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STATUS OF GREEN RIVER COLORADO PIKEMINNOW 1359 FIGURE I.-Green River basin study area, Utah and Colorado; RK = river kilometer. relatively low-gradient, depositional valley reaches were interspersed with higher-gradient, erosive, canyon- bound reaches. Depositional reaches were braided, had mostly sand or gravel substrate, meandered to some extent within the floodplain, and were dominated by run and pool habitat. Canyon reaches were typically constrained, single-thread channels that had a mix of riffles, runs, and pools and substrate that ranged from sand to boulder-sized particles. River flows were highest in spring and early summer during snowmelt runoff and declined to base levels by midsummer (Figure 2). During elevated-flow periods, main-channel velocities were swifter and low-velocity areas were limited, often consisting of flooded tributary mouths and canyons washes, shorelines, and a few large backwaters. During lower-flow periods in all reaches, low-velocity habitat was near shore, in small eddies and pools, and behind or adjacent to midchannel sand or gravel bars. Methods Sampling and fish handling.-Sampling for abun- dance estimation was conducted in spring in each year from 2000 to 2003 in the Yampa River, White River, and middle Green River study reaches and in each year from 2001 to 2003 in the Desolation-Gray Canyon and lower Green River reaches. Using Pollock's robust design as a guide (Pollock 1982; Pollock et al. 1990), we conducted three or four relatively short-term 2001 -+-2002 -1964-1999 450 400 350 ~ 300 g 250 " 200 ~ ~ 150 is 100 50 0 ,,' "o,' o}'1, .;:,' ~, ,)yo,' ~o,<:l 'I{?<:l ~'1,'" ~'1,'b ,~o{j> ,,,fi- ,~fi- Date FIGURE 2.-Average daily discharge of the Green River near Jensen, Utah (gauge 09261000), in the period 1964-1999 and the study years 2001 and 2002.