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. o o tv W . . CHAPTER 2 - SALINITY OF THE RIVER Overview The Colorado River drains 246,000 square miles (approximately 157 million acres) of the western United States and a small portion of northern Mexico. Its waters serve some 5,5 million people within the United States' portion of the Colorado River Basin, and through export provides full or supplemental water supply to another 22.3 million people outside the Basin, The regional economy is based on irrigated agriculture, livestock grazing, mining, forestry, manufacturing, oil and gas production, recreation and tourism, About 3.5 million acres are irrigated within the Colorado River Basin and hundreds of thousands of additional acres are irrigated by waters exported from the Basin, Hydroelectric power facilities along the Colorado River and its tributaries generate approximately 12 billion kilowatt-hours annually which is used both inside and outside of the Basin, The Colorado River also serves about 2.3 million people and 500,000 irrigated acres in Mexico, Salinity has long been recognized as one of the major problems ofthe river. The Colorado, like most western rivers, increases in salinity from its headwaters to its mouth, carrying an average salt load of approximately nine million tons annually past Hoover Dam, the uppermost location at which numeric criteria have been established, In addition to total salt load which measures the total mass of salt carried in the River (tons/year), this report also examines salinity in terms of concentration as expressed in milligrams per liter (mg/L), The salts in the Colorado River system are indigenous and pervasive, Many of the saline sediments of the Basin were deposited in prehistoric marine environments, Salts deposited with the sedimentary rocks are easily eroded, dissolved, and transported into the river system, The Colorado River Basin Salinity Conn:.~l Program is designed to prevent a portion ofthis abundant salt supply from moving into the river system, In a 1971 study!, the EPA analyzed salt loading in the Colorado River Basin and divided it into two categories, naturally occurring and human-caused, The EP A concluded that about half (47 percent) of the salinity concentration measured in water arriving at Hoover Dam is from natural causes including salt contributions from saline springs, ground water discharge into the river system (excluding irrigation return flows), erosion and dissolution of sediments, and the concentrating effects of evaporation and transpiration, The natural causes category also included salt contributions from nonpoint (excluding irrigated agriculture) or unidentified sources or from the vast, sparsely-populated regions of the drainage, much of which is administered by the BLM or other governmental agencies, Gfthe land within the Colorado River Basin, about 75 percent is owned and adrninistered by the Federal Government or held in trust for Indian tribes. The greatest portion of the naturally-occurring salt load originates on these federally-owned and adrninistered lands, Human 'The Mineral Qualitv Problem in the Colorado River, Surrunary Report, Environmental Protection Agency, Regions VIII and IX, 65 pp" 1971. 2-1