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I I I I I I I I I I I I I I I I I I I Shafer Canyon and at the Upper Loop (RM 11.0), while vertical wall habitat was common at the Loop between RM 5 and 9. A large rock slide at RM 1.5 on the Colorado River ("The Slide") had constricted the channel, forming a compression riffle and a large, deep eddy. Habitat at this locale was somewhat unique and diverse as compared to the rest of the region due to the presence of higher water velocities and deeper, turbulent eddies. Sampling in this location produced a variety of species including Colorado squawfish and humpback chub, as well as two razorback suckers in a previous investigation (Valdez et a1. 1982). Two major drainages flow into the. Colorado River within this region including Indian Creek (RM 16.5) and Salt Creek (RM 3.6). Several smaller ephemeral drainages also empty into this region, including Lockhart, Shafer, Buck, Lathrop, and Elephant Canyons. Flow of the Colorado River in this region from 1985 to 1988 varied from about 2,000 to 42,000 cfs (See Section 2.6). The Colorado River above this region is not impounded by any large mainstem dams, but its flow is affected by irrigation, municipal, and indust.rial withdrawals as well as the flow of several major tributaries including the Gunnison and Dolores Rivers. 2.3 Region 3: Confluence to Brown Betty This region (Figure 5) extended from the confluence of the Colorado and Green Rivers (RM 216.5) downst.ream to Rapid #1 (RM 212.5, 'Brown Betty Rapid'). It received the flow of both rivers and was characterized by a deep, swift, and voluminous channel. The gradient was st.eeper (5 feet per mile) than the areas above the confluence and the channel remained constricted by the talus slopes and small rock slides, except for the area of Spanish Bottom where the river widened before entering Cataract Canyon (Photos A-13 through A-16). Fish habitat in this region was provided by deep runs, eddies, backwaters, cobble bars, and shorelines. Only two types of shoreline habitat were present including boulder talus and tamarisk/willow habitat. The boulder talus shoreline, which was interspersed with stretches of sand and silt, formed many eddies and slackwaters. Tamarisk/willOW habitat was found primarily in the area of Spanish Bottom. Several backwaters formed in this region as flows dropped in early summer. Most. were relatively small but deep and were habitat for YOY Colorado squawfish and chubs as well as many other species of native and non-native fishes. The only tributary in this region of the study area was Lower Red Lake Canyon. Alluvial deposits from this ephemeral drainage were relatively extensive and formed a large, shallow cobble area below the mouth. Flows of the Colorado River below the confluence from 1985 to 1988 varied from 3,000 to 70,000 cfs (See Section 2.6). 2.4 Region 4: Cataract Canyon This was the principal study region of this investigation (Figure 6). It encompassed 11 miles of deep channel from 'Brown Betty Rapid', Rapid #1 (RM 212.3) to the top of 'Ten Cent Rapid', Rapid #26, (RM 201.5). Depending on the level of Lake Powell, 24 to 26 major rapids occurred within this region. The gradient averaged 8 feet per mile with short sections of up to 30 feet per mile. This region of the Colorado River had the steepest gradient in the upper basin as well as the deepest water, recorded at 92 feet below Rapid #2 (RM 211.8) by Valdez et al. (1982). Before Lake Powell, Cataract Canyon was 41 miles long with 62 major rapids (Dellenbaugh 1908), and extended from its present upstream margin at RM 212 downst.ream to RM 171, about 1.5 miles above the confluence of the Dirty Devil River. The character of the river within this region was dominated by rapids (Photos A-17 through A-24), and their nature was controlled by river flows. Fish sampling was best at flows of less than 10,000 cfs, 9