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

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STRATUM
Figure 27. Box plots showing the range in bankfull flow in specific reaches of the Colorado River.
DISCUSSION
Most contemporary models of fluvial systems describe the longitudinal changes in rivers as an
orderly progression in which the channel characteristics adjust smoothly downstream to carry
increasing amounts of water and sediment. Empirical and theoretical relations for hydraulic
geometry indicate that the bankfull width and depth increase downstream as power functions of
discharge (Church, 1992; Hey and Thorne, 1986; Parker, 1979; Simons and Albertson, 1960;
Leopold and Maddock, 1953), while the slope and grain size are observed or assumed to decrease
as exponential functions of distance (Pizzuto, 1992; Snow and Slingerland, 1987; Brierly and
Hickin, 1985; Bradley et al., 1972). These changes in width, depth, slope and sediment size are
sometimes accompanied by changes in planform and sinuosity (Schumm, 1985). The ecological
implications of downstream transitions in channel characteristics have been discussed in many
papers, notably those by Vannote et al. (1980) and Stanford et al. (1996).
The results of this study show that, while the physical characteristics of the Colorado River do
indeed change systematically downstream, the changes do not occur in the manner that is often
described in the geomorphological literature. Specifically, we observe that the bankfull depth of the
Colorado River increases faster downstream than the bankfull width, resulting in a nearly constant
width-depth ratio. This observation contrasts with results from many other studies (summarized by
Knighton, 1998) showing that an increase in width-depth ratio is typical of most alluvial rivers. In
addition, our sediment data show that the bed material of the Colorado River changes, slowly
downstream- the D50 of the surface sediment decreases by a factor of only 2 over a distance of
more than 250 km. Such slow changes in grain size are not expected, and are explained in this case
by the influence of ephemeral tributaries and hillslopes which deliver coarse sediment to the main
stem without supplying much additional water. In order to satisfy the basic requirement of carrying
this sediment, the Colorado River has developed a deep, narrow channel that provides sufficient
shear stress to carry the imposed load without much additional flow.
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