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The area consists of several intermountain valleys and mesas drained by the Uncompahgre and the Gunnison Rivers and their tributaries. The Continental Divide through the Elk Mountains and the Cimmarron Ridge form the area's east and southeast boundaries. The Grand Mesa forms the north boundary. The Uncompahgre Plateau and the San Juan Mountains comprise the west and south boundaries, respectively. Physiography and Geology The Lower Gunnison Unit contains two million acres. About 42 percent of this area is privately owned and consists of 182,500 acres of irrigated cropland including the 171,000 acres within the study area, and 663,500 acres of nonirrigated rangeland. The irrigated land, mostly within the 0-2 and 2-5 percent slope groups, developed from alluvium, glacial outwash and shale. Forest land managed by ~he U.S. Forest Service makes up 36 percent of the area and is considered a low producer of salts and sediment. Rangeland, highly susceptible to erosion, managed by the Bureau of Land Management, comprises 22 percent of the area. Mancos shale underlies most of the valley surrounded by older Dakota sandstone and younger volcanic rock as shown in Figure IV-2. The Mancos formation, a marine shale with high salt content, consists mainly of gray silty clay. It is the principal source of salt in the Lower Gunnison Unit. Salts are con- centrated in certain areas within the shale and the younger alluvial deposits. Lenses of crystalline salt often are exposed during excavation into shale. Elevation in the area ranges from 4,550 feet above mean sea level where the Gunnison River joins the Colorado River, to 14,300 feet on Uncompahgre Peak in the southern end of the basin. Because of the arid climate below about 7,000 feet elevation, salts have not been leached naturally from the soil. The leaching process is greatly accelerated by applying irrigation water to these salt laden soils. Notable sites of scenic importance within the project area include Box Canyon, the San Juan Mountains, Young's Peak--a chimney rock near Crawford--and the Black Canyon of the Gunnison River. Climate The mean annual temperature at Delta is 510F and ranges from a mean maximum temperature for July of 940F to a mean minimum temperature for January of 140F. The mean annual temperature for all Lower Gunnison weather stations is 49.60F. The average annual precipitation at Delta is 8 inches and 9 inches at Montrose. About one half is in the form of winter snow. The average length of the growing season is 146 days near Delta with elevation of 4,960 feet above mean sea level and 152 days at Montrose, elevation 5,800. The estimated length of growing season at Crawford and other locations at higher elevation is about 105 days. The area below 6,200 feet elevation is characterized by low precipitation, low humidity, abundant sunshine, and a wide range in annual and daily temperatures. Water Supply Projects built by the Bureau of Reclamation supply water to 99,000 acres, 54 percent of the irrigated land in the valley. The largest, the IV-26