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I I I I I I I I I I I I I I I I I I I During this st.udy period, main channel water temperatures generally peaked in July and August. at 25 to 300c and often dropped to OOC for extended periods in January and February depending on ambient conditions (See Section 2.6). Specific conductivities in this region ranged from 255 umhos/cm, during spring runoff to 3,250 umhos/cm in late summer and early fall. Flows in the Green River during this investigation ranged from a maximum of 35,000 cfs during peak runoff in late May and early June to a low of 1000 cfs in late summer and early fall. Flows in this region were controlled largely by Flaming Gorge Dam, located about 410 miles upstream from the confluence, as well as input from the Vamps, White, Duchesne, and San Rafael Rivers. Periodic rain st.orms in late summer and early fall caused dramatic flash floods that resulted in'short-term increases in flow greatly reducing fish sampling efficiency. The opportunity to observe the effects of one such periodic storm event came during a trip on August. 26-27, 1988. A high intensity thunderstorm dropped 1 to 4 inches of rain in the canyonlands area within a 4-hour period on August. 26, 1988. Subsequent rain freshets triggered large rock falls in Cataract Canyon and a high volume flash flood in Horse Canyon (RM 14.4) as well as smaller freshets from other less extensive drainages on the Green River. Increases in water level of 1.5 to 3 feet were observed in the Colorado River below the confluence over a 15-hour period following the rain, and it was estimated that the volume of water in the river doubled, from 5,000 to 10,000 cfs, during the peak of the flood although this was not reflected in any stream gage since it occurred downstream of the USGS gage near Cisco, Utah, (Colorado River) and the USGS gage near Green River, Utah, (Green River). Debris carried by the flood choked the Green River channel, and was deposited along shorelines or washed downstream to Lake Powell. The unusually high numbers of YOY Colorado squawfish observed in lower Cataract Canyon shortly afterward were probably transported by this flood from the lower Green River (See Section 4.3.4). The effects of the flash flood at the mouth of Horse Canyon were inspected 2 weeks later. Flood waters from Horse Canyon had been sufficiently st.rong to deposit a large alluvial fan that probably dammed the Green River temporarily. This short-lived dam breached and created a minor rapid that persisted through 1988. 2.2 Region 2: Colorado River above the Confluence This region of the st.udy area (Figure 4) encompassed the Colorado River from near the Potash boat launch (RM 50.0) downstream to its confluence with the Green River (RM 0). The canyon area included in this region, although not formally named, was designated as Meander Canyon in a recently published river guide (Baars 1987). The Colorado River in this region was similar to the Green River above the confluence with some exceptions (Photos A-9 through A-12). Both rivers were characterized by a low gradient of about 3 feet per mile, and a meandering channel lined with tamarisk/willow, talus slopes, or vertical rock walls and ledges. However, the subst.rate of the lower Colorado River was predominantly sand, whereas that of the lower Green River was silt, probably accounting for differences in fish composition and species densities (See Section 4.3.6). Gravel and cobble substrate were also found in localized areas around the mouths of tributaries, but these were fewer than on the Green River. The largest deposits of cobble in this region were at Salt Creek (RM 3.6) and Elephant Canyon (RM3.0). The only razorback sucker captured in the 4 years of this study was on the cobble bar at the mouth of Salt Creek. Fish habitat in this region was the same as that described for the Green River above the confluence, except for a difference in the relative abundance of habitat types. This region provided the second highest density of backwaters of any in the study area. Bottomlands were less extensive on the Colorado River than on the Green River. Consequently, tamarisk/willow habitat, although still the most prevalent shoreline habitat, was less abundant on the Colorado. Conversely, talus slopes, rock ledges, and vertical walls were more common in this region. Rock ledges occurred primarily at RM 35.6, near 7