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0G??O Basin Characteristics The White River Basin in northwestern Colorado includes most of Rio Blanco County and parts of Mof- fat and Garfield Counties (fig. 1). The principal struc- tural elements that affect the direction and extent of streamflow in the basin (figs. I and 2) are as follows: 1. The White River uplift. A regional uplift that raised the eastern third of the basin to eleva- tions ranging from 6,000 to 12,000 ft. Streams flow mostly north and west from the uplift. 2. The Grand Hogback monocline. The Grand Hogback is in the east-central part of the basin and is a north-south trending monoclinic ridge that developed during the White River uplift. The ridge is characterized by erosion-resistant strata that dip steeply to the west. Most surface runoff from the eastern third of the basin flows west via the White River through an erosional cut in the Grand Hogback 2 mi west of Meeker. 3. The Piceance structural basin. The Piceance Basin is the principal geologic structure in the west-central part of the White River Basin. Sedimentary strata in the basin are rich in oil shale and alkaline minerals. Stream courses are controlled by fracture patterns; most streams in the Piceance Basin flow north or northeast. 4. The Douglas Creek arch. The arch is a regional anticline near the western border of Colorado. The axis of the arch trends north- south, and surface runoff near the arch flows east and north into the White River or west into Utah. I. In addition, smaller structures such as the Meeker dome, 3 mi east of Meeker, locally affect hydrologic patterns. Surface geology in the White River Basin (fig. 2) is mostly sedimentary rocks ranging in age from the Paleozoic Era to the Cenozoic Era. Paleozoic and Mesozoic Era sedimentary rocks are most common in the eastern third of the basin; Mesozoic and Cenozoic Era sedimentary rocks dominate in the northern, cen- tral, and western parts of the basin. During the last half of the Cenozoic Era, extrusives of mostly basaltic com- position intermittently covered exposed rocks along the crest of the White River uplift. These volcanics are still evident as resistant rock layers that cap older strata in the eastern parts of the basin. Other resistant rocks in this area are from the Paleozoic Era and the Triassic, Jurassic, and Cretaceous Periods. Cretaceous and Ter- tiary shales and siltstones are common in the central and western part of the basin and are less resistant to erosion than the rocks of the White River uplift. The White River originates in the high mountain elevations of the White River uplift in eastern Rio Blanco and Garfield Counties. The White River flows from an alpine climate westward through transitional climates near Meeker into a semiarid climate in west- ern Rio Blanco County. Tributary streams to the White River east of Meeker are mostly perennial or intermit- tent. Except for Piceance, Yellow, and Douglas Creeks, tributary streams west of Meeker are mostly ephem- eral. About 14 mi west of Rangely, the White River enters Utah and subsequently is tributary to the Green River. The drainage area of the White River at the Colorado-Utah State line is 3,680 mi'. Drainage areas for the White River at sites 1-6 are listed in table 1. Natural vegetation cover in the basin at eleva- tions generally greater than 7,000 ft primarily consists of conifer and aspen forests; pinon pines, junipers, mixed grasslands, and sagebrush predominate at eleva- tions generally less than 7,000 ft. The conifer and aspen forests are common in the eastern parts of the basin and the high elevations along the rim of the Piceance Basin. Pinon pines, junipers, mixed grass- land, and sagebrush are common in the central parts of the basin; sagebrush, sparse growths of grasses, pinon pines, and juniper are typical in the western parts of the basin. Irrigated and dry-farm crops of grains, mixed grasses, and alfalfa hay are grown in the central basin and along stream valleys throughout much of the White River Basin. Ranching and agriculture, recreation, and energy-resource mining are the primary land uses in the basin. A general discussion of the White River Basin is presented in Boyle and others (1984). Surface-Water Hydrology Average annual precipitation in the White River Basin ranges from 9 in. in the west to 22 in. in the east- ern parts of the basin (National Climatic Data Center, 1982). The principal source of water in the White River originates from snowpack that accumulates in the mountainous areas in the eastern parts of the basin. Streamflow patterns for water years 1975-88 for site 4 (fig. 3) generally are representative of streamflow pat- terns for sites 1-6. Peak flows and most stream dis- charge occur during the months of May and June (fig. 3) when runoff from melting snowpack is at a maximum. Occasional intense thunderstorms may temporarily increase flow in the White River during INTRODUCTION 5 ,~.