8274

• UCREFRP Catalog Number: 8274
• Author: Pitlick, J. and R. Cress.
• Title: Longitudinal Trends in Channel Characteristics of the Colorado River and Implications for Food-Web Dynamics.
• USFW Year: 2000.
• USFW - Doc Type: Recovery Implementation Program Project 48-C,
• Copyright Material: NO

The geomorphology of the study reach was characterized using closely spaced measurements of
channel gradient, bed material and cross section morphology. These measurements indicate that the
characteristics of the Colorado River change systematically downstream, but not in the manner
expected from geomorphological studies of other rivers. Specifically, we observe that the grain size
of the substrate sediment decreases rather slowly in comparison to the average gradient. This
observation contrasts with the common assumption that substrate sediment size and slope are
highly correlated. In addition, we observe that the average bankfull depth increases downstream
more rapidly than the average bankfull width. Over the 300-km length of the study area, the
bankfull depth increases by roughly 100% (doubles) while the bankfull width increases by only
about 30%. This result contrasts with results from many other studies showing that the bankfull
width of an alluvial river typically increases downstream faster than the bankfull depth.
Thresholds for coarse-sediment transport were estimated on the basis of flow modeling at 10 sites,
and measurements of channel characteristics throughout the study area. These results indicate that
the framework gravels of the Colorado begin moving at flows somewhere between 35 and 55% of
the bankfull discharge; flows higher than this occur on average about 30 days per year. Flows
equal to the bankfull discharge are assumed to rework most of the bed material; the frequency of
these flows varies from several (5-7) days per year in upper reaches to a few (1-2) days per year in
lower reaches. The magnitudes of these threshold flows are shown to be similar when individual
reaches are grouped according to the two major tributaries in the area. For the three reaches above
the Gunnison River, we estimate that discharges of 211 to 278 m3/s (7500-9800 ft3/s) will initiate
motion, while discharges of 580 to 623 m3/s (20,500-22,000 ft3/s) will reach the bankfull level; for
the reaches between the Gunnison River and the Dolores River, discharges of 497 to 548 m3/s
(17,500-19,300 ft3/s) will initiate motion, and discharges of 979 to 1320 m3/s (34,600-46,600 ft3/s)
will reach the bankfull level; and for the reaches below the Dolores River, initial motion occurs at
discharges ranging from 561 to 659 m3/s (19,800-23,300 ft3/s), and bankfull flow occurs at
discharges ranging from about 1500 to 2000 m3/s (54,500-71,000 ft3/s).
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, with framework
gravels moving at flows equal to about half the bankfull discharge and higher. These flows are an
important prerequisite for maintaining mobile bed features and in-channel 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 results
indicate that suspended sediment concentrations (turbidity) are typically higher in the lower reaches
of the study area than they are in the upper reaches. 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, 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)
become important in the lower reaches (see related reports by Lamarra, 1999 and Osmundson, 1999
for complete descriptions of the aquatic community structure in the Colorado River).
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