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<br />Objective Planning Team under leadership of the Water and Power Resources <br /> <br />Service. Participants in this team effort included personnel from the Water <br /> <br />and Power Resources Service, Soil Conservation Service, U. S. Fish and <br /> <br />wildlife Service, U.S. Geological Survey, Colorado Water Conservation Board, <br /> <br />Colorado Department of Health and the Delta County Soil Conservation <br /> <br />District. <br /> <br />Mancos shale, a marIne formation with high salt content, underlies most <br /> <br />of the valley (See Figure 111-2) and is the principal source of salt in the <br /> <br />Lower Gunnison Unit. Lenses of crystalline salt often are exposed during <br /> <br />excavation into shale. Because of the arid climate salts have not been <br /> <br />leached naturally and applying irrigation water to the soil greatly accele- <br /> <br />rates the leaching process. <br /> <br />The Gunnison River contributes about 1.4 million tons of salt annually <br /> <br />to the Colorado River. About 600,000 tons come from natural sourCes and <br /> <br />about 840,000 tons are attributed to irrigated agriculture which was first <br /> <br />introduced into the valley about 100 years ago. Most of the salt is leached <br /> <br />from the soil and underlying Mancos shale and carried to the river by deep <br /> <br />percolation from irrigation and by seepage from earthen ditches. <br /> <br />Improved management of irrigation water including land leveling, measur- <br /> <br />ing water onto the fields and lining onfarm and off'farm ditches could <br /> <br />reduce the river's salt load by as much as 500,000 tons reducing salinity <br /> <br />concentration of the Colorado River at Imperial Dam by 52 milligrams per <br /> <br />liter. <br /> <br />001197 <br /> <br />1-2 <br />