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1981). Once the larvae enter the flow, they are transported rapidly downstream in the drift (e.g., Haynes et al 1984, Nesler et al. 1988, Tyus and Haines 1991). As is typical of oviparous fishes (cf., Balon 1984), initiation of exogenous feeding is a critical event in the development of larval pikeminnow. The larvae are capable of feeding after about six days post- hatch, and significant mortality occurs at two weeks if feeding has not been initiated (Bestgen 1996). Thus, there is a relatively narrow time window (probably not more than 10 days) when the drifting larvae must reach habitat suitable for feeding. In nature, the larvae are transported in a matter of days from high gradient spawning areas to lower gradient, alluvial reaches where they enter backwaters (Nesler et al. 1988, Tyus and Haines 1991). Although the young fish move between backwaters and other habitats (Tyus 1990), 84% of them occupy backwaters (Tyus and Haines 1991), where they feed on benthic organisms and some plankton (Muth and Snyder 1995). Pikeminnow larvae may be displaced or move from one backwater to another and especially downstream during the late summer and fall seasons (Tyus and Haines 1991). However, they tend to stay in backwater habitat of low-gradient river sections until the following spring when backwaters are flooded. Larvae are able to locate backwater habitats and move freely into and out of them, in part, in response to environmental change (Tyus 1991). The life history of the razorback sucker is less well-known (reviewed by U.S. Fish and Wildlife Service [USFWS] 1998). Extant populations are declining due to low or no recruitment. In general, the early life history of the razorback sucker is similar to the Colorado pikeminnow, but razorback sucker spawns earlier in the year and the larvae may use inundated floodplains in addition to backwaters (USFWS 1998). Colorado pikeminnow larvae apparently select larger (average = 94 m2 surface area) and deeper (average water depth 32 cm) backwaters (Tyus and Haines 1991). Backwater habitat is dynamic and ephemeral due to river stage fluctuations. When the river is rising, the volume of each backwater expands, eventually to the point of being incorporated in the river at high flows. Conversely, when river stage is falling, each backwater shrinks, ultimately to the point of becoming an isolated pool that may completely desiccate. In the historic Colorado River, unpredictable changes in river stage could occur on short time scales, no doubt affecting backwater availability. In the present system, locations downstream of reservoirs can experience flooding, isolation or dewatering of backwaters on a more frequent basis. To be most successful, organisms that live in backwater habitat must cope with changing water levels, yet virtually nothing is known about such mechanisms. Visual cues of changing conditions are a possibility, but presumably useful only in daylight hours. How then do larval fishes cope with changing backwater conditions? The present study was an assessment of the responses of larval Colorado pikeminnow and razorback sucker to flows in a series of simulated backwaters. Although the work was performed exclusively in a laboratory setting, the findings will be 526