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6 <br />DESCRIPTION OF STUDY AREA <br />The primary collection sites (Figure 2) were located at Echo Park <br />(3) and the Walter Walker Wildlife Area (8). Sites farther up the <br />Yampa River at Lily Park (1) and Castle Park (2) were also extensively <br />sampled, but no razorback suckers were collected. Island Park (4) <br />was sampled twice and Horseshoe Bend (5), Ouray (6) and Sand Wash (7) <br />were sampled once during exploratory sampling for razorback suckers. <br />The Yampa River at Echo Park and Castle Park flows through <br />narrow canyons with sections of deep, fast water (Figure 3). The <br />shoreline and river bottom are composed of rocks ranging from a few <br />millimeters to 20 meters in diameter. Quiet eddies are created behind <br />these large boulders. Lily Park on the Yampa River and Island Park, <br />Horseshoe Bend, Ouray and Sand Wash on the Green River, are reaches <br />of river that flow through open valleys. The water velocity is slow <br />and the river is shallow with sand or silt as the main substrate. <br />The riparian vegetation along the Yampa and Green Rivers charac- <br />teristically consists of box elder (Ater negundo), cottonwoods (Populus <br />spp.), willows (Salix spp.) and tamarisk (Tamarix pentandra), with <br />rabbit brush (Chrzysothamnus spp.), big sagebrush (Artemisia tridentate) <br />and Utah juniper (Juniperus osteosperma) on the hillsides. Eolrngren <br />[1962] provides a thorough discussion of the vegetation in this area. <br />The region is extremely arid, with an average precipitation of 25 cm <br />per year [Vanicek, 1967]. Air temperatures range from a maximum of <br />40 C in summer to a minimum of -25 C in winter. Water temperatures <br />range from 0 C in winter to 30 C in late summer [Vanicek, 19671.