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GREEN RIVER STUDY PLAN 26 February 2, 2007 by the river. These floodplains provide shelter from swift river currents, are highly productive, and provide warm sheltered habitats for growth. Larval and age-O bonytails may also use these habitats as nurseries. Additionally, juvenile and adult razorback suckers, Colorado pikeminnows, and bonytails use these habitats for feeding and shelter. Floodplains tend to be. warmer than the main channel in the spring, and gonadal maturation is enhanced in adult fish that use them. Reach 2, especially from Split Mountain to the White River confluence, is the most important reach of the Green River for floodplain habitat (Valdez and Nelson 2004). The only known spawning bar for wild razorback suckers is located near the upstream end of this reach, and downstream floodplains are vital nurseries for entrained larvae. The quality of these floodplains is important to the survival, growth, and subsequent movement of subadult razorback suckers to the mainstem. The importance of a floodplain as nursery habitat is thought to be related to the river stage at which it connects to the mainstem, depth of water remaining after peak flows recede, water quality, length of time water sufficient to support fish is retained, frequency of connection from year to year, rate of sediment deposition and erosion in the habitat, larval entrainment rates into the habitat, survival and growth of young fish in the habitat, the ability of fish to move into the mainstem once they have reached sufficient size, and species and numbers of nonnative fish that occur in the habitat. Recent studies of floodplains in both the Upper Basin and Lower Basin have yielded much information about various aspects of floodplains, but information needs remain that must be filled before flow and temperature regimes can be developed to assist in species recovery. Ongoing and planned studies help to address some of these needs, but information needs remain that preclude best management of flow and temperature for properly functioning floodplains. The positive response by nonnative fish to floodplain restoration continues to be problematic and should be considered and addressed as part of any habitat restoration effort. Nonnative fish are an integral factor in determining the quality of a floodplain. Floodplain inundation for larval entrainment, rearing, and subsequent movement of subadult razorback suckers into the mainstem in Reach 2 were identified as the most important issues related to evaluation of the flow and temperature recommendations. The upper end of Reach 3 (i.e., White River confluence to Sand Wash) also has large floodplain terraces that may be important razorback sucker nurseries. Floodplain nursery habitats in Reach 3 downstream of Desolation/Gray Canyons are not plentiful and are only available at very high flows (> 39,000 cfs). Nine high priority hypotheses related to floodplains were identified for evaluation (Table 2). The recommended studies that address each identified anticipated effect, uncertainty, and information need are identified in the remainder of this section. Recommended studies include continuation of ongoing studies that need to be completed before decisions on future studies are made through an adaptive learning process. Recommended studies fall into three subject areas: (l) evaluation of survival and recruitment, (2) entrainment rates, and (3) the timing of larval drift.