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<br /> <br />Reservoir (12%) <br /> <br /> <br />l!\J" <br />~ <br />~ <br /> <br />Chapter 3 <br />CAUSES AND IMPACTS OF SALINITY <br /> <br />CAUSES OF SALINITY <br /> <br />Nearly half of the salinity in the Colorado River System is from natural sources. <br />Saline springs, erosion of saline geologic formations, and runoff all contribute to this <br />background salinity. Irrigation, reservoir <br />evaporation, and municipal and industrial <br />(M&I) sources makeup the balance of the <br />salinity problem in the Colorado River Basin <br />(Basin). Figure 2 shows the relative amounts <br />each source contributes to the salinity <br />problem. The Environmental Protection <br />Agency (EPA)[l] estimated the natural <br />salinity at Hoover Darn was 334 milligrams <br />per liter (mg/L). In 1993, the salinity at <br />Hoover Darn was reported to be 660 mg/L, a <br />326-mg/L increase. <br /> <br />Natural (47%) <br /> <br />Figure 2,-Sources of salinity. <br /> <br />The natural or background salinity of the Colorado River has been increased by the <br />development of water resources in two major ways: (1) the addition of salts from <br />water use and (2) the consumption (depletion) of water. <br /> <br />The combined effects of water use and consumption have had a significant impact on <br />salinity in the Colorado River Basin. Concern over the damaging levels of salinity <br />prompted the Basin States and the Federal Government to adopt salinity standards <br />and an implementation plan to limit further increases in salinity that are discussed <br />later in chapters 5 and 6. <br /> <br />Agricultural Sources of Salinity <br /> <br />Irrigated agriculture is the largest user of water in the Colorado River Basin and a <br />major contributor to the salinity of the system. Iorns[2] found that irrigated lands in <br />the Upper Colorado River Basin contribute about 3.4 million tons of salt per year <br />(37 percent of the salinity of the river). Irrigation increases salinity (figure 3) by <br />consuming water and by dissolving salts found in the underlying saline soils and <br />geologic formations, usually marine shales. <br /> <br />Deep percolation mobilizes the salts found naturally in the soils, especially if the <br />lands are over irrigated. Through salinity control practices, these contributions to the <br />river system can be dramatically limited, helping maximize the future beneficial uses <br />of the river. <br /> <br />Irrigation development in the Upper Basin took place gradually from the beginning of <br />settlement in about 1860 but was hastened by the purchase of land from the Indians <br />in 1873. About 800,000 acres were being irrigated by 1905. Between 1905 and 1920, <br /> <br />9 <br />