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INTRODUCTION Many nonnative, warmwater fish species, that have become established in the Upper Colorado River Basin, may occupy restored floodplain habitat. The Recovery Program endeavor at restoration of floodplain habitat is to provide productive nursery areas for larval and juvenile razorback sucker. However, there is a great potential for some nonnative fishes to flourish in the floodplain habitat and adversely impact larval and juvenile razorback suckers through predation and competition. Although razorback sucker still spawn and produce larvae in the Green River, poor survival during the early life stages results in poor recruitment. Poor survival is attributed, in part, to predation and competition from nonnative fishes and and possibly starvation during the critical l.arval period. Present nursery areas consist of backwaters along the main channels that are dominated by nonnative fish species. Zooplankton production in the main channels is extremely low in the upper basin and backwaters do not appear to produce sufficient numbers of zooplankton that are needed for survival of the razorback sucker during their critical larval period (Grabowski and Hiebert 1989; Mabey and Shiozawa 1993; Papoulious and Minckley 1990). While the production of zooplankton is excellent in off-channel flooded bottomland habitats, these habitats have been disconnected from the main channels by levee construction. One current Recovery Program thrust is to augment or reintroduce razorback suckers in formerly occupied habitat of the Upper Colorado River Basin by reconnecting floodplain with rivers. This would provide nursery habitat for rearing fish until they reach a size when they are less vulnerable to predation and competition. If this is achieved, razorback sucker may be able to survive the recruitment bottleneck. Enhancing or restoring floodplain habitats, however, will also be beneficial to nonnative, warmwater fishes that may adversely affect razorback suckers through predation or competition. The objectives of this issue paper were to: 1. Review available information on the composition of nonnative fish species in selected reaches of the Upper Colorado River Basin. 2. Identify nonnative fish species that are likely to become established in restored floodplain habitats and adversely affect larval and juvenile razorback suckers. 3. Explore ways to prevent or reduce nonnative fishes in restored floodplain habitats. The geographic area considered in this issue paper include the mainstems of the Green River (RM 0-345), Colorado River (RM 0-185), and Gunnison River (RM 0-75). METHODS Maps that documented the distribution and relative abundance of nonnative fish species in the Upper Colorado River Basin in 1982 (Tyus et al, 1982) were updated from information in various studies on the river reaches defined in the geographic area considered in this issue paper, Native species and