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14 Stratum v: Salt Creek to Dolores-San Miguel confluence. This was the longest stratum sampled on the Dolores River (23 miles). High, sheer canyon walls constitute most of the riparian topography. Instream features include several small rapids in the upper portions of the stratum while small riffles were prevalent downstream. River widths ranged from 82 to 115 ft. The mean depth was approximately 6.5 ft with a maximum depth of 33 ft. Conductivity ranged fro@ 700 to 2100 umhos cm-l and averaged 1400 umhos cm-1 in the stratum. The San Miguel River had a conduct- ivity of 500 umhos cm-1 in May 1981. Salinity values were 0-1 ppth and dissolved oxygen concentrations were 7-9 mg/l. Stratum W: Dolores-San Miguel confluence to Paradox Valley. This was the uppermost stratum sampled by the GJFS. TIle riparian topography is similar to stratum V with high canyon walls. River features include many small rapids approximately 330-500 ft apart. Stratum W was sampled only once during May 1981. After this time flows were extremely low (<50 cfs) due to upstream diversions for irrigation. River widths ranged from 80 to 100 ft. Conductivity was 1100 umhos cm-1 in May 1981, but dissolved oxygen and salinity data were not collected from this stratum. Species Composition Fish compos~t~on of the Gunnison and Dolores rivers differed in several respects. GJFS collected 18 species from the Gunnison River and 16 from the Dolores River (Tables 2 and 3). All 16 species taken from the Dolores River were also found in the Gunnison River. Northern pike and Colorado squawfish were caught in the Gunnison River, but not in the Dolores River. The two rivers differed considerably in percentage species compos~t~on. Sand shiner, red shiner and fathead minnow were the most abundant species in Dolores River. Non-native species comprised 87.9% of -the total catch in the Dolores River. Contrastingly, native fishes such as bluehead sucker, roundtail chub and flannelmouth sucker comprised 73.1% of the catch from the Gunnison River. The differences in species compos~t~on can be attributed to variations in water quality, flow regimes, and the relative degree of man-made modifications and perturbations. The lower Gunnison (down to Redlands Diversion at RM 3) receives relatively constant flows throughout the year, due in part to upstream regulated releases from the Curecanti Storage Unit. Few backwaters or other lentic habitats exist within the study area. The Dolores River, however, loses much of its water during the summer via the Montezuma Valley Irrigation Company diversion. The Dolores below this structure becomes a series of slow-flowing pools and shallow riffles, with some small backwaters. It appears that non-native fishes prefer still-water habitats, thus the greater number of lentic