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jz f 1 I I 1 I 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 warming trend (Fig. 3). This was accompanied by a decrease in river discharge from a peak of 28,300 cfs (cubic feet per second) on June 26 to about 10,000 cfs on July 20. The maximum drift ot' 1.91 fish per 1000 cfw was seen during the week of July 25-31, when water temperature varied from 16 to 17 C and discharge was about 9,750 cfs. During the second week of high drift density of 1.65 fish per 1000 cfw, water temperatures ranged from 17 to 18 C, and discharge averaged 8230 cfs. In the third week of high drift density of 1.21 fish (August 8-14), water temperatures ranged from 18 to 20 C and discharge averaged 7375 cfs. Drift decreased dramatically when the average midchannel water temperature exceeded 20 C. Either the fish actively avoided these temperatures during drift or the numbers of available drifting fish was depleted. Diel Dri ft Daily sampling by 4-hour periods, for most of the 9-week study. revealed a definite pattern in diel drift (Fig. 4). Highest drift rates at the Parachute Site (1.05 fish per 1000 cfw) occurred during the period 2000 to 2400 hours, while the lowest density (0.25 fish) occurred in the preceeding period of 1600 to 2000 hours. Although this represents a four- fold increase in drift density, analysis of variance revealed no significant differences (p)0.05) between any time periods. The largest numbers of drifting larvae were noted during very dark evenings (2000-2400 hours) preceeding the appearance of a full moon. This observation and the high night-time drift densities suggest that the larval fishes are photosensitive, and many are actively capable of. entering and escaping the surface drift currents. The absence of a significant diel drift pattern may be attributed to the effect of frequent turbidity and sediment load on instream light penetration and intensity. Gale and Mohr (1978) speculated that riverine larval fish drift during high levels of turbidity among floating particles and debris to avoid high light intensity and coincidentally escape detection by predators. Geen et a1. (1966) found white suckers drifting primarily on very dark nights and in turbid water. The lack of significance in diel drift at the Parachute Site may also be attributed to pooling of samples collected over a 9-week period. This caused high seasonal variation in densities and thus, high diel variation. To reduce thi s effect, the d i el sampl e effort at the Pal i sade Site was designed to sample drift over a short time period (see Results of Palisade Study) . Length Of Drifting Fishes A comparison of the sizes and developmental phases of larvae collected in drift nets with those collected concurrently in shoreline seine hauls indicates that, for certain species, only a well-defined size group was 177