Colorado River Shortage: Tiers, Triggers, and Delivery Cuts

Colorado River shortage tiers are the federal government’s system for cutting water deliveries to Arizona, Nevada, California, and Mexico when Lake Mead drops too low. Each tier is tied to a specific projected elevation at Lake Mead on January 1, and each triggers a set volume of mandatory reductions. For 2026, the Bureau of Reclamation’s August 2025 study projected Lake Mead’s January 1 elevation between 1,075 and 1,050 feet, placing the Lower Basin in a Tier 1 shortage for the calendar year.1Bureau of Reclamation. Most Probable 24-Month Study: August 2025 The Bureau announced an expected elevation of roughly 1,056 feet, about 20 feet below the 1,075-foot shortage trigger.2Bureau of Reclamation. Interior Announces 2026 Colorado River Operating Conditions

What Triggers a Shortage Declaration

The Bureau of Reclamation runs a monthly simulation called the 24-Month Study, which forecasts reservoir levels from snowpack, precipitation, and projected demand. For shortage purposes, only the August version matters. It projects Lake Mead’s water surface elevation as of the following January 1, and that projection locks in the operating tier for the entire upcoming calendar year.3Bureau of Reclamation. Draft 2026 Annual Operating Plan for Colorado River Reservoirs

Once finalized, the August projection feeds into the Annual Operating Plan, the single reference document that governs how the Colorado River’s reservoirs will be managed for the year. Actual reservoir readings can drift from the projection as the year progresses, but the tier itself does not change mid-year.4Bureau of Reclamation. Lower Colorado River Operations – Lake Mead Elevation

The Elevation Bands That Define Each Tier

The Lower Basin’s operating status is defined by the projected water surface elevation at Lake Mead on January 1. Each tier corresponds to a specific elevation band:

  • Tier 1: projected elevation at or below 1,075 feet and at or above 1,050 feet. The first formal shortage level.
  • Tier 2: projected elevation below 1,050 feet and at or above 1,025 feet.
  • Tier 3: projected elevation below 1,025 feet. The most severe shortage category under the 2007 Interim Guidelines.

The 2019 Drought Contingency Plan added a pre-shortage band sometimes called Tier 0, covering elevations between 1,090 and 1,075 feet. At Tier 0, no formal shortage exists under the 2007 Guidelines, but states are required to make conservation contributions to slow the reservoir’s decline.5Bureau of Reclamation. Drought Contingency Plans

Delivery Cuts Under the 2007 Interim Guidelines

Under normal conditions, Arizona is apportioned 2.8 million acre-feet per year, Nevada gets 300,000 acre-feet, and California receives 4.4 million acre-feet. When a shortage is declared, Arizona and Nevada’s allotments shrink at every tier while California’s stays intact:6Bureau of Reclamation. Colorado River Decree Accounting – 2007 Interim Guidelines

  • Tier 1 (1,075–1,050 feet): Arizona loses 320,000 acre-feet; Nevada loses 13,000 acre-feet.
  • Tier 2 (1,050–1,025 feet): Arizona loses 400,000 acre-feet; Nevada loses 17,000 acre-feet.
  • Tier 3 (below 1,025 feet): Arizona loses 480,000 acre-feet; Nevada loses 20,000 acre-feet.

California’s exemption reflects the priority system built into the 1928 Boulder Canyon Project Act. California holds the most senior apportionment of the three Lower Basin states, so its 4.4 million acre-feet is the last to face reduction. Arizona, with the most junior rights (particularly through the Central Arizona Project), absorbs the deepest cuts first.6Bureau of Reclamation. Colorado River Decree Accounting – 2007 Interim Guidelines

Drought Contingency Plan Contributions on Top

The 2019 Lower Basin Drought Contingency Plan layered additional conservation requirements on top of the 2007 Guidelines. It introduced earlier action triggers and, for the first time, pulled California into the reduction framework at lower elevations.5Bureau of Reclamation. Drought Contingency Plans

The DCP volumes work on a more granular sliding scale than the three-tier 2007 structure. At elevations between 1,090 and 1,045 feet, Arizona contributes 192,000 acre-feet and Nevada contributes 8,000 acre-feet to Lake Mead storage. Once the reservoir drops below 1,045 feet, those contributions rise to 240,000 acre-feet for Arizona and 10,000 for Nevada. California begins contributing at 1,045 feet with 200,000 acre-feet, and its contribution ramps up in 50,000 acre-foot increments as the reservoir falls further, reaching 350,000 acre-feet below 1,025 feet.7Congressional Research Service. Management of the Colorado River: Water Allocations, Drought, and the Federal Role

These DCP amounts stack on top of the 2007 Guidelines reductions. At the worst-case scenario, with Lake Mead below 1,025 feet, the combined impact would be 720,000 acre-feet cut from Arizona (480,000 from the Guidelines plus 240,000 from the DCP), 30,000 acre-feet from Nevada, and 350,000 from California. The legal structure treats DCP amounts as contributions to Lake Mead storage rather than permanent losses of water rights, meaning states could theoretically recover stored water if the reservoir rises significantly in future years.5Bureau of Reclamation. Drought Contingency Plans

Mexico’s Cuts Under Minute 323

The United States and Mexico share the Colorado River under a 1944 treaty that guarantees Mexico 1.5 million acre-feet per year. Minute 323, a 2017 amendment, requires Mexico to accept delivery reductions tied to the same Lake Mead elevation triggers that govern the U.S. Lower Basin states:8International Boundary and Water Commission. Minute 323 – Extension of Cooperative Measures and Adoption of a Binational Water Scarcity Contingency Plan in the Colorado River Basin

  • Tier 1 (1,075–1,050 feet): Mexico’s delivery is reduced by 50,000 acre-feet.
  • Tier 2 (1,050–1,025 feet): reduction increases to 70,000 acre-feet.
  • Tier 3 (below 1,025 feet): reduction reaches 125,000 acre-feet.

Minute 323 also created a Binational Water Scarcity Contingency Plan that triggers additional savings at elevations between 1,090 and 1,025 feet. Unlike the mandatory shortage reductions, these savings are recoverable: Mexico can claim them back when Lake Mead’s projected elevation exceeds 1,110 feet.8International Boundary and Water Commission. Minute 323 – Extension of Cooperative Measures and Adoption of a Binational Water Scarcity Contingency Plan in the Colorado River Basin

Who Actually Loses Water Inside Arizona

Arizona absorbs the largest cuts by far, but they do not fall evenly. The Central Arizona Project, which delivers Colorado River water through a 336-mile canal system to central and southern Arizona, has an internal priority hierarchy that determines who loses water first.

The system works from the bottom up. Excess pool water, used for aquifer recharge and by some Pinal County farms, is the first to disappear. Non-Indian Agriculture allocations come next, and they get reduced to zero once CAP’s total available supply drops below about 990,000 acre-feet. Only after agricultural users are fully cut off do Municipal and Industrial users and Indian priority holders begin to see reductions.9Central Arizona Entitlements and Access Tool. Shortage Priority Basics

The practical result: a Tier 1 shortage can devastate agricultural users while leaving urban taps in Phoenix and Tucson largely untouched. Farming communities in Pinal County have already seen their CAP surface water allocations reduced to zero in recent shortage years, forcing growers to pump more groundwater or fallow fields.10Central Arizona Project. Economic Impacts to Central Arizona of Reductions in CAP Deliveries

The Infrastructure Floors Below the Tiers

Below all of the operational tiers sit two critical thresholds. At roughly 950 feet, Hoover Dam loses the ability to generate hydroelectric power.11Bureau of Reclamation. Post-2026 Operational Guidelines Draft Environmental Impact Statement Executive Summary At 895 feet, known as dead pool, water can no longer flow through the dam by gravity, cutting off all downstream deliveries to Arizona, California, and Mexico.12Southern Nevada Water Authority. Intake 3 and Low Lake Level Pumping Station The entire tier structure is designed to keep the reservoir well above those points.

Even above the power cutoff, reduced water pressure means the turbines produce less power per unit of flow. At the 2026 operating elevation around 1,060 feet, Hoover Dam generates substantially less than its roughly 2,000 megawatt nameplate capacity. Southern Nevada has hedged against extreme scenarios by completing a third intake and low lake level pumping station that can deliver up to 900 million gallons per day even if the reservoir falls below dead pool. That station began operating in April 2022.12Southern Nevada Water Authority. Intake 3 and Low Lake Level Pumping Station

Why 2026 Is the Last Year of These Rules

Both the 2007 Interim Guidelines and the 2019 Drought Contingency Plan expire at the end of 2026. The Bureau of Reclamation is running a multi-year process under the National Environmental Policy Act to establish replacement rules that could govern the river for decades. In January 2026, the Bureau released a Draft Environmental Impact Statement analyzing five alternatives ranging from minimal federal coordination to aggressive supply-driven management.13Bureau of Reclamation. Alternatives Development – Post 2026 Colorado River Operations The public comment period closed in March 2026.14Bureau of Reclamation. Colorado River Post 2026 Operations

Under several of the proposed alternatives, mandatory cuts could fall far more heavily on Arizona and Nevada than the current framework requires. The post-2026 rules will also determine whether California continues to enjoy its historical exemption from shortage reductions or begins sharing cuts at higher reservoir elevations. Until a final Record of Decision is issued, the tier structure and cut volumes described above remain in effect. They are interim rules that were always designed to be replaced.