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N. ~ ~ ~, adopt salinity standards and an implementation plan to limit further increases in salinity that are discussed later in chapters 5 and 6, Chapter 3 Causes and Impacts of Salinity CAUSES OF SALINITY Nearly half of the salinity in the Colorado River system is from natural sources, Saline springs, erosion of saline geologic formations, and runoff all contribute to this background salinity, Irrigation, reservoir evaporation, and municipal and industrial (M&I) sources make up the balance of the salinity problem in the Basin, Figure 2 shows the relative amounts each source contributes to the salinity problem, The EPA[I] estimated the natural salinity at Hoover Dam was 334 mgIL, In 1991, the salinity at Hoover Dam was reported to be 634 mg/L, a 300 mgIL increase. Ifrigalion (37%) Natural (47%) Reservoir (12%) Evaporation Figure 2,-Sources of salinity, The natural or background salinity of the Colorado River has been increased by the development of water resources in two major ways: (1) the addition of salts from water use and (2) the consumption (depletion) of water, The combined effects of water use and consumption have had a significant impact on salinity in the Colorado River Basin, Concern over the damaging levels of salinity prompted the Basin States and the Federal Government to Agricultural Sources of Salinity Irrigated agriculture is the largest user of water in the Colorado River Basin and a major contributor to the salinity of the system, lorns[2] found that irrigated lands in the Upper Colorado River Basin contribute about 3.4 million tons of salt per year (37 percent of the salinity ofthe river), Irrigation increases salinity (figure 3) by consuming water and by dissolving salts found in the underlying saline soils and geologic formations, usually marine shales, Deep percolation mobilizes the salts found naturally in the soils, especially if the lands are overirrigated, Through salinity control practices, these contributions to the river system eM be dramatically limited, ensuring the future beneficial uses of the river, Irrigation development in the Upper Basin took place gradually from the beginning of settlement in about 1860 but was hastened by the purchase ofland from the Indians in 1873, About 800,000 acres were being irrigated by 1905, Between 1905 and 1920, 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 over 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 development in the Lower Basin began at about the same time as in the Upper