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I I I I I I I I I I I I I I I I I I I 2.1 RIVER FLOWS AND WATER TEMPERATIJRES FISh habitat in the study area was largely determined by the surface elevation of Lake Powell. In early spring (April) lake levels were unusually low (3620 ft above sea level), creating a slow flowing riverine channel constricted by silt/sand bars. At this level the Colorado River entered Lake Powell just below the Rite Bridge (RM 170). In late summer (August) runoff had brought lake levels up 47 ft to a surface elevation of 3667 ft. At this level the rapid at Imperial Canyon (RM 200) was mostly inundated, consisting of a series of small standing waves. Low velocity lake habitat dominated the study area at this reservoir level. Flows of the Green River, Colorado River above the confluence, and Colorado River in Cataract Canyon for 1993 are presented in Figure 2. Colorado River flows at the Lake Powell inflow during pre-runoff sampling ranged from 14,600 to 16,400 cfs. Post-runoff flows ranged from 7500 to 8500 cfs. Spring runoff in Cataract Canyon was high in 1993 with peak flow reaching 70,900 on May 30 (provisional USGS data). Colorado River mainchannel temperatures during pre-runoff sampling ranged from 11.5 to 12.5 DC. During post-runoff sampling Colorado River temperatures ranged from 24.0 to 25.0 DC, and Lake Powell surface temperatures averaged 26.0 DC. 80 - Green River - Colorado River - Cataract Canyon 60 en - u 0 0 0 ..,.... 40 x :s: 9 u.. --.... ~ ________ ............---.-. ------..... -....--oO-- ._______ Pre-runoff Sampling Post-runoff Sampling 20 - - - - - - - - - - - - - . . - - - - - - o JAN FEB MAR APR MAY JUN JUL AUG SEP Figure 2. Flows of the Green River, Colorado River and Cataract Canyon for 1993, as recorded at Green River and Cisco, Utah, USGS data (Cataract Canyon flows estimated from sum of Cisco and Green River gages). 3