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I I I I I I I I I I I I I I I I I I I STUDY AREA The Little Snake River is located primarily in northwestern Colorado, but much of the watershed is in southwestern Wyoming (Figure 1). The river drains two very different physiographic regions, the Southern Rocky Mountains and the arid Wyoming Basin (Hunt 1 974) . Total drainage area is 9,700 km2 and mean annual runoff is 412,400 acre-feet/year (Ugland et al. 1995). Discharge is both seasonal and extreme. For example, mean-monthly discharge is 40 m3/sec in May, but base flows regularly approach zero between July and November (Ugland et al. 1995). Highest measured peak flow was 473 m3/sec on May 18, 1984 (Ugland et aJ. 1995). Most of the warm-water habitat in the Little Snake River occurs in Colorado. Within Colorado we identified three different reaches, each with unique geomorphological characteristics. These characteristics included the local topography and geology, river gradient, and predominant substrate. Locations were identified from U.S. Geological Survey (USGS) topographic maps of Moffat County (1 :50,000 scale). Specific locations on the river were identified on the map by delineating river kilometers (RK) starting at the confluence with the Yampa River. An upper reach from RK 160 to RK 80 contained gravel and cobble substrates and riffle-pool habitats that maintained highly diverse fish habitats at all flow levels. At runoff; riffles, pools, and eddies were abundant in this reach and at baseflow large refugia pools were common. A middle reach between RK 80 and RK 15 was located in a broad, unconfined riverbed with low gradient and predominately sand substrate. Habitat diversity in this reach was low at all water levels. During runoff, most of the habitat in this reach was homogeneous run with a few eddies that provided some velocity refuge for fish. During baseflow, much of the habitat in this reach was marginal because flow dispersed through the broad sand bed providing little refugia for large fishes. The riverbed in the lower reach (RK 15 to RK 5) was confined by a canyon that not only constricted the river 3