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The Colorado River winds through canyons at sunset

The Colorado River

Southern Nevada gets nearly 90 percent of its water supply from the Colorado River, which begins as snowmelt in the Rocky Mountains. The snowmelt travels through a series of tributaries into the river, which winds its way south for 1,450 miles before emptying into the Gulf of California in Mexico.

The river serves more than 40 million people across seven western states and Mexico. The river is divided into two major districts: the Upper Basin and the Lower Basin, and it is governed by a series of compacts, laws and court decisions collectively known as the Law of the River.

How Colorado River water is allocated

A complex body of laws, court cases, and regulations known as the "Law of the River" control the use of Colorado River water and the operation of its dams.

In the 1800s, states diverted water from the Colorado River and its tributaries without restrictions. As the diversions increased, a long battle over apportionment evolved. Today, the Colorado River is among the most controlled, controversial, and litigated rivers in the world.

Nevada receives 300,000 acre-feet per year (AFY) of Colorado River water under the Law of the River compacts. An acre-foot is equivalent to 325,851 gallons of water.

At the time of the 1928 Boulder Canyon Project Act, negotiators felt 300,000 acre-feet was more than enough. Southern Nevada had no significant agricultural or industrial users, groundwater seemed plentiful, and the area’s small population was not anticipated to grow significantly. Negotiators focused instead on hydro-electricity and secured one-third of the electricity generated by Hoover Dam.

The compact also referenced Mexico's right to Colorado River water. In 1944, the United States signed a treaty in which it agreed to deliver an annual quantity of 1.5 million acre feet per year to Mexico.

Colorado River apportionment

In 1922, seven western states negotiated the Colorado River Compact, which divided the states into two basins: Upper Basin and Lower Basin. The Upper Basin includes Colorado, New Mexico, Utah and Wyoming. The Lower Basin includes Arizona, California and Nevada.

A total of 16.5 million acre feet per year (MAFY) are apportioned as follows:

Upper Basin allocation Million acre-feet per year (MAFY)
Colorado 3.9 MAFY
Utah 1.7 MAFY
Wyoming 1 MAFY
New Mexico 0.85 MAFY
Lower Basin allocation Million acre-feet per year (MAFY)
Arizona 2.85 MAFY
California 4.4 MAFY
Nevada 0.3 MAFY
Additional allocations Million acre-feet per year (MAFY)
Mexico 1.5 MAFY

Unused apportionment

As part of its 1992 Colorado River contract, the Southern Nevada Water Authority has a right to the unused apportionment of other Nevada Colorado River contract holders. The Water Authority anticipates some of this water will be available for use in the planning horizon, and intends to utilize this water if and when it is available.

The Water Authority also may choose to leave a portion of Nevada's unused allocation in Lake Mead to help alleviate the impacts of drought conditions and avoid critically low Lake Mead levels.

An aerial of Hoover Dam with Lake Mead in the background

Hoover Dam

The 1922 Colorado River Compact provided the legislative stimulus to harness the Colorado River. Congress authorized the building of Hoover Dam in 1928, and construction began in 1931. Completed on March 1, 1936, the dam was the first step toward controlling the rampaging river.

Average river flows

Compact negotiators estimated the flow of the river to be at least 17 MAFY and allocated 16.5 MAFY per year for use by the seven western states and Mexico.

Today's records indicate an average flow of 14.9 MAFY at Lee's Ferry, just below Lake Powell.

Consequently, the sum of the actual compact apportionments and the Mexican treaty exceed the flow of the river in most years.

In addition, a sustained drought has reduced significantly the average runoff into the Colorado River. 

Despite the reduction in water flowing into the river, there is little support among the seven states sharing the water to renegotiate the compact.

A new agreement would require approval from each state’s legislature and the U.S. Congress.

Colorado River System Conservation Pilot Program

The Southern Nevada Water Authority, Department of the Interior, and other Colorado River water users have committed to fund up to $30 million between 2015 and 2018 for water conservation projects that will benefit the Colorado River system by conserving additional water in Lake Powell and Lake Mead.

Potential projects include land fallowing, split season irrigation, irrigation efficiency, desalination, reuse, and municipal conservation, among others.

Unlike other water resources in the SNWA Water Resource Portfolio, water conserved as a part of this agreement will benefit the entire Colorado River System by increasing reservoir elevations; these resources cannot be recovered by any individual water user.

Drought response actions

The Water Authority, Department of Interior, and other lower basin water users and states set a goal of developing 1.5 to 3 million acre-feet of water in Lake Mead before 2020 to serve as a "protection volume." This water is intended to help stabilize water levels.

As part of a 2014 memorandum of understanding, the parties will use their best efforts to create a total of 750,000 acre-feet between 2014 and 2017. The SNWA's commitment to the program is 45,000 acre-feet. Southern Nevada's current water use is well below the state's 300,000 AFY Colorado River allocation and SNWA plans to meet its commitment by forgoing offstream banking of its unused apportionment.

Minute 323 to the 1944 Treaty

Minute 323, a binational operational agreement under the 1944 water treaty between the U.S. and Mexico, defines Colorado River water deliveries to Mexico under high- and low-reservoir conditions and permits storage of Mexico's deferred Colorado River water deliveries in Lake Mead. The minute also allows U.S. water agencies to invest in water efficiency projects in Mexico and, in turn, receive a share of the water saved.

The Southern Nevada Water Authority has agreed to fund projects yielding a minimum of 51,025 and maximum of 78,300 acre-feet. For its contribution to the program, the Water Authority receives nearly 24,000 acre-feet of water to meet the Southern Nevada community's future water needs. Overall, the provisions outlined in Minute 323 will help bolster water levels in Lake Mead, delay potential shortage conditions on the river, and provide more certainty for Colorado River water users.

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