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~c ~" - '- ..... -....'. "~:i ,~.^. }" GEOLOGIC AND HYDROLOGIC SETTING Rock units underlying the study area are the Precambrian crystalline complex, unconformably over- lain by the Morrison Formation of Late Jurassic age, the Dakota Sandstone of Early Cretaceous age, and the Mancos Shale of LatS Cretaceous age. The forma- tions strike about N. 2 W. and dip about 60 E. The oldest formation exposed is the Morrison, which crops out along Iron Creek downstream from the damsite. The Dakota Sandstone is exposed along the western edge of the reservoir site except where it has been incised by Iron Creek and overlain by alluvium. At the damsite the Dakota is dissected and forms steep canyon walls. West of Iron Creek the Dakota caps a northeast-tilted plateau that is on the east flank of the Gunnison uplift, a northwest-plunging anticline. Except where it is incised by streams, the plateau is continuous in most of the Smith Fork drainage basin downstream from the Iron Creek conflu- ence. The Mancos Shale forms the bedrock in most of the reservoir site and in much of the Smith Fork drainage basin upstream from the Iron Creek confluence. The Mancos underlies a scalloped land surface, which rises at a moderate gradient eastward. Several miles east of the reservoir site, igneous rocks of Tertiary age overlie older rocks to form the West Elk Mountains. Snowmelt from the mountains is the principal source of most of the surface water in the Smith Fork drainage basin. Streams reach maximum flows in May and decline 75 to 90 percent by late September. The Crawford area is semiarid; the average annual precipitation is less than 11 inches. The azerage annual air temperature ranges from 320 to 60 F.