2.1. The Rio Grande/Bravo Basin (RGB)
The U.S. and Mexico share a nearly 3200 km long border that crosses the Colorado River (CR), the Tijuana River (TR), and the Rio Grande/Bravo (RGB). The Rio Grande River is longest of these rivers and forms the international boundary for 2034 km (Figure 1
The Rio Grande, as it is known in the U.S., or the Río Bravo, as it is called in Mexico, is the 20th largest river in the world, with a total length of 3059 km. It carries less water than rivers of similar size because it runs through arid areas for most of its length (the mean flow is 25 m3
/s at Matamoros, 1955–2006 [1
]; by comparison, the Danube, River, a river of similar length, has a mean flow of 6486 m3
/s just before the delta [2
]). Higher precipitation only characterizes its headwater (>800 mm) and mouth regions (650 mm). The river basin covers an area of 924,300 km2
, divided almost in half between the two countries.
The headwaters of the RGB are located in the U.S. state of Colorado. The river flows south through New Mexico (NM) arriving well south of Albuquerque at two major reservoirs on its course, Elephant Butte (EB) and the Caballo dams. Downstream of these, the flow almost entirely depends on releases from the reservoirs. Starting from the cities of Ciudad Juarez (Chihuahua, Mexico) and El Paso (Texas, USA?) until its mouth, the RGB forms the international border between Texas and the Mexican states of Chihuahua, Coahuila, Nuevo León and Tamaulipas. Due to the semi-arid climate and the extensive withdrawal of water for irrigation, the flow of the river in the section downstream of Ciudad Juarez/El Paso is severely depleted and is aptly called the Forgotten Reach
. The Rio Conchos, in the Mexican state of Chihuahua, replenishes the RGB which then flows through the ecologically unique, mountainous area of the Big Bend Reach, protected in the form of several national parks. The Pecos River, flowing through the U.S. states of NM and Texas, enters the RGB at the Amistad Reservoir. Downstream in the vicinity of the twin cities of Laredo, Texas and Laredo, Tamaulipas, Falcon Reservoir constitutes the primary water storage and supply structure on the river’s lower reach. The river’s flow in these lower parts is continuous thanks to the more humid, subtropical climate, tributaries’ inflows, and agricultural return flows to the river. The RGB then joins the Gulf of Mexico [3
The RGB Basin is characterized by a low level of rainfall and a high load of pollutants, leading to a dual dilemma of water scarcity and diminished water quality. These twin problems, exacerbated by population and economic growth, drive competition between different users and degrade the riparian ecosystem [6
2.1.1. Water Scarcity
The RGB is a water scarce basin with an average precipitation greater than 800 mm in less than 5% of the RGB basin’s area. While the headwaters are fed by snowmelt from the San Juan mountains in Colorado, most of the basin is located in the Chihuahuan desert, with an average precipitation of less than 500 mm [12
]. Scarce precipitation leads to limited water availability. From this limited supply, significant withdrawals for human utilization are made basinwide. Large irrigation withdrawals in Colorado and New Mexico have greatly reduced the streamflow in the RGB mainstem, historically [13
], and currently [14
]. Historic increased water diversions along the border have also diminished the streamflow in the RGB mainstem, making the RGB basin one of the top 10 rivers at risk in 2007 [15
]. The situation is sufficiently critical that some river sections are at times completely dry. No water flows through the Forgotten Reach and the river is barren for the six-month non-irrigation season near Las Cruces in NM.
To date, no binational measures have been taken to sustain instream flows for environmental purposes even though some studies have demonstrated the hydrologic and economic feasibility of doing so in portions of the RGB mainsteam, namely in the Big Bend reach [16
]. However, a first step towards ensuring a healthy river was taken by the U.S. state of Texas as it adopted environmental flow standards in 2014 [19
2.1.2. Water Quality
A considerable array of water quality issues are found on the river and in many sections water quality is seriously degraded. Here, we provide a brief overview of these. The greatest and most widespread problems are fecal contamination and high salt levels [20
]. Fecal contamination carries pathogens and originates from municipal wastewater discharges as well as livestock and wildlife manure. It is mostly prevalent in and downstream of large urban centers. High salt levels (dissolved solids), including chloride and sulphate, originate principally from irrigation return waters and present challenges for the aquatic life as well as human uses such as municipal supply and crop cultivation [21
]. Nutrients originate from fertilizers and wastewater discharges. Higher nutrient levels are found in several sections and cause eutrophication. Ammonia, a potentially toxic nutrient, is also found. High salt, depressed oxygen and high nutrient concentrations may be associated with blooms of toxic golden alga (e.g., Prymnesium parvum
), leading to extensive fish kills [21
]. Additionally, agrochemicals and other industrial organic compounds as well as several metals (e.g., copper, nickel, zinc, mercury, and arsenic) have been detected in surface waters, sediments and in some cases even in edible fish tissue. These compounds and elements are all harmful for aquatic life and human health [22
The above described problems of water quantity and water quality are inherently linked. On one hand, small volumes of receiving waters impede the river’s natural cleaning processes, or recycling of nutrients. On the other hand, if the available water is highly polluted, it is not suitable for municipal or agricultural usage, or only at much higher treatment costs.
2.1.3. Ecosystem Degradation
Channelization and dams coupled with pollution have greatly altered the natural water regime with adverse impacts on the local ecosystems, both riparian and aquatic. Several native species are endangered, such as the silvery minnow (Hybognathus amarus
) and the southwestern willow flycatcher (Empidonax traillii extimus
). At the same time, exotic, invasive species dominate several areas, such as the salt cedar (Tamarix ramosissima
) or the giant cane (Arundo donax
) on the banks of the upper stretches to the Big Bend area or the water hyacinth and the hydrilla in waters downstream of the International Amistad Reservoir [21
]. With such a degraded environment, the RGB Basin’s capacity to provide ecosystem services [23
], such as nutrient cycling and sustaining fish stocks, has been greatly compromised.
2.1.4. What Is Fuelling These Problems?
The region has seen a rapid population and economic growth in the past decades [4
] (pp. 159–177). Today, 14 million people live in the southwest of the U.S. and the north of Mexico, predominantly in the large, so-called sister cities
: San Diego–Tijuana; Calexico–Mexicali; Yuma–San Luis; Nogales–Nogales; Naco–Naco; Douglas–Agua Prieta; Columbus–Puerto Palomas; El Paso–Sunland Park–Ciudad Juárez–Ysleta del Sur Pueblo; Presidio–Ojinaga; Del Río–Ciudad Acuña; Eagle Pass–Piedras Negras; Laredo–Nuevo Laredo; McAllen–Reynosa; Weslaco–Rio Bravo; Brownsville–Matamoros [24
]. In order to supply the needs of this population, the largest quantities of water are allocated to irrigation as the first priority, and to municipal water supply as the second. The number of irrigation and municipal facilities has rapidly increased without real coordination among the users and uses of water [6
Municipal wastewater discharges, irrigation return flows and industrial discharges are the key sources of pollution. Infrastructure development has lagged behind the rapid growth of the region. Adequate treatment of wastewaters to significantly reduce pollution loads is available at some municipalities, while other border settlements lack these facilities [21
]. Water scarcity means irrigation water is repeatedly reused, enhancing salt loads, especially in the western part of the basin. This growth is partially driven by more than 3000 assembly plants, so-called maquiladoras
, that increase the potential for toxic contamination [21
, impoverished rural border settlements in the U.S., are home to 432,000 people in Texas and NM [26
] and represent a special case. With little or no infrastructure, they contribute to water pollution. At the same time, they are highly exposed to water quality issues as they rely on fish and untreated water for human consumption [3
2.2. The Legal Instruments Shaping the Binational Institutional Framework for Water Allocation
The RGB water allocation legal framework is based on international agreements
between the U.S. and Mexico and compacts
at the national level among the U.S. riparian states. As an ensemble, these instruments translate into a set of formal rules regulating stakeholder behavior, and facilitating cooperation [27
]. In the case of international agreements
, both federal states have the authority to enter into agreement. Compacts
are provided for in Article I, section 10, clause 3 of the U.S. Constitution, which establishes the basis for negotiating these interstate agreements: “No state shall, without the consent of Congress […] enter into any agreement or compact with another state or with a foreign power […]” [28
]. Taking into consideration that the first step for a compact
is the Congress authorization, a compact
has been defined in American Western water right law as “an agreement between two or more states dealing with competing demands for a water resource beyond the legal authority of one state alone to solve” [29
]. A compact
may regulate surface and/or groundwater flows. These legal instruments across the U.S. portion of the RGB Basin regulate surface water and provide a framework to manage problems resulting from stakeholder interdependencies [3
] (p. 88).
The RGB legal framework is composed of two federated states’ compacts (the Rio Grande Compact (RGC) and the Pecos River Compact (PRC)), and two binational agreements (the 1906 Convention, and the 1944 Water Treaty), which aim to regulate, control, and manage water resources allocation among the signatories. Figure 2
provides a view on the legal instruments regulating water resources in the RGB Basin.
Each of these legal instruments is discussed here in chronological sequence. The Convention between the United States and Mexico providing for the equitable distribution of the waters of the Rio Grande for irrigation purposes (the 1906 Convention) is an international legal instrument which defines the amount of water to be delivered by the U.S. to Mexico for the primary purpose of irrigation [31
]. The 1906 Convention envisions the distribution of surface waters of the RGB basin, within the international segment of the river located between El Paso, Ciudad Juarez and Fort Quitman [3
]. The U.S. must deliver a total of 60,000 acre-feet/year (74 million m3
/year) to Mexico at the diversion point called Acequia Madre, located close to Ciudad Juarez, Mexico. However, in case of drought, the amount of 60,000 acre-feet/year can be proportionally reduced according to the specific conditions.
The second instrument is the Rio Grande Compact (RGC)
]. Signed in 1929 and revised in 1939, the Compact provides for the equitable interstate apportionment [35
] of the Rio Grande waters—at a level intended to protect water use as it existed from 1928 to 1937—between the U.S. states of Colorado, New Mexico and Texas [36
]. The RGC allocates water among the three states after the Mexican 1906 allocation is satisfied. It permits an average normal release from Elephant Butte Reservoir of 790,000 acre-feet/year (974 million m3
/year) for use on lands in New Mexico downstream of Elephant Butte Reservoir and on lands in Texas and to comply with the obligations of the 1906 Convention [37
]. This release is primarily for irrigation purposes [38
]. The RGC provides for debts and credits to be carried over from year to year until relinquished under the provisions of this agreement for the U.S. states [3
] (p. 89).
The third instrument is the Treaty between the United States of America and the United Mexican States relating to the utilization of the waters of the Colorado and Tijuana Rivers, and of the Rio Grande (Rio Bravo) known widely as the 1944 Water Treaty [39
]. Signed in 1944, and ratified by each country in 1945, the 1944 Treaty aims “to obtain the most complete and satisfactory utilization of shared waters” [33
] based on the equitable distribution between the two countries of the waters of shared river systems. The 1944 Treaty establishes RGB water allocations for the U.S. and Mexico and joint use of its international waters. The Treaty also authorized the construction and operation of two reservoirs, Amistad and Falcon, along the mainstem of the RGB. The Treaty allocates one-third of the water reaching the RGB mainstem from 6 tributaries originating in Mexico to the U.S. and two-thirds to Mexico. The U.S. third shall not be less than 350,000 acre-feet/year (432 million m3
/year), calculated as an average over a treaty cycle of five consecutive years [33
]. Under the 1944 Treaty, the International Boundary Commission (IBC), established in 1889 to rectify and maintain the riparian boundary of the Rio Grande and Colorado Rivers [40
], became the International Boundary Water Commission (IBWC, hereinafter called the Commission), with specific mandates on shared water resources [41
] (pp. 72–75). The Treaty establishes the Commission’s authority as an international body and endows it with considerable flexibility in addressing extant and emerging issues through the Minute procedure [33
]. The Treaty specifies that if “there are provisions in this Treaty for joint action or joint agreement by the two Governments […] the particular matter in question shall be handled by or through the Department of State of the U.S. and the Ministry of Foreign Relations of Mexico” [33
] using the Minute process (presented in Section 2.3
) and taking advantage of its role in supporting continuing and emerging issues not explicitly included in the Treaty. This provides for a project or action in the Treaty to be carried out through the foreign ministries—in effect the IBWC—and not some other agency pertaining to the governments.
Finally, the water of the Pecos River, the largest U.S. tributary of the RGB, is allocated between New Mexico and Texas through the Pecos River Compact
(PRC) signed in 1948 [3
]. Its purpose is to promote inter-state collaboration and remove the causes of current and future water resources controversies. Both the RGC and the PRC aim to promote development within the U.S. states and facilitate the construction of infrastructure for the recovery of water, its effective use, and protection against floods [34
It is important to note that the scope and the jurisdiction of the two binational instruments between the U.S. and Mexico, the 1906 Convention and the 1944 Treaty, do not overlap. This is a relevant aspect, which ensures the accurate implementation of both treaties without conflicts of law. However, as opposed to the 1906 convention that solely addresses water quantity, the 1944 Treaty also addresses transboundary water sanitation problems.
A common feature of the aforementioned agreements is their failure to address environmental objectives associated with joint management of surface and groundwater, and yet this water governance framework has the potential to address inappropriate or uncontrolled water uses, conflicts among water users, and damages to the environment if both countries choose to adapt the treaties and compacts to better address current river basin challenges.