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Weekly Drift Densities And Patterns Although larval and juvenile fishes were found in the drift for a period of 9 weeks, nearly 72 percent were found in a 3-week period from July 25 to August 15, 1983, (Figure 3). Mean weekly densiIies over the 9-week period varied from 0.0 to 0.067 fish per cubic meter (m ) of water. Fish larvae were first caught in the drift nets in late July when the main channel water temperature first reached 16 C, during a rapid wanning trend (Figure 3). This was accompanied by a decrease in river discharge from a pea k of 804 cms (cub ic meters per second) on June 26 to about 284 cms on July 20. The maximum drift of 0.067 fish per cubic meter was seen during the week of July 25-31, when water temperature varied from 16 to HOC and discharge was about 277 ems. During the second week of high drift density of 0.058 fish per cubic meter, water temperatures ranged from 17 to 180C, and discharge averaged 234 cms. In the third week of high drift density of 0.042 fish (August 8-14), water temperatures ranged from 18 to 200C and discharge averaged 209.5 cms. Drift decreased dramatically when the average midchannel water temperature exceeded 200C. Either the fish actively avoided these temperatures during drift or the number of available drifting fish was depleted. - ~ ..! u "- .. ~ " :: ~ . :: ~ ~ " 11 ! c .. c~ ~ ~ ~ .. - 0 ~ . !=-~ " '\ I lO 0.07 1 \ Dlscho'go \/ \ 0.06 "- ...... , 15 0.05 ....~~' , , \ ,'- , \ , , 0.04 , .' \ 10 O.OJ \ .. 0.02 0.01 0 0 .. .. ~ 2- :: D goo 7S0 ,---......... " " .... ;t!. "'.__- /" ~w.ter ..... T e.perl ture 600 CSO JOO --..... ........ -- -........ ........-- __ ISO o AUGUST FIGURE 3. Mean weekly densities of drifting larval fish compared to water temperature and discharge at the Parachute Site on the upper Colorado River, 1983. 570