Colorado River Water Distribution: Which States Get How Much and Why
The 1922 Colorado River Compact and later agreements divide the river among seven states and Mexico, setting fixed shares that no longer match current flows or needs.
- Seven states plus Mexico share the Colorado River under a 1922 compact that splits the basin into upper and lower halves.
- California receives the largest single allocation at 4.4 million acre-feet per year, followed by Arizona and Colorado.
- Severe drought has dropped Lake Powell and Lake Mead levels, prompting proposals to tap upper-basin storage for lower-basin users.
- The system relies on average flows that have not occurred in decades, creating legal and political tension over shortages.
The Colorado River Compact of 1922 apportions the waters of the Colorado River and its tributaries among the seven basin states and Mexico. It divides the river into an Upper Basin (Colorado, Wyoming, Utah, New Mexico) and a Lower Basin (Arizona, Nevada, California) with each basin entitled to 7.5 million acre-feet annually; Mexico later received a 1.5 million acre-feet guarantee under a 1944 treaty.
How the 1922 Compact and Subsequent Laws Allocate Water
The compact itself does not list exact state-by-state amounts; those were set later. The Upper Basin states must deliver 75 million acre-feet over any ten-year period at Lee Ferry to the Lower Basin. Within the Lower Basin, the 1928 Boulder Canyon Project Act assigned California 4.4 million acre-feet, Arizona 2.8 million acre-feet, and Nevada 300,000 acre-feet. Upper Basin shares were fixed in the 1948 Upper Colorado River Basin Compact: Colorado 51.75 percent, Utah 23 percent, Wyoming 14 percent, New Mexico 11.25 percent of whatever water remains after meeting the Lee Ferry obligation.
Reservoirs, Priority Systems, and Shortage Rules
Lake Powell stores Upper Basin water behind Glen Canyon Dam; Lake Mead stores Lower Basin water behind Hoover Dam. Both are operated under the 2007 Interim Guidelines and the 2019 Drought Contingency Plans. These rules impose voluntary and mandatory cutbacks when reservoir levels fall below trigger elevations. California holds the most senior rights in the Lower Basin because of early 20th-century development, so its 4.4 million acre-feet is protected until shortages become extreme. Arizona and Nevada face earlier cuts. Recent proposals to move water from Lake Powell downstream reflect the fact that the Lower Basin now consumes more than its 7.5 million acre-feet share in dry years.
The allocation system matters today because measured natural flows at Lee Ferry have averaged only about 12.4 million acre-feet since 2000, well below the 16.5 million acre-feet the compacts assumed. Agriculture uses roughly 70 percent of the water, cities about 20 percent, and evaporation and losses account for the rest. With climate change reducing snowpack and increasing evaporation, the fixed shares create recurring legal disputes and emergency negotiations among the states.
| State | Annual Allocation (million acre-feet) | Basin |
|---|---|---|
| California | 4.4 | Lower |
| Arizona | 2.8 | Lower |
| Colorado | 3.86 (approx.) | Upper |
| Nevada | 0.3 | Lower |
| Utah | 1.71 (approx.) | Upper |
| New Mexico | 0.84 (approx.) | Upper |
| Wyoming | 1.04 (approx.) | Upper |
Sources
- U.S. Bureau of Reclamation Colorado River Basin reports
- 1922 Colorado River Compact text
- 2007 Interim Guidelines for Lower Basin Shortages
