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,...\l 0, . l\:) c.,) r-:. ~ INTRODUCTION Background The Colorado "River Basin encompasses portions of seven States: Colorado, Wyoming, Utah, Nevada, New Mexico, California, and Arizona. The river is 1,400 miles long, with its headwaters in Wyoming and Colorado. It empties into the Gulf of California and serves some 14.5 million people on its way. It is one of the most physically developed and regulated rivers in the Nation. The river flow is apportioned among Upper and Lower Basin states and the Republic of Mexico. There are some 2.6 million acres of private irrigated land and some 44 million acres of nonfederal forest and rangeland within the United states (U.S.) 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 Secretary of the United States Department of the Interior (USDI) is assigned leadership responsibilities which he has delegated to the Water and Power Resources Service (WPRS) of that Department. The United States Department of Agriculture (USDA) and the Environmental Protection Agency (EPA) are named as cooperating" agencies. The Act has two major components. One is to deliver to Mexico the quantity of water agreed to in 1944 at the quality standard agreed to in 1973. This is covered in Title I of the Act which includes a large desalting plant to treat drainage return flows from the nellton-Mohawk Irrigation and Drainage District near Yuma, Arizona. The second component deals with the salinity concentration in the River aoove Imperial Dam and the controls necessary to meet U.S. water quality standards. This is covered in Title II of the Act. The Salinity Problem The salinity of the water delivered to Mexico increased to nearly 1500 milligrams per liter (mgfl) in 1961. This was partially due to the highly saline drainage return flows from the Well ton-Mohawk area. Title I of the Act deals with this problem. The total salt load in the river at Imperial Dam near Yuma, Arizona, is estimated to be 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 mgfl in the headwaters to about 820 mgfl at Imperial Dam. A future salinity level of 1,150 mgfl at Imperial Dam is projected for the year 2000 as additional upstream development takes place, assuming no corrective action. The long-term average annual salinity concentration at Imperial Dam was 757 mgfl for the 1941-1970 period. Each mgfl increase in salinity concentration causes approximately $0.5 million per year in economic damages to downstream agricultural, municipal, and industrial water users within t~e U.S.