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826 NATURAL RESOURCES JOURNAL [Vol. 40 Fall 2000] MANAGING ECOSYSTEM CONSERVATION 827 mesquite, deer and wild boar, wild geese and ducks, doves, quail, and fish, providing a subsistence lifestyle that required a healthy Delta ecosystem.25 C. The Delta Transformed 1963 to 1981 as Lake Powell filled behind the newly-constructed Glen Canyon Dam in Arizona (see figure 2).29 Today, with these reservoirs near capacity, the dams are used to regulate flows so that water can be reliably apportioned among users. Most flood flows can be contained, regulated, and added to the river's capacity to supply agriculture and urban centers. Floodwaters, known as "space- C'" building" or "spill" flows, are released from Lake Mead, the largest ~ reservoir on the river, only when the U.S. Bureau of Reclamation (BOR), the ~ agency managing the dams, predicts flows that exceed the system's capacity Q for use and storage. 0 The Colorado River is now one of the most highly regulated and diverted rivers in North America. Virtually every drop is accounted for in the allocation of water among nine states (seven in the United States and two in Mexico) and the 27 native tribes that have rights to use it. 30 The river irrigates more than 3.7 million acres of farmland in the southwestern United States and Mexico, and supplies water to nearly 30 million people. While irrigated agriculture tops the list of Colorado River water uses in the United States and Mexico, the second largest consumption of water is evaporation from reservoirs.31 Diversions out of the Colorado basin, such as water delivered to Los Angeles, are the third largest use, followed by municipal and industrial uses. In addition to providing water for consumptive use, the dams along the Coiorado River in the United States provide hydroelectric power to the states in the U.S. Southwest, with a total generating capacity of about 4425 megawatts.32 In years without flooding, the only Colorado River water to cross the border is the 1.5 million acre-feet allotted by treaty to Mexico,33 slightly more than 10 percent of current estimates of the river's average annual The physical transformation of the Colorado River delta is the result of numerous local and basin-wide developments. By the nineteenth century, the Delta was open for navigation, and steamboats consuming riverside cottonwoods for fuel traveled from Yuma, Arizona, through the Delta to the Gulf of California, in an active river trade,26 By the early 1900s, farmers in the Mexicali Valley had begun to clear the land and irrigate their fields. Irrigators in the United States, subjected to the river's annual cycle of spring floods and low summer flows, demanded that the federal government control the Colorado River to provide a consistent and reliable supply of water .27 Water's power to transform the dry desert landscape, and its power to generate electricity, would make Colorado River water an irresistibly valuable resource throughout the twentieth century. As the West's population and need for water' have grown, the Colorado River has been tapped through a system of dams and diversions. Over its 1400-mile course, 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 Nevada in the 1930s marks the beginning of the modem 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, and the riparian zone of the river from Morelos Dam to the junction with the Rio Hardy was a dry ecosystem, dominated by widely spaced mesquite trees.28 As Lake Mead filled, the river flow was perennial below the junction of the two rivers due to the discharge of agricultural wastewater from the Mexicali Valley and tidewater entering from the Gulf of California. The marked decrease of water in the mainstem from Morelos Dam to the confluence with the Rio Hardy recurred from 29. See INT'L BoUNDARY &; WA'IU COMM'N,WESTERN WA'IU BUU.E'J1N: PLow Of1HE COLORAOO RIvER AND OTHER. WESTERN BoUNDARY STREAMS AND RELATED DATA (1960-1998); U.S. GEOLOGICAL SURVEY, GEOLOGICAL SURVEY WA'IU-SUPPLY PAPER 1313, COMPlLATION Of REcoRDS Of SURPACE WATERS Of 11iIl UNITED STA'reS 1HROUGH 5EPTEMBER 1950, PART 9: CoLORAOO RlvER BASIN 709-29 (1954). 30. There are 34 tribes In the Colorado River basin. of which 27 claim rights to Colorado River water. See PONTIUS, supra note 1, at 72. 31. A11ocations znade under the lawl and compacts that make up the Law of the River do not account for 1.5 miWonac:re-feet in annual evaporative losses from malnstem ~ervoirs. See PoNTIUS, supm note I, at 10. 32. Set Larry MacDoN\e1l &; Bruce Driver, Rethinking Colorado Riwr Gowrnance, 1996 PRocEEDINGS REPoRT FROM nm COLORAOO RlvER WORKSHOP 181, 190. 33. See Treaty on the Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande, Feb. 3, 1944, U.S.-Mex., 59 Stat. 1219. 25. See Anita Alvarez de Williams, Cocopd, in 10 HANoBOOKOf NORni AMERICAN INDIANS 99 (Alfonso Ortiz ed., 1983). 26. See id.; GoDfREY SYKES, THE COLORAOO DELTA 30-34 (1937). See generally MARKK. BRIGGS, RIPARIAN EcOSYSTEM REcOVERY IN ARID LANDs (1996). 27. See NORRlSHUNDLEY, JR., W ATERANDnm WBST: lHl!COLORADO RIvER COMPACf AND niB Pouncs Of W A'IU IN 1HE AMERICAN WEST 5-10 (1975). 28. These observations are based on inspection of 1972 aerial photographs and interviews with residents. See V ALOes-cASlLLAS ET AL., supm note 12, at 5.