Bureau of Reclamation Lower Colorado Region Banner
Reclamation Home             Regional Offices             Newsroom             Library             Projects & Facilities

Hoover Dam
Frequently Asked Questions and Answers

The Colorado River

Scenic view of Lower Colorado RiverThe Colorado River begins its journey in the snowcapped mountains of north central Colorado and zigzags southwest for more than 1,400 miles toward the Gulf of California.

The river and its tributaries - including the Green, Gunnison, San Juan, Virgin, Little Colorado, Bill Williams, and Gila rivers - and the lands these waters drain are all part of the "Colorado River Basin." The rivers drain 242,000 square miles in the United States, or one-twelfth of the country's continental land area, and 2,000 square miles in Mexico.

Seven western states and Mexico share the waters of the Colorado River Basin.

What are the seven western states that have interests in the Colorado River?

The Colorado River Basin states are: Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming. Each state is party to the Colorado River Compact entered into in Santa Fe, New Mexico, on November 24, 1922.

How is the Colorado River Basin divided?

The Colorado River Compact divided the Colorado River Basin into the Upper Basin and the Lower Basin. The division point is Lees Ferry, a point in the mainstem of the Colorado River about 30 river miles south of the Utah-Arizona boundary, just downstream of Glen Canyon Dam.

The "Upper Basin" includes those parts of the states of Arizona, Colorado, New Mexico, Utah, and Wyoming within and from which waters naturally drain into the Colorado River system above Lees Ferry, and all parts of these States that are not part of the river's drainage system but may benefit from water diverted from the system above Lees Ferry.

The "Lower Basin" includes those parts of the states of Arizona, California, Nevada, New Mexico, and Utah within and from which waters naturally drain into the Colorado River system below Lees Ferry, and all parts of these States that are not part of the river's drainage system but may benefit from water diverted from the system below Lees Ferry.

How is Colorado River water apportioned?

Little Colorado River flowing into the Colorado River at the Grand CanyonThe Colorado River Compact apportioned to each basin the use of 7,500,000 acre-feet of water per year from the Colorado River system in perpetuity.

How much water is apportioned to each State in the Colorado River Basin?

The Colorado River Compact did not apportion water to any State.

On October 11, 1948, the Upper Basin States entered into the Upper Colorado River Basin Compact, which apportioned use of the Upper Basin waters among them. The compact permits Arizona to use 50,000 acre-feet of water annually from the upper Colorado River system, and apportioned the remaining water to the Upper Basin States in the following percentages: Colorado, 51.75 percent; New Mexico, 11.25 percent; Utah, 23 percent; and Wyoming, 14 percent.

The Lower Basin States of Arizona, California, and Nevada were not able to reach agreement on their apportionments. So in 1952, Arizona filed suit in the United States Supreme Court to determine how the waters of the Lower Basin should be divided. In October 1964, the Court issued a decree that of the first 7,500,000 acre-feet of mainstem water in the Lower Basin, California is entitled to 4,400,000 acre-feet, Arizona 2,800,000 acre-feet, and Nevada, 300,000 acre-feet.

The Boulder Canyon Project Act of 1928 designated the U.S. Secretary of the Interior as lower basin "water master" responsible for distributing all Colorado River water below Hoover Dam. Major water users on the Colorado River must contract for water with the U.S. Secretary of Interior for annual deliveries. The United States has contracted with the States of Arizona and Nevada and with various agencies in Arizona and California for the delivery of Colorado River water. These contracts make delivery of the water contingent upon its availability for use in the respective States under the Colorado River Compact and the Boulder Canyon Project Act.

What are Mexico's rights to Colorado River water?

The United States and Mexico entered into a treaty on February 3, 1944, which guarantees Mexico 1,500,000 acre-feet of Colorado River water annually. This entitlement is subject to increase or decrease under circumstances provided for in the treaty.

Last Reviewed: December 2008