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O~2~35 Valley is about 20 to 30 in., and most winters have snow cover on the ground only for short periods. Precipitation increases with increasing elevation to almost 40 in. on Grand Mesa north of Delta and in the San Juan Mountains southwest of Ouray. Winters are cool with occasional sub-zero temperatures, but severe cold is not typical. Summers are quite hot, and afternoon temperatures frequently exceed 90 of. The growing season varies from 112 to 148 days in the river valley. Geology The following discussion about geology in the study area was summarized from Meeks (1950) and Brooks and Ackerman (1985). Geologic maps for the study area are in reports by Williams (1964) and Tweto and others (1976). The major structural feature near the study area is the Uncompahgre uplift, an anticline plunging northwest and southeast. This feature forms the Uncompahgre Plateau shown in figure 1. Most of the irrigated land west of the Uncompahgre River is composed of soils and outcrops derived from alluvial deposits of Quaternary age and the Dakota Sandstone of Cretaceous age. The alluvial deposits are valley-fill material composed of clay, sand, gravel, and cobbles and glacial outwash material composed of sand, gravel, cobbles, and boulders. The Dakota Sandstone consists of sedimentary deposits of sandstone, shale, and coal. Underlying the Dakota Sandstone is the Morrison Formation of Jurassic age, which is exposed in a few canyons on the west side of the valley. The Morrison Formation is composed of mudstone, sandstone, and siltstone. Much of the irrigated land east of the Uncompahgre River is composed of soils and outcrops of the Mancos Shale of Cretaceous age. The Mancos Shale is a gray marine shale that-has thin beds of sandstone and siltstone and contains gypsum. Compared to other sedimentary rocks, shales often are enriched in certain trace elements, including arsenic, boron, chromium, mercury, selenium, uranium, vanadium, and zinc (Brownlow, 1979). The Mancos Shale is exposed in a few areas west of the river. There also are scattered alluvial deposits east of the river. Soils and surface deposits around Sweitzer Lake and deposits underlying the lake primarily were derived from the Mancos Shale. Soils and Land Use Soils in the Uncompahgre River valley are separated into four major types: terrace or mesa soils, Mancos soils, floodplain soils, and Dakota soils (U.S. Bureau of Reclamation, 1984). About 60 percent of irrigated land served by the Uncompahgre Project is on mesa soils, mostly west of the Uncompahgre River. Mesa soils were derived from terrace deposits and are mostly medium textured and have a low salt content. Mesa soils are productive and extensively farmed. Soils derived from Mancos Shale (also called adobe soils) compose about 34 percent of the irrigated area and are mostly east of the Uncompahgre River. Mancos soil is calcareous, medium to fine texture, and has a maximum depth of 9