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One of the main objectives of this study was to examine the relation between physical channel <br />characteristics and the productivity of aquatic organisms that support the food base for native <br />fishes. Primary productivity is influenced by several factors, including nutrient availability, water <br />temperature and sunlight intensity. In turbid rivers, light intensity attenuates rapidly with depth, <br />thus conditions favorable for primary productivity may exist only where the flow is relatively <br />shallow, such as along the edge of the channel or over riffles. Our observation that the width of the <br />Colorado River changes little downstream, while the depth changes considerably means that, at a <br />given discharge, the wetted perimeter of the channel is roughly the same upstream as it is <br />downstream. However, because the flow is deeper downstream than it is upstream, proportionally <br />less of the bed receives sufficient sunlight to allow primary production, which in turn would affect <br />the amount of food available for benthic invertebrates (see Lamarra, 1999). <br />Our field observations and modeling results indicate that mobile gravel substrates are present <br />throughout the study area. Clean gravel substrates can potentially provide habitat for periphyton <br />and benthic invertebrates, and spawning sites for adult Colorado pikeminnow. However, our <br />analysis of suspended sediment trends suggests that it may be more difficult to maintain clean <br />gravel substrates in the lower reaches of the study area (especially below the Dolores River) <br />because, on average, suspended sediment concentrations are higher here than they are upstream. <br />This observation, coupled with the disproportionate increase in channel depth and higher sand <br />loads, suggests that there is a relatively abrupt transition in the physical condition of micro-scale <br />habitats in the lower reaches. Samples obtained by Lamarra (1999) indicate that the unit biomass of <br />periphyton and macro-invertebrates decreases rapidly from upper to lower parts of the study area, <br />reaching asymptotically low levels in strata 1 through 5. Similarly, data presented by Osmundson <br />(1999) show that fish that could serve as forage for Colorado pikeminnow are much less abundant <br />in the reaches below Westwater Canyon than they are in the reaches above. Thus, in a broad sense, <br />there appears to be a clear link between the physical habitat characteristics of the Colorado River <br />and the structure of the aquatic community. <br />Although human activities have influenced many aspects of the Colorado River, it is unlikely that <br />the basic geomorphic characteristics of these reaches were radically different prior to human <br />occupation. For example, given the climate and geology of the region, it is likely that the Colorado <br />River has carried high suspended sediment loads for much of the Holocene. We can also reason <br />that over shorter time scales (say, several hundred years) the average slope of the river has changed <br />little, because a large amount of net deposition is required to change the average slope of a river by <br />even a few percent. Whether the size of the sediment supplied to the river has changed appreciably <br />in historic time is very hard to say, however, exposures in the numerous gravel pits and terraces in <br />the area provide abundant evidence that the Colorado River has always carried a coarse sediment <br />load. Thus, the characteristics of the river that we see today have probably existed for some time, as <br />have the differences in geomorphology. <br />The contrast between upper reaches, characterized by a laterally active shallow channel, versus lower <br />reaches, characterized by a stable, more-incised channel, has likely been an important influence on <br />the fish community structure of the Colorado River (Osmundson, 1999). Adult Colorado <br />pikeminnow appear to favor habitats in the upper reaches, while other fish such as humback chub <br />(Gila cypha) favor habitats in the canyon-bound reaches. Although these segments of the river are <br />different in character, they are linked by the flow of water, sediment and nutrients, thus the recovery <br />program should continue to take a system-wide approach of improving environmental conditions <br />that benefit the aquatic community as a whole, rather than one species. <br />40