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CHAPTER II - SALINITY OF THE RIVER w -...] The Colorado RiveL drains 244,000 square miles of the western ~ United states and a small portion of northern Mexico. Its waters ~ serve some 2.5 million people within the United states' portion of the basin and through export provides full or supplemental water supply to another 18.0 million people outside the basin. The regional economy is based on irrigated agriculture, Ii vestock grazing, mining, forestry, manufacturing, oil and gas production, and tourism. About 2.5 million acres are irrigated within the basin and hundreds of thousands of acres are irrigated by waters exported from the basin. The Colorado River also serves about 1.7 million people and 500,000 irrigated acres in Mexico. Salini tyl has long been recognized as one of the maj or problems of the river. The Colorado, like most western rivers, increases in salinity from its headwaters to its mouth, carrying a salt load of about 9 million tons annually past Hoover Dam, the upper most location at which numeric criteria has been established. In addition to total salt load (tons), this report also examines salinity in terms of concentration (mg/l). It must be emphasized that all of the salts in the Colorado River System are natural. Many of the sedimentary rocks of the basin were deposited in environments which were saline. Sal ts deposited with the rocks can be dissolved and transported by the river system. This makes the strategies and planning related to salinity control much different than in river systems where man is inducing a non-natural pollutant into ~llG ~ystem at point sources. In the Colorado River drainage, salt is pervasive; it is literally everywhere. The salinity control program is designed to prevent a portion of this almost limitless salt supply from becoming dissolved and moving through the river system. The Environmental Protection Agency (EPA) analyzed the salt loading and for cop"enience divided it into two categories: natural and human-caused. They concludeG ~:1at about half the salt loading to the river is from natural causes. This division of salts into a "natural causes" category, however, may be an oversimplification, and the classification may be misleading. This natural causes category includes 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 government agencies. Man's activities in these vast regions, however, do influence the rate 1 Salinity is a measvre of the total dissolved solids of a water sample including ~11 inorganic material in solution, whether ionized or not. The principal constituents found in Colorado River water are: calcium, magnesium, sodium, sulfate, chloride and bicarbonate. The terms "salinity" and "total dissolved solids" are considered equivalent. 5