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<br />CHAPTER 2 - SALINITY OF THE RIVER <br /> <br />o <br />1'\.) <br />W <br />N <br /> <br />Overview <br /> <br />The Colorado River drains 246,000 square miles (approximately 157 million acres) of the <br />western United States and a small portion of northern Mexico. Its waters serve some 7.8 million <br />people within the United States' portion of the Colorado River basin, and through export provides <br />full or supplemental water supply to another 23 million people outside the basin. The regional <br />economy is based on irrigated agriculture, livestock grazing, mining, forestry, manufacturing, oil and <br />gas production, recreation and tourism. About 3.5 million acres are irrigated within the Colorado <br />River Basin and hundreds of thousands of additional acres are irrigated by waters exported from the <br />Basin. Hydroelectric power facilities along the Colorado River and its tributaries generate <br />approximately 12.billion kilowatt-hours annually which is used both inside and outside of the Basin. <br />The Colorado River also serves about 2.3 million people and 500,000 irrigated acres in Mexico. <br /> <br />Salinity has long been recognized as one of the major problems of the river. The Colorado, <br />like most western rivers, increases in salinity from its headwaters to its mouth, carrying an average <br />salt load of approximately nine million tons annually past Hoover Dam, the uppermost location at <br />which numeric criteria have been established. In addition to total salt load which measures the total <br />mass of salt carried in the River (tons per year), this report also examines salinity in terms of <br />concentration as expressed in milligrams per liter (mg/L). <br /> <br />The salts in the Colorado River system are indigenous and pervasive. Many of the saline <br />sediments of the Basin were deposited in prehistoric marlD.e environments. Salts contained within <br />the sedimentary rocks are easily eroded, dissolved, and transported into the river system. The <br />Colorado River Basin Salinity Control Program is designed to prevent a portion of this abundant salt <br />supply from moving into the river system. <br /> <br />In a 1971 study, the EPAanalyzed salt loading in the Colorado River Basin and divided it <br />into two categories, naturally occurring and human-caused. The EP A concluded that about half <br />(47 percent) of the salinity concentration measured in water arriving at Hoover Dam is from natural <br />causes including salt contributions from saline springs, ground water discharge into the river system <br />(excluding irrigation return flows), erosion and dissolution of sediments, and the concentrating <br />effects of evaporation and transpiration. The natural causes category also included salt contributions <br />from non-point (excluding irrigated agriculture) or unidentified sources or from the vast, <br />sparsely-populated regions of the drainage, much of which is administered by the BLM or other <br />governmental agencies. Qfthe land within the Colorado River Basin, about 75 percent is owned and <br />administered by the Federal Government or held in trust for Indian tribes. The greatest portion of <br /> <br />8The Mineral Ouali1y Problem in the Colorado River, Summary Report, Environmental Protection Agency, <br />Regions VlII and IX, 65 pp., 1971. <br /> <br />2-1 <br /> <br />.jo,_. <br />