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suitable habitats for larval fishes than main channel habitats in spring and summer. They found cladocerans and copepods much more abundant in the open water habitats of Old Charley Wash and Wyasket Lake (mean of>45 organisms/l) than backwaters and main channel river habitats (< 1.0 organisms/I) between June and September. In a laboratory study, Papoulias and Minckley (1990) observed significant starvation mortality of larval razorback sucker when Artemia densities were less than 50 zooplankters/l. In experimental ponds, no differences in survival of razorback sucker larvae occurred when prey densities ranged between 13 and 43 organisms/l, however, significant differences occurred in fish growth between prey densities (Papoulias and Minckley 1992). Welcomme (1985) reported that zooplankton densities are higher in floodplain wetlands/lakes than in main channel habitats. He concluded that zooplankton densities were influenced more by water volume than turbidity, dissolved oxygen, or conductivity. Zooplankton densities in Old Charley Wash were one or two orders of magnitude greater than reported in main channel habitats in the Green River, however, densities in the wetland decreased with depth (i.e. time). The decrease in zooplankton was coincident with observed rapid aquatic macrophyte growth. Macrophytic growth, particularly Polygonum sp. around the margins (mostly depths < 1 m) of the wetland and Potamogeton pectinatus in the deeper (> 1 m) areas in the middle ofthe wetland increased rapidly in July. Bonetto (1975, in Welcomme 1985) observed an inverse relationship of zooplankton numbers with macrophyte density in floodplain habitats of the Parana River. McVea and Boyd (1975) reported that macrophytes compete with phytoplankton through shading and uptake of phosphorus, thus reducing food base for pelagic zooplankton. In addition to a nutrient availability or shading effect from macrophytes, zooplankton numbers may also have been reduced by grazing from the abundant age-O fishes in the wetland. Most likely, both factors contributed to the decline of open water zooplankton in Old Charley Wash. Despite the significant reduction in zooplankton densities in June and July, no noticeable reduction in growth of age-O carp was detectable in length frequencies. It is likely that alternate prey sources associated with macrophytic growth were available to maintain growth. The timing and duration of the nursery environment provided by floodplain wetlands like Old Charley Wash could be critical to the growth and survival of razorback sucker larvae in the spring. During May and early June, main channel habitats were colder, zooplankton densities were low, and few velocity refuges existed. Larval razorback sucker first appear in the main channel at approximately peak flows (Tyus 1987, Muth et al. 1997) and were collected for at least six weeks afterward in the main channel (Muth et al. 1997). Most floodplain habitat in the Ouray area is lost below 447 m3/s (minimum flows that inundate floodplain in the Ouray area, Flo Engineering 1996, although flows overtop Old Charley Wash dikes through a ditch at approximately 396 m3/s). In the absence of off-channel wetlands, most larval fish in floodplains would be forced into the main channel after flows receded below approximately 447 m3/s. In 1995, floodplains were disconnected from the main channel of the river (447m3/s) 36 d before the appearance of most backwaters (i.e. 109 m3/s, i.e. Pucherelli et al. 1988). In 1996, the first razorback sucker larvae were collected in the river after flows receded below 447 m3/s. The number of days between the first and last razorback sucker larvae collected in 1996 (June 3 to 27; Bruce Haines, personal communication) and the time flows reached 109 m3/s was 37 d and 13 d, respectively. Once backwaters appeared, these habitats would provide favorable temperatures, but lack vegetative cover found in floodplains and wetlands and have marginal prey densities. Papoulias and Minckley (1990) reported that larval razorback sucker die if they do not feed 41