Physical features.--The Washita River heads in the Texas Panhandle, in Hemphill, Roberts, and Wheeler Counties. It flows generally east-southeastward through southwestern and south-central Oklahoma to its confluence with the Red River above Denison, Texas, at a point about 4 miles northwest of Cartwright, Oklahoma. That point of confluence is now beneath the waters of Lake Texoma, into which the river flows near Tishomingo in southern Johnston County.
The average gradient of the Washita River is about 3.7 feet per mile, beginning with an elevation of about 2,800 feet at its source, and ending with an elevation of about 510 feet at its mouth. The river is about 626 miles in length.
The total area of the Washita River drainage basin is about 7,945 square miles, of which about 463 square miles is in Texas. The 7,482 square miles of the basin in Oklahoma is in the 17 counties of Roger Mills, Beckham, Comanche, Grady, Stephens, McClain, Custer, Washita, Caddo, Kiowa, Garvin, Carter, Bryan, Murray, Pontotoc, Johnston and Marshall. The river has a winding, sinuous course. In some reaches, such as near Anadarko, it flows in broad meanders 3 or 4 miles across which have incised the bedrock. Throughout its course in Oklahoma the river meanders within its alluvial valley, so that the valley is much shorter than the river channel. In Texas and at the western edge of Oklahoma the Washita flows in a valley cut in rocks of the High Plains; much of the coarse gravel and sand of the alluvium is derived from these rocks. About nine-tenths of the Oklahoma portion of the valley, from western Roger Mills County to near Davis in northern Murray County, is cut in Permian redbeds. These red beds consist largely of shale, siltstone, and fine-grained sandstone. In the western part of the basin, down to about Mountain View in northeastern Kiowa County, they contain irregular layers of gypsum. The gypsum locally forms steep valley walls, such as those along State Highway 152 east of Cordell.
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