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(RM 102.0) to Mineral Bottom boat launch (RM 52.0). The Green River reach has mostly cobble substrate near the top of the reach that is gradually replaced by sand downstream. The San Rafael River ~ enters the Green River at RM 97.0 and is the only major tributary flowing into the reach. Many other dry washes enter the reach, which are flooded at high water creating low-velocity habitat. The Green River was sampled during spring high flows; therefore, most of the available low-velocity habitats were flooded ~ wash mouths, often in excess of 500 m long and 2 m deep. Frequently sampled habitats were referred to by name (Table 1). On the Colorado River, habitats within three disjunct 10-mile reaches were treated. The Dewey ~ reach (RM 97-87) contains mostly main-channel backwaters on the upstream or downstream ends of sandbars. The Dolores River enters the Colorado River in the upper part of this reach. The Moab reach (RM 65-55) is a slower reach of river with a few large flooded wash habitats and seasonally abundant ~ main-channel backwater habitats. This reach includes Moab Wash, which flows seasonally beside and under the Atlas Uranium Mine tailings pile and contributes high levels of ammonia into the river (United States Geological Survey [USGS] 2002), and Mill Creek which is a small perennial tributary that flows into , the Colorado River through The Nature Conservancy's Matheson Wetlands Preserve. The Lathrop reach (RM 30-20) contains primarily main-channel backwaters. The lower two reaches were chosen because they were extensively sampled during a nursery habitat study from 1992 to 1996 (Trammell and Chart 1999c). 2.2 Sampling In each river reach, removal trips were conducted weekly for four or five weeks. All backwaters were sampled on each trip. In each sampled backwater for each one-day sampling occasion, several seine hauls were made to deplete the fish (removal passes). The number and physical configuration of seine hauls used to deplete the backwaters changed during the study as described in the river-specific ~ sections. A 4.8 mm mesh seine was used to target fish greater than 25 mm TL to minimize capture of native sucker larvae. At each backwater, a seine was used to block the sampled habitat from the river, while the seinable portion of the backwater was depleted. The seinable portion of the habitat depended ~ -3- 1