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INSIGHTi
Responding to Drought in the Colorado River
Basin
Updated March 11, 2024
The Colorado River Basin (Figure 1) covers more than 246,000 square miles in seven U.S. states and
Mexico. Basin waters are governed by multiple documents, known collectively as the Law of the River.
The Colorado River Compact of 1922 established the framework to apportion water supplies between the
river’s Upper and Lower Basins, with each basin allocated 7.5 million acre-feet (MAF) annually. The
Bureau of Reclamation (Reclamation) plays a prominent role in basin water management due to the many
federally authorized projects in the basin.
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Figure 1. Colorado River Basin Allocations
(Upper Basin allocations in percentages of overall allocation, Lower Basin allocations in million acre-feet [MAF])
Source: CRS, using data from U.S. Geological Survey ESRI Data & Maps, 2017, Central Arizona Project, and ESRI World
Shaded Relief Map.
Notes: 7.5 MAF in Upper Basin allocations assumes ful allocations under the Colorado River Compact. Due to
uncertainty about how much water would remain after meeting obligations to the Lower Basin and Mexico, most Upper
Basin compact apportionments are in terms of percentages.
When federal and state governments originally approved the Colorado River Compact, it was assumed
that river flows would average 16.4 MAF per year. Actual annual flows from 1906 to 2023 were
approximately 14.6 MAF, and these flows have averaged significantly less (12.4 MAF per year) since
2000. Several studies have projected lower annual runoff volumes in the future compared with the
historical baseline.
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The imbalance between basin water supplies and demand has depleted storage in the basin’s two largest
reservoirs—Lake Powell in the Upper Basin and Lake Mead in the Lower Basin—and threatens water
supplies for millions in the Southwest. Reclamation makes operational decisions for basin reservoirs in
monthly 24-month studies, which project operational conditions for upcoming years (Figure 2, Figure 3).
Figure 2. Lake Powell Storage Elevations and Projections
(January/February 24-month study inflow scenarios)
Source: Bureau of Reclamation, 24-Month Study Projections, https://www.usbr.gov/lc/region/g4000/riverops/24ms-
projections.html.
Note: DROA = Drought Response Operations Agreement; maf = mil ion acre-feet; WY = water year.
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Figure 3. Lake Mead Storage Elevations and Projections
(January/February 2024 24-month study inflow scenarios)
Source: Bureau of Reclamation, 24-Month Study Projections, https://www.usbr.gov/lc/region/g4000/riverops/24ms-
projections.html.
Notes: DROA = Drought Response Operations Agreement; maf = mil ion acre-feet; WY = water year.
Mitigating Drought in the Colorado River Basin
Previous efforts to improve the basin’s water supply outlook resulted in agreements in 2003, 2007, and
2019. The agreements reduced Lower Basin deliveries based on operational “tiers” for Lake Mead,
authorized additional water conservation efforts, and implemented a framework to coordinate Upper
Basin operations and protect hydropower generation at Glen Canyon Dam.
Despite these efforts, storage levels declined. Since 2020, Reclamation has curtailed water deliveries for
Arizona and Nevada based on annual hydrologic conditions (Table 1) and implemented Upper Basin
operational changes in 2021 and 2022. Storage at both lakes rebounded in 2023, but there remains
widespread concern about the long-term water supply outlook.
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Table 1. Lower Colorado River Basin Operational Tiers, 2020-2024
(water delivery cutbacks in thousand acre-feet [KAF])
Cumulative Delivery Cutbacks (percentage of
total deliveries)
Operational
Year
Tier
Arizona
California
Nevada
2020
Zero
192 (6.8%)
—
8 (2.6%)
2021
Zero
192 (6.8%)
—
8 (2.6%)
2022
One
512 (18.2%)
—
21 (7.0%)
2023
Two
592 (21.1%)
—
25 (8.3%)
2024 (forecast)
One
512 (18.2%)
—
21 (7.0%)
Source: CRS, based on Bureau of Reclamation data, 2019-2023.
Near-Term Operations
At a July 2022 congressional hearing, Reclamation asked states to come up with plans to conserve an
additional 2-4 MAF in 2023 and 2024, and in October 2022 Reclamation noticed its intent to analyze
near-term operational changes through 2026. In 2023, California and the six other basin states each
responded to Reclamation with proposals with competing water conservation proposals.
On April 11, 2023, Reclamation released draft modeling for two federal alternatives that both would have
imposed new Lower Basin cuts (0.020-2.900 MAF per year in water years 2024-2026, depending on Lake
Mead elevations). One alternative would have apportioned reductions based on water rights, whereas the
other would have imposed percentage-based delivery reductions tied to evaporation and other factors. In
lieu of these plans, on May 22, 2023, Reclamation announced a consensus proposal in which Lower Basin
states agreed to conserve a total of 3 MAF prior to 2026, with 2.3 MAF of these cuts compensated by the
federal government via $4.0 billion in drought response funds appropriated to Reclamation in P.L. 117-
169. Reclamation finalized this plan on March 5, 2024 (Figure 4).
Figure 4. Near-Term Lower Colorado River Water Delivery Cuts After 2024 SEIS
Acre-feet (af) in thousands
Source: Bureau of Reclamation, Near-term Colorado River Operations, Final Supplemental Environmental Impact
Statement, March 5, 2024, https://www.usbr.gov/ColoradoRiverBasin/documents/NearTermColoradoRiverOperations/
20240300-Near-termColoradoRiverOperations-FinalSEIS-508.pdf.
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Notes: SEIS= Supplemental Environmental Impact Statement; 2007 ROD= 2007 Record of Decision for Lower Basin
Operations; DCP= 2019 Drought Contingency Plans. Proposed action modeling for 2024 SEIS does not represent state-
level commitments.
Post-2026 Operations
Most existing plans to conserve Colorado River Basin water supplies expire in 2026; thus, Reclamation is
also analyzing post-2026 operational alternatives for the system. To date, Upper and Lower Basin states
have been unable to agree on a consensus-based plan for post-2026 operations. The two sub-basins
submitted competing plans to Reclamation in early March 2024. The Lower Basin’s plan proposes a new
paradigm in which total basin storage dictates water cuts, with equal cuts across the basin at lower storage
levels and cuts to only Lower Basin users at higher levels. The Upper Basin’s plan proposes a separate set
of changes, including Lake Powell water releases to the Lower Basin dictated by Lake Powell storage
conditions (i.e., in lieu of the compact’s 7.5 MAF).
Questions for Congress include whether Reclamation’s authorities for basin management are sufficient,
how competing operational proposals would affect water supplies, and what ongoing federal funding
commitments are necessary to support conservation plans by Reclamation and the basin states.
Author Information
Charles V. Stern
Specialist in Natural Resources Policy
Disclaimer
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