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After a period of stagnation through the 1920's, the Bureau of Reclamation launched its major dam-building era (the second phase of its conquest of lhe river) with passage of the Boulder Canyon Project Act in 1929 (Fradkin 1981). That law and the Upper Colorado River Basin Compact apportioned waters among states in the lower and upper basins, respectively, and Mexico was guaranteed water from the Colorado River by the Mexican Water Treaty (Graf 1985). High dams in the upper basin were authorized by the Colorado River Storage Project Act of 1956. Plans to construct dams at Echo Park in Dinosaur National Monu- ment and at Bridge and Marble canyons in Grand Canyon were scuttled when public opinion stimulated by Wilderness Society and Sierra Club advertising reached U.S. congress- men (Watkins 1969; Nash 1970; Coats 1984) . Welsh (1985) described the prolonged controversy over the Cen- tral Arizona Project and withdrawal of funding for Orme Dam. The Bureau entered its third phase of conquest, construc- tion of large aqueducts to deliver water to local areas, with passage of the Colorado River Basin Project Act of 1968. Fradkin's (1981) fourth phase, designed to "clean up the mess" caused by preceding phases, was launched by pas- sage of the Colorado River Basin Salinity Control Act in 1974. The final phase will involve increasing the amount of water in the Colorado River Basin by imports from other basins or augmenting runoff through weather modification, strategies which will require additional study and negotia- tion (Pillsbury 1981; Harris 1983). Since closure of Hoover Dam, the Colorado River System has become one of the most intensively controlled in the United States (Petts 1984). Although high dams provide security and profit for humans in the southwestern United States (Graf 1985), they have also transformed most of the streams in the Colorado basin into man-made rivers (Petts 1984). Modifications documented in Table 3 B represent only the "tip of the iceberg" of man-induced changes in the Colorado River System. Bishop and Porcella (1980) reported some 117 reservoirs with individual storage capac- ities over 0.001 km3 in the upper basin alone and about 40 trans-mountain canals and tunnels exporting waler from that region. Water from the Colorado River Basin has long been diverted to and used in the Arkansas, Platte, and Rio Grande river basins and the Great Basin. On the lower Colorado, channels have been shortened by excavation, banks rip-rapped, and channels deepened by dredging (Minckley 1979). Lowering of water tables due to ground- water pumping, diversion and modification of spring runs, channelization, and impoundments have reduced fish habitats in desert portions of the Colorado basin (Pister 1981; Meffe et al. 1983; Williams et al. 1985). Reservoir construction and diversion of water (often in open canals) from streams in the Colorado River Basin have enormously increased evaporative water loss from the sys- tem (Welsh 1985). Twelve percent ofthe Colorado's annual flow evaporated from reservoirs by the 1970's, and another 3 % was diverted from the basin (Graf 1985). Weatherford and Brown (1986) noted that 6.16 km3 of water are exported from the basin annually. These phenomena team with crop irrigation and effects of municipal and industrial water uses to exacerbate natural downstream salinity increases (Carlson and Carlson 1982). High salinity, the 230 river's most serious water quality problem, increases costs of water use and adversely affects agricultural productivity (Graf 1985). By 1974, salt concentrations in the lower river had reached maximum levels recommended for agriculture and human consumption, and basin states agreed upon a pro- gram to maintain salinity at or below levels measured near lower-basin mainstream dams in 1972. The Colorado River Basin Salinity Control Act restricted salinity of water deliv- ered to Mexico at Morelos Dam (Pillsbury 1981). A canal was constructed to divert highly saline water from the Welton-Mohawk Irrigation District to the Gulf of Califor- nia, and the Bureau of Reclamation is building a desalting plant at Yuma, Arizona, to ensure delivery of required flows and salinities to Mexico. Such projects are expected to reduce TDS by 130 mg. L -1 at Imperial Dam (Paulson and Baker 1983). Increased use of Colorado River water has also caused local problems associated with high heavy metals concentra- tions, radioactive materials, acid mine drainage, and oxygen levels near waste-treatment facilities (Bishop and Porcella 1980; Graf 1985). Additional energy development in the upper basin will increase impacts related to mining and rapid urbanization (Spofford et aI. 1980; Jacobsen 1982; Adams and Lamarra 1983), and acid deposition may become a serious problem. Watkins (1969) and Carothers and Johnson (1983) recog- nized the conflict between increasing visitation to National Park System areas in the Colorado River Basin and preser- vation of natural qualities of such lands. The National Park Service, Forest Service, and Bureau of Land Management now impose quotas on river rafters (Coats 1984) and enforce strict regulations on camping areas, use of fires, and waste management on lands they administer (Graf 1985). Nash (1986) emphasized the need to protect the Colorado's remaining wilderness. The Colorado River Today The Colorado River Basin is largely an area of very low human population density. It has few large cities but is highly urbanized; only 20 % of its population is rural (Graf 1985). Most of the land in the basin is administered by the United States government, primarily as Indian reservations, National Park System lands, nalional forests, and Bureau of Land Management (BLM) areas. The basin includes nine national parks, four recreation areas, 25 national monu- ments, and huge tracts of national forest and BLM- administered grazing or mineral-bearing lands (Graf 1985). The present-day Colorado River supplies more water for consumptive use than any other river in the United States (Pillsbury 1981) despite having the lowest unit-area dis- charge (28575 m3.km-2) of any United States river basin (Bishop and Porcella 1980). Stanford and Ward (1986a) referred to the basin as one of the driest in the world. The long-term (1896-1984) estimated annual average vir- gin flow of the Colorado River at Lee Ferry was 18.26 km3, but 1983 and 1984 levels were 29.60 and 30.22 km3, respectively (Upper Colorado River Commis- sion 1984). Stream regulation in the basin has reduced high spring flows and resulted in relatively high summer flows; drastic daily variation is common (Graf 1985). The Colorado River below Glen Canyon Dam may rise as much