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(G. robusta) and the most abundant nonnative species included reside shiner (Richardsonius balteatus) and red shiner (Cyprinella lutrensis). Large bodied nonnative species including gamefish were extremely rare in the Little Snake River in all years. Reasons for the paucity of nonnative were not studied but might be related to the extreme habitat conditions in the Little Snake River. The high variability of discharge (high in spring and extremely low in fall) influences related physico-chemical characteristics of the river such as water temperature and sediment transport that directly and indirectly affect fish survival and growth. During baseflow, fish tend to concentrate in refugia pools where conditions are often less than ideal. Pools are subjected to extreme diel temperature fluctuations, loss of water quality, and storm spates that displace fish and carry and deposit large amounts of sediment. There may be other unobserved causal agents for the scarcity of nonnative species but the events related to the hydrograph probably playa significant role in structuring the fish community in the Little Snake River. Based on captures and telemetry data, individuals of both Colorado pikeminnow and humpback chub from the Yampa River move into and occupy the lower 15 km of the Little Snake River between May and July. Colorado pikeminnow probably move into the Little Snake River from the Yampa River to increase their condition prior to their spawning migration and exploit abundant prey fishes and warmer water temperatures found in the Little Snake River. Most Colorado pikeminnow leave the Little Snake River and return to the Yampa River just before their spawning migration; however, some Colorado pikeminnow may remain in the Little Snake River year round, based on historical accounts and the reported capture of one in late August near the Colorado-Wyoming state line during baseflow. Humpback chub may also use the Little Snake River for reasons similar to those of Colorado pikeminnow, but the following circumstantial evidence suggests VII