their flows; in this regard, the Colorado likely surpasses all other western rivers. With rapid increases in population and water demand in the Colorado River region in the past 25 years, issues of water management, science, and law have assumed tremendous importance and prominence.

The tree-ring-based Colorado River flow reconstructions issued in the late 1990s and early 2000s represent a key advance in scientific understanding of the region’s climate and hydrology. General findings from these reconstructions—that sustained, severe droughts have recurred for centuries across the region, that the 1890-1920 period was exceptionally wet, and that the long-term mean flow of the Colorado River is lower than the 15 million acre-feet per year reflected in the Lees Ferry gaged record (and less than the framers of the Colorado River Compact assumed)—are important in themselves. They are also important because their publication coincided with severe drought conditions in the late 1990s and early 2000s, and during a period of increasing population growth and water demands. The convergence of these findings and trends induced great concern among water managers in federal, state, regional, and municipal agencies across the Southwest. Recent water conservation and management initiatives, such as the Department of the Interior’s Water 2025 program, acknowledge the challenges and conflicts that will inevitably attend increasing water demands, limited (and likely decreasing) water supplies, and recurrent drought.

Urban water use has always been part of the context of the Law of the River and the operations of Lakes Powell and Mead, but for many decades it was overshadowed by large-scale irrigation development. The 1968 National Research Council report on water and choice in the Colorado River basin, for instance, noted that while population was growing rapidly in the region, “Much of the Colorado basin is almost uninhabited.” Large portions of the basin’s interior and arid regions remain sparsely populated today, but over the past 40 years, and especially since the mid-1980s, urban water demands within the basin and in water delivery areas outside of the basin have grown in importance in the context of Colorado River water storage and operational decisions. In earlier times, concerns regarding hydroclimatic variability and the Colorado River largely centered on the Bureau of Reclamation’s Annual Operating Plan and operational specifics derived from the Law of the River, such as equalization of storage levels between Lakes Powell and Mead. With today’s rapidly

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