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INTRODUCTION General Historically, upper Colorado River basin floodplains were inundated annually by flows during spring runoff, but today floodplains are not regularly connected to the river because of channelization by levees, dikes, or rip-rap near population centers and in agricultural areas. Introduced salt cedar or tamarisk (Tamarix spp.) has become established along the shoreline of major rivers resulting in sediment deposition and stabilized banks that further reduce the connectivity of the river with historic flooded bottomlands (Graf 1978). The periodicity of out-of-channel flooding in the upper Colorado River has dramatically decreased following the onset of transmountain water diversions, irrigation diversions, and the construction of mainstem dams (Osmundson and Kaeding 1991). The construction of dikes coupled with the reduction of high spring flows has altered the natural hydrograph and either reduced or eliminated regular flooding of bottomlands. It is believed that flooded bottomlands may have served as nursery areas for the razorback sucker' (McAda 1977; Osmundson and Kaeding 1991). In addition, Osmundson and Kaeding suggested that oxbow lakes and flooded pastures in the Grand Valley near Grand Junction, Colorado, were historically the primary spawning habitats of the razorback sucker. They also suggested that low velocity, off-channel habitats were used by adult Colorado squawfish during high streamflows. Numerous studies (Grawboski and Hiebert 1989; Tyus and Karp 1989; Wydoski and Wick 1994) have suggested the importance of seasonal flooding to river productivity. Other studies have suggested that flooded bottomlands were important to adult razorback sucker for feeding prior to and after spawning and for the rearing of their young (Tyus and Karp 1989). Adult fish may have used these off-channel habitats for "velocity shelters" to escape the high water velocities from the spring runoff. The turbid rivers in the upper basin are not very productive for zooplankton that are essential for survival during the early life stages of the razorback sucker. When compared to the riverine environment and river backwaters, inundated bottomlands produce the highest densities of zooplankton (Grabowski and Hiebert 1989; Mabey and Shiozawa 1993). Although predation has been documented to be a limiting factor in survival of larval razorback sucker in the lower basin (Minckley et al. 1991), starvation may also limit survival (Marsh and Langhorst 1988; Papoulias and Minckley 1990). It is hypothesized that the loss of these productive flooded bottomland habitats appears to be limiting the recruitment of the razorback sucker in the upper Colorado River basin because of the lack of the right sizes of food organisms at the right time and in sufficient quantity (Wydoski and Wick 1994). Flooded bottomland habitats occur in broad valleys along low gradient stream reaches. Bottomlands are off- or out-of-channel habitats that include ' Scientific names and two-letter codes of all fishes mentioned in this report are given in Appendix A; Table A.1. Only common names for these fishes are used in the text.