Water law and policy

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This page summarizes the laws and policies governing the allocation and management of water in the Colorado River Basin. These laws and policies bound the decision space for the basin’s water managers and thus influence how scientific research and related technical practice inform current and future management, such as with the upcoming renegotiation of the 2007 Interim Guidelines. The evolving scientific understanding of the basin (e.g., climate change impacts on streamflows) can also lead to reconsideration of assumptions underpinning existing policy.

Prior appropriation

Much of the water in the West is allocated and managed according to the doctrine of prior appropriation. The prior appropriation system was developed in California in response to the need for water for gold mining and is tailored to watersheds with highly variable seasonal flows and limited water resources. It differs dramatically from the Riparian Doctrine of water law that dominates in the eastern United States. The four guiding principles of the prior appropriation system are:

1. Water rights are established by diverting water for a beneficial use. These water rights are usufructuary: they grant the ability to use the water--a public resource--but not to own it.

2. Earlier (senior) water rights have priority over later (junior) water rights during times of shortage and the order of water rights is maintained by a priority list. When a water user places a call on the river, all upstream water rights junior to the water user who placed the call are restricted until the call is removed. This principle is commonly referred to as “first in time, first in right.”

3. Water can be transported away from the stream for distant use, and ownership of land in the watershed is not mandated for use of water from the watershed.

4. Established water rights can be sold without changing the priority date.

The Law of the River

The “Law of the River” refers to the interlocking body of compacts, treaties, laws, regulations, and court decisions that guides water allocation and use in the Colorado River Basin among the seven basin states, Mexico, and sovereign Indian nations.

While the doctrine of prior appropriation still generally applies to the administration of water rights and use within each basin state, the overall effect of the Law of the River is that prior appropriation does not fully apply between states, or between the U.S. and Mexico, or between the U.S. or the states and tribal nations. In particular, many exceptions have been made to the stipulation that water rights are established by actually diverting water; starting with the Colorado River Compact of 1922, water allocations and rights were granted well in excess of the then-existing diversions and uses.

Colorado River Compact (1922)

The Colorado River Compact is the keystone of the Law of the River and allocates water between the two sub-basins: the Upper Basin and the Lower Basin, which are divided at Lee Ferry, located 20 miles downstream from Lake Powell. The Upper Basin (aka Upper Division) states are Wyoming, Colorado, Utah, and New Mexico (a very small part of Arizona is located above Lee Ferry). The Lower Basin (aka Lower Division) states are Arizona, Nevada, and California.

The negotiators of the Colorado River Compact assumed that the annual mean flow of the Colorado River at Lee Ferry was 17 or 18 million acre-feet (maf) per year of water based on the 20-30 years of gage records to that point. However, the long-term mean natural flow at Lee Ferry (1906-2021) has been 14.7 maf/yr, and from 2000-2021, the mean flow was only 12.3 maf/yr.

The Compact allocates 7.5 maf per year to the Upper Basin states and 7.5 maf per year to the Lower Basin states. The Compact also grants the Lower Basin the right to increase its water use by one million acre-feet annually and states that the Upper Basin “will not cause the flow at Lee Ferry to be depleted below an aggregate of 75,000,000 acre-feet for any period of ten consecutive years.” The Upper Basin has so far consistently met the terms of this non-depletion clause, albeit while using far less than its full 7.5 maf/yr allocation: about 4.5 maf/yr since 2015. While the Colorado River Compact of 1922 did not include allocations to Mexico, it stipulated that any future water obligations to Mexico would be shared by the Upper and Lower Basins equally.

Further allocation of water

While the Colorado River Compact of 1922 was clear in its division of water between the Upper Basin and the Lower Basin, it did not specify the allocations to each state. The allocations that were subsequently made or quantified to the states and Mexico are summarized in the table below.

Who? Sub-region Allocation Legal Basis
Wyoming Upper Basin 11.25% of available Upper Basin supply (1.043 maf of 7.5 maf) Upper Colorado River Basin Compact (1948)
Colorado Upper Basin 51.75% (3.860 maf of 7.5 maf) Upper Colorado River Basin Compact (1948)
Utah Upper Basin 23% (1.710 maf of 7.5 maf) Upper Colorado River Basin Compact (1948)
New Mexico Upper Basin 11.25% (0.838 maf of 7.5 maf) Upper Colorado River Basin Compact (1948)
Arizona Upper Basin 0.050 maf Upper Colorado River Basin Compact (1948)
Arizona Lower Basin 2.800 maf Boulder Canyon Project Act (1928) &
Arizona v. California decree (1964)
Nevada Lower Basin 0.300 maf Boulder Canyon Project Act (1928) &
Arizona v. California decree (1964)
California Lower Basin 4.400 maf
(+ not more than 50% of surplus)
Boulder Canyon Project Act (1928) &
Arizona v. California decree (1964)
Mexico n/a 1.500 maf
(+ 0.2 maf in years of surplus)
U.S.-Mexico Water Treaty (1944)

Tribal water rights

While the 1922 Colorado River Compact did not explicitly grant any tribal water rights, its Article VII says that the Compact should not be interpreted as affecting the federal obligations to the tribes that were established by the Winters v. U.S. (1908) Supreme Court decision. Starting in 1963, under many separate court decisions and settlement acts, a total of over 3 maf/yr of diversion rights to the Colorado River water have been quantified and allocated to 18 of the 30 Tribes with reservations in the Colorado River Basin. Over half of these diversion rights by volume are allocated to three tribal nations: the Navajo Nation, the Ute Indian Tribe of the Uintah & Ouray Reservation (Utah), and the Colorado River Indian Tribes (Arizona).

Another three Tribes receive Colorado River water under contracts with the federal government, but without formal quantification of their rights. One dozen Tribes have outstanding claims to water rights that have not been quantified at all, and five tribes have partially quantified claims. Tribal water rights are quantified based on the “practically irrigated acreage” of tribal lands, per Winters (1908) and Arizona v. California (1963); however, tribes are able to use the water for purposes other than agriculture after it has been allocated.

Not all of these diversion rights are exercised each year; many tribes lack sufficient water infrastructure to fully utilize their rights. The water that is actually consumed under these diversions rights--which may be as little as half of what was diverted, depending on location and use--is counted as part of the overall allocation made to the state in which the reservation is located. Some of the water allocated to tribes is leased to non-tribal water users--agricultural, municipal, and industrial. This 2021 policy brief by the Water & Tribes Initiative contains much more detail about the status and usage of tribal water rights in the basin.

Summary of selected laws and treaties that make up the Law of the River

Adapted from the "Summary of the Law of the Colorado River" tables, pp. 21-27, in Kwon and Gimbel (2021).

Law Key provisions/effects Relevance
Colorado River Compact (1922)
7.5 maf/yr allocated to each of the Upper Basin and the Lower Basin.
Upper Basin and Lower Basin are equally responsible for any future allocation to Mexico if not met by surplus above their own 7.5 maf/yr allocations.
The Upper Basin will not cause the flow at Lee Ferry to be depleted below 75 maf for any consecutive ten year period.
The Lower Basin has fully developed its Colorado River allocation; the Upper Basin has not.
A future failure of the Upper Basin to meet the “non-depletion” clause could trigger a “Compact Call” by the Lower Basin.
Boulder Canyon Project Act (1928)
Congressionally ratified the Colorado River Compact and agreed to limit California to 4.4 maf.
Authorized the building of the Hoover Dam and the All-American Canal; authorized the Secretary of the Interior as the Water Master in the Lower Basin.
Suggested Lower Basin apportionments:
- CA: 4.4 maf
- AZ: 2.8 maf
- NV: 0.3 maf
Serves as the foundation for the management of Lower Basin water.
U.S.-Mexico Water Treaty (1944)
Appropriates 1.5 maf/yr to Mexico; an additional 0.2 maf if surplus supply; Mexico must share in shortage in case of “extraordinary drought.” Considered the first priority that must be met on the Colorado River
U.S.-Mexico Water Treaty Minutes (1946-present)
Minutes are used to clarify the implementation of the terms of the 1944 Treaty. A total of 150 minutes have been signed to date, notably:
242: Salinity Control
306: Environmental considerations
319: Cooperative measures to address variable Colorado River water supplies
323: Extends and clarifies Minute 319 to 2026
Minutes 319 and 323 are especially significant as package agreements for Mexico to accept reduced deliveries (Minute 323), participate in shortages with the Lower Basin states (Minutes 319 and 323), and establishing the mechanism for accomplishing a “pulse flow” through the Colorado River Delta (Minute 319).
Upper Colorado River Basin Compact (1948)
Sets forth Upper Basin apportionment:
- Arizona receives 50,000 acre-feet
- Colorado: 51.75% of UB total
- New Mexico: 11.25%
- Wyoming: 14%
- Utah: 23%
Establishes Upper Colorado River Commission.
The actual water supply available to these percentages changes based on hydrology and water storage. Arizona is the only state to receive a flat water volume.
Colorado River Storage Project Act (1956)
Congress commits funding to develop Colorado River water in the Upper Basin.
Approved Glen Canyon Dam (Lake Powell), Flaming Gorge Dam and Reservoir, Navajo Dam and Reservoir, Blue Mesa Dam and Reservoir.
Authorized participating projects and hydropower revenues.
Directive for federal assistance to develop water in the Upper Basin. Serves to authorize storage facilities and promote water use.
Arizona v. California (1963, 1964)
1963 Supreme Court Decision
Affirms the allocation of Lower Basin water made by the 1928 Boulder Canyon Project Act.
Determines the allocation of reserved waters for five Tribes in the basin.
1964 Decree
Affirms Secretary of the Interior’s operational authority over water in the Lower Basin.
Any surplus water to be split:
CA: 50%
AZ: 46%
NV: 4%
Even under shortage conditions, CA still receives full 4.4 maf/yr.
Established the expectation that Lower Basin states will not have reservoir evaporation or water conveyance losses (currently ~1.2 maf/yr) counted against their 7.5 maf/yr allocation.

Also created the expectation, by some, that the use of tributaries in the Lower Basin is separate from entitlements from the Colorado River mainstem. The Upper Basin states do not subscribe to this interpretation.

Colorado River Basin Project Act (1968)
Congress authorizes construction of the Central Arizona Project.
Establishes coordinated operations of federal storage facilities in the Boulder Canyon Project Act (e.g., Mead) and Colorado River Storage Project Act (e.g., Powell) by setting release priorities:
Releases to supply half of any deficiency for the Mexico Treaty
Releases to provide 7.5 maf to the Lower Basin
Storage of water in the Upper Basin to assure the two first two priorities are met in the future.
Releases to meet additional uses in Lower Basin so long as Powell storage is not less than Mead, to equalize storage in Powell and Mead, or to avoid fill-and-spills from Powell.
The calculation for determining the storage in (3) is the subject of disagreement between the Upper and Lower Basins.
Excess releases beyond those needed to meet priorities (1-3) are known as “equalization releases.”

NOTE: The later 2007 Interim Guidelines further allowed for releases greater than or less than 8.23 maf as needed to better balance reservoir storage between Lakes Powell and Mead between 2008 and 2026.

Long Range Operating Criteria (1970)
Reclamation establishes criteria for the coordinated operation of the Colorado River Storage Project (e.g., Powell) and Lake Mead.
Sets the “minimum objective release” from Lake Powell at 8.23 maf.
8.23 maf determined by prorating the the Upper Basin non-depletion requirement of 75 maf over 10 years (7.5 maf/yr), subtracting tributary inflows below Glen Canyon Dam and above Lee Ferry (0.02 maf), and adding half of the U.S.-Mexico Treaty allocation (0.75 maf).
Affirmed notion of “normal”, “surplus”, and “shortage” years set up by 1968 CRBPA act, but silent as to how “surplus” and “shortage” years would be identified.
Interim Guidelines for Lower Basin Shortages and Coordinated Operations of Lake Powell and Lake Mead (2007)
Clarifies the definition of a “shortage” condition under which less than 7.5 maf/yr would be released from Lake Mead for Lower Basin users.
Sets storage tiers for Powell under which different volume releases will be made, contingent on levels of Powell and Mead.
Authorizes mechanisms for the storage in Lake Mead and future delivery of water that is conserved by Lower Basin users (“Intentionally Created Surplus”).
The Interim Guidelines came about after rapidly declining levels of Powell and Mead in the wake of severe drought from 2000-2004 showed inadequacies of the 1970 Long Range Operating Criteria.
Under the Guidelines, Reclamation made several releases >8.23 maf from Powell to balance storage with Mead in the late 2000s and 2010s when both reservoirs were below full capacity. While allowable under the Guidelines, such releases have proven controversial within the Upper Basin.
In 2022, deliveries to AZ and NV were reduced a combined 333 kaf in accordance with the Guidelines.
Drought Contingency Plans (DCPs; 2019)
Interstate agreements
Separate plans for the Upper and Lower Basin provide an overlay to the 2007 Interim Guidelines. Lower Basin DCP commits to implementing conservation measures or to accepting additional delivery reductions from Lake Mead.
Upper Basin DCP (i) promotes weather modification to augment supplies; (ii) protects minimum power pool elevation at Lake Powell through upstream reservoir releases; (iii) investigates ‘demand management’ program to reduce UB use and store conserved water in Powell.
Under the Lower Basin DCP, AZ and NV had a combined delivery reduction of 200 kaf in 2020, 2021, and 2022; in this last year, the DCP-imposed 200 kaf reduction was in addition to the 333 kaf reduction imposed under the 2007 Interim Guidelines. Future DCP-imposed delivery reductions may be greater and also include California, depending on Lake Mead levels.

Under the Upper Basin DCP (ii), Reclamation made additional releases from Flaming Gorge (2021 and 2022) and Blue Mesa (2021) to help protect critical elevations at Lake Powell. Pilot efforts to inform demand management investigations have occurred throughout the Upper Basin, consistent with the Upper Basin DCP (iii), but a broader program has not yet been developed or implemented.

Development of new guidelines for post-2026 operations

By 2005, declining reservoirs and increasing concern about the over-allocation of Colorado River water led to ongoing discussions regarding basin shortage guidelines. In 2007, after two years of development and a formal NEPA (National Environmental Policy Act) process resulting in an Environmental Impact Statement (EIS), a set of interim guidelines was signed by the Secretary of the Interior to implement shortage guidelines regarding water in the Colorado River basin through 2026.

The Interim Guidelines have these four purposes (i) managing shortages in Lake Mead and Lower Basin states; (ii) optimizing Lake Powell and Lake Mead operations, (iii) creating an intentional surplus in Lake Mead; and (iv) modifying and extending the existing interim surplus guidelines. However, the shortage measures in the Interim Guidelines have proven insufficient to cope with the impacts of continuing poor hydrology and underlying basinwide imbalances. Large drawdowns in Powell and Mead during severe drought years in 2012-13 and 2018, with no recovery in intervening near-average flow years, spurred the development of the 2019 Drought Contingency Plans (DCPs).

In preparation for developing new guidelines for post-2026 operations, in 2020 Reclamation completed a review of the implementation of the 2007 Interim Guidelines, evaluating what has worked and not worked.

In November 2022, recognizing that still-declining reservoir levels could necessitate management actions beyond those laid out in the 2007 Interim Guidelines and 2019 DCPs, Reclamation filed a Federal Notice of Intent to prepare a Supplemental Environmental Impact Statement (SEIS) for Near-term Colorado River Operations. This SEIS, when completed in late 2023, will supplement the 2007 Interim Guidelines EIS, and serve as a bridge until the development of the new set of guidelines for post-2026 operations.

In June 2023, Reclamation began scoping for the post-2026 NEPA/EIS process. In October 2023, Reclamation released the Scoping Report for Post-2026 Colorado River Reservoir Operations and the Proposed Federal Action based on that scoping report. Reclamation's Post-2026 Operations web page has more information about the NEPA process and its current status. For more background on the technical tools and approaches that Reclamation will use in the process, see the archived presentations to the Integrated Technical Education Workgroup (ITEW).

In December 2023, Reclamation released the Post-2026 Operations Explorations Web Tool, which provides an interactive interface to the Colorado River Simulation System (CRSS) model, allowing stakeholders to develop operational strategies (i.e., alternatives) using built-in customizable options and actually test them in the agency's policy model. In March, April, and May 2024, nine alternatives or supplemental "concepts for consideration" were submitted to Reclamation by basin states, Tribal nations, environmental NGOs, and other basin stakeholders.

In May 2024, Reclamation released the Record of Decision (ROD) for the SEIS for near-term Colorado River Operations, implementing the Preferred Alternative identified in the Final SEIS. It addresses the potential for low-runoff conditions in the Basin between now and 2026 through limited adjustments to the 2007 Interim Guidelines. Due to the improved runoff conditions in water years 2023 and 2024, it is now highly unlikely that the adjustments implemented by the SEIS will be needed.

References and additional resources

Quenching Thirst in the Colorado River Basin by Kwon and Gimbel (2021)

Colorado River Basin by Lawrence MacDonnell (2021)

Congressional Research Service Report on the River (2022)

Reclamation homepage for Colorado River Basin

Summary of 2007 Interim Guidelines by Reclamation

Tribes and Water in the Colorado River basin by the Colorado River Research Group (2016)

The Status of Tribal Water Rights in the Colorado River Basin by the Water and Tribes Initiative (2021)