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" 5 COLORADO RIVER SETTING ~ Much of the irrigated land in the west has been irrigated only 50 (D to 100 years. This is a relatively short time period when ~. compared to the Nile Valley and parts of India and China where ~ irrigation has been going on for over 4000 years. Concentration of salinity in the subsurface return flow remains essentially constant as the geo-hydro-chemical interaction of water passing through the underlying aquifer has reached a steady state condition. The salinity concentration can be a few hundred mg/L to several thousand mg/L depending on the characteristics of the aquifer. Soil lab analysis of saline soils(SCS, ) indicate concentrations of up to 35 , gypsum and lime in the soil profile after 50 to 100 years of irrigation. Soluble salts are leached quickly from the soil profile but the less soluble salts are leached slowly. The less soluble salts precipitate out first from irrigation or soil water, and gypsum and lime are what is usually visible on irrigated fields when they appear white. Highly soluble salts are leached out of the root zone soil in the first two or three years of initial reclamation when adequate drainage is provided. The supply of less soluble salt in the underlying aquifers is essentially unlimited for the next several thousand years, where irrigation is over ancient lake and seabeds. "In these cases, reducing the drainage water may be expected to reduce the salt emissions proportionately. II (Tanji 1990 p.477)." Therefore, the salt pickup from irrigation is directly proportional to amount of deep percolating return flow. Irrigation water management by the farmer is key to how much salt is added to downstream streamflow. In several drainages of the Colorado River, applied irrigation water carries less than 0.5 ton of salt per acre foot(100 to 400 mg/L) and drainage return flows can carry about 5 tons of salt in each acre-foot(3000 to 4000 mgfL). The objective of the salinity control program is to reduce the salt pickup by irrigation return flow by one and a half million tons per year or 30 percent. The beautiful and rugged Colorado River provides water for life to over 18 million people. However, due to heavy withdrawals, the River stopped flowing to the Gulf of California in 1964. Salts are concentrated by irrigation consumptive use and exports from the basin in the upper reaches. The Republic of Mexico receives the last 10% of the flow of the river with severe salinity pollution. In 1973 the U.S. agreed with Mexico to reduce salinity. This led to passage of The Colorado River Basin salinity Control Act of 1974, Public Law 93-320, without a dissenting vote in Congress. The Act illustrates the pUblic's awakening to the significance of off-site effects of irrigated agriculture. Water quality standards for salinity have been set by the states on the Colorado River to reduce present and future downstream salinity damages and to meet the requirements of the Clean Water Act, PL 92-500.