prerequisite for maintaining habitats such as riffles and gravel bars.  Perhaps just as important are the conditions favoring primary and secondary productivity.  Biological processes at these lower trophic levels are affected by a variety of factors, including nutrient availability, water temperature, turbidity and interstitial void space.  Our analysis of sediment data indicates that suspended sediment concentration (turbidity) increases downstream.  In addition, our cross section data show that the channel becomes considerably deeper downstream without getting much wider.  The net effect of the changes in sediment concentration and cross-section shape is a more box-like channel where proportionally less and less of the bed receives sufficient sunlight to allow primary production.  As a result, the factors that enhance primary and secondary productivity (clear, shallow water) are optimized in the upper reaches, whereas factors that might limit these processes (turbid, deep water) dominate in the lower reaches [see Osmundson et al., 2002]. 

Our analyses of channel geometry and sediment-transport thresholds indicates that the Colorado River has adjusted its bankfull width and depth such that bed load begins moving at about half the bankfull discharge.  Flows exceed this value for about 30 days per year, on average. The consistency in dimensionless shear stress suggests that a given flow level will produce roughly the same intensity of bed load transport along most reaches of the river.  This result provides some assurance that flows designed to maintain or improve in-channel habitat (e.g. flushing flows) will likely achieve the desired effect on a widespread basis.

Osmundson, D.B., R.J. Ryel, M.E. Tucker, B.D. Burdick, W.R. Elmblad, and T.E. Chart, 1998, Dispersal patterns of subadult and adult Colorado squawfish in the upper Colorado River, Trans. Am. Fish. Soc., v. 127, p. 943-956.

Osmundson, D.B., R.J. Ryel, V.L. Lamarra, and J. Pitlick, 2002, Flow-sediment-biota relations: Implications for river regulation effects on native fish abundance, Ecological Applications, v. 12, p. 1719–1739.

Pitlick, J. and R. Cress, 2002, Longitudinal trends in the channel characteristics of a large gravel-bed river, Water Resources Research, v. 38(10), 1216, doi:10.1029/2001WR000898

Pitlick, J. and M.M. Van Steeter, 1998, Geomorphology and endangered fish habitats of the upper Colorado River 2. Linking sediment transport to habitat maintenance, Water Resources Research, v. 34, p. 303-316.

Van Steeter, M.M. and J. Pitlick, 1998, Geomorphology and endangered fish habitats of the upper Colorado River 1. Historic changes in streamflow, sediment load and channel morphology, Water Resources Research, v. 34, p. 287-302.