A bilateral 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 also states that the countries agree to give preferential attention to the solution of all border sanitation problems, such as flows of raw sewage and industrial wastewater in the Tijuana River Valley (TRV). The binational International Boundary and Water Commission (IBWC), established in 1944 pursuant to the treaty, administers agreements on river flows and sanitation issues and is the principal venue for addressing related disputes between the United States and Mexico. The IBWC develops rules and proposed decisions, called minutes, regarding treaty execution and interpretation.
During the 119th Congress, U.S.-Mexico water issues have been receiving attention, including Mexico's deliveries in the Rio Grande basin, U.S. approval of diversions from the Colorado River to Mexico, and sanitation concerns related to the TRV. Congressional considerations include how Congress and the executive branch are shaping responses to these matters and the effect on U.S.-Mexico cooperation.
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). On multiple occasions since 1994, Mexico has not met its Rio Grande delivery obligations within a cycle. Almost 4½ years of the current cycle (which started October 25, 2020) have passed. As of April 19, 2025, Mexico's deliveries were estimated at roughly 0.53 MAF for the cycle, based on IBWC's deliveries reporting. If Mexico fails to meet its minimum Rio Grande flow obligations for a cycle because of extraordinary drought—a term not defined in the 1944 Water Treaty or in any minute—it must replace the deficiency during the next five-year cycle.
Concerns about Mexico's repeated occasions of not meeting five-year cycle delivery requirements, and the consequences for the Texas economy, have led some U.S. stakeholders to support the establishment, monitoring, and application of mechanisms to achieve a more reliable and predictable water-delivery regime. Article 24(d) of the 1944 Water Treaty provides mechanisms for dispute resolution if Mexico fails to meet its minimum flow obligations and the countries dispute that an extraordinary drought existed. In early April 2025, President Trump raised the prospects of sanctions and tariffs when discussing Mexico's water deliveries on social media. Other stakeholders identify drought conditions in portions of Mexico's Rio Grande basin during some of the current cycle, as well as potential shifts in weather and climate patterns, as potential factors contributing to delivery shortfalls under recent cycles. On April 28, 2025, the U.S. announced an agreement with Mexico to increase deliveries to the United States and reduce the current shortfall by October 2025 (i.e., the end of the current cycle).
Historically, Congress has been involved in Rio Grande water-sharing issues through oversight and, at times, direction for enhanced State Department reporting on deliveries and efforts to improve compliance. In 2024, various Members of Congress from Texas proposed withholding certain funds that the United States provides to Mexico to push Mexico to meet its treaty obligations. Considerations for the 119th Congress include what executive branch mechanisms are being employed to influence Mexico's deliveries under the current and next cycles and whether additional minutes—beyond Minute 331, "Measure to Improve the Reliability and Predictability of Rio Grande Water Deliveries to Benefit the United States and Mexico," approved in November 2024—are warranted.
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 (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. Hydrology on the Colorado River has deteriorated 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 had been announced as of early 2025.
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, which typically account for a small portion of Mexico's total Colorado River apportionment (averaging 1,678 acre-feet per year from 2020 to 2024), have been regularly extended over five-year increments and currently are authorized through 2027 (Minute 327) but 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 nine minutes to address TRV transboundary pollution. 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).
More recently, in Minute 328 from 2022, Mexico agreed to provide $144 million and the United States $330 million to support projects to address transboundary pollution in the TRV. 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 in Mexico. A 2022 IBWC press release stated that Minute 328 projects are expected to be completed in 2027 "that would result in a 50% reduction in the number of days of transboundary wastewater flow." Minute 328's expansion of the SBIWTP is supported by $300 million provided by the United States-Mexico-Canada Agreement Implementation Act (P.L. 116-113). The U.S. contribution to other Minute 328 projects has been supported by appropriations for the U.S. Environmental Protection Agency. Minute 328 states that a future minute will identify each country's contribution to these projects' O&M.
While both countries take action to fulfill their Minute 328 commitments, wet-weather events have led to increased transboundary flows in the TRV, overwhelming the existing aging infrastructure and resulting in public health concerns in San Diego County. These events led to two Biden Administration supplemental funding requests for $310 million for SBIWTP in FY2024 and in FY2025. Congress provided supplemental funding for SBIWTP in the American Relief Act (P.L. 118-158, Division B), discussed below. Recent appropriations for the IBWC have focused on addressing transboundary pollution in the TRV. P.L. 118-47 provided $156.1 million in FY2024 appropriations for IBWC's construction account "to address urgent water management and water quality improvement programs of the ... [U.S.] IBWC, including the rehabilitation and expansion" of SBIWTP. In addition, P.L. 118-158 provided $250 million to IBWC, which some have identified as being for "emergency water infrastructure repairs and to address sewage contaminating United States communities." (For FY2025, P.L. 119-4 Section 11203 provides $78 million for IBWC's construction account, although no amount is specifically identified for TRV.)
As a part of Minute 328's development, stakeholders identified long-term wastewater infrastructure projects, with a total cost of $327 million, to reduce transboundary wastewater flows in the TRV. These projects and their rates of implementation, including those supported by Mexico, raise a number of congressional considerations.
Until recently, Mexico's Rio Grande deliveries, Colorado River cooperation, and TRV sanitation have been addressed as independent issues. Some U.S. stakeholders, however, argue that two or all three of these issues should be linked, and a few have raised the prospect of water treaty renegotiation. Congress may assess the risks and benefits of connecting Rio Grande deliveries to actions in other binational water basins (i.e., the Colorado River basin) and what actions it wants to support, such as new minute negotiations or other measures. It also may consider the linkage of water issues to the broader U.S.-Mexico relationship (e.g., tariffs, foreign aid dependent on water deliveries). Among other things, new proposed authorities associated with future operations in the Colorado River basin could be affected by the broader status of international water management negotiations between the United States and Mexico.
For the TRV, Congress may assess the appropriate level of funding for the additional wastewater infrastructure projects discussed above. Another consideration may pertain to the mechanisms available for ensuring Mexico's financial contribution to Minute 328 projects and for O&M.
For an introduction to bilateral relations, see CRS In Focus IF12765, Mexico: Political Overview and U.S.-Mexican Relations, by Clare Ribando Seelke and M. Angeles Villarreal. For background on water management, see CRS Report R45546, Management of the Colorado River: Water Allocations, Drought, and the Federal Role, by Charles V. Stern, Pervaze A. Sheikh, and Kristen Hite; and CRS Report R45430, Sharing the Colorado River and the Rio Grande: Cooperation and Conflict with Mexico, by Nicole T. Carter, Steve P. Mulligan, and Charles V. Stern.