WSP07972 - Laserfiche WebLink

Home Browse Search

~ w ~ N CHAYI'ER 2 - SALINITY OF THE RIVER OVERVIEW The Colorado River 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, livestock 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. Salinity! has long been recognized as one of the major problems of the river. The Colorado, like most western rivers, increases in salinity from its headwaters to its mouth, carrying an average salt load of 9 million tons annually past Hoover Dam, the uppermost location at which numeric criteria have been established. In addition to total salt load (tons/yr), 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 sediments of the basin were deposited in marine environments which were saline. Salts deposited with the sedimentary rocks are easily eroded, dissolved, and transported into the river system. The salinity control program is designed to prevent a portion of this abundant salt supply from moving into the river system. The EPA (1971) analyzed the salt loading and for convenience divided it into two categories: naturally occurring and human-caused. They concluded that about half of the salinity concentration measured at Hoover Dam is from natural causes. Natural causes include salt contribution 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 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. Human activities in these vast regions influence the rate of natural salt movement from the rocks and soils to the river system. Human activities which influence the "natural" contribution include livestock grazing, wildlife management, logging, mining, oil exploration, road building, recreation and others. IFor this report, the terms "salinity" and "total dissolved solids" (TDS) are used interchangeably. 2-1