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four mlnlma. This indicates that drift densities at the Palisade Site varied with time of day, with the highest densities between 2000 and 0400 hours. Vertical Distribution Of Drift Paired sets were made with shoreline drift nets during a 24-hour sample effort to assess the vertical distribution of larval drift. A total of 16 samples were taken from each of two levels in the vertical water column. Level one was the uppermost 30 em of water, including the water surface. Level two extended from 30 to 90 cm below the surface of the water with the net frame resting on the river bottom. An analysis of variance revealed no significant difference (p=O.15) between the mean drift density in the upper (0.39 fish per 1000 cfw) and lower (0.26 fish per 1000 cfw) levels. It appears from this analysis that there is uniform mixing of drifting larval fishes to a depth of at least 30 cm in the vertical water column along the shoreline. DISCUSSION Drift of young fishes in the Upper Colorado River was observed soon after hatching in mid to late July during independent studies in 1983 and 1984. The same 8 species, 5 natives and 3 non-natives, were found drifting at both sites. The drift was dominated by the intermediate phase (mesolarvae) of the larval stage of development, although the initial (protolarvae) and final (meta1arvae) phases were also represented. Very few juveniles of the native species (flannelmouth sucker, bluehead sucker, speckled dace, roundtail chub, mottled sculpin) were found in the drift, but the non-native fathead minnow drifted in abundance as a juvenile. The other two non-native species in the drift (white sucker and green sunfish) were not abundant enough for this assessment. Although the species composition of shoreline surface drift and midchannel surface drift was similar, fish densities in the former were nearly seven times greater, suggesting that the majority of larval fish drifting in the Upper Colorado River do so along the shoreline. However, in neither of these studies was the midchannel benthic zone sampled. Diel patterns in densities of surface drift indicate that the larvae may drift on the surface during darkness (2000-0400 hours) and descend to dri ft in the benthic zone or to rest in the substrate during daylight. Use of the midchannel benthic zone will not be known until effective quantitative drift samples can be taken in this deep, high-velocity area. Drifting larvae were first seen at both sites 3 weeks before peak densities when the water temperature first reached 16 C during a rapid warming period. Over 60% of the drift occurred during a 2 to 3-week period in mid to late July when water temperatures ranged from 18 to 21 C, and river discharge dropped rapidly after spring runoff. Drift lasted for 9 to 10 weeks from the first of July to the first of September. From these observations, it appears that water temperature greatly influences the 184 * l: i ....-....'",.,,--'. ,-~""-,-'~---'-,,,,*~"'ljI,--~.- ~