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INTRODUCTION Native Colorado River fishes evolved in a fluvial environment characterized by seasonal-extremes of flow volume and velocity, water temperature, and sediment load. .This environment was created by snowmelt driven hydrologic regimes. Many life history stages of these native fishes are timed to particular hydrologic signals (Tyus and Karp. 19.88), thus the-fish .appear superbly adapted to these environmental extremes. -Over the past lflfl years the construction of mainstream dams, irrigation diversions, irrigation return flows, municipalities, dikes to prevent seasonal flooding, agriculture•on floodplains and river banks and other human activities have changed the-very dynamic fluvial environment of most Colorado River tributaries to a more seasonally homogenous hydrologic regime.. In addition, non-native fishes including carp, catfish, red shiners and others have been introduced to this-river system. The result is that four of the largest native fish species :.are rare today, including the razorback sucker, humpback chub, bonytail-chub,-and-Colorado River squawfish. The U.S, Fish and Wildlife Service is coordinating the Recovery; Program-for the Endangered.Fishes of ,the Upper Colorado River. This report is a part of the recovery program for the razorback sucker. Critical life history and flow requirements of these fishes has recently been reviewed by"Stanford-(1993).- Since his review provides a useful-overview of the literature, no additional synthesis will be attempted here. Razorback suckers are-known to spawn-at or before the seasonal peak flow in May or early June and their larvae, which hatch several days later,.-are washed downstream. Spring runoff "water is cold, relatively sterile with .regard to food resources for larval fishes, and fast flowing. Larvae that remain in the current can be washed dozens or hundreds of river miles downstream and are faced with .few opportunities for feeding. In addition, predatory fishes are .hungry due to the limited food . available in the river. Thus, predation on the larvae may. be high. It has been hypothesized-that floodplain wetlands may have provided critical habitat for .larval, as well as adult, razorback suckers in-the past. Razorback larvae that drift into a wetland -would experience warmer,-less turbid water which has much greater zooplankton densities than the river channel and provide the- - larvae the opportunity to grow large enough to avoid predators. -Five candidate sites have been identified during 1993 as having potential for flooded bottomlands enhancement in the upper Colorado -and Green-River basins. These sites are;. 1} Escalante 2