Colorado River Compact
The Colorado River Compact is a 1922 agreement among seven states in the southwestern United States that fall within the drainage basin of the Colorado River. The pact governs the apportionment of the river's flow between the upper and lower division states.The agreement, originally proposed by attorney Delph Carpenter, was signed at a meeting at Bishop's Lodge, near Santa Fe, New Mexico, by representatives of the seven states the Colorado river and its tributaries pass through on the way to Mexico. The Colorado River is managed and operated under numerous compacts, federal laws, an international treaty, court decisions and decrees, contracts, and regulatory guidelines collectively known as "the Law of the River". Provisions
The Compact divides the river basin into two areas, the Upper Division (comprising Colorado, New Mexico, Utah and Wyoming) and the Lower Division (Nevada, Arizona and California). The Compact requires the Upper Basin states not to deplete the flow of the river below  during any period of ten consecutive years. Based on rainfall patterns observed in the years before the treaty's signing in 1922, the amount specified in the Compact was assumed to allow a roughly equal division of water between the two regions. The states within each basin were required to divide their  share allotment among themselves. The Compact enabled the widespread irrigation of the Southwest, as well as the subsequent development of state and federal water works projects under the United States Bureau of Reclamation. Such projects included the Hoover Dam and Lake Powell. The annual allotments in the Lower Basin were established in 1928 as part of the Boulder Canyon Project, while the annual allotments in the Upper Basin were established by the Upper Colorado River Basin Compact of 1948. They are:

In addition to this,  of Colorado River water is allocated to Mexico, pursuant to the treaty relating to the use of waters of the Colorado and Tijuana rivers and of the Rio Grande, signed February 3, 1944, and its supplementary protocol signed November 14, 1944. Also, the lower basin can get an additional  in surplus conditions. The 1922 Colorado River Compact was the fruit of several years of negotiations among the states. The seven states had previously formed the League of the Southwest in 1917 to promote development along the river. In 1921, Congress authorized the states to enter into a compact for allocation of the river resources. The agreement, also known as the Santa Fe Compact, was approved by Congress in 1922, the same year it was signed by the delegates who negotiated the agreement. The agreement was subject to the ratification of the seven states. Six states ratified the agreement by the end of January 1923. Arizona did not sign until 1944. The Compact determined that the water would be shared equally among the upper and lower basin. Prior to the Compact, the name of the river was standardized along its length. Previously the portion of the river upstream from its confluence with the Green River had been known locally as the "Grand River". The change was opposed by many local residents in Utah and Colorado, and the new name was enforced locally by acts of the state legislatures in both states in the early 1920s. One of the major ongoing concerns since the 1920s was the expanding population, which increased the demand for water, particularly in California. In more recent years, mainly because of Las Vegas, Nevada has been looking for more use of the Colorado River. The Compact did not address a number of issues, including Indian or Mexican water rights, or how evaporation would be shared among the basins. Later studies of flow found that the Compact apportioned more water than would be reliably delivered at the boundary between the two basins. The Compact allowed use of surplus flows by downstream states, but did not provide clear rules addressing shortages. The Compact was controversial at the time. During the Compact negotiations, Governor-elect Hunt laid out his determination to protect Arizona’s rights to irrigate her lands and obtain a portion of the electrical power revenue that would be generated by future dams. There was also fear that the surplus water would be usurped by Mexico, as treaty rights had not yet been defined and agricultural development of the Mexican delta was underway. As a result of these concerns and others, the 1923 Arizona legislature refused to ratify the Compact. A 1927 effort among the lower basin states to agree how to share shortages and surplus failed. Arizona feared that during times of shortage, she would have to release stored water from the Gila River system to Mexico. Led by Fred Colter, the state refused to sign the Compact until two months after Colter's death in 1944. In 1944, Mexico and the U.S. negotiated a treaty granting Mexico the right to 1.5 million acre-feet of Colorado River water. The 1928 Boulder Canyon Project Act provided a way for the nation to move ahead with construction of dams and diversions without the approval of Arizona. It invited the three lower basin states to divide the waters of the Colorado with 2.8 million acre-feet to Arizona, 300,000 acre-feet to Nevada and 4.4 million acre-feet to California. It gave Congressional pre-approval to such allotments, and to sharing any surplus equally between California and Arizona.The specific state allotments in the Boulder Canyon Act were disputed by Arizona until the United States Supreme Court upheld the amount in the 1963 decision in Arizona v. California. Arizona took other steps as well to protect its water and keep California from gaining too large a share. In 1934, Arizona, unhappy with California's decision to dam and divert the river, called out the National Guard and even commissioned a two-boat "navy." The contentions over water sharing and the provisions of the Boulder Canyon Project Act were eventually settled in court through a series of four Arizona v. California Supreme Court decisions.The case lasted 11 years and cost over $5 million, requiring the work of over 50 lawyers. Arizona v. California found, for the first time, that Congress had the power to allocate water to the states and had done so through the Boulder Canyon Act, for the lower states (which are California, Nevada, and Arizona). While the Court ultimately ruled in favor of Arizona, it agreed with California's interpretation of how it should receive surplus water supplies.The 1963 court decision ended many years of dispute, clearing the way for the Central Arizona Project, authorized by Congress in 1968. Tribal claims and contributions to flows
The 1908 Supreme Court decision Winters v. United States recognized a federal obligation to ensure water for tribal homelands. Under prior appropriation, these federally reserved rights date to when the reservations were established. In theory, the early priority dates of tribal water rights should mean that these rights are among the most valuable and reliable. In practice, many tribes cannot use the Colorado River water that flows through their lands. Delays in quantifying water rights have meant that tribal water has been used by others for decades. The Compact 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" but the federal and state governments have long delayed actions to meet those obligations. It was not until 1963 that Arizona v. California established how Indian water rights should be quantified. In 2001, Arizona’s Supreme Court rejected the standard set in Arizona v. California for yet a different method. In negotiations, Arizona has established other conditions that tribes find reprehensible, a factor which has also delayed use or delivery of water to reservations. There are 30 federally recognized tribes in the Colorado River basin, 12 of which still struggle to get all of their water rights. The Navajo Nation has the largest water right in the Colorado River basin that is yet to be quantified. The Navajo sued in 2003 for their water rights; the Supreme Court decided in 2023 that the federal government cannot be forced to act in a timely way to quantify or settle their claims to the river. The Colorado River Indian Tribes have one of the largest and oldest rights to the flow of the Colorado River. By volunteering to leave part of their nearly 720,000-acre-foot allocation in Lake Mead, they forestalled deeper cutbacks for others along the Colorado River system under the 2019 Drought Contingency Plan. Over-use, climate change, and other issues 
Since the development of the Colorado River Compact, California (and Arizona) have been using the surplus water that has been left over from other states. With increasing population growth in the Southwest, there was concern that this surplus will soon not exist for California use. In 2001, Secretary of the Interior Bruce Babbitt signed an interim agreement determining how water surplus from the Colorado River would be allocated between the states and creating a 15-year period to allow California time to put conservation methods in place to reduce the state's water usage and dependence on Colorado River water. The Lower Basin has received a total of 10 million acre-feet of water above and beyond the Compact requirements since January 2000, but the levels of Lake Mead have been dropping. The steady depletion of water at Lake Mead due to over-allocation of the flows is called the “structural deficit”. According to Thomas McCann, former Deputy Manager for the Central Arizona Project, the deficit is caused by 1) avoiding any allocation of evaporation costs to the Lower Basin states, and 2) meeting Mexican Treaty obligations by taking water out of Lake Mead storage rather than out of Arizona and California's allocations. Continued structural deficit at Lake Mead puts the reliability of the Lower Basin’s water and hydroelectric power at risk if lake levels should drop too low. Nevada, with the smallest water allocation in the lower river basin, became concerned that the decline in Lake Mead levels would jeopardize the water intake for the Las Vegas area. In 2008 Southern Nevada Water Authority began constructing a new water intake at a lower level in Lake Mead. General Manager Pat Mulroy said she did not support water reallocation. Instead, Las Vegas and the Water Authority have worked assiduously on water conservation to support their region's growing population. Overuse from Lake Mead is not the only issue. There has been a megadrought in the Southwestern US from 2000 to 2023. Climate change will likely decrease the river’s flow (from its mid-20th century average) by 20 percent by 2050. A more recent study estimated 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 allocation of water was reserved for environmental functions. Using and reusing the river water, as well as frequent damming, results in an unfavorable environment for native fish species. Dams block fish passage, reduce flood peaks alter natural sediment transported to flood plains and deltas, and change water temperaturesall negatively impacting the natural ecosystem. Dams and diversions have also caused the Colorado River Delta, located in Mexico, to deteriorate significantly. Once a lush and green area, a vast ecosystem has nearly disappeared. It was originally estimated to have 17 million acre-feet a year, but prior to the drought, flows have often been less than a third of that. A coalition of U.S.-based organizations is working with the Mexican government to restore some flows to the delta. Estimated flows
The amount of water allocated via the Compact was based on an expectation that the river's average flow was   per year (641 m³/s). Subsequent tree ring studies, however, have concluded that the long-term average water flow of the Colorado is significantly less. Estimates have included   per year (516 m³/s),   per year (528 m3/s), and   per year (559 m3/s). Many analysts have concluded that when the Compact was negotiated, the period used as the basis for "average" flow of the river (1905–1922) included periods of abnormally high precipitation. The book Science be Dammed finds that politicians and boosters repeatedly ignored engineering estimates of the 1920s that provided lower values of flow. Shortages 
Since 2000, dwindling Colorado River flows and consumption rates in excess of natural replenishment have provoked a need to plan for shortages. Because of its lower priority, shortages should fall primarily upon the users of the Central Arizona Project, however the consequences for some Arizona farmers and urban sectors could be dire. If the federal government acts unilaterally to restrict other uses of water, they run the risk of litigation. For these and other reasons, river users are exploring voluntary measures address shortages. Some have called for amending or re-interpreting the Compact in light of its past deficiencies. The widespread dropping of reservoir levels in the region, in particular at Lake Powell, created by the Glen Canyon Dam in 1963, has prompted calls for the reservoir to be permanently drained and decommissioned, or at least operated to fill only after Lake Mead levels are brought up. In December 2007, a set of interim guidelines on how to allocate Colorado River water in the event of shortages was signed by the Secretary of the Interior. The guidelines extend through 2026, and, "acknowledging the potential for impacts due to climate change and increased hydrologic variability," interim guidelines provide "the opportunity to gain valuable operating experience for the management of Lake Powell and Lake Mead, particularly for low reservoir conditions...whether during the interim period or thereafter." The agreement specifies three levels of shortage conditions, depending on the level of Lake Mead:  
Light shortage. When the surface elevation at Lake Mead is below  relative to mean sea level but above , the Lower Basin states will receive   per year:  to California,  to Arizona, and  to Nevada. Heavy shortage. When the surface elevation of Lake Mead is below  but above ,  per year will be delivered to the Lower Basin states:  to California,  to Arizona, and  to Nevada. Extreme shortage. The most severe shortage considered in the interim guidelines is when the level of Lake Mead drops below , in which event  per year will be delivered to the Lower Basin states:  to California,  to Arizona, and  to Nevada. A Drought Contingency Plan signed by seven Western states in 2019 used voluntary cutbacks to forestall federal reductions in use of the Colorado River amid a 19-year drought. In May 2023, the states agreed to further reduce allocations temporarily. In June 2023, U. S. Bureau of Reclamation began preparing an Environmental Impact Statement for further modifications of dam operations at Hoover and Glen Canyon to respond to deepening drought conditions. Flows to Mexico 
On November 20, 2012, the International Boundary and Water Commission of the United States and Mexico signed an agreement termed "Minute 319," which updated the Law of the River to address how the  of Colorado River water that Mexico receives every year would be affected by surplus or drought conditions. Under surplus conditions (when the surface elevation of Lake Mead is above  relative to mean sea level) the annual flow to Mexico will increase by . In cases where the surface elevation of Lake Mead is higher than that, the extra deliveries to Mexico progressively increase, reaching a maximum of an additional  per year. Minute 319 also specifies that deliveries to Mexico will be reduced under drought conditions. Starting from the base allocation of  per year, annual deliveries will be reduced by  if the surface elevation of Lake Mead is between  and  above mean sea level; reduced by  acre-feet if the elevation is  to ; and reduced by  if the surface elevation is below . See also
Colorado River dispute
Colorado River Storage Project
Colorado River Board of California
Course of the Colorado River

References

Further reading
 Norris Hundley. Water and the West: The Colorado River Compact and the Politics of Water in the American West (University of California Press, 2009) 415 pp. the standard scholarly history; 
 
 Jason Robison. "Review of Hundley, Water and the West," H-Water, H-Net Reviews. September, 2010. online
 Sandra Postel. The Colorado River Delta Series, National Geographic. March through May, 2013. online
 Daniel Tyler. Silver Fox of the Rockies: Delphus E. Carpenter and Western Water Compacts, (Norman: University of Oklahoma Press) 2003. History of the American West
History of the Southwestern United States
United States interstate compacts
Colorado River
Water management authorities in the United States
Water in Colorado
Water law in the United States
Colorado law
New Mexico law
Utah law
Wyoming law
Nevada law
Arizona law
California law