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3-1 3.0 THE RIVER REACH 3.1 General Description Fisheries investigations are planned by FWS within a IS-mile reach of the Colorado River from the Grand Valley Canal at RM (river mile) 185.0 downstream to the mouth of the Gunnison River at RM 170.0. The river in this reach leaves Debeque Canyon and enters the large flat agricultural- residential valley of Grand Junction and vicinity. In 1981 (Valdez et al. 1982), it was described as having an extensively braided river channel with many vegetated gravel islands. These islands and the adjacent alluvium attract many gravel-extraction operations; the gravel is for domestic and industrial uses around Grand Junction. The result of this extraction is a series of inundated gravel pits that pockmark the river bank. Some of these gravel pits that are connected to the river, such as Clifton Ponds, seem to attract a variety of riverine fishes, including the endangered species. Al though these pits have been the subject of recent studies (Berry et ale 1985), they do not appear to be the focus of the current proposed studies. This IS-mile reach of river is suitable for Colorado squawfish and razorback sucker. Not enough is known about the bony tail chub to assess its potential use of the area, and the deep, rocky habitat usually associated with the humpback chub is not present. The predominate substrate in this reach of river is gravel and rubble, and the habitat is mostly runs and riffles with some eddies, backwaters, and side channels (Valdez et al. 1982). Maximum depth in this reach during studies in 1979-81 was 18 feet, with an average of 3.9 feet. Water temperatures ranged from 14 C in April and June to 26 C in August, and conductivity ranged from 250 to 1100 umhos/cm. A series of canals, ditches and siphons divert substantial volumes of water from the river at the upstream end of the subject reach. A schematic of these diversions, as prepared by FWS, is provided to deta i1 the water rights of each of the users (Figure 3-1). Since the Grand Valley Water Users Association, which diverts water through the Grand Valley Canal, has the senior water right in the Upper Colorado River Basin, their demand of 640.3 cfs must be satisfied before all other demands. Most of the water withdrawn at this point is used for agricultural irrigation, and is returned to the Colorado River through a series of lateral collectors that feed into Mack Wash and Salt Creek, which flow into the river at RM 144.0. Major diversion of water through the complex illustrated in Figure 3-1 occurs annually from May to October. During dry years, the Colorado River is largely dewatered from late July to October in the reach inmediately downstream of the Grand Valley Canal at RM 185.0. Accretion and some irrigation returns provide an increasing volume of water progressively downstream for 15 miles to the mouth of the Gunnison River, where flow is supplemented. On some years, there remain but a few pool s connected by a trickle of water between the Grand Valley Canal and the mouth of the Gunnison River.