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INTRODUCTION o CD to CJ1 Description of the Basin The Colorado River Basin includes 632 000 km' in the Western United States and northern Mexico (fig. 1). The average unregulated flow of the Colorado River below Lees Ferry, Arizona, is 16.0-18.5 km3/a, which is small compared with that of other North American rivers with similar size basins. However, the river is an important source of water for more than 12 million people and approximately 1 million ha of irrigated agri- cultural land [1]. * The headwaters of the Colorado and its major tributaries, the Green and San Juan Rivers, lie in the high peaks ofthe Rocky Mountains, where the annual precipitation is normally between 100 and 150 cm. Most of its course, however, crosses the semiarid Colorado Plateau and the Sonoran Desert, where the average annual precipitation is only 6 em [2]. Many of the geologic formations in this part of the basin are of marine origin and contain sodium chloride (halite) and calcium sulfate (gypsum) salts. Natural springs and man-made wells intercept saline ground waters associated with these formations and discharge into the river system. Soils in much ofthe basin have developed residually on gypsum-bearing shales. Irrigation water applied to this land promotes weathering and dissolution of salts from the soil and under- lying shales. As a result this water returns to the river with a greater salt load than was diverted [3]. Irrigation also increases dissolved mineral con- centrations in the river by depleting the streamflow volume. As a result, TDS (total dissolved solids) increases from approximately 50 mg/L at the headwaters to 800 mglL(1977-1981 average)[2] at Imperial Dam, the final diversion point on the Colorado River in the United States. Water resource development in the basin began when the pioneers settled there in the 1860's. By the 1920's, wt.1en the USGS (U.S. Geological Survey) began monitoring surface water quality, much of the present irrigation development was already in place, and several reservoirs and trans- basin diversions had been constructed in the upper reaches of the basin. The first major multi- purpose reservoir, Lake Mead, began filling in 1935. Development in the basin upstream of Lake Mead continued gradually, though mainstem flow was essentially unregulated until the early 1960's. Between 1962 and 1966, the storage capacity above Lees Ferry increased from 3 to 45 km3, * Numbers in brackets refer to entries in the bibliography. mostly as a result of the construction of the Colorado River Storage Project reservoirs: Navajo Reservoir on the San Juan River (began filling in November 1962), Blue Mesa Reservoir on the Gunnison River (November 1965), and Lake Powell on the Colorado River(March 1963). Reservoir and transbasin diversion development in the basin upstream of Lake Mead is outlined in table 1 of appendixes B through P. Colorado River Water Quality Improvement Program During the period of accelerated development, the USGS initiated an appraisal of the water supply in the basin [3]. Its purpose was to determine whether development would be limited by "legal, physical, and economic factors." Much of the study focused on the impacts of human activities on salinity, It concluded that, by 1957, approximately half the average annual TDS concentration at Lees Ferry was caused by domestic, industrial, and agri- cultural activities within the basin and transbasin diversions. By 1970, salinity in the lower Colorado River was recognized as a basinwide problem [4]. TDS stan- dards for lower basin gage locations were adopted by the basin States in response to amendments to the Federal Water Pollution Control Act of 1972. These standards were set at the 1972 average concentrations, thus establishing a policy of non- degradation. To offset the effects of future water development, the Colorado River Basin Salinity Control Act of 1974 authorized the USBR to construct 4 salinity control projects and investigate the feasibility of cpnstructing 12 others. The location of active projects upstream of Hoover Dam is shown in figure 1. At the same time, computational methods were developed to predict the effects of planned water resources development. Early projections indi- cated that salinity increases observed at Imperial Dam between 1949 and 1970 would continue, with TDS concentration eventually reaching 1200 mg/L [4, 5, 6]. However, contrary to these projections, salinity has actually decreased since 1970 (see fig, 2). It is not clear whether this decrease in salinity is part of a long-term cycle or is indicative of a permanent change somewhere in the system. Other investigators [7, 8] have argued that new conditions created by impoundment of large reservoirs have permanently altered chemical processes in the river system and may be a factor 2