The Colorado River agreement is not one document. The river is managed under a stack of compacts, federal statutes, a Supreme Court decree, and an international treaty known collectively as the Law of the River, anchored by the Colorado River Compact of 1922. That compact split the river between an Upper Basin and a Lower Basin at 7.5 million acre-feet each per year, and every later agreement builds on that division. The operating rules that govern shortages expire at the end of 2026, and the seven basin states, the federal government, Mexico, and thirty tribal nations are now negotiating what replaces them.
What the Law of the River Includes
The Law of the River is the full body of legal authority governing the Colorado. The main pieces are the 1922 Compact, the Boulder Canyon Project Act of 1928, the 1944 treaty with Mexico, the Upper Colorado River Basin Compact of 1948, the Supreme Court’s 1963 decree in Arizona v. California, the 2007 Interim Guidelines, the 2019 Drought Contingency Plan, and the 2023 Lower Basin conservation agreement.
No single agency runs the system. The Bureau of Reclamation, inside the Department of the Interior, handles day-to-day operations: annual release schedules, reservoir monitoring, and shortage declarations.1Bureau of Reclamation. Colorado River Basin Which rights and obligations apply to any given user depends on which layer of the Law of the River is in play.
How the Water Is Divided
The 1922 Compact splits the river at Lee Ferry, Arizona, roughly one mile below the Paria River. Article III apportioned 7.5 million acre-feet per year of beneficial consumptive use to the Upper Basin (Colorado, New Mexico, Utah, and Wyoming) and 7.5 million acre-feet per year to the Lower Basin (Arizona, California, and Nevada). The Lower Basin also received the right to use an additional one million acre-feet annually.2Bureau of Reclamation. Colorado River Compact, 1922
The Compact did not divide the Lower Basin’s share among its three states. The Boulder Canyon Project Act of 1928 did. California agreed to limit its consumptive use to 4.4 million acre-feet of the Lower Basin’s basic apportionment, Arizona received 2.8 million acre-feet, and Nevada 300,000 acre-feet.3Office of the Law Revision Counsel. Title 43, Chapter 12A – Boulder Canyon Project The Supreme Court confirmed those figures in its 1963 decree in Arizona v. California and held that the Secretary of the Interior has authority to apportion water among the three Lower Basin states in times of shortage or surplus.4Justia Law. Arizona v California, 376 US 340 (1963)
Nevada’s 300,000 acre-feet reflected the state’s small early-twentieth-century population. The Las Vegas metropolitan area now serves more than two million people on that same allocation, which is part of why Nevada has become one of the most aggressive water recyclers in the basin.
What Mexico Gets
The 1944 Water Treaty between the United States and Mexico commits the U.S. to deliver 1.5 million acre-feet of Colorado River water to Mexico annually.5U.S. Department of Agriculture. Mexico Agrees to Meet Water Treaty Obligations for Farmers in the American Southwest The International Boundary and Water Commission administers the treaty.6International Boundary and Water Commission. Treaties Between the US and Mexico
Article III(c) of the 1922 Compact addressed this in advance. If the U.S. recognized any Mexican right to Colorado River water, the supply was to come first from surplus flows beyond the 16 million acre-feet apportioned to the two basins; if surplus proved insufficient, both basins would share the burden equally.2Bureau of Reclamation. Colorado River Compact, 1922 In practice, the river rarely produces enough surplus to cover Mexico’s share without affecting domestic allocations.
Tribal Water Rights
Thirty federally recognized tribes hold water rights within the Colorado River Basin. Tribes with quantified rights hold approximately 3.2 million acre-feet annually. Including unresolved claims, the total tribal share could reach roughly 3.6 million acre-feet, which would make tribal nations collectively one of the largest water rights holders in the basin.7Water and Tribes Initiative. The Status of Tribal Water Rights in the Colorado River Basin
About a dozen tribes still have unquantified claims, so the total volume of legally recognized rights in the basin could grow as those claims settle. Many tribes with quantified rights have not yet developed the infrastructure to use their full allocation, so actual tribal consumption sits well below the paper entitlement. As water becomes scarcer, the gap between paper rights and actual use is a central tension point in every negotiation.
Tribes are also active in conservation. In September 2024, the Gila River Indian Community signed long-term conservation agreements with the Department of the Interior that could conserve over 73,000 acre-feet within the next decade, the first long-term tribal conservation deals under the current federal framework.8U.S. Department of the Interior. Biden-Harris Administration Marks Major Progress for Colorado River System Health, Signs Five New Water Conservation Agreements
Shortage Tiers and Mandatory Cuts
The 2007 Interim Guidelines, supplemented by the 2019 Drought Contingency Plan, set mandatory delivery reductions tied to Lake Mead’s elevation. The Secretary of the Interior declares a shortage for the following year when projections show Lake Mead at or below 1,075 feet on January 1. Cuts escalate as the reservoir drops.
Nevada’s reductions show how the tiers work:9Southern Nevada Water Authority. Water Shortages
- At or below 1,090 feet: 8,000 acre-feet per year in Drought Contingency Plan contributions.
- At or below 1,075 feet (Tier 1 shortage): 21,000 acre-feet per year total (13,000 in shortage reductions plus 8,000 in DCP contributions).
- Below 1,050 feet (Tier 2): 25,000 acre-feet per year total.
- At or below 1,045 feet: 27,000 acre-feet per year total.
- Below 1,025 feet: 30,000 acre-feet per year total.
Arizona absorbs the largest mandatory cuts by volume at each tier, reflecting its larger base allocation. California, with the most senior rights in the Lower Basin, faces no mandatory reductions until the most severe shortage levels. That seniority structure is one of the sharpest sources of friction in the post-2026 talks.
A Tier 1 shortage is in effect for 2026 operations, with Lake Mead forecasted between 1,050 and 1,075 feet as of January 1, 2026.9Southern Nevada Water Authority. Water Shortages As of mid-May 2026, Lake Mead sat at approximately 1,051.5 feet, with a projected end-of-year elevation near 1,037 feet.10Bureau of Reclamation. Lower Colorado Weekly Hydrologic Update If that projection holds, 2027 operations would trigger deeper cuts.
The 2023 Lower Basin Conservation Deal
In May 2023, Arizona, California, and Nevada agreed to conserve at least 3 million acre-feet in Lake Mead through the end of 2026, with at least half of that total conserved by the end of 2024.11U.S. Department of the Interior. Lower Basin Plan Letter It was the most significant voluntary conservation commitment in the river’s history.
The deal built in an emergency backstop. If Bureau of Reclamation projections showed Lake Mead dropping below 1,025 feet, the Lower Basin states had 45 calendar days to propose an implementation plan to keep the reservoir above 1,000 feet. If no acceptable plan emerged, Reclamation could act unilaterally.12U.S. Department of the Interior. Lower Basin Plan LetterLower Basin Proposal Adopted by Federal Government Stabilizes Colorado River System Through 2026
Federal money made the target politically viable. Section 50233 of the Inflation Reduction Act appropriated $4 billion through fiscal year 2026 for drought mitigation across the 17 western reclamation states, and Section 50231 provided $550 million for domestic water supply projects in disadvantaged communities.13Congressional Research Service. Bureau of Reclamation Funding in the Inflation Reduction Act Interior estimated about $3.5 billion would be obligated to Colorado River Basin projects by the end of fiscal year 2026, roughly $2 billion for infrastructure and $1.5 billion for short-term conservation.14U.S. Department of the Interior. Buy America General Applicability Public Interest Waiver for Colorado River Basin Drought Projects By early 2024, 24 conservation agreements in California and Arizona were expected to conserve up to 1.58 million acre-feet through 2026 using about $670 million in IRA funds, with another $500 million supporting Upper Basin efforts.
What Happens After 2026
The 2007 Interim Guidelines and the 2019 Drought Contingency Plan both expire at the end of 2026. The Bureau of Reclamation is running a National Environmental Policy Act process to write new post-2026 operating rules, and its Draft Environmental Impact Statement analyzes five alternatives:15Bureau of Reclamation. Post 2026 Colorado River Operations
- No Action Alternative: current guidelines expire with no replacement.
- Basic Coordination Alternative: focuses on the Secretary’s existing federal authorities.
- Enhanced Coordination Alternative: a hybrid approach expanding those authorities.
- Maximum Operational Flexibility Alternative: emphasizes cooperative conservation.
- Supply Driven Alternative: ties operations more directly to actual inflows and system storage.
The core fight is between the Upper and Lower Basins over who takes the cuts. The Lower Basin states have proposed that reductions beyond 1.5 million acre-feet be shared equally between the two basins. The Upper Basin states have resisted committing to specific reduction targets. From the Lower Basin’s view, that approach places all reduction responsibility on Arizona, California, and Nevada while the Upper Basin states make no binding commitments to cut their own use.16Arizona Department of Water Resources. FAQs Lower Basin States Alternative for Post-2026 Colorado River Operations
The Draft EIS puts the harder problem plainly: under critically dry futures, even large and unprecedented reductions may not stabilize storage.17Bureau of Reclamation. Post-2026 Operational Guidelines and Strategies for Lake Powell and Lake Mead Draft EIS If the states cannot agree, the Secretary of the Interior retains authority under the Arizona v. California decree to allocate water unilaterally among the Lower Basin states, a power no Secretary has ever fully exercised but one that gives the federal government real leverage at the table.