What Is the 1944 Water Treaty Between the U.S. and Mexico?

The 1944 Water Treaty between the U.S. and Mexico is the binding international agreement that divides the waters of the Rio Grande, the Colorado River, and the Tijuana River between the two countries. Signed on February 3, 1944 and in force since November 8, 1945, it guarantees Mexico 1,500,000 acre-feet of Colorado River water each year and requires Mexico to deliver an annual average of at least 350,000 acre-feet of Rio Grande water to the United States, measured over repeating five-year cycles.¹ Its formal name is the Treaty Respecting Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande, and it remains the framework for transboundary water management along the border more than 80 years after adoption.²

What the Treaty Covers

Three river systems fall under the treaty, but not all of each one. The Colorado River provisions apply to all waters from any source within the basin. The Tijuana River provisions cover the whole system where it interacts with the international boundary. The Rio Grande provisions are narrower and cover only the stretch from Fort Quitman, Texas downstream to the Gulf of Mexico.³ Water rights on the Rio Grande upstream of Fort Quitman are not governed by this treaty at all.

The agreement also reconstituted the earlier International Boundary Commission as the International Boundary and Water Commission (IBWC), gave it responsibility for shared dams and diversion works, and set the framework the two countries still use to resolve disputes and adjust operations.

How the Rio Grande Is Divided

Article 4 of the treaty splits Rio Grande flows using a tributary-based formula. The United States receives, in full, all water that reaches the river from named U.S. tributaries: the Pecos and Devils Rivers, Goodenough Spring, and Alamito, Terlingua, San Felipe, and Pinto Creeks.⁴

The United States also receives one-third of the flow reaching the main channel from six named Mexican tributaries: the Conchos, San Diego, San Rodrigo, Escondido, and Salado Rivers, plus the Las Vacas Arroyo. That one-third must average at least 350,000 acre-feet per year over each five-year cycle. If the one-third share happens to exceed 350,000 acre-feet in a cycle, the United States keeps the larger amount, but it cannot claim more than one-third of those flows.⁴ The United States additionally receives half of unallocated flows in the main channel, including water from unmeasured tributaries between Fort Quitman and the lowest major international storage dam.⁵

Mexico keeps the remaining two-thirds of the six named Mexican tributaries, all flows from any Mexican tributaries not on the U.S. list, and half of the unallocated main-channel flows. Because the Conchos is by far the largest Mexican tributary, its flows drive most of the accounting, and drought in the Conchos basin is what typically puts Mexico behind on its 350,000 acre-foot obligation.

The Five-Year Cycle and Current Shortfalls

The five-year cycle is the treaty’s flexibility mechanism. Mexico does not have to deliver exactly 350,000 acre-feet each year. It has to deliver a cumulative 1,750,000 acre-feet over the cycle, so a wet year can make up for a dry one. If Mexico is short at the end of the cycle, the deficit rolls into the next one.¹

Mexico has accumulated water debts in multiple cycles, particularly during extended droughts in northern Mexico. In December 2025, the USDA announced that Mexico had agreed to release 202,000 acre-feet to address shortfalls, with deliveries beginning the week of December 15, 2025, and the two governments began negotiating a plan for full repayment of the outstanding deficit from the previous cycle.⁶ IBWC Minute 331, signed in November 2024, established measures aimed at improving the reliability and predictability of Rio Grande deliveries.⁷

How the Colorado River Is Divided

Article 10 gives Mexico a guaranteed annual quantity of 1,500,000 acre-feet from the Colorado River, delivered at the international boundary or at agreed diversion points such as Morelos Dam.¹ This is a firm obligation, and the U.S. must operate its reservoir system, including Lake Mead and Lake Powell, to meet it.

When the U.S. Section of the IBWC determines that a surplus exists beyond U.S. needs and Mexico’s guaranteed volume, Mexico may take additional water up to a total of 1,700,000 acre-feet in that year. Mexico acquires no permanent right to that extra 200,000 acre-feet; the surplus provision resets each year.¹

Drought Reductions

The treaty has its own drought clause. If extraordinary drought or a serious accident to the U.S. irrigation system makes it difficult to deliver the full 1,500,000 acre-feet, Mexico’s allotment drops in the same proportion as consumptive uses are reduced within the United States.⁴ Both countries share the shortfall equally on a percentage basis. The treaty does not define “extraordinary drought,” which is why later Minutes have spelled out specific reservoir-elevation triggers.

How the Tijuana River Is Handled

The Tijuana River is treated differently from the other two rivers. Rather than a fixed allocation formula, Article 16 sets up a study-and-recommend framework: the IBWC investigates the system and submits recommendations to both governments on equitable water distribution, storage and flood control, cost estimates, and division of operation and maintenance responsibilities.¹ Both governments must approve any plan before construction, and they split costs of jointly operated infrastructure equally.

In practice, the dominant issue in the Tijuana River region has been transboundary sewage, not water allocation. Under IBWC Minute 333, signed in December 2025, the two countries agreed to a new wastewater treatment plant in Tijuana, sediment removal from the Tijuana River channel, and a sediment basin in Matadero Canyon.⁸ Mexico has obligated $59 million in state and federal funding for border sewage infrastructure in 2026, above its $46 million commitment under a July 2025 memorandum of understanding.⁹

The IBWC and the Minutes System

Article 2 gave the International Boundary and Water Commission the status of an international body, with a U.S. Section and a Mexican Section, each headed by a Commissioner appointed by its own government and funded separately.⁵ The IBWC operates and maintains international dams and flood control infrastructure, resolves disputes over treaty interpretation, and coordinates physical delivery of water. In U.S. law, the treaty is cited as 59 Stat. 1219 (Treaty Series 994), and Congress has declared that satisfying Mexico’s Colorado River allotment is a national obligation.¹⁰

The treaty’s most important adaptive feature is the IBWC “Minutes” process. Minutes are formal, binding agreements signed by both Commissioners that supplement the treaty without requiring renegotiation. They function as operational amendments, letting the two countries address conditions the 1944 drafters could not have anticipated. As of early 2026, the IBWC has issued 333 Minutes covering topics from dam safety to environmental restoration to drought contingency planning.⁷

Water Quality and the Salinity Fix

The 1944 treaty guaranteed Mexico a volume of Colorado River water but said nothing about its quality. By the 1960s, agricultural drainage from the Wellton-Mohawk Irrigation District in Arizona had raised the salinity of water arriving in Mexico to levels that damaged crops, producing a serious diplomatic dispute.

The fix came through IBWC Minute 242, adopted in 1973. It set a permanent salinity standard: the roughly 1,360,000 acre-feet of Colorado River 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, U.S. count) above the salinity of water arriving at Imperial Dam.¹¹ Water delivered through the All-American Canal counts as if delivered upstream of Morelos Dam for that calculation.

Modern Drought Agreements on the Colorado

Since the early 2000s, the Colorado River basin has been in a prolonged drought that pushed Lake Mead and Lake Powell to historic lows. The treaty’s original vague drought clause proved insufficient, and the Minutes process filled the gap.

Minute 319, signed in 2012, ran through 2017 and was the first comprehensive binational agreement on Colorado River drought. It tied delivery reductions to reservoir conditions, created a mechanism called Intentionally Created Mexican Allocation (ICMA) that lets Mexico defer part of its allotment for later use, and allowed Mexico to store water in U.S. reservoirs.¹²

Minute 323, signed in 2017, extended and expanded those measures through December 31, 2026. It set specific shortage-sharing tiers keyed to Lake Mead’s elevation. When Lake Mead falls below 1,090 feet, Mexico reduces its annual deliveries on a sliding scale: 41,000 acre-feet at elevations between 1,090 and 1,075 feet, 70,000 acre-feet below 1,075, 100,000 acre-feet below 1,050, and 124,000 acre-feet below 1,045 feet. Minute 323 also committed the United States to provide $31.5 million for water conservation projects in Mexico, with resulting savings split evenly between returning water to the Colorado River system and staying with Mexico.¹³ With Minute 323 expiring at the end of 2026, negotiations on a successor agreement are underway.

What the Treaty Does Not Cover

The 1944 treaty governs surface water only. Transboundary aquifers, the groundwater resources that cross the border, have no formal recognition or regulation at the binational level. Researchers have identified as many as 72 shared aquifer systems, and none is subject to an IBWC Minute or any other binding agreement. Pumping on one side can deplete supplies on the other, and as of 2026, no binding binational instrument addresses that gap.

1
International Boundary and Water Commission. Treaty Series 994 Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande
2
International Boundary and Water Commission. Treaties Between the U.S. and Mexico
3
International Boundary and Water Commission. Treaties Between the U.S. and Mexico – Section: Treaty of February 3, 1944
4
U.S. Bureau of Reclamation. The Mexican Water Treaty of 1944
5
FAOLEX. Treaty Between the United States of America and Mexico Signed at Washington February 3, 1944
6
U.S. Department of Agriculture. Mexico Agrees to Meet Water Treaty Obligations for Farmers in the American Southwest
7
International Boundary and Water Commission. Minutes Between the United States and Mexican Sections of the IBWC
8
International Boundary and Water Commission. Commission News Brief 2026 Winter Edition
9
U.S. Environmental Protection Agency. USMCA Tijuana River Watershed Public Updates
10
Office of the Law Revision Counsel. 43 U.S. Code 1512 – Mexican Water Treaty
11
International Boundary and Water Commission. Minute 242 – Permanent and Definitive Solution to the International Problem of the Salinity of the Colorado River
12
U.S. Bureau of Reclamation. Minute 319
13
International Boundary and Water Commission. Minute 323 – Extension of Cooperative Measures and Adoption of a Binational Water Scarcity Contingency Plan in the Colorado River Basin