Divining the Future, Dividing the River By Doug Kenney University of Colorado Natural Resources Law Center

Interstate Obligations As the primary headwaters state for the Arkansas, Colorado, Platte and Rio Grande rivers, the impact of climate change in Colorado has the potential to affect 18 downstream states and the Republic of Mexico. Despite a lack of regional climate change studies for basins other than the Upper Colorado and the ongoing challenge of identifying clear trends regarding precipitation in the state, the majority of recent hydrologic studies suggest future declines in runoff for most of Colorado’s river basins. By the terms of nine interstate water compacts and two United States Supreme Court equitable apportionment decrees, Colorado is limited to consuming approximately one-third of the water produced by its stream systems in an average year. The extent to which the influence of climate change on interstate apportionments is problematic for Colorado water users depends greatly on site specific circumstances, such as the seniority of rights; the availability of reservoir storage capacity; growth pressures; and the amount of water, if any, legally available for new developments. Current levels of water use in the Arkansas, Platte, and Rio Grande basins are already at or very near the state’s full apportionment, while in the Colorado River basin, some additional water may be left for Colorado. The Colorado River and its tributaries—including the Yampa, Gunnison, and San Juan rivers—are of particular concern to the water management community on both sides of the 30

Continental Divide, as this is the only water supply source for West Slope users, and is a significant source of water exports to the South Platte, Arkansas, and to a lesser extent, the Rio Grande basins. Front Range projects importing Colorado River water include Denver Water’s Moffat Tunnel and Roberts Tunnel systems, the Northern Colorado Water Conservancy District’s Colorado-Big Thompson Project, the Homestake Project that serves Aurora and Colorado Springs, and the Frying Pan-Arkansas Project that supplies southeastern Colorado. These and other transmountain diversions, some in operation for more than 100 years, are crucial to present and future Front Range agricultural, municipal, commercial, recreational and environmental uses. These flows are equally valued by growing mountain communities, many of which already find it difficult to secure legal rights to waters flowing through local streams. Finding water for West Slope recreational and environmental purposes, including for endangered fish protected by the Upper Colorado Recovery Implementation Program, is also an ongoing challenge magnified by changes in streamflow amounts, timing and character. With the water needs of potential energy development factored in, climate change on the Colorado River resonates through every region and sector of the state. It is also an issue of particular concern to the six additional basin states—Wyoming, Utah, New Mexico, Nevada, Arizona and California—and Mexico that share a

hydrologic, cultural and legal connection to the river. Climate Change on the Colorado River Determining how much Colorado River water remains for increased use and how best to use it are questions central to—and greatly complicated by—an era of climate change. Over the past century of record keeping, flows on the Colorado River have averaged almost 15 million acre feet, or MAF, per year at Lee Ferry, the official measuring point. Flows range from a high of 24.5 MAF in 1983 to a low of 5.6 MAF in 1934. Interstate apportionment provided by the 1922 Colorado River Compact and the 1944 U.S.-Mexico Treaty are based on the assumption of average flows of at least 16.5 MAF each year, divided among the states of the Upper Basin and Lower Basin, 7.5 MAF each, and Mexico, 1.5 MAF. In fact, a total of 17.5 MAF is actually apportioned, with the extra 1 MAF reserved for the Lower Basin if available. The error derives, in part, to unusually wet conditions before the Colorado River Compact negotiations in the 1920s, which provided negotiators with an unrealistic expectation of future flow levels. The Colorado River Compact calls on the Upper Basin states—Colorado, Wyoming, New Mexico and Utah—to prevent flow depletions at Lee Ferry below an aggregate of 75 MAF in 10 consecutive years. Under certain circumstances, the Upper Basin states bear the burden of one-half the deficiency of the Mexican obligation. If the river yields an average annual flow of 15 MAF, rather than 16.5 MAF,

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