A water treaty from 1944—the Treaty on Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande (1944 Water Treaty)—between the United States and Mexico and other binational agreements guide how the two governments share flows of the Rio Grande and Colorado Rivers. The 1944 Water Treaty states that the countries agree to give preferential attention to solving all border sanitation problems (e.g., flows of raw sewage and industrial wastewater in the Tijuana River Valley [TRV]). The binational International Boundary and Water Commission (IBWC), which was established in 1944 pursuant to the treaty and administers agreements on river flows and sanitation issues, is the principal venue for addressing related disputes between the United States and Mexico. The IBWC develops rules and proposed decisions, called minutes, on treaty execution and interpretation. Congressional considerations during the 119th Congress may include executive branch actions on U.S.-Mexico water matters and their role and effect on U.S.-Mexico cooperation, congressional responses to these actions and actions by Mexico, and forthcoming recommendations on addressing water management and sanitation issues.
The 1944 Water Treaty addresses the Rio Grande basin below Fort Quitman, TX. Among other things, it establishes that the United States has a right to flows from tributaries that feed the Rio Grande in the United States and one-third of specified Mexican tributaries flows, which must average at least 0.35 million acre-feet (MAF) per year, measured in five-year cycles (1.75 MAF total). It also provides for the construction of international storage dams on the Rio Grande. Among other infrastructure, IBWC operates two international Rio Grande dams and their reservoirs. On multiple occasions since 1994, Mexico has not met its Rio Grande delivery obligations for a cycle. As of late October 2025, at the end of the previous five-year cycle, Mexico had delivered 0.88 MAF, according to IBWC. The Consolidated Appropriations Act, 2026 (P.L. 119-75, Division F, Section 7045(h)(1)), conditioned some funds provided therein on the Secretary of State certifying Mexico's delivery of Rio Grande water "owed" to the United States, as prescribed in the Treaty. In February 2026, the U.S. Department of State announced that Mexico had committed to make annual deliveries of at least 0.35 MAF and to develop a plan to address the 2020-2025 cycle's shortfall.
Mexico not meeting five-year cycle delivery requirements, and the consequences for the Texas economy, has led some U.S. stakeholders to support mechanisms to achieve more reliable and predictable water deliveries. Although Mexico's internal water management influences deliveries, stakeholders have identified various other factors that may contribute to delivery shortfalls under recent cycles, including drought conditions in portions of Mexico's Rio Grande Basin and weather and climate patterns. In 2022, the U.S. section of the IBWC commissioned a study on the challenges to improving reliability and predictability of water deliveries and stakeholders' proposed solutions, resulting in the December 2022 white paper—The Rio Grande/Rio Bravo Water Deliveries Under the 1944 Treaty: A Compendium of Ideas. It identified the following among proposals for expanding water supplies: salinity improvements and desalination; conservation (e.g., canal lining, aqueduct construction); and "storage projects, both small and large, in Texas and in Mexico."
U.S. delivery of Colorado River basin waters to Mexico is part of a broader allocation of basin waters pursuant to the Colorado River Compact, a seven-state agreement signed in 1923 that apportioned 7.5 MAF annually to both the Upper and Lower Colorado River basins in the United States and delineates how additional waters are to be sent to Mexico pursuant to a subsequent treaty. In the 1944 Water Treaty, the United States agreed to deliver to Mexico 1.5 MAF of Colorado River water per year, plus an additional 0.2 MAF when a surplus is declared. During drought, the United States may reduce deliveries to Mexico in similar proportion to reductions of U.S. consumptive uses. The 1.5 MAF obligation is generally split equally between Upper and Lower Basins.
The United States typically has met its Colorado River delivery requirements to Mexico pursuant to the 1944 Treaty. Colorado River average flows have decreased significantly since 2000. During this time, the two countries have negotiated multiple minutes (e.g., Minute 319 in 2012, Minute 323 in 2017) that, among other things, reduced deliveries to Mexico under specified conditions and increased Mexico's ability to conserve and store water in U.S. reservoirs. Recent negotiations over future water curtailments in U.S. basin states have included speculation on the potential for similar changes for Mexico, although no new minutes have been announced.
Since 1972 (Minute 240), deliveries of a portion of Mexico's Colorado River water have been provided to the Mexican city of Tijuana through U.S. diversions at Parker Dam, through Southern California and across the international border near San Diego. These diversions, typically account for a small portion of Mexico's total Colorado River apportionment (averaging 1,678 acre-feet per year from 2020 to 2024). They have been regularly extended over five-year increments and currently are authorized through 2027 (Minute 327). The deliveries must be requested by Mexico and approved annually by the United States. In March 2025, the United States denied Mexico's delivery request for the first time in the treaty's history. A State Department post explaining the denial cited "continued shortfalls in [Mexico's] water deliveries," under the 1944 Water Treaty.
In addition to Tijuana's water supply and the Colorado River, U.S.-Mexico bilateral cooperation has involved efforts to address transboundary pollution in the TRV. Raw sewage and industrial wastewater entering the United States via the Tijuana River, San Antonio de los Buenos Creek, and cross-border tributaries have caused health, economic, environmental, and recreational problems in Southern California. The IBWC has acted under at least 10 minutes to address TRV sanitation. For example, under Minute 283, agreed to in 1990, IBWC constructed the South Bay International Wastewater Treatment Plant (SBIWTP) in San Ysidro, CA, which treats Mexican wastewater on the U.S. side of the border. Both countries support SBIWTP's operation and maintenance (O&M).
In 2022's Minute 328, Mexico agreed to provide $144 million and the United States $330 million to support projects to address TRV transboundary pollution. These projects include expanding the SBIWTP's capacity; rehabilitating pumping plants, pump stations, and collectors and pipelines in Mexico; and constructing a new wastewater treatment plant (WTP) in Mexico. Minute 328's expansion of the SBIWTP is supported by $300 million provided by P.L. 116-113 to the U.S. Environmental Protection Agency (EPA) that was transferred to IBWC by Section 7069 of P.L. 117-328. The U.S. contribution to other Minute 328 projects has been supported by EPA appropriations. Minute 328 states that a future minute will identify each country's O&M contributions.
Some recent appropriations for the IBWC have focused on addressing transboundary pollution in the TRV. (Appropriations for IBWC are included in the Department of State, Foreign Operations, and Related Programs [SFOPS] appropriations measure.) P.L. 118-47 provided $156.1 million in FY2024 appropriations for IBWC's Construction account, which the explanatory statement directs "to address urgent water management and water quality improvement programs of the ... [U.S. IBWC], including the rehabilitation and expansion" of SBIWTP. For FY2025, P.L. 118-158 provided $250 million in supplemental appropriations to IBWC, which some identified as being for "emergency water infrastructure repairs and to address sewage contaminating United States communities." (For FY2026, P.L. 119-75 provides $78 million for IBWC's Construction account, although no amount is specifically identified for TRV in the law or in the explanatory statement. P.L. 119-75 also dedicated $12.5 million from IBWC's salaries and expenses account to O&M of the pipeline that conveys wastewater from Nogales, Mexico, to the Nogales International WTP in Arizona, as authorized by P.L. 118-31, Section 5602(b)(3).)
In July 2025, Mexico's Secretary of the Environment and Natural Resources and EPA entered into an agreement that identifies timeframes to "initiate" or "facilitate completion" of TRV projects, including some from Minute 328, and identified that a future minute would be finalized by the end of 2025. Agreed to on December 15, 2025, Minute 333 outlines that the U.S. and Mexico intend to form a work group to assess the feasibility of certain projects and to develop scopes of work for projects to address various aspects of TRV water management. It also requires Mexico to construct a new WTP in Tijuana, among other actions. Minute 333 contains other commitments intended improve binational information sharing (e.g., notification protocol for discharges of wastewater in the Tijuana River). Minute 333 directs the creation of a binational O&M work group to develop strategies to maintain sanitation infrastructure, such as the creation of an O&M account at the North American Development Bank. In addition, it states that EPA indicated that its funding to initiate new projects in Tijuana, other than those identified in the July agreement and Minute 328, is contingent on confirmation that certain projects are on schedule for completion.
Congressional deliberations on activities to fund in future appropriations may be shaped by a report that Congress requested in the explanatory text accompanying IBWC's FY2026 appropriations. The text called for IBWC to report recommendations for improving water "storage, water movement, and deliveries in the regions affected by the 1944 Treaty," including "the feasibility of constructing the third international reservoir." The extent to which Mexico meets the annual Rio Grande delivery commitments it made in February 2026 and makes scheduled progress on TRV projects may inform congressional appropriations and oversight on these matters. This may include oversight on executive branch TRV actions, such as decisions to condition funding on Mexico's progress on certain TRV projects, and responses to work group recommendations.
Congress may consider related legislation; for example, S. 3120 and H.R. 6386 would link Rio Grande deliveries to consideration of certain Colorado River delivery requests by Mexico. Until recently, actions in the Rio Grande, Colorado River, and TRV largely had been addressed as independent issues in the context of binational water cooperation. Congress may assess the risks and benefits of connecting actions and negotiations across two or three river basins (e.g., the effect of denying Colorado River diversion requests based on Rio Grande deliveries on binational water cooperation in Colorado River Basin). Congress and executive branch policymakers also may weigh interest by some House and Senate Members in including or addressing the 1944 Water Treaty during the review of the United States-Mexico-Canada Agreement.
For background on the Colorado River, see CRS Report R45546, Management of the Colorado River: Water Allocations, Drought, and the Federal Role. For more on the trade agreement review, see CRS Report R48787, USMCA Joint Review: Process and Role of Congress.