How to share water has long been a complex issue for the U.S.-Mexican border region, and it has been the cause of both tension and cooperation in the broader U.S.-Mexican relationship. The two countries share a nearly 2,000-mile border. Multiple rivers cross or form the border at various points. The principal shared river basins are the Colorado River and the Rio Grande. This report is a primer on U.S. and Mexican water sharing. It focuses on the sharing of surface waters. After presenting an introduction to U.S.-Mexican water sharing, this report describes the legal obligations and processes under the treaty on Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande (hereinafter Treaty or 1944 Water Treaty). It then describes developments in the Colorado River and Rio Grande basins. In the 19 th century, questions arose regarding the boundaries between the two countries and the sharing of international rivers. Early agreements, starting in 1848, sought to clarify the location of the border. The two countries entered into the Convention of March 1, 1889, to apply agreements related to the nations' borders that were defined by international waters. Starting in 1906, binational water agreements began to emerge; a 1906 convention on the sharing of Rio Grande for irrigation purposes distributed water in the vicinity of El Paso, TX. In 1944, the two countries entered into the 1944 Water Treaty. The Treaty designated the International Boundary and Water Commission (IBWC) responsible for facilitating resolution of issues arising during application of U.S.-Mexican treaties. The two principal rivers shared by the U.S. and Mexico are the Colorado River, which is predominantly in the United States and crosses the Mexican border on its way to the Gulf of California ( Figure 1 ), and the Rio Grande, with major tributaries in the United States and Mexico and whose riverbed is the U.S.-Mexican border in Texas ( Figure 2 ). Water stored in aquifers generally is not addressed by the U.S.-Mexican binational water-sharing agreements. Groundwater may play a significant role in water supply in some parts of the U.S.-Mexican border, especially during dry conditions. For more on shared groundwater, see the box "Border Aquifers Are Largely Not Addressed by Binational Agreements." The 1944 Water Treaty establishes water allocations for the United States and Mexico and creates the current governance framework for the IBWC to resolve disputes arising from the Treaty's execution. In April 1945, the Senate provided its advice and consent to the Treaty, subject to certain "understandings." Similar to treaty reservations and declarations, understandings are interpretative statements made by the Senate that clarify or elaborate on provisions in a treaty but do not alter them. In November 1945, President Truman ratified the 1944 Water Treaty, subject to the Senate's understandings. In March 1, 1889, the United States and Mexico signed the convention that established the International Boundary Commission (IBC) to apply border agreements. The 1944 Water Treaty reconfigured the IBC and expanded its role, creating the International Boundary and Water Commission (IBWC). The IBWC is charged with addressing issues that arise during application of the boundary and water treaties. The IBWC is an international body consisting of U.S. and Mexican Sections, each led by a commissioner, two principal engineers, a legal adviser, and a foreign affairs secretary. The U.S. Section of the IBWC (USIBWC) is headquartered in El Paso, TX, and the Mexican Section is located across the border in Ciudad Juárez. The USIBWC is a federal government agency that operates under the foreign policy guidance of the Department of State, and it is included in the Department of State's budget. The commissioner to the USIBWC is appointed by the President. The position historically has not been subject to Senate confirmation, and the tenure of U.S. commissioners has ranged from a few months to 27 years. IBWC funding typically is provided through annual Department of State, Foreign Operations, and Related Programs appropriations. A 2005 report from the Office of Inspector General at the Department of State concluded that the IBWC has resolved most border water disputes since 1944, although its processes may be slow to reach resolution. The IBWC employs a combination of technical expertise and diplomacy (backed by the U.S. Department of State and Mexico's Foreign Ministry) to find solutions and agreement, which are formalized in amendments to the 1944 Water Treaty called minutes . For the United States, the executive branch has the authority to approve or disapprove the proposed minutes to the 1944 Water Treaty. The Treaty authorizes the IBWC to develop rules and to issue proposed decisions, called minutes, regarding matters related to the Treaty's execution and interpretation. Once issued, a proposed minute is forwarded within three days to the government of each country for approval. If neither country announces its disapproval within 30 days, the minute is considered adopted. If either government disapproves, the matter is removed from IBWC control and the two governments negotiate the issue. If an agreement is reached between the governments following negotiation, the IBWC must take any further actions "as may be necessary to carry out such agreement." The Department of State is the U.S. agency responsible for responding to proposed minutes and attempting to negotiate a resolution if either government disapproves. Minutes that have been adopted pursuant to the 1944 Water Treaty have addressed a range of issues, including the operation and maintenance of cross-border sanitation plants, water conveyance during droughts, construction of dams, water salinity problems, and many others. In many cases, minutes are adopted (or not objected to) by the Secretary of State without involvement from Congress or the Senate. Because these minutes are agreed to by the executive branch pursuant to the authority of the 1944 Water Treaty, they are considered binding agreements between the United States and Mexico called executive agreements, and the power to enter into them lies within the executive branch. In providing its consent to 1944 Water Treaty, however, the Senate included an understanding that the IBWC and the Secretary of State could not commit the United States to build any "works" at the United States' expense without prior approval by Congress. Accordingly, Congress has passed legislation authorizing the construction of public works and projects pursuant to the Treaty. The 1944 Water Treaty defines the basic water distribution arrangements as follows: For the Colorado River basin, the United States is to provide Mexico annually with 1.5 million acre-feet (AF) of water. For the Rio Grande basin below Fort Quitman, TX: Mexico has the rights to two-thirds of the flows that feed into the Rio Grande from the six major tributaries that enter from Mexico: the Conchos, San Diego, San Rodrigo, Escondido, and Salado Rivers and the Las Vacas Arroyo (stream). the United States receives all flows from Rio Grande tributaries in the United States and one-third of flows from the six Mexican tributaries. Mexico's water delivery from these six tributaries must average at least 350,000 AF per year, measured in five-year cycles. If Mexico fails to meet its minimum flow obligations for a five-year cycle because of "extraordinary drought"—a term not defined in the 1944 Water Treaty or in any minute—it must make up the deficiency during the next five-year cycle with water from the Mexican tributaries. Minute 234 established that Mexico may repay a water debt using three sources of water: (1) excess water from its tributaries; (2) a portion of its allotment from its tributaries; or (3) a transfer of its stored water in international reservoirs, such as the Falcon Dam and Amistad Dam, located on the Rio Grande on the border of Texas and Mexico. If Mexico fails to meet its minimum flow obligations for a five-year cycle and there is no agreement that an extraordinary drought existed, Article 24(d) of the 1944 Water Treaty provides certain mechanisms for dispute settlement. First, the IBWC has the authority "to settle all differences that may arise between the two Governments with respect to ... application of the Treaty, subject to the approval of the two Governments." However, if the commissioners are unable to reach agreement on a dispute, the dispute is to be settled through diplomatic channels between the United States and Mexico. Article 24 also provides that the countries may seek recourse in any "general or special agreements which the two Governments have concluded for the settlement of controversies." However, it does not appear that these mechanisms have been necessary in the past. As discussed throughout this report, the United States and Mexico previously have used the minute process and diplomatic efforts to reconcile disputes regarding the 1944 Water Treaty. Article 9 of the 1944 Water Treaty provides the IBWC with some flexibility regarding the diversion of water from the Rio Grande. For example, in cases of extraordinary drought occurring in one of the countries, the IBWC may permit water to be withdrawn from the other country in order to help alleviate drought conditions. Further, the IBWC may allow one country to use water allocated to the other country if it can be done "without injury to the latter and can be replaced at some other point on the river." However, if the IBWC authorizes temporary diversions of water from one country to another, the use of such water does not establish a permanent right to divert. Under Article 9, the IBWC also is responsible for keeping records concerning the water belonging to both Mexico and the United States. The 1944 Water Treaty establishes a hierarchy of uses for the water: (1) domestic and municipal uses; (2) agriculture and stock-raising; (3) electric power; (4) other industrial uses; (5) navigation; (6) fishing and hunting; and (7) any other beneficial uses which may be determined by the commission. A frequent critique of this hierarchy is that it does not include an obligation to maintain water for ecological purposes.