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LOCATION-NUMBERING SYSTEM Streamflow Stations Stations where streamflow is measured or sampled on a regular basis are assigned eight-digit numbers, such as 09212500. The first two digits (09) identify the major drainage in which the site is located; in this case the Colorado River drainage. The remaining six digits identify the relative location of the site, with numbers increasing progressively in the downstream direction. Miscellaneous Sites Sites at which only a few streamflow measurements or samples have been obtained are not assigned regular downstream station numbers. Instead, these sites are identified by a 15-digit number, such as 421229109252701. The first six digits designate latitude of the site, the next seven digits designate longitude, and the last two digits are sequence numbers to distinguish between sites that may have the same latitude and longitude. DESCRIPTION OF THE STUDY AREA The Big Sandy River drains an area of 1,785 square miles on the western slope of the Wind River Mountains in western Wyoming (fig. 1). Topography in the basin ranges from rugged mountains in its headwaters to relatively flat, desert-like terrain at the mouth (figs. 2-6). Eleva- tions range from more than 10,500 feet above NGVD of 1929 in the head- waters to 6,240 feet at the mouth. Tributaries of the Big Sandy River form a dendritic drainage pattern. Stream gradients range from 8 feet per mile near the mouth to 365 feet per mile in the headwaters. Longi- tudinal profiles of selected streams are shown in figure 7. Geology, Physiography, and Soils The Big Sandy River basin lies within the geomorphic province known as the Wyoming Basin (Hunt, 1974). The headwaters are underlain mainly by crystalline and metamorphic rocks and glacial deposits. The remainder of the basin is underlain mainly by sedimentary rocks. The major geo- logic units within the basin are shown in figure 8, and a description of the lithology of each unit is given in table 1. Canyons and distinctive ridges and buttes dominate much of the landscape along the Continental Divide. Topographic relief in this area is from 400 to 600 feet. The western boundary of the Big Sandy basin is characterized by tablelands dissected by gullies, canyons, and to a lesser extent by broad ancient valleys. Topographic relief in this area is from 200 to 400 feet. 2