The Colorado River Compact is a 1922 agreement that divides water among seven states in the Southwestern United States. The agreement covers the area where the Colorado River flows.

The agreement was first suggested by attorney Delph Carpenter. It was signed during a meeting at Bishop's Lodge, near Santa Fe, New Mexico, by representatives from the seven states through which the Colorado River and its tributaries flow on their way to Mexico. The agreement was supported by Secretary of Commerce Herbert Hoover.

The Colorado River is managed by many agreements, laws, treaties, court decisions, contracts, and rules known as "the Law of the River."

There are ongoing disagreements about tribal water rights on parts of the river, with many rights not fully measured or determined. The original agreement estimated more water flowed through the river than actually did, causing shortages. These shortages have worsened because of drought. The Colorado River Delta in Mexico has mostly disappeared due to less water flowing into it. However, agreements exist for the water that flows into Mexico, and efforts to restore the area are continuing.

Provisions

The Colorado River Compact divides the river basin into two parts: the Upper Basin (Colorado, New Mexico, Utah, and Wyoming) and the Lower Basin (Nevada, Arizona, and California). The compact requires the Upper Basin states to ensure the river’s flow does not drop below 75,000,000 acre-feet (93 km³) over any ten-year period. This amount was based on rainfall patterns observed before the 1922 treaty was signed, which was expected to allow fair water sharing between the two regions. Each basin’s states must divide their 7.5 million acre-feet per year (293 cubic meters per second) share among themselves. The compact made it possible to irrigate large areas in the Southwest and led to water projects like Hoover Dam and Lake Powell, managed by the United States Bureau of Reclamation.

In 1928, the Lower Basin’s annual water share was set by the Boulder Canyon Project. The Upper Basin’s share was established by the Upper Colorado River Basin Compact in 1948. These allocations are:

Additionally, 1.5 million acre-feet per year (59 cubic meters per second) of Colorado River water is given to Mexico under a 1944 treaty. The Lower Basin may also receive an extra 1.1 million acre-feet per year (43 cubic meters per second) during times of surplus water.

The 1922 Colorado River Compact resulted from years of discussions among states. In 1917, the seven states formed the League of the Southwest to promote river development. Congress approved the states’ agreement in 1921, and the compact was signed in 1922. The states had to ratify the agreement. Six states did so by January 1923, but Arizona did not sign until 1944.

The compact ensured equal water sharing between the Upper and Lower Basins. Before the compact, the river’s name was changed from “Grand River” to “Colorado River” upstream from where it meets the Green River. Many residents in Utah and Colorado opposed the name change, but state legislatures enforced it in the early 1920s. Since the 1920s, population growth, especially in California, has increased water demand. Recently, Nevada has sought more use of the river, partly due to Las Vegas’s growth.

The compact did not address issues like water rights for Native American tribes or Mexico, or how evaporation would be shared. Later studies showed the compact allocated more water than could reliably reach the boundary between the basins. The compact allowed downstream states to use surplus water but did not clearly define how to handle shortages.

The compact was controversial. During negotiations, Arizona’s Governor-elect Hunt emphasized protecting Arizona’s right to irrigate its land and share revenue from future dams. There were fears that Mexico might claim surplus water, as treaty rights were unclear. These concerns led Arizona to refuse ratifying the compact until 1944.

A 1927 effort by Lower Basin states to agree on water shortages and surpluses failed. Arizona feared it might have to release water from the Gila River system to Mexico during shortages. Arizona did not sign the compact until two months after Fred Colter, a leader in the effort, died in 1944. In 1944, the U.S. and Mexico signed a treaty granting Mexico 1.5 million acre-feet of river water annually.

The 1928 Boulder Canyon Project Act allowed dam and diversion projects to proceed without Arizona’s approval. It set water shares for the Lower Basin states: 2.8 million acre-feet to Arizona, 300,000 acre-feet to Nevada, and 4.4 million acre-feet to California. Congress approved these shares and allowed surplus water to be split equally between California and Arizona. Arizona disputed these shares until the U.S. Supreme Court upheld them in the 1963 case Arizona v. California.

Arizona took steps to protect its water rights. In 1934, Arizona called out the National Guard and created a two-boat “navy” after California began damming the river. Legal disputes over water sharing and the Boulder Canyon Act were settled in court through four Arizona v. California Supreme Court cases. The case lasted 11 years, costing over $5 million and involving more than 50 lawyers. The 1963 ruling confirmed Congress’s power to allocate water and cleared the way for the Central Arizona Project, authorized by Congress in 1968.

Tribal claims and contributions to flows

In 1908, the Supreme Court ruled in Winters v. United States that the federal government must provide water to tribal lands. These water rights, based on the creation of reservations, are protected under a system called prior appropriation. In theory, these rights should be valuable and reliable because they were established early. However, many tribes cannot use the water from the Colorado River that flows through their lands. Delays in determining the exact amount of water rights have allowed others to use tribal water for many years.

The agreement mentions tribal water rights in Article VII: "Nothing in this compact shall be construed as affecting the obligations of the United States of America to Indian Tribes." However, the federal government delayed actions to fulfill these obligations for a long time. It was not until 1963 that the case Arizona v. California established how to measure Indian water rights. In 2001, Arizona’s Supreme Court rejected the method from Arizona v. California and chose a different approach. During negotiations, Arizona has included conditions that tribes consider unacceptable, which also delayed water use or delivery to reservations.

There are 30 federally recognized tribes in the Colorado River basin, and 12 of them still struggle to secure all their water rights. The Navajo Nation holds the largest unquantified water right in the Colorado River basin. The Navajo Nation filed a lawsuit in 2003 for their water rights. In 2023, the Supreme Court ruled that the federal government cannot be forced to act quickly to determine or settle these claims.

The Colorado River Indian Tribes have one of the oldest and largest water rights to the Colorado River’s flow. By choosing to leave part of their nearly 720,000-acre-foot allocation in Lake Mead, they helped avoid deeper reductions for others in the Colorado River system under the 2019 Drought Contingency Plan.

Over-use, climate change, and other issues

Since the Colorado River Compact was created, California and Arizona have used extra water left by other states. As the population in the Southwest grew, people worried that this extra water might soon run out. In 2001, Secretary of the Interior Bruce Babbitt signed a temporary plan to decide how the extra water would be shared among the states. This plan gave California 15 years to develop ways to use less water and rely less on the Colorado River.

Since January 2000, the Lower Basin states have received 10 million acre-feet of water beyond what the Compact required. However, water levels in Lake Mead have been falling. This drop in water levels, caused by using more water than is available, is called a "structural deficit." Thomas McCann, a former official for the Central Arizona Project, explained that this deficit happens because 1) the Lower Basin states do not pay for water lost to evaporation, and 2) water is taken from Lake Mead to meet agreements with Mexico instead of using water already allocated to Arizona and California. A continued structural deficit could harm the reliability of water and electricity supplies in the Lower Basin if Lake Mead levels drop too low.

Nevada, which has the smallest share of water in the Lower Basin, became worried that falling Lake Mead levels might threaten the water supply for Las Vegas. In 2008, the Southern Nevada Water Authority started building a new water intake at a lower point in Lake Mead. General Manager Pat Mulroy said she did not support changing how water is shared. Instead, Las Vegas and the Water Authority focused on using water more efficiently to support their growing population.

Overusing Lake Mead is not the only problem. A long and severe drought has affected the Southwest since 2000. A study from the U.S. Environmental Protection Agency in 1993 warned that the Colorado River Basin’s water system is highly sensitive to changes in climate. This study said that changes in water flow could happen, and the current system might not be able to handle the worst effects. Recent studies agree and predict that climate change could reduce the river’s flow by about 20% by 2050. Another study found that rising temperatures reduced the river’s flow by around 10% between 2000 and 2021.

When the Colorado River Compact was signed in 1922, no water was set aside for nature. Using and reusing river water, along with building dams, has harmed native fish. Dams block fish from moving, reduce flood levels, change how sediment moves to floodplains and deltas, and alter water temperatures. These changes harm the natural environment.

Dams and water diversions have also damaged the Colorado River Delta in Mexico. Once a rich and green area, the delta has nearly disappeared. It was once estimated to receive 17 million acre-feet of water each year, but flows have often been less than a third of that amount. A group of U.S. organizations is working with the Mexican government to restore some water flow to the delta.

Estimated flows

The amount of water given out through the compact was based on the idea that the Colorado River's average flow was 16,400,000 acre-feet (20.2 km³) per year (641 m³/s). Later studies using tree rings showed that the river's long-term average flow is much lower than expected. Some estimates suggest the average is 13,200,000 acre-feet (16.3 km³) per year (516 m³/s), 13,500,000 acre-feet (16.7 km³) per year (528 m³/s), or 14,300,000 acre-feet (17.6 km³) per year (559 m³/s). Many experts believe the time used to calculate the river's "average" flow (1905–1922) included years with unusually high rainfall. The book Science be Dammed states that politicians and supporters often ignored engineering reports from the 1920s that showed lower flow estimates.

Shortages

Since 2000, the Colorado River has had less water flowing through it, and people have used more water than the river can naturally refill. This has led to the need for planning how to handle water shortages. Because the Central Arizona Project has lower priority for water use, shortages would mainly affect its users. However, some Arizona farmers and cities might face serious problems. If the federal government acts alone to reduce water use by others, it could face legal challenges. For these reasons, water users are looking for ways to address shortages without forced actions.

Some people have suggested changing or reinterpreting the Colorado River Compact, which has had past problems. Lower reservoir levels, especially at Lake Powell, created by the Glen Canyon Dam in 1963, have led to calls for permanently draining or decommissioning the reservoir, or at least filling it only after Lake Mead levels rise.

In December 2007, the Secretary of the Interior signed interim guidelines to help manage Colorado River water during shortages. These guidelines apply until 2026 and recognize the effects of climate change and changing water patterns. They provide experience in managing Lake Powell and Lake Mead, especially during low water levels. The guidelines define three shortage levels based on Lake Mead’s elevation:

• Level 1 shortage: If Lake Mead’s surface is below 1,075 feet (328 m) but above 1,050 feet (320 m), the Lower Basin states will receive 7,167,000 acre-feet (8.840 km³) per year: 4,400,000 acre-feet (5.4 km³) to California, 2,480,000 acre-feet (3.06 km³) to Arizona, and 287,000 acre-feet (0.354 km³) to Nevada.
• Level 2 shortage: If Lake Mead’s surface is below 1,050 feet (320 m) but above 1,025 feet (312 m), the Lower Basin states will receive 7,083,000 acre-feet (8.737 km³) per year: 4,400,000 acre-feet (5.4 km³) to California, 2,400,000 acre-feet (3.0 km³) to Arizona, and 283,000 acre-feet (0.349 km³) to Nevada.
• Level 3 shortage: If Lake Mead’s surface drops below 1,025 feet (312 m), the Lower Basin states will receive 7,000,000 acre-feet (8.6 km³) per year: 4,400,000 acre-feet (5.4 km³) to California, 2,320,000 acre-feet (2.86 km³) to Arizona, and 280,000 acre-feet (0.35 km³) to Nevada.

A Drought Contingency Plan signed by seven Western states in 2019 used voluntary reductions in water use to avoid federal limits during a 19-year drought. In May 2023, the states agreed to temporarily reduce water allocations further. In June 2023, the U.S. Bureau of Reclamation began preparing an Environmental Impact Statement to consider changes in dam operations at Hoover and Glen Canyon to address worsening drought conditions.

The Bureau of Reclamation must decide how to manage the river after 2026, when the current guidelines expire. In March 2024, Reclamation received five proposals for guidelines after 2026. By December 2024, Reclamation will share a draft of these guidelines and ask for public feedback. Around December 2025, Reclamation may release the final version of the guidelines. On January 1, 2027, the Interim Guidelines and Drought Contingency Plan, which have managed the river since 2007, will no longer be in effect. The new guidelines after 2026 will then determine how Reclamation manages the river.

Flows to Mexico

On November 20, 2012, the U.S.-Mexico International Boundary and Water Commission signed an agreement called "Minute 319," which updated the Law of the River to explain how 1,500,000 acre-feet (1.9 km³) of Colorado River water that Mexico receives each year would be affected by surplus or drought conditions. When there is more water than needed (surplus conditions), and the surface elevation of Lake Mead is above 1,145 feet (349 m) compared to the average sea level, the annual water delivery to Mexico increases by 40,000 acre-feet (0.049 km³). If the elevation is even higher, the additional water deliveries to Mexico increase gradually, reaching a maximum of 200,000 acre-feet (0.25 km³) per year.

Minute 319 also explains that water deliveries to Mexico decrease during drought conditions. Starting from the base allocation of 1,500,000 acre-feet (1.9 km³) per year, deliveries are reduced by 50,000 acre-feet (0.062 km³) if Lake Mead’s elevation is between 1,075 feet (328 m) and 1,050 feet (320 m) above the average sea level; reduced by 70,000 acre-feet (0.086 km³) if the elevation is between 1,050 feet (320 m) and 1,025 feet (312 m); and reduced by 125,000 acre-feet (0.154 km³) if the elevation is below 1,025 feet (312 m).