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INSIGHTi
Responding to Drought in the Colorado River
Basin: Federal and State Efforts
Updated November 1, 2023
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 full 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
(October 2023 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: MAF = million acre-feet; WY = water year.
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Figure 3. Lake Mead Storage Elevations and Projections
(October 2023 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; WY = water year.
Mitigating Drought in the Colorado River Basin
Previous efforts to improve the basin’s water supply outlook included agreements in 2003, 2007, and
2019 (approved by Congress in P.L. 116-14). These agreements reduced Lower Basin deliveries based on
operational “tiers” for Lake Mead storage, authorized additional water conservation efforts, and
implemented a framework to coordinate Upper Basin operations to prevent losing 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). In the Upper Basin, Reclamation
implemented operational changes in 2021 and 2022. Storage at both lakes rebounded in 2023, but
widespread concerns about long-term water supplies remain.
Table 1. Lower Colorado River Basin Operational Tiers, 2020-2024
(water delivery cutbacks in thousand acre-feet [KAF])
Cumulative Delivery Cutbacks by KAF
(percentage of total deliveries)
Operational
Year
Tier
Arizona
California
Nevada
2020
Zero
192 (6.8%)
—
8 (2.6%)
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Cumulative Delivery Cutbacks by KAF
(percentage of total deliveries)
Operational
Year
Tier
Arizona
California
Nevada
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 (expected)
One
512 (18.2%)
—
21 (7.0%)
Source: CRS, based on Bureau of Reclamation data, 2019-2023.
At a 2022 congressional hearing, Reclamation asked states to conserve an additional 2-4 MAF in 2023
and 2024 and noted that it was prepared to act unilaterally if these targets were not met. No new state
commitments were announced, and in October 2022 Reclamation noticed its intent to study, via an
environmental impact statement (EIS), revised “near-term” operations through 2026. In early 2023,
California and the six other basin states each responded to this action with their own proposals.
On April 11, 2023, Reclamation released its draft EIS modeling for two federal action alternatives that
both would have imposed equal amounts of new Lower Basin delivery reductions (0.020-2.900 MAF per
year in water years 2024-2026, depending on Lake Mead elevations). The primary difference between the
two alternatives was the approach to apportioning reductions among those contracted to receive Colorado
River water; one alternative would have apportioned reductions based on water rights priority, whereas
the other would have imposed percentage-based delivery reductions based on evaporation and other
factors.
Subsequent developments have shifted this discussion. 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. In October 2023, Reclamation released a new draft
supplemental environmental impact statement (SEIS) analysis of the Lower Basin’s consensus proposal.
Reclamation noted that previous modeling was based on 2022 hydrology and that improved 2023
hydrology had significantly decreased the chances of both reservoirs reaching critical levels prior to 2026
(Figure 4).
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Figure 4. Improved Hydrological Conditions at Lakes Mead and Powell
(chances of reservoirs reaching “critical elevations” through 2026 under 2023 SEIS alternatives)
Source: Bureau of Reclamation, Department of the Interior, Near-Term Colorado River Operations, Revised Draft
Supplemental Environmental Impact Statement, October 2023, https://www.usbr.gov/ColoradoRiverBasin/documents/
NearTermColoradoRiverOperations/20231019-Near-termColoradoRiverOperations-RevisedDraftEIS-508.pdf.
Notes: SEIS = Supplemental Environmental Impact Statement. Action Alternatives 1 and 2 refer to April 2023 draft SEIS
alternatives. Lower Division Proposal refers to April 2023 proposal from Lower Basin states. “Percent of Traces”
represents percentage of modeled scenarios.
Parallel to this study process, Reclamation is studying long-term (post-2026) operational alternatives. In
October 2023, Reclamation released a scoping report detailing public comments on potential long-term
operational changes.
Questions facing Congress include how potential changes would affect long-term water supplies, how
state-level commitments would be met if contractors were unwilling to participate in voluntary actions,
and whether federal funding commitments would accompany future water delivery curtailments.
Author Information
Charles V. Stern
Specialist in Natural Resources Policy
Congressional Research Service
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Disclaimer
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan shared staff
to congressional committees and Members of Congress. It operates solely at the behest of and under the direction of
Congress. Information in a CRS Report should not be relied upon for purposes other than public understanding of
information that has been provided by CRS to Members of Congress in connection with CRS’s institutional role.
CRS Reports, as a work of the United States Government, are not subject to copyright protection in the United
States. Any CRS Report may be reproduced and distributed in its entirety without permission from CRS. However,
as a CRS Report may include copyrighted images or material from a third party, you may need to obtain the
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