US-Mexico Water Treaty: Deliveries, Debt, and IBWC Minutes

The US-Mexico Water Treaty of 1944 divides the Colorado River, the Rio Grande, and the Tijuana River between the two countries, guaranteeing Mexico 1.5 million acre-feet of Colorado River water each year and requiring Mexico to deliver an average of 350,000 acre-feet annually from named tributaries to the Rio Grande. Signed on February 3, 1944, and in force since November 8, 1945, it remains the binding legal framework that governs shared surface water on the border.¹ The treaty has no expiration date, but the cooperative agreements that make it workable in a drying climate do, and the most important of those expires at the end of 2026.

What Mexico Gets From the Colorado River

The United States guarantees Mexico 1,500,000 acre-feet of Colorado River water per year, measured at the international boundary. When surplus water exists beyond what all U.S. users need, Mexico may receive an additional 200,000 acre-feet, raising the potential total to 1,700,000 acre-feet.² These figures are fixed obligations written into the treaty text. They do not rise or fall with snowpack.

Most of Mexico’s allotment arrives through the main river channel, where Mexico diverts it at Morelos Dam into its canal system. A smaller portion reaches Mexico through a connection between the All-American Canal and Mexico’s Alamo Canal near San Luis, Sonora.¹

The Tijuana River is handled differently. Rather than assigning fixed volumes, the treaty calls for joint investigations to determine how both countries can best share those waters and build the diversion and storage works needed to serve nearby cities and farms.²

What Mexico Owes on the Rio Grande

The Rio Grande allocation covers the river from Fort Quitman, Texas, to the Gulf of Mexico. Here the flow of obligation reverses: Mexico owes water to the United States from tributaries that drain north into the main channel.¹

The treaty gives each country full ownership of specific tributaries. The United States receives all water from the Pecos and Devils Rivers and several smaller streams. Mexico receives all water from the San Juan and Alamo Rivers.¹

The core Rio Grande rule involves six Mexican tributaries that feed the main channel: the Conchos, San Diego, San Rodrigo, Escondido, and Salado Rivers, plus the Las Vacas Arroyo. The United States is entitled to one-third of their combined flow, and Mexico keeps two-thirds. The treaty guarantees the U.S. share will average no less than 350,000 acre-feet per year, calculated over rolling five-year cycles.¹ Unallocated flows in the main channel are split evenly.

The five-year accounting means Mexico must deliver 1,750,000 acre-feet across each cycle. The structure gives Mexico flexibility in dry years as long as it catches up in wetter ones. If a cycle ends with a deficit, that shortfall carries into the next cycle and must be repaid from Mexican tributaries. There is one release valve: if the international reservoirs on the Rio Grande fill to capacity before a cycle ends, the cycle is considered satisfied and any outstanding debt is cancelled.¹ Prolonged drought has kept reservoir levels well below capacity, so that clause has offered little relief in recent decades.

Drought Provisions

Each river has its own drought rule. On the Colorado, Article 10 addresses what happens when the United States cannot physically deliver Mexico’s full 1,500,000 acre-feet because of extraordinary drought or a serious failure of irrigation infrastructure. Mexico’s allotment then shrinks in the same proportion that U.S. consumptive uses are reduced.¹ If American users take a 20 percent cut, Mexico takes 20 percent as well.

On the Rio Grande, Article 4 works differently. If Mexico cannot deliver the required volume because of extraordinary drought, any shortfall at the end of a five-year cycle rolls into the next cycle, and Mexico must repay it from its tributaries once conditions improve.² The treaty never defines “extraordinary drought,” and that ambiguity has been a recurring source of disagreement. Mexico has invoked drought as a reason for shortfalls; U.S. officials have argued that Mexican water management decisions, not just weather, share responsibility.

Mexico’s Rio Grande Water Debt

The five-year system works in theory but has produced repeated friction. Mexico first missed its deadline in 1997 and carried the debt forward. It missed again in 2002, prompting lawsuits, interim agreements, and investments in upstream water conservation on the Mexican side. Payments lapsed again in 2015. Mexico repaid that debt in 2016 but nearly missed the 2020 target, transferring water three days before its deadline.

The consequences for Texas farmers have been significant. Mexico’s persistent shortfalls have contributed to hundreds of millions of dollars in agricultural losses over the past decade, according to industry estimates. Texas lost its last sugar mill in 2024, which officials linked to the water dispute, and the state’s South Texas citrus industry faces similar pressure.³

The current cycle runs through October 2025. By mid-2025, Mexico had delivered roughly 30 percent of the water it owed. In December 2025, Mexico agreed to release 202,000 acre-feet to the United States, and both governments are negotiating a broader repayment plan covering the current and previous cycle’s deficits, with finalization expected by January 31, 2026.³

How the Treaty Is Administered: The IBWC and Its Minutes

The International Boundary and Water Commission administers the treaty. It has two sections, one American and one Mexican, each led by an Engineer-Commissioner appointed by their respective president.⁴ The commission operates the international dams, tracks deliveries, and settles disputes over how to interpret the treaty’s language.

Its most important tool is the Minute, a formal document that functions as a binding supplement to the treaty. When the commission identifies an operational problem or a chance to cooperate, the two Commissioners investigate jointly, draft findings in English and Spanish, and submit the Minute for approval. Once both governments approve a signed Minute, it carries the same legal force as the treaty itself.⁴ This is how the 1944 framework has adapted to problems its drafters never anticipated, from Colorado River salinity to multi-year mega-droughts.

The Salinity Fix

The original treaty said almost nothing about water quality. By the 1960s, agricultural drainage from Arizona’s Wellton-Mohawk Irrigation District was sending highly saline water into the Colorado just before it crossed into Mexico, damaging Mexican farmland. The IBWC addressed this in 1973 with Minute 242, which set a permanent salinity standard: water delivered to Mexico upstream of Morelos Dam must have an annual average salinity no more than 115 parts per million (plus or minus 30 ppm) above the salinity of Colorado River water arriving at Imperial Dam.⁵ The U.S. cannot count salty drainage water toward Mexico’s allotment.

Modern Colorado River Cooperation: Minutes 319 and 323

The original treaty’s drought provisions were blunt. They said Mexico’s delivery should shrink proportionally in extraordinary drought but did not define triggers or build a structured framework for a basin in long-term decline. Two recent Minutes filled that gap.

Minute 319, signed in 2012, tied shared shortages and surpluses to Lake Mead’s water level. When the lake dropped below certain elevations, both countries took reductions. At 1,075 feet, the U.S. cut 375,000 acre-feet while Mexico cut 50,000, with mandatory consultation if the level fell below 1,025 feet. The agreement also let Mexico conserve water and store up to 375,000 acre-feet in U.S. reservoirs for later delivery, and it committed both countries to an environmental pulse flow of 105,392 acre-feet to the Colorado River delta in the spring of 2014, the first intentional flow to reach the delta since 1997.⁶

Minute 323, signed in September 2017, extended and expanded those measures through December 31, 2026. It established a Binational Water Scarcity Contingency Plan that mirrors the U.S. Drought Contingency Plan for the Lower Colorado River Basin. Under this framework, Mexico takes progressively larger delivery reductions as Lake Mead drops, starting at 41,000 acre-feet when the lake sits at 1,090 feet and increasing to 200,000 acre-feet if it falls below 1,025 feet.⁷

Mexico can also defer part of its annual Colorado River allotment, storing it in Lake Mead as “Intentionally Created Mexican Allocation” up to 250,000 acre-feet at any one time, with the same evaporation losses charged to other conserved water in the reservoir. Minute 323 committed a further 210,000 acre-feet to environmental flows in the delta, funded jointly by the U.S., Mexico, and a coalition of environmental organizations, and directed $16.5 million in U.S. investment into binational water conservation projects in Mexico, with the conserved water split evenly.⁷

What Happens Next

The treaty itself has no expiration date and remains in force until replaced by a new agreement between the two governments.¹ Minute 323 expires at the end of 2026, and the next agreement will have to address Lake Mead elevations that were unthinkable in 1944, ongoing disputes over Rio Grande deliveries, and whether environmental flows in the delta become a permanent feature rather than an experiment. The Rio Grande repayment plan the two governments expect to finalize by January 31, 2026 will resolve the immediate compliance dispute; the Colorado River successor agreement will shape how both countries share a shrinking river for the decades that follow.

1
U.S. Bureau of Reclamation. Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande
2
International Boundary and Water Commission. Treaty Series 994 – Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande
3
U.S. Department of Agriculture. Mexico Agrees to Meet Water Treaty Obligations for Farmers in the American Southwest
4
International Boundary and Water Commission. About Us
5
International Boundary and Water Commission. Minute No. 242 – Permanent and Definitive Solution to the International Problem of the Salinity of the Colorado River
6
International Boundary and Water Commission. Minute No. 319 – Interim International Cooperative Measures in the Colorado River Basin
7
International Boundary and Water Commission. Minute No. 323 – Extension of Cooperative Measures and Adoption of a Binational Water Scarcity Contingency Plan in the Colorado River Basin