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SUMMARY The Quality of Water, Colorado River Basin, Progress Report is prepared every 2 years to summarize the status of water quality in the Colorado River Basin. The Colorado River provides municipal and industrial water for more than 18 million people in seven Basin States and irrigation water to more than 1. 7 million acres of land. The threat of salinity is a major concern in both the United States and the Republic of Mexico. Increases in salinity are important in the Colorado River Basin because of its effect on crops and on municipal and industrial users. Damages in the Upper Basin and Mexico are unquantified, but damages from 1976-85 in the Lower Basin were estimated by the Milliken Chapman Research Group at $311 million per year. These damages will increase to over $1 billion per year as salinity levels approach normal levels under the salinity standards. The Colorado River Basin Salinity Control Act (Salinity Control Act) (Public Laws 93-320 and 98-569) authorizes the Secretaries of the U.S. Department of the Interior (Interior) and U.S. Department of Agriculture (USDA) to enhance and protect the quality of water available in the Colorado River for use in the United States and the Republic of Mexico. Title I of the Salinity Control Act authorizes the construction and operation of a desalting plant, brine discharge canal, and other features that will enable the United States to deliver water to Mexico having an average salinity no greater than 115 parts per million (ppm) +/- 30 ppm over the annual average salinity of the Colorado River at Imperial Dam. Title II of the Salinity Control Act authorizes the Secretary of the Interior to construct five salinity control units and authorizes the Secretary of Agriculture to implement an onfarm salinity program. Further, the Secretary of the Interior was directed to undertake research on additional methods to control salinity and to cooperate with the Department of Agriculture and others. The 1993 Review, Water Quality Standards for Salinity, Colorado River System found that the "flow-adjusted" salinity levels at the Hoover, Parker, and Imperial stations were at or slightly below the numeric criteria levels of the salinity standards (table 12, page 86). Without salinity control, salinity would have exceeded the numeric criteria. Compliance with the standard was due to the Interior and USDA salinity control programs. By 1995, these programs will be removing more than 500,000 tons of salt annually from the river system. Without further controls, the salinity standards will be exceeded. Reclamation estimates that more than a million tons of salt per year will ultimately need to be removed from the river system to maintain average salinity below the numeric criterion level of 879 milligrams per liter (mg/L) at Imperial Dam. To meet this objective, several new salinity control units will need to be authorized and constructed. Even with these controls in place, there will still be temporary excursions above the numeric criterion of 879 mg/L due to the natural variations in climatic conditions, hydrology, and water usage.