to the river’s flow. The Colorado River system reservoirs have a total of roughly 60 million acre-feet of storage capacity, approximately four times the Colorado’s average annual flow. Although the basin’s major storage reservoirs have dampened the effects of climate and hydrologic variability, the amount of water in storage remains sensitive to climate fluctuations. Given the strong reliance that steadily increasing populations are placing on the river and its water storage system, variations in Colorado River flows and climate across the basin are as important as they have ever been.

The substantial economic value of Colorado River water has fostered competition—and at times intense animosity—among states and prospective water users. Over the decades, negotiations and legislation involving the Colorado’s water resources have added to a considerable body of laws, compacts, treaties, and agreements that partition and allocate its waters; significantly, much of this legal corpus has been designed to accommodate hydrologic and climate variations. Collectively known as the Law of the River, key components of this legal and institutional framework include the 1922 Colorado River Compact, the 1928 Boulder Canyon Project Act, the 1944 Mexico-United States Treaty, the 1948 Upper Colorado River Basin Compact, the Colorado River Storage Project Act of 1956, the 1963 U.S. Supreme Court decision in Arizona v. California, the 1968 Colorado River Basin Project Act, the 1973 Minute 242 agreement between Mexico and the United States, the 1992 Grand Canyon Protection Act, and other statutes, court decisions and decrees, contracts, and administrative decisions.

The Law of the River’s legal structure is complemented by an elaborate physical infrastructure of dams, reservoirs, levees, canals, aqueducts, tunnels, pumping stations, penstocks, pipes, and ditches. Most of the larger water control structures on the Colorado River and its tributary streams were constructed and today are operated by the U.S. Bureau of Reclamation. The two largest dams across the Colorado River are Hoover Dam, located near Las Vegas, Nevada, and Glen Canyon Dam, located 15 miles south of the Arizona-Utah border. Respectively, these dams impound Lake Mead and Lake Powell, the basin’s two primary storage reservoirs. With storage

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