Most of the runoff in the Colorado River Basin, especially in the Upper Basin, is the product of snowmelt occurring during the period of late spring to summer. For the period 1906-2008, 92% of the Colorado River Basin's mean annual natural runoff occurred above Lees Ferry. In this location, the mean annual flow is close to 14.9 million acre-feet (MAF), with annual flow ranging from 5.4 MAF to 24.3 MAF. The most important tributary to the basin is the Green River with a total contribution of 33% to the mean annual flow.
In 1922, the states and the federal government created the Colorado River compact, which divided the Colorado River Basin into the Upper (Wyoming, Utah, Colorado, and New Mexico) and Lower (Nevada, California, and Arizona) Basin; each basin receiving 7.5 million MAF of water rights entitlements. Based on the agreements of the 1928 Boulder Canyon Project Act, in the lower basin, Nevada, California, and Arizona are entitled to 0.3 MAF, 4.4 MAF, and 2.8 MAF respectively. As a result of Arizona vs California (1968), California holds senior water right priority over Nevada and Arizona. The Upper Colorado River Basin Compact of 1948 defined the apportionment on a percentage basis: Colorado received 51.75%, New Mexico received 11.25%, Utah received 23%, and Wyoming received 14%. The compact included a clause delivering 50,000 AF to Arizona. Mexico and the US signed the 1944 treaty to share waters of the Colorado and Rio Grande Rivers. As such, Mexico is entitled to 1.5 MAF per year, or lower during extraordinary drought conditions.
- Water Supply (chart of annual natural flow at Lee Ferry)
- Map of Basin and Delivery Service Area (others)
- Water Rights by State
- Monthly Colorado River Supply report