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predation pressure was realized. It should be noted that had centrarchid numbers been significantly reduced and YOY Colorado pikeminnow catch rates had increased, we might hypothesize a cause-and-effect relationship, but drawing any such conclusions would be difficult given the possibility for alternative explanations. One potentially important observation was that green sunfish were often found concentrated along shorelines containing rip-rap. Several backwaters were adjacent to banks rip-rapped with large rocks or chunks of concrete and green sunfish seemed to prefer the crevices associated with this material, particularly when other cover was lacking. Submerged sticks in beaver dams or lodges served a similar function and evidently provided enhanced habitat conditions for both green sunfish and largemouth bass. Although little can be done to reduce beaver in the floodplain, construction of dikes that contain rip-rap extending into the water should be discouraged. In addition to structure, another important factor that may have enhanced conditions for centrarchids during the project period was the lack of large flushing flows through the side-channels. Spring flows were below average during 1998-2001 and backwaters that might normally have stayed connected to the river became seasonally isolated allowing for pond-like conditions to develop. Although most of these channels flowed during spring, flows were apparently not high enough to flush fish, accumulated sticks or rooted vegetation. In addition to the high number of green sunfish and largemouth bass that were found inhabiting upper Colorado River backwaters, there was also a surprisingly high number of non-native black bullhead and white sucker. Bluegill, though not abundant, were also found in higher numbers than anticipated. These high numbers of non-native fish species, once thought rare, may have resulted from the use of a gear type not used in past backwater sampling or may have resulted from real increases in these populations in recent years, or perhaps resulted from a combination of these two factors. Channel catfish, a ubiquitous member of the main-channel fish community, was found in surprisingly low numbers in backwaters during late summer, fall, and early spring. Based on degree of piscivory known from other studies (Osmundson 1987, Nesler 2002) and relative abundance documented here, the greatest threat to survival of 22