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5-6 before the maximum river temperature of 21.5 C was reached. No drifting fishes were observed the sixth week of sampling suggesting that drift activity was completed. But, a second pulse of drifting fishes was observed in late August and early September as densities rose to 1.3310 fishes/lOOO cubic feet of water. This pulse appears to be related to a slight increase in river temperature observed about the same time. Drift density and corresponding river temperature decreased after the first week of September. Valdez and Carter (1983) observed a similar second pulse of drifting fishes with a short-term temperature increase. A similar plot of mean weekly drift densities for the three most common larval fishes (bluehead sucker, flannelmouth sucker, and roundtail chub) shows the contribution of each of these species to the total drift (Figure 4). Details on sample size, means, and standard deviations for mean weekly drift densities for the five most common larval species are presented in Table B-7. Each of the three species contributed equally to the first major drift pulse observed in late July and early August, but the second pulse in early September was made up of primarily bluehead suckers. The drift of all three species decreased substantially in mid August. Based on the results of this study and those of Valdez and Carter (1983), it appears that larval fish drift in this region of the Upper Colorado River occurs during the accelerated warming period when river temperatures rise sharply from about 16 to 21 C during a continual decrease in river discharge. Subsequent pulses in larval drift might occur with short-term rises in river temperature of as little as 0.5 C. 5.3 Diel Drift Drift net sampling over a 24-hour period was conducted at the proposed JH-1 site to evaluate diel patterns of drifting fishes. Paired samples