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~ ~ ~ ... CHAPTER I (.. DESCRIPTION OF THE BASIN Location and Drainage Area The Yampa River basin is located in the northwest corner of Colorado and southcentral part of Wyoming. The basin land area is approximately 2,811 square miles in Wyoming and 6,719 square miles in Colorado, for a total of 9,530 square miles. The Colorado portion contains most of Moffat and Routt Counties and small parts of Rio Blanco, Garfield, and Grand Counties. In Wyoming it covers parts of Carbon and Sweetwater Counties. The basin is 128 miles long from west to east and 75 miles in width. It is bounded on the north by the Great Divide basin, on the east by the Continental Divide, and on the south and west by the White River basin. The Yampa River is the largest tributary of the Green River. It flows about 200 miles from its headwaters in the Park and Gore Ranges and Flattops Moun- tains of central and northern Colorado to its confluence with the Green River in Dinosaur National Monument. The Green River is the largest tributary of the Colorado River and joins the Colorado River in Canyonlands National Park, Utah. The Little Snake River baSin, with a drainage area of 3,730 square miles (39 percent of the total basin), joins the Yampa River near its mouth and about 4 miles upstream of the Dinosaur National Monument. Other major tributaries to the Yampa include the Williams Fork and Elk River. There are also numerous smaller tributaries, many of which are intermittent streams. Physiography Most of the Yampa River basin lies within the southern part of the Wyoming Basin physiographic province, a plateau area underlain by widespread de- posits of relatively soft sedimentary rocks, isolated mountain ridges, and bordered in part by abrupt mountain slopes. Narrow areas along the eastern and southeastern margins of the basin are occupied by the Park Range and White River Plateau portions of the Southern Rocky Mountains physiographic province. The Uinta Mountains portion of the Middle Rocky Mountains physio- graphic province comprises an area at the southwestern edge of the basin. Elevations vary from 5,000 feet at the confluence of the Yampa River with the Green River to 12,493 feet on Flattop Mountain in the southeastern part of the basin. The Park Range and White River Plateau areas form the headwaters for most of the basin's major streams. The Park Range, extending along the eastern edge of the basin, consists mainly of broad mountain slopes about 10,000 feet in elevation. The high mountain valleys which drain these slopes are generally broad and open. For a distance of about 20 miles extending south of the Wyoming border, the crest of the Park Range is a strongly glaciated ridge with peaks rising to above 12,000 feet elevation. The White River Plateau, lying along the southeast margins of the basin, consists mainly of basalt uplands between 10,000 and 11,500 feet in eleva- tion, with a few peaks above 12,000 feet. The upland surface of the plateau 1