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~ ~ cropland and some 44 million acres of nonfederal forest and rangeland within ~ the United States' portion of the Basin. ~ Salinity control in the Colorado River Basin is addressed in the Colorado River, Basin Salinity Control Act of 1974, Public Law 93-320~ The Act has two major components. one is to maintain the quality standard agreed to,on August 30, 1973, in Minute 242 of International Boundary and Water Commission for water delivered to Mexico under the Mexican Water Treaty of 1944. This is covered in Title I of the Act which includes a large desalt- ing plant to treat drainage return flows from the Well ton-Mohawk Irrigation and Drainage District near Yuma, Arizona. The second component, covered in Title II of the Act, deals with the salinity concentration in the River above Imperial Dam and the controls necessary to meet U.S. water quality standards established by the seven basin States and EPA. USDI is assigned leadership responsibilities which has been delegated to the USBR of that Department. USDA and the EPA are named as cooperating agencies and USDA's involvement is documented in a MOU with Interior. The Salinity Problem The salinity ,of the w~r delivered to Mexico increased from an'annual average of about 800 mg/l to nearly 1,500 mg/l in 1962. This was partially attributed to the highly saline drainage return flows from the Wellton-Mohawk area which empties into the Colorado River below Imperial Dam. Title I of the Act deals with this problem. The total salt load in the river entering Lake Mead above Boover Dam is estimated to average 10 million tons per year. To meet the salinity control objective of Title II, it is necessary to remove some 2.8 million tons per year of this salt load. The present average annual salinity concentration of the river varies from about 50 mg/l in the headwaters to about 820 mg/l at Imperial Dam. The USDI projects a future salinity level of 1,141 mg/l at Imperial Dam for the year 2000 as additional upstream development takes place, assuming no corrective action. The long-term'average annual salinity concentration at Imperial Dam is 875 mg/l under current devel- opment conditions. Each mg/l increase in salinity concentration causes approximately $469,000 per year (first quarter 1981 dollars) in economic damages to downstream agricultural, municipal, and industrial water users within the U.S. The control of salinity in the river at the historic level, while the States continue to develop compact apportions of water supply, is required by an agreement made between the seven Basin States and EPA for implementation of Public Law 92-500, the Clean Water Act. Average onfarm irrigation and distribution system efficiencies, especially in the Upper Basin, are generally low. Low irrigation efficiencies generally indicate high surface runoff and/or overirrigation. Overirrigation can result in excessive deep percolation which leaches excess salts from the soil into the river. This greatly contributes to the salinity problem. 4