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water exchange between the mainchannel and backwaters and allowed for greater backwater warming and stability. Alternatively, greater diet overlap and diversity in the lower reach might have been a reflection of a difference in backwater availability between the upper and lower reaches. Tyus and Haines (1991) reported about 150% more backwaters per kilometer in the upper than lower reach. Fishes in the lower reach might have been more crowded in available backwaters resulting in greater shared use and broader intraspecific use of available food. McAda and Tyus (1984) attributed their observations of reductions in diet overlap between Colorado squawfish > 40 mm TL and red shiner or channel catfish to Colorado squawfish's decreased consumption of immature dipterans and increased consumption of fish. However, Ruppert et al. (1993) reported fish larvae in digestive tracts of 15% of adult red shiner (36-79 mm TL) collected from ephemeral shoreline embayments near the confluence of the Green and Yampa rivers. This suggests that high diet overlap between red shiner and young Colorado squawfish might re-occur or continue with larger red shiner even after Colorado squawfish shift to a more piscivorous diet. Although we documented high diet overlap between young Colorado squawfish > 10 mm TL and other fishes in backwaters of the Green River, especially channel catfish (Table 2), only red shiner, because of its extreme abundance (Haines and Tyus 1990), is likely to be a serious competitor for food with young Colorado squawfish. Red shiner has been implicated often in the decline of native fishes of the American Southwest (e.g., Minckley and Deacon 1968, Deacon and Bradley 1972, Minckley 1973, Greger and Deacon 1988, Rinne 1991). Competition might also be a factor between smaller specimens of both Colorado squawfish and other fishes. Few specimens < 21 mm TL, other than red shiner and fathead minnow 11-20 mm TL, were available for comparisons with Colorado squawfish. However, 14