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<br />7 <br /> <br />III, PROBLEMS AN[J NEEDS <br /> <br /><'l;"it'y P",,"]"'" <br />..::...~.'-.:...:.___~'"-..::.._~2'- <br /> <br />The problems with salinity on irriguted cropland are ge~eral1y related <br />to poor drainage, and sizeable areas of formerly irrigated cropland in <br />the Lower Uncompahgre Subbas in hdV(; been ab" ,dolled. There a"e few cases <br />of harmful sa] inity accumulation on irrigated lan0; with good drainage, <br />because the irrigation water is generally of high quality and most <br />fields are irrigated in excess nf consumptive use requirements. How- <br />ever, the irrigation return flo\'J'::i frC;rf1 dr'ea~ underlain :)y Mancos shale <br />contain large quantities of total dissolved sol ids and seriously degrade <br />the qual ity of water returning to the Colorado River system. The <br />Gunnison River discharges 1.4 million tons of salt annually, About 0.6 <br />mill Ion tons are estimated to be the normal inflow and 0.8 ~ill ion ton_ <br />are estimated to be related to man's activities. Most of this is caused <br />by the irrigation of 184,700 acres in the Lower Gunnison Unit. T~e <br />average increase in salt loading is over four tons per acre irrigate~ <br />per year. <br /> <br />Irrigation Systems and Management Improvement Needs <br /> <br />Critical erosion and sediment problems exist on the shale badlands or <br />shale-derived alluvium entrenched by gullies. Most of these critical <br />areas are also associated with irrigation canal waste or field tailwater <br />return systems. High sediment yields are generally associated with soils <br />underlain at shallow depths by shales of the Mancos Formation. These <br />areas generally contain soils with a sal ine substratum and through the <br />erosion process are responsible for a large part of the salt pickup by <br />irrigation return flows. The very high sediment yield suppl ies a sus- <br />pended load of clay and silt particles, and a salt loading of calcium, <br />sulphate, sodium and chlorides. Figure 2 is a general sediment yield <br />map of t~e Lower Gunnison Basin Unit. <br /> <br />Also, serious streambank erosion occurs on about 180 streambank miles <br />and moderate erosion on about 170 streambank miles in the Lower Gunnison <br />Basin Unit. This results in an estimated 408,000 tons of soil loss <br />annually. <br /> <br />The Uncompahgre Subbasin from above Montrose downstream to near Delta Is <br />the area of largest potential for improvement because of its very high <br />sediment yield, large increases in salt loading and serious streambank <br />erosion. This area Is mostly within the USBRs Uncompahgre Project <br />Improvement program scheduled for completion in FY 1977, <br /> <br />? <br /> <br />o <br /> <br />003055 <br />