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'~.)~:::..::.- ., .' the region and growing pressures on water, land, and other resources will intensify the strain on the delta. At some point, water users both north and south of the border may be forced to make some difficult choices about Colorado River allocations. The authors' long- term view is that the delta's ecological, social, and economic values will come to be widely recognized in future deliberations over the allocation of surplus waters, and that in-stream flows may be dedicated to sustain it. THE DELTA OF YESTERDAY The Colorado River meets the Gulf of California in Mexico, where the states of Baja California and Sonora share a border. The delta once covered over 3,000 square miles (7770 km2), an area the size of Rhode Island. Highly variable flood cycles on the Colorado created a dynamic and unstable delta populated by a rich array of adapt- able and resilient plant and animal species and human communities that lived off this bounty. For eons, as much as 70 percent of the Colorado River's silt load was carried to the delta,4 bringing nutrients and spreading the delta ever wider into the upper Gulf. The delta's richness is further increased by the action of tides typically 13 feet (4 m) or higher, an unusually high ebb and flow that extends the tidal estuary 34 miles (56 km) or more upriver (Payne et aI., 1992). The interaction of these tidal flows with freshwater from the Colorado River creates a rich breeding ground for the Gulf's marine life, among other benefits. The delta once sup- ported an estimated 200 to 400 species of vascular plants (Ezcurra et aI., 1988). Early explorers reported jaguars, beavers, deer, and coyotes in addition to the legendary abundance of waterfowl, fish, and other marine and estuary organ- isms (Spamer, 1990; Leopold, 1948). Early explorers also encountered local people known as the Cucapa, or "the people of the river." The Cucapa are descendants of the Yuman-speaking Native Americans and have inhabited the delta for nearly a thousand years. Spanish explorer Hernando de Alarcon made the first recorded contact with the Cucapa in 1540 and reported seeing many thou- sands. The Cucapa used the delta floodplain extensively, for harvesting Palmer's saltgrass, a wild grain, and for cultivating corn, beans, and squash. Other foods included mesquite - ground into a meal or made into a drink - deer and wild boar, wild geese and ducks, doves, quail, and fish. They lived in rectangular and round huts, and relied on dugouts and rafts for river travel (Williams, 1983). THE COLORADO BASIN TRANSFORMED By the nineteenth century, the delta was open for navi- gation, and steamboats traveled from Yuma, Arizona, through the delta to the Gulf, in an active river trade. This trade ceased in 1877, when the Southern Pacific rail line reached the Colorado River (Sykes, 1937; Williams, 1983). By the early 1900's, farmers in the Mexicali Valley had begun to irrigate their fields, and the desert bloomed. Water's power to transform the dry desert landscape-and its power to create electricity-would make Colorado River water an irresistibly valuable resource through the entire twentieth century. [See Figure 1.] As the West's population and need for water have grown, the Colorado River has been tapped through a system of dams and diversions that begin close to its source in the mountains of Colorado and Wyoming. Over its 1400-mile (2300 km) course, from its head- waters to the end of its main channel at Montague Island in the Gulf of California, the Colorado is interrupted by more than 10 major dams. More than 80 major diversions carry water away from the river for agriculture and other uses. The construction of Hoover Dam in the 1930's marks the beginning of the modern era for the Colorado delta. For six years, as Lake Mead filled behind the dam, virtually no freshwater reached the delta. Even spring flooding was captured. This ecologically devastating event was repeated from 1963 to 1981 as Lake Powell filled behind the Glen Canyon Dam (Glenn et aI., 1996). With these reservoirs now filled, the dams are used to regulate flow so that water can be reliably apportioned among users and its use maximized. Most flood flows can be contained, regulated, and added to the river's capacity to sustain the West's urban centers and agri- culture. Floodwaters are released only when the Bureau of Reclamation, the agency managing the dams, predicts flows that exceed the system's capacity for use and storage. 4 Between 45 million and 455 million metric tons of silt per year were transported through the Grand Canyon between 1922 and 1935 (Minckley, 1991). 2