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<br />Salt Sources <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Uncompahgre Project, diverts water from the Uncompahgre River and from the <br />Gunnison River through the seven mile long Gunnison Tunnel completed in 1908. <br />The Uncompahgre is a full service project. Taylor Dam and Reservoir, completed <br />in 1929, stores water for late season use within the Uncompahgre Project area. <br />Other projects for irrigation water supply include the Fruitgrowers Dam, the <br />Paonia Project, the Smith Fork Project, the Bostwick Park Project, and the <br />Dallas Creek Project wherein the Ridgeway Dam currently is under construction. <br />Water for the balance of the irrigated land is supplied by numerous private <br />and group ditches and storage reservoirs, which depend on direct runoff and <br />cannot be relied upon to meet late season water requirements. <br /> <br />Analysis of water quality data collected by the U.S. Geological <br />Survey (USGS) indicates that the Lower Gunnison River contributes about 1.44 <br />million tons of salt annually to the Colorado River. Most of these salts are <br />leached from the soil and underlying Mancos shale and carried into the river <br />by deep percolation from irrigation application and by seepage from water <br />delivery and tailwater collection systems. <br /> <br />Both natural runoff and irrigation contribute to the problem, by salt concen- <br />tration and by salt loading. Salt concentration is caused by removal of water <br />from the river system through consumptive use by irrigated crops and other <br />vegetation, and by evaporation; mineral constituents are concentrated in the <br />water that remains. Salt loading occurs as ground\water dissolves subsurface <br />minerals while flowing through the salt laden shale layers. Although both <br />salt concentration and salt loading are at work, salt loading is the major <br />cause of the salinity increase. <br /> <br />Salt loading from irrigated cropland is related to subsurface return flows. <br />Most fields are irrigated in excess of normal crop needs and deep percolation <br />of excess irrigation water causes substantial return flows through layers of <br />Mancos shale that contain large quantities of undissolved salts (solids). The <br />concentration of dissolved salts transferred to the percolating water seriously <br />degrades the quality of water returning to the Colorado River system. <br /> <br />Of the 1.44 million tons of salt delivered annually to the Colorado River from <br />the Gunnison some 600,000 tons come from natural sources, about 440,000 tons <br />from onfarm irrigation systems and practices, and about 400,000 tons from <br />off-farm canals and laterals (See Table IV-4). The salt load from onfarm <br />sources can be reduced by as much as 335,000 tons (76 percent) depending on <br />which alternative plan is implemented. An additional reduction of 50,000 tons <br />can be attained by lining 109 miles of smaller off-farm laterals. Lining <br />another 100 miles of the off-farm laterals, while not cost effective, would <br />achieve an additional 115,000 ton reduction. <br /> <br />Salt Loading <br /> <br />Salt pickup through deep percolation from field irrigation, and <br />seepage from onfarm ditches account for salt loading. Irrigation water delivered <br />to the farms has an average salt concentration of 260 milligrams per liter <br />(mg/l). Base flows returning to the river in the drains and washes have mineral <br />concentrations ranging from 175 to 7650 mg/l with an average of 2650 mg/l. <br />Salts are concentrated in soil moisture as plants extract water and minerals <br /> <br /> <br />IV-28 <br />