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focuses on these latter two taxa since they constitute the most abundant prey items in the upper basin. Grabowski and Hiebert (1989) reported 0.2 to 0.136 planktonic crustaceans (cladocerans and copepods) per liter in the middle Green River channel and 0.02 to 0.289 organisms per liter in backwaters during 1987 and 1988 (Table 1). Mabey and Shiozawa (1993) documented zooplankton densities in the middle Green River as 0.3 to 1.3 organisms per liter, 1.5 to 7.1 in the Ouray backwater, 63.4 at Intersection Wash (a large backwater), and 206 to 690 in Old Charlie Wash (an off-channel impoundment also known as Woods Bottom) along the Green River on the Ouray National Wildlife Refuge, located downstream from Vernal, Utah (Table 2). In an open water habitat of the Moab Slough, immediately upstream on the Colorado River from Moab, Utah, the density of planktonic crustaceans (cladocerans and copepods) averaged about 2 organisms per liter for backwater sites and about 36 organisms per liter for an open water wetland during the summer of 1993 (Table 2; Cooper and Severn 1994a). Samples of planktonic crustaceans (cl adocerans and copepods) from the Escalante Ranch site on the middle Green River, upstream from Jensen, Utah, contained 0 organisms per liter for the main channel, a mean of 41 organisms per liter for backwaters, and a mean of 71 organisms per liter for an open water wetland (Table 2; Cooper and Severn 1994b). The mean number of cladocerans and copepods from a backwater of the Gunnison River at the Escalante State Wildlife Area, about 5 miles downstream from Delta, Colorado, was 11 organisms per liter of water and the mean number from an open water wetland was 25 organisms per liter (Table 2; Cooper and Severn 1994c). Zooplankton (cladocerans and copepods) samples from a bottomland (Old Charlie Wash) on the Ouray National Wildlife Refuge contained a mean of 31 organisms per liter of water in 1993 (Table 2; Cooper and Severn 1994d). Cooper and Severn reported that samples taken from the mi ddl e Green River and a backwater on the refuge in 1993 did not contain any cladocerans or copepods. The density of cladocerans was 26 organisms per liter and the density of copepods was 28 organisms per liter in a gravel-pit pond along the Colorado River in October. 1993 (Table 2; Cooper and Severn 1994e). These studies demonstrate that the highest zooplankton densities are in off-channel habitats such as flooded bottomlands. Too date, studies have not been conducted to document the success i on of zooplankton speci es or dynami cs of the zooplankton dens i ties in the Upper Colorado River Basin. V. FACTORS AFFECTING DISTRIBUTION OF BENTHIC INVERTEBRATES Benthi c invertebrates are important to the endangered Colorado Ri ver fi shes because they are used as food by juvenile as well as adult fish. The drift of macroinvertebrates exhibits marked diurnal and seasonal periodicity in temperate waters (Hynes 1970) so utilization of benthic invertebrates by the endangered Colorado River fishes would be opportunistic based on availability. The Colorado squawfish is an exception to that statement because wild juveniles over 100 mm in total length become entirely piscivorous in their food habits (Vanicek 1967). Vari ous factors affect the mi crodi stri but i on of benthi c insects in streams including water velocity and substrate size such as gravel, silt, and detritus (Rabeni and Minshall 1976). The field experiment by Rabeni and Minshall was conducted using trays filled with varying sizes of gravel as substrate to test the importance of water velocity and substrate size in the distribution and relative abundance of benthic invertebrates. Benthic insects were 31% more abundant in riffles than in pools. The amount of colonization for most taxa of 6