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