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<br />N <br />Ul <br />OJ <br />~ <br /> <br />the development of irrigated land <br />increased at a rapid rate, and by 1920, <br />nearly 1.4 million acres were being <br />irrigated, The Upper Colorado Region <br />Comprehensive Framework Study, <br />June 1971 reported that more than <br />1.6 million acres were in irrigation in <br />1965. Since that time, development <br />has leveled off and decreased <br />somewhat because of physical, <br />environmental, and economic <br />limitations. About 1.4 million acres <br />are currently being irrigated. <br /> <br />Irrigation Sources of Salinity <br /> <br />Colorado River <br /> <br />IRRIGATED <br />FIELD <br /> <br />IRRIGATION <br />DITCHES <br /> <br /> <br />...:.::::::::::t'::::::::. <br /> <br />Figure 3,-Schematic of agricullural salt pickup. <br /> <br />Irrigation development in the Lower <br />Basin began at about the same time as in the Upper Basin but was slow due to the <br />difficulty of diverting water from the Colorado River with its widely fluctuating flows. <br />Development of the Gila area began in 1875 and the Palo Verde area in 1879, <br />Construction of the Boulder Canyon Project in the 1930's, and other downstream <br />projects since that time, has provided for a continued expansion of the irrigated area. <br />In 1970, an additional 21,800 acres were irrigated by private pumping either directly <br />from the Colorado River or from wells in the flood plain. In 1980, nearly 400,000 <br />acres were being irrigated along the Colorado River mainstream; total irrigated lands <br />for the entire Lower Basin were about 1.5 million acres. <br /> <br />Natural Sources of Salinity <br /> <br />Flow and quality records reveal that along certain reaches of the Colorado River, large <br />increases in salt loads occur that cannot be attributed to irrigation or other <br />development-related activities. These increases are mainly due to natural diffuse <br />sources and saline springs. <br /> <br />Natural diffuse sources of salt occur gradually over long reaches of the river system. <br />Salt pickup occurs over large surface areas from underlying soils, geologic formations, <br />and from stream channels and banks. Salt pickup is difficult to identify, measure, or <br />control; yet, diffuse sources contribute the largest overall share of the salts to the <br />Colorado River. The natural salt load for the Colorado River at Lees Ferry, Arizona, <br />was estimated to be about 5.3 million tons per year[2,3]. Natural point sources are <br />saline springs where the contribution of salt and water is easily identified, issuing <br />from single or concentrated sources. The Glenwood-Dotsero Springs Unit and the <br />Paradox Valley Unit are two examples of point source salinity control. <br /> <br />Municipal and Industrial Sources of Salinity <br /> <br />Salts contributed to the Colorado River System by M&I sources are generally minor, <br />totaling about 1 percent of the Basin salt load (or 3 percent of the salinity). Iorns[2] <br />estimates that municipal and industrial users increased salinity by about 100 tons per <br />1,000 people in the Basin. The population reported for 1985 for the Upper Basin was <br /> <br />10 <br />