A Regal River Runs Dry

Rising from the snow-capped San Juan Range of Colorado’s Rocky Mountains, the Rio Grande River descends through New Mexico, nips the western tip of Texas, then snakes eastward, marking the Mexico–US border for hundreds of miles before finally emptying into the Gulf of Mexico. In the sixteenth century, Spanish explorers christened the Rio Grande “the Fierce River of the North” (El Rio Bravo del Norte) out of respect for the turbulent cataracts in its upper reaches. These days, however, the once-regal river has been dethroned by a confluence of forces. Explosive population growth, increasing demand for irrigation and industrial uses, a decades-long “mega-drought,” and rising temperatures attributed to climate change have conspired to dramatically diminish the river’s flow. In June 2021, observers watched with horror as New Mexico’s Elephant Butte Reservoir, which had brimmed over during the 1980s and 1990s, dropped to a paltry 5 percent of its full capacity. Other reservoirs that feed the Rio Grande have experienced similar distress due to rising temperatures and growing demand, forcing regulators in Mexico and the United States to adopt extreme rationing policies that restrict the lower basin to about 2 percent of its historic annual flow. Residents of Albuquerque, El Paso, and Ciudad Juarez have seen the Rio Grande run dry for longer periods each year, its mirrored surface replaced with a parched sandbar.Footnote ¹

Dwindling fresh water threatens vital interests on both sides of the Mexico–US border. Over six million people rely on the Rio Grande and its tributaries for agriculture, industry, and municipal uses, and population centers in the region continue to expand. In the past, farmers in southern New Mexico and El Paso County could count on forty inches of water to irrigate their onions, corn, and green peppers; by 2021, some received as little as four inches. Across the border, Mexican pecan growers have complained likewise that water rationing is imperiling their groves.Footnote ² Less water to go around means fewer acres of arable farmland, shorter growing seasons, and mounting pressure to transition to more drought-resistant (but generally less lucrative) crops – all of which equates to fewer jobs and diminishing profits. Meanwhile, ecologists have expressed concern that hydroelectric dams are imperiling the basin’s flora and fauna. Hundreds of native plants and animals have been decimated by the region’s changing ecology, including the endangered silvery minnow and cottonwood trees that have evolved to rely on the basin’s natural hydrological cycle.Footnote ³ To make matters worse, scientists predict that current drought conditions are likely to get worse over time, as climate change continues to dry out the Rio Grande basin while inundating other regions with floods and hurricanes.Footnote ⁴ As the fresh water crisis in the Rio Grande Basin has dragged on, regulators on both sides of the border have confronted difficult tradeoffs regarding the competing interests of farmers, industrialists, and ordinary residents, as well as the moral imperative to practice prudent stewardship of the natural environment.

If these domestic policy dilemmas were not daunting enough, the crisis at the Rio Grande also poses a serious diplomatic challenge. Because the river passes through and collects tributary water from both Mexico and the United States, both states have legitimate claims to the river’s limited flow. This clash of national interests might appear to be a recipe for intractable conflict or hegemonic domination. Yet, remarkably, the opposite is true: for well over a century, the Rio Grande has emerged as a success story for international cooperation. Even as climate change has depleted reservoirs, raising the political and economic stakes on both sides of the border, thus far a spirit of cooperation and compromise has sustained the bilateral partnership.Footnote ⁵

This chapter tells the story of how cooperation has flourished at the Rio Grande in the face of formidable challenges. International law plays a central role in this history. By the middle of the twentieth century, international courts and tribunals were promoting cooperation in disputes over transboundary rivers by decisively rejecting the absolutist conception of national sovereignty. They declared that general principles of equity required riparian states to cooperate with one another to reconcile conflicts between their sovereign rights. These developments in international law greased the wheels for international cooperation over transboundary rivers, helping to avert and defuse conflicts during an era when demand for fresh water was increasing exponentially.

Abandoning Absolute Sovereignty

The Rio Grande is one of 276 transboundary rivers that collectively serve 2.8 billion people – approximately 40 percent of the world’s population. Transboundary river basins, also known as “international watercourses,”Footnote ⁶ occupy nearly half of the earth’s land surface and account for roughly 60 percent of global river flow.Footnote ⁷ The fresh water that flows through these rivers is vital to human survival and economic development, facilitating agriculture, industry, transportation, electricity generation, and other uses.

Transboundary rivers have been at the center of international disputes for millennia. The earliest recorded international treaty addresses tensions between two Ancient Sumerian city-states over the Euphrates River in approximately 2,500 BCE. Since then, political communities have concluded thousands of agreements on the use and management of transboundary rivers.Footnote ⁸ Disputes over these agreements have also led to international arbitration and adjudication, including an advisory opinion and two judgments in contentious cases from the Permanent Court of International Justice (PCIJ), the first international tribunal with general jurisdiction (i.e., authority to hear any kind of case brought before it that is amenable to international adjudication).Footnote ⁹

Notwithstanding the importance of transboundary rivers to human flourishing, the international legal norms governing them were unsettled at the turn of the twentieth century. This uncertainty was attributable in part to a paradox at the heart of the then-prevailing theory of absolute sovereignty. Recall from Chapter 1 that Grotius conceptualized states as possessing exclusive authority and proprietary interests over their own sovereign territory, as well as the prerogative to judge the legality of their own actions for themselves. In disputes over transboundary rivers, upstream states invoked the absolutist conception of territorial sovereignty to support their position that they could use transboundary rivers however they pleased, including by diverting water for domestic irrigation. But downstream states also relied on territorial sovereignty to defend their national interests. In their view, sovereignty entailed the right to absolute territorial integrity, including the unimpaired natural flow of fresh water from upstream sources. How to choose between these dueling theories of sovereign rights – known today as “absolute territorial sovereignty” and “absolute territorial integrity” – was by no means obvious under the prevailing Grotian theory of sovereignty. Consequently, upstream and downstream riparians could each plausibly claim that international law authorized them to dictate how a transboundary river running through their borders should be utilized.

A nineteenth-century dispute between Mexico and the United States over the Rio Grande illustrates this dilemma.Footnote ¹⁰ In July 1848, the United States and Mexico concluded the Mexican-American war by signing the Treaty of Guadalupe Hidalgo.Footnote ¹¹ The treaty designated the deepest channel of the Rio Grande as the boundary line between the two countries for 1,255 miles, and it prohibited both parties from constructing “any work that may impede or interrupt” navigation along this section of the river without the other party’s consent.Footnote ¹² As time wore on, diversions of the river by American farmers depleted the Rio Grande to the point that Mexico felt constrained to file a formal diplomatic protest. In October 1894, Mexico’s chief representative to the United States, Matías Romero, sent a note to US Secretary of State Walter Q. Gresham asserting the “urgent necessity” to address the United States’ “taking of water from the Rio Bravo (Rio Grande) del Norte, which has so seriously affected the existence of the frontier communities for several miles below [Ciudad Juárez]” that they “may be annihilated.” Romero argued that further discussions between the countries should focus on maintaining “the equitable division of the waters of the river.”Footnote ¹³

Responding two weeks later, Gresham disputed Romero’s factual assertion that American irrigation – rather than natural drought – had diminished the Rio Grande’s flow.Footnote ¹⁴ At Gresham’s request, US Attorney General Judson Harmon provided a legal opinion that vigorously defended the United States’ sovereign right to use the Rio Grande as it saw fit, without regard to Mexico’s rights and interests. Harmon explained that the Treaty of Guadalupe Hidalgo did not prohibit the United States from diverting water for irrigation, as long as these diversions did not impede navigation. Turning next to principles of general international law, Harmon emphasized that the “fundamental principle of international law is the absolute sovereignty of every nation, as against all others, within its own territory.”Footnote ¹⁵ He then quoted a passage from Chief Justice John Marshall’s opinion for the US Supreme Court in Schooner Exchange v. McFaddon:

In Harmon’s view, the United States’ prerogative to use as much of the Rio Grande as it wished reflected two basic principles of international law: (1) the “necessarily exclusive and absolute” nature of its territorial sovereignty, and (2) the voluntarist principle (later associated with The Lotus Case) that any limitations on state sovereignty must be attributable to state consent. Thus, because the United States had not consented to limit its use of the Rio Grande for irrigation, it was not violating international law.

Harmon’s defense of absolute territorial sovereignty – the so-called “Harmon Doctrine” – supplied legal cover at a moment when the United States was anxious to preserve existing irrigation works and parry Mexican claims for monetary reparations. The opinion might have advanced these objectives, but its legal analysis was highly problematic from the start. If every state had “exclusive and absolute” jurisdiction over its own territory, as Harmon supposed, then what about Mexico’s territorial sovereignty? Surely Mexico could also claim “full and complete” authority to ensure its own self-preservation by guaranteeing the Rio Grande’s uninterrupted flow through its territory? Allowing the United States to divert the river without Mexico’s consent would, in the Chief Justice’s words, “imply a diminution of [Mexico’s] sovereignty to the extent of that restriction, and an investment of that sovereignty to the same extent in [the United States]” – a result antithetical to Mexico’s sovereignty. Yet, how could Mexico and the United States both exercise “exclusive and absolute” jurisdiction over the Rio Grande’s flow? The only way to resolve this dilemma without violating the territorial sovereignty of either state would be for Mexico and the United States to either negotiate an agreement or submit to international arbitration.

In fact, this is precisely what happened. Well before Harmon weighed in with his now-infamous opinion, the US State Department pursued negotiations with Mexico.Footnote ¹⁷ US President Grover Cleveland delivered an address to Congress in which he argued that tensions over “the storage and use of the waters of the Rio Grande for irrigation should be solved by appropriate concurrent action of the two interested countries.”Footnote ¹⁸ Congress apparently agreed. Expressing concern that US irrigation practices were “rendering the lands [downstream] arid and unproductive, to the great detriment of the citizens of the two countries who live along its course,” Congress adopted a joint resolution in 1890 urging the President to pursue negotiations with Mexico to resolve the issue.Footnote ¹⁹ Thus, in stark contrast to Harmon’s legal opinion, US foreign policy throughout this period demonstrated respect for Mexico’s sovereign rights and interests and a commitment to resolve the issue through bilateral cooperation.

Rather than proceed unilaterally, the two governments asked the International Boundary Commission, which they had established in 1889, to evaluate the facts on the ground and help them determine “the best and most feasible mode” of regulating the Rio Grande “so as to secure to each country concerned and to its inhabitants their legal and equitable rights in said waters.”Footnote ²⁰ Guided by the Commission’s recommendations, Mexico and the United States concluded a treaty in 1906 “to provide for the equitable distribution of the waters of the Rio Grande for irrigation purposes, and to remove all causes of controversy between them in respect thereto.”Footnote ²¹ Under the treaty, the United States committed to deliver 60,000 acre-feet of water from the Elephant Butte Dam annually, and Mexico agreed to waive legal claims arising from the United States’ diversions.Footnote ²²

Mexico and the United States have continued this cooperative relationship into the present. In 1944, the two states concluded a new treaty in which the United States expressly recognized Mexico’s sovereign interest in utilization of the Rio Grande.Footnote ²³ Each of the states committed to contribute a defined volume of water to the Rio Grande from tributaries within their respective jurisdictions.Footnote ²⁴ The treaty established an International Boundary and Waters Commission (IBWC) to coordinate the planning, construction, and operation of works along boundary waters, including hydropower dams, storage and diversion structures, and flood control measures.Footnote ²⁵ It also contemplated that the parties would work through the IBWC to develop additional bilateral agreements – described as “minutes” – to facilitate continued cooperation and resolve future disagreements related to their transboundary waters.Footnote ²⁶ In the 1960s and 1970s, as demand for water accelerated and both countries struggled to meet their contribution commitments, the IBWC’s minutes focused largely on issues of equitable apportionment. By the 1990s, drought management and ecological preservation became more salient.Footnote ²⁷ Thus, cooperation between Mexico and the United States evolved over time in response to the states’ shifting priorities.

A dispute between Mexico and the United States illustrates the resiliency of this cooperative framework. In September 2020, Texas Governor Greg Abbott delivered a letter to US Secretary of State Mike Pompeo formally complaining that Mexico was close to defaulting on its obligations under the 1944 treaty. At the time, Mexico owed the United States “almost a year’s worth of water” – over 300,000 acre-feet – with only six months remaining until the deadline. When Mexico announced that it would honor its treaty obligation, thousands of pecan farmers converged on the Bolaquilla Dam in northern Mexico. Brandishing sticks and stones, the protesters seized control of the dam and closed its valves in an attempt to preserve the dam’s scarce water for their own parched orchards. Rather than appease the farmers, the Mexican government directed its National Guard to reclaim the dam by force and reopen the valves, leading to violent clashes that claimed the life of a protestor, Yesica Silva.Footnote ²⁸ In the end, despite intense regional resistance, the cooperative relationship between Mexico and the United States prevailed, allowing water to continue to flow to the lower Rio Grande. Although Mexico has since fallen further behind in its obligations under the 1944 treaty and political tensions between communities on either side of the river continue to run high, thus far the two sides have managed to prevent the situation from escalating into further violence.Footnote ²⁹

Reconciling Conflicting Rights

The robust Mexico–US partnership at the Rio Grande demonstrates that international cooperation is possible even when states have divergent interests with respect to scarce resources. It does not yet show, however, that international law affirmatively requires such cooperation, much less what the grounds of such a requirement would be. Beginning early in the twentieth century, however, national courts and international courts and tribunals began to recognize that general principles of law do, in fact, obligate states to cooperate with one another to resolve disputes over transboundary rivers. Through these decisions and supportive state practice, mandatory cooperation became firmly enshrined in the customary international law of transboundary rivers.

In the early twentieth century, the US Supreme Court decided a series of cases supportive of mandatory cooperation involving disputes between states of the United States.Footnote ³⁰ In 1907, Kansas sued Colorado for diverting water from the Arkansas River, allegedly depriving Kansas and its inhabitants of the full volume of water to which they were accustomed.Footnote ³¹ The Court observed that “because Kansas and Colorado are states sovereign and independent in local matters, the relations between them [arguably] depend … upon principles of international law.”Footnote ³² The Court then proceeded to identify the applicable legal principles:

In light of these principles, the Court held that the equal rights of Colorado and Kansas required an “equitable division” of water between the two states.Footnote ³⁴

Similarly, in a 1931 case, New Jersey v. New York,Footnote ³⁵ the Court considered New Jersey’s request for an injunction to prevent New York from channeling water away from the Delaware River and its tributaries toward New York City. In an opinion authored by Justice Oliver Wendell Holmes, the Court acknowledged that

According to Justice Holmes, the goal of reconciling sovereign rights in this manner “always is to secure an equitable apportionment without quibbling over formulas.”Footnote ³⁷

Eight years later, the Italian Court of Cassation confirmed that equitable principles also apply to states under international law. In the Convention of December 17, 1914, France and Italy had established a joint framework for utilization of the transboundary Roya River. France later objected to an Italian firm’s plan to construct a power station on the Italian side of the border.Footnote ³⁸ In deciding in favor of the Italian firm, the Court described the applicable law as follows:

Applying these principles, the Court concluded that international cooperation was required under international law to reconcile the states’ conflicting sovereign rights relative to the Roya.

An arbitral tribunal eventually delivered the most influential statement on mandatory cooperation in the 1957 Lake Lanoux Arbitration.Footnote ⁴⁰ The dispute concerned the rights of France and Spain respectively to waters flowing from Lake Lanoux in the Pyrenees.Footnote ⁴¹ Lake Lanoux is situated entirely in France, but its waters flow into the Carol River, which enters Spanish territory after traveling roughly twenty-five kilometers through France.Footnote ⁴² The Treaty of Bayonne of 1866 established the Franco-Spanish border and contained provisions regulating the use of transboundary watercourses like the Carol River.Footnote ⁴³ Article 8 of the Treaty affirmed that “standing and flowing waters … are subject to the sovereignty of the State in which they are located.”Footnote ⁴⁴ Anticipating that situations could arise in which works in one country might affect waters in the other, Article 11 provided that any proposal “to construct works or to grant concessions which might change the course of the volume of a [transboundary] watercourse” could proceed only after “prior notice” to the other state. The purpose of this requirement was to enable aggrieved states to raise timely objections, thus ensuring that “the interests that may be involved on both sides will be safeguarded.”Footnote ⁴⁵

In 1950, France announced that it would divert some of the waters from Lake Lanoux to develop a hydroelectric project on its own soil. France proposed to return the diverted waters to the Carol River (the natural destination of those waters) through an engineered tunnel.Footnote ⁴⁶ Spain objected to the proposed project, arguing that Article 11 of the Treaty of Bayonne required France to obtain Spain’s consent before proceeding.Footnote ⁴⁷ In response, France invoked Article 8, which affirmed the parties’ sovereign jurisdiction over “standing and flowing waters” within their borders. France argued that in any event Spain would suffer no harm from the project because all of the diverted water would return to the Carol River before it crossed into Spain.Footnote ⁴⁸

The Tribunal emphatically rejected the suggestion that France must obtain Spain’s consent before diverting water from Lake Lanoux. The Tribunal expressed concern that if Spain had the right to veto such works in French territory, Spain could dominate its neighbor, suppressing France’s “exercise of territorial jurisdiction” through “unconditional and arbitrary opposition” to new development projects.Footnote ⁴⁹ Spain’s argument would exalt territorial integrity to the detriment of territorial sovereignty, subjecting upstream riparians to the unilateral power of downstream riparians. This would place the sovereign rights of France and Spain on an unequal footing in violation of the principle of sovereign equality.

Although the Tribunal rejected Spain’s argument, it confirmed that France was legally obligated to cooperate with Spain to provide equitable utilization of the disputed waters. Accordingly, France could not treat Spanish interests with indifference; rather, it must consult and negotiate with its downstream neighbor in good faith.Footnote ⁵⁰ In this context, “the reality of the obligations thus undertaken is incontestable and sanctions can be applied in the event, for example, of an unjustified breaking off of the discussions, abnormal delay, disregard of the agreed procedures, systematic refusals to take into consideration adverse proposals or interests, and, more generally – in cases of violation of the rules of good faith.”Footnote ⁵¹ In particular, the Tribunal stressed the “necessity to reconcile [conflicting interests] by mutual concessions,”Footnote ⁵² and that to this end “there would thus appear to be an obligation to accept in good faith all communications and contracts which could, by a broad comparison of interests and by reciprocal good will, provide States with the best conditions for concluding agreements.”Footnote ⁵³

More specifically, the Tribunal held that it would not be enough for France, the upstream state, to engage in a mere box-ticking exercise “such as taking note of complaints, protests or representations made by the downstream State.”Footnote ⁵⁴ Instead, “the upstream State is under the obligation to take into consideration the various interests involved, to seek to give them every satisfaction compatible with the pursuit of its own interests, and to show that in this regard it is genuinely concerned to reconcile the interests of the other riparian State with its own.”Footnote ⁵⁵ France had satisfied these obligations by negotiating with Spain in good faith and giving reasonable consideration to Spanish interests by agreeing to restore waters diverted to the Carol River in a manner that would best accommodate Spanish agriculture.Footnote ⁵⁶

Because France’s efforts to cooperate with Spain were carried out within a mandatory legal framework, Spain was not subject to unilateralism on the part of France, notwithstanding the fact that France was exercising sovereign rights over the waterway. Although Spain did not obtain a veto over France’s hydroelectric project, the Tribunal concluded that Spain was entitled to meaningful consultation and equitable accommodation, thus providing a legal bulwark against French unilateralism. In particular, the Tribunal emphasized that French sovereignty over the upstream waterway must not be used to interfere arbitrarily with Spanish interests or to threaten such interference.Footnote ⁵⁷ Spain had argued that diverting water into a hydroelectric project was wrongful because it would make Spain dependent on France for the continued delivery of water from Lake Lanoux.Footnote ⁵⁸ The Tribunal did not reject this argument on principle, but on the grounds that the facts did not bear it out: because France could not shut off the flow of water without causing serious injury to its own interests, any threat to interfere with Spain’s access to water from Lake Lanoux would be an empty one.Footnote ⁵⁹ Accordingly, there was no genuine prospect of France using the proposed hydroelectric project to dominate Spain or interfere with Spain’s territorial integrity. The Tribunal concluded that France had adequately respected Spain’s sovereign interests by notifying Spain of the proposed project, engaging in extensive consultation and negotiation, submitting to arbitration, and giving due regard to Spain’s sovereign interests in the shared watercourse.

Legal scholars today often characterize Lake Lanoux as affirming a doctrine of “limited sovereign rights” over international watercourses, but this characterization requires qualification.Footnote ⁶⁰ As noted previously, the Treaty of Bayonne expressly preserved the states-parties’ sovereign authority over their respective territories.Footnote ⁶¹ The Tribunal did not question this proposition. Accordingly, the Tribunal left the parties’ sovereign rights undisturbed but held that each of the parties must exercise their rights in accordance with equitable principles that respect the other’s equal sovereignty.

This approach to reconciling conflicts of sovereign rights resonates with equity’s operation in national legal systems. As Henry Smith has observed, equity constitutes a form of “second-order law” or “meta-law” that formally preserves legal rights but conditions how those rights may be exercised in some contexts.Footnote ⁶² Equitable principles characteristically apply to disputes involving conflicting rights, where the exercise of a right by one party could prejudice others’ rights.Footnote ⁶³ Equity’s response to conflicting rights is to impose second-order “in personam” obligations that “leave the presumptive rights in place and to reconcile them ex post based on an equitable, context-sensitive style of reasoning.”Footnote ⁶⁴ Equity accommodates conflicting rights by constraining how parties may exercise rights to prevent what the civilian legal tradition calls an “abuse of right.” Hence, a state may not abuse its sovereign authority over a transboundary river by unilaterally imposing regulations that effectively subordinate another riparian state’s sovereign authority. Nor may a state divert water from a transboundary river in a manner that would disproportionately impact another state’s sovereign rights to the same resource. Consistent with the dominant proportionality-based approach to adjudicating conflicts of rights,Footnote ⁶⁵ international law calls for apportioning water between riparian states in a manner that accounts for the states’ respective equitable interests.Footnote ⁶⁶ This distinctively equitable approach to conflicts of rights has become “the cornerstone of customary international law” governing the use of transboundary rivers.Footnote ⁶⁷

By placing equitable principles at the heart of its regime governing transboundary rivers, international law aims to mitigate practical inequities generated by its allocation of territory among states. If equitable principles did not apply, upstream states (e.g., France, the United States) might invoke their own territorial sovereignty as a trump justifying the unilateral diversion of water that would otherwise flow to downstream states (e.g., Mexico, Spain). Or economically and militarily powerful downstream states (e.g., Egypt) might cite territorial integrity as a justification for coercing upstream states to refrain from using an equitable portion of transboundary waters (e.g., Burundi, Ethiopia, Uganda). By affirming that equitable principles apply to international rivers, international law bars states from dominating their neighbors by unilaterally dictating the terms upon which these shared resources will be governed and apportioned.Footnote ⁶⁸

Establishing a Community of Interest

Recognizing the uncertainty that surrounded the international law of international watercourses at the dawn of the twentieth century, the eminent Swiss jurist Max Huber identified four possible legal paradigms: absolute territorial sovereignty, absolute territorial integrity, limited territorial sovereignty, and community of interest. The last option – community of interest – would treat an international watercourse (including the primary river and all tributaries) holistically as a unitary common resource.Footnote ⁶⁹ Proponents of the community interest paradigm tend to link the idea to concepts of “internationalization” or “denationalization,” suggesting that transboundary rivers are subject to the joint-stewardship of the entire community of riparian states.Footnote ⁷⁰ Under this approach, no member of the community could claim unilateral jurisdiction over a transboundary river, but each would have an equal right to use the river, and each would have equal voice in collective governance.

In its first case on the merits, the PCIJ applied the community of interest paradigm to the Oder, a river that presently traverses the territory of the Czech Republic, Germany, and Poland. Under the Treaty of Versailles, an international commission was responsible for regulating navigation on the Oder.Footnote ⁷¹ The question presented in The River Oder Case was whether the commission’s jurisdiction extended to certain tributaries of the Oder in Polish territory. Purporting to summarize “the principles governing international fluvial law in general,” the Court indicated that

Decades later, the International Court of Justice (ICJ) would cite this passage from The River Oder Case with approval in Gabčikovo-Nagymaros v. Hungary, asserting that the subsequent “development of international law has strengthened this [community of interest] principle for non-navigational uses of international watercourses as well.”Footnote ⁷³

The community of interest paradigm aptly describes some treaty regimes, like the Mexico–US partnership at the Rio Grande, which establish international commissions to govern watercourses holistically for the shared benefit of riparian states. It also helps to make sense of watercourse treaties (or provisions thereof) that are designed to promote environmental conservation, sustainability, and intergenerational equity. The purpose of these agreements is to remove matters of common concern from the unilateral jurisdiction of individual states and commit those matters instead to the joint stewardship of a community of riparian states.Footnote ⁷⁴ Within a community of interest regime, each state must cooperate with the rest of the community to achieve the regime’s objectives, while sharing equitably in the associated benefits and burdens. Significantly, because community of interest regimes are premised on sovereign equality, they are subject to the same equitable principles that apply in conflict of rights cases like Lake Lanoux. In both contexts, states must share relevant information, seek consensus through consultation and negotiation, and, if all else fails, resort to peaceful third-party dispute resolution.

Most transboundary rivers have not been fully internationalized through treaties that establish a community of interest.Footnote ⁷⁵ The reason why states have not concluded more treaties is not hard to imagine. States are often reluctant to relinquish their territorial sovereignty. They do not necessarily trust neighboring states as prospective partners. They might hesitate to conclude agreements that could lock in long-term disadvantages or strengthen a competitor. When states decline to establish a community of interest regime for these or other reasons, Lake Lanoux’s customary default rule applies: each state retains its sovereign rights but must exercise those rights in a manner that satisfies equitable principles.

Codifying Mandatory Cooperation

International watercourse treaties often codify aspects of Lake Lanoux’s customary default rules. In some instances, these agreements have sustained productive international partnerships even among otherwise bitter regional rivals. A striking example is the Indus Waters Treaty of 1960.Footnote ⁷⁶ Originating on the Tibetan plateau, the Indus River travels 1800 miles through India and Pakistan, sustaining the lives and livelihoods of approximately 215 million people throughout its basin. Following the 1947 partition of India and Pakistan, the World Bank averted possible hostilities between the two states by brokering an agreement that has held firm for over sixty years.Footnote ⁷⁷ Rather than treat the Indus River Basin as a unitary system under a community of interest regime, India and Pakistan elected to divide the basin’s waters equitably, with India taking the three eastern rivers (the Ravi, Beas, and Sutlej) and Pakistan the three western rivers (the Indus, Jhelum, and Chenab), subject to India’s right to use some water for irrigation, hydropower generation, and other purposes before the western rivers enter Pakistan.Footnote ⁷⁸ The two states also agreed to develop dams, canals, and other works (bankrolled largely by the World Bank) to increase the total volume of water available for agriculture, electricity generation, and other uses.Footnote ⁷⁹ The treaty establishes an orderly process for information-sharing, consultation, and dispute resolution through a Permanent Indus Commission.Footnote ⁸⁰ If the commissioners fail to reach agreement, the parties may jointly submit unresolved “differences” either to a neutral expert who is a highly qualified engineer or to the Permanent Court of Arbitration (PCA).Footnote ⁸¹ Against all odds, this cooperative framework between India and Pakistan has survived three wars (1965, 1971, 1999), a nuclear arms race, intensifying nationalism, and a host of other challenges,Footnote ⁸² though its future remains fragile in light of enduring tensions between the two historic rivals.Footnote ⁸³

Another important codification of mandatory cooperation is the UN Economic Commission for Europe’s 1992 Convention on the Protection and Use of Transboundary Watercourses and International Lakes, also known as the “Helsinki Convention.”Footnote ⁸⁴ Following the devastating 1986 Sandoz chemical spill, which dyed the Rhine River blood red, wiping out flora and fauna for hundreds of miles, European countries cooperated to establish an “ecosystem-oriented, holistic approach to the sustainable use and integrated management of freshwater resources in Europe.”Footnote ⁸⁵ The keystone of this effort is the Helsinki Convention, which requires state-parties to “prevent, control, and reduce pollution of waters causing or likely to cause transboundary impact”; “ensure that transboundary waters are used with the aim of ecologically sound and rational water management, conservation of water resources and environmental protection”; “ensure that transboundary waters are used in a reasonable and equitable way”; and “ensure conservation and, where necessary, restoration of ecosystems.”Footnote ⁸⁶ The Convention also obligates states-parties to “cooperate in the conduct of research into and development of effective techniques for the prevention, control and reduction of transboundary impact,”Footnote ⁸⁷ and to practice the “widest exchange of information, as early as possible, on issues covered by the provisions of this Convention.”Footnote ⁸⁸ The Helsinki Convention makes international cooperation mandatory by declaring that states-parties “shall on the basis of equality and reciprocity enter into bilateral or multilateral agreements or other arrangements” on a wide range of issues “in order to define their mutual relations and conduct regarding the prevention, control and reduction of transboundary impact.”Footnote ⁸⁹ Each of these features of the Helsinki Convention affirms states’ joint stewardship over the sustainable use and environmental protection of transboundary rivers.

Perhaps the most important codification of mandatory cooperation for transboundary rivers is the landmark 1997 Convention on the Law of Non-Navigational Uses of International Watercourses (the UN Watercourses Convention).Footnote ⁹⁰ Article 5 endorses the principle of equitable and reasonable utilization by providing that states-parties “shall in their respective territories utilize an international watercourse in an equitable manner … taking into account the interests of the watercourse States concerned, consistent with adequate protection of the watercourse.”Footnote ⁹¹ Factors relevant to the equitable apportionment of water include a river’s natural characteristics, the social and economic needs of riparian states, how usage affects other states, and the need for conservation and efficient use of water resources.Footnote ⁹² The Convention also obligates states “when utilizing an international watercourse in their territories, [to] take all appropriate measures to prevent the causing of significant harm to other watercourse States.”Footnote ⁹³

To foster international equity and prevent avoidable harm, the UN Watercourses Convention obligates states to “cooperate on the basis of sovereign equality, territorial integrity, mutual benefit and good faith in order to attain optimal utilization and adequate protection of an international watercourse.”Footnote ⁹⁴ This general obligation entails specific duties to exchange data and information on the conditions of a watercourse,Footnote ⁹⁵ including planned works that might adversely affect other riparian states.Footnote ⁹⁶ In addition, the Convention requires states-parties to consult and negotiate with one another “in good faith,” respecting the principle of sovereign equality by “pay[ing] reasonable regard to [others’] rights and legitimate interests.”Footnote ⁹⁷ If consultation and negotiation do not produce agreement, states-parties must “seek a settlement of the dispute by peaceful means.” Options for peaceful dispute resolution include recourse to an impartial factfinder; the good offices, mediation or conciliation of a third party; submission to a joint watercourse institution; international arbitration; or adjudication in the ICJ.Footnote ⁹⁸ These features of the UN Watercourses Convention codify the customary duties of mandatory cooperation.Footnote ⁹⁹

Managing Entrenched Disputes

When states fail to agree on mutually acceptable terms of cooperation for a transboundary river, this can lead to a protracted stalemate. Entrenched disputes over transboundary rivers are not necessarily the product of bad faith. In some cases, neighboring states may disagree in good faith about the ingredients of an equitable solution. They may prefer different dispute-resolution mechanisms (e.g., mediation, arbitration, adjudication) or institutions (e.g., ICJ, PCA, regional courts). While international dialogue can help to resolve discord between states, in some instances it can harden points of disagreement.Footnote ¹⁰⁰ When this occurs, transboundary river disputes can easily drag on without the expectation of an immediate resolution.

Some of the world’s most important watercourses have become mired in entrenched disputes. The iconic Nile River Basin, which covers over three million square miles and feeds 250 million people, illustrates this problem.Footnote ¹⁰¹ By 2050, the combined population of Nile Basin riparian states is expected to reach one billion, straining the basin’s capacity to meet the population’s basic needs for fresh water.Footnote ¹⁰² For generations, the eleven states within the Nile River Basin have debated how to apportion its finite waters. Citing historical practice, colonial-era treaties, and its continuing needs, Egypt has asserted the right to use over 55 percent of the Nile’s annual flow. Sudan has also claimed preferential rights to Nile Basin waters.Footnote ¹⁰³ However, as upstream riparians have grown in population, political influence, and the ability to attract foreign investment for hydropower and irrigation projects, the reaching claims of Egypt and Sudan have encountered increasingly stiff resistance.

In 2010, multilateral discussions among Nile Basin states produced the Nile Basin Cooperative Framework Agreement (CFA), a treaty that seeks to “promote integrated management, sustainable development, and harmonious utilization of the water resources of the Basin, as well as their conservation and protection for the benefit of present and future generations.”Footnote ¹⁰⁴ Modeled after the UN Watercourses Convention, the CFA embraces the general principles of equitable and reasonable utilization, avoiding significant harm, the preservation of endangered ecosystems, good faith consultation and cooperation, and mandatory dispute resolution.Footnote ¹⁰⁵ The CFA specifically affirms “[t]he principle of cooperation between States of the Nile River Basin on the basis of sovereign equality, territorial integrity, mutual benefit and good faith in order to attain optimal utilization and adequate protection and conservation of the Nile River Basin and to promote joint efforts to achieve social and economic development.”Footnote ¹⁰⁶ Notably, the CFA also invokes the “community of interest” concept as a guiding principle, calling into question the sovereignty-based claims of Egypt and Sudan.Footnote ¹⁰⁷ To date, only seven states have signed on to the CFA (Burundi, Ethiopia, Kenya, Rwanda, South Sudan, Tanzania, Uganda), leaving four holdouts (Democratic Republic of the Congo, Egypt, Eritrea, and Sudan).Footnote ¹⁰⁸ The fact that so many downstream states have declined to endorse the CFA leaves the Nile’s future in limbo, despite growing acceptance on both sides that equitable principles constitute the applicable international lawFootnote ¹⁰⁹ and despite burgeoning cooperation with respect to some issues of common concern, such as flood control and environmental conservation.Footnote ¹¹⁰

Another well-known example of an entrenched dispute is the Gabčikovo-Nagymaros Case.Footnote ¹¹¹ In 1993, Hungary and Slovakia sought the ICJ’s assistance in resolving a dispute over Hungary’s unilateral suspension of a joint-venture between the two countries to develop a system of locks and hydroelectric power plants on the Danube River.Footnote ¹¹² Hungary argued that the project negatively impacted its legal interest and the international community’s collective interest in preserving the river’s ecological health.Footnote ¹¹³ Slovakia contended, in turn, that Hungary’s actions violated a treaty between the two countries.Footnote ¹¹⁴ After concluding that both parties had violated their international legal obligations, the ICJ sent the dispute back to the parties with the instruction to “find an agreed solution” through “good faith” negotiation.Footnote ¹¹⁵ Although Slovakia eventually agreed to discontinue proceedings in the ICJ, for nearly two decades the two sides made little progress in negotiations, raising the specter of perpetual deadlock.Footnote ¹¹⁶

International law responds to entrenched disputes like these by imposing special requirements of mandatory cooperation. First, as long as disputes remain unresolved, states must continue to explore dispute resolution mechanisms in good faith until they find a successful avenue for achieving an equitable resolution.Footnote ¹¹⁷ Second, although states may take provisional steps to secure their legal interests once it becomes clear that an impasse has been reached,Footnote ¹¹⁸ they must notify other interested parties of such measures,Footnote ¹¹⁹ maintain a continuous process of dialogue to ensure that their provisional measures do not foreclose a multilateral solution,Footnote ¹²⁰ and avoid any actions that would “establish a fait accompli capable of prejudicing the outcome of negotiations.”Footnote ¹²¹ The last requirement is especially critical as applied to transboundary rivers: states must refrain from constructing dams, diverting tributaries, and engaging in other invasive works that could cause permanent and irreversible harm to other riparian states’ interests and the natural environment.Footnote ¹²² These obligations are necessary to reconcile sovereign equality and joint stewardship with the often-slow pace of international dispute resolution.

The Promise and Perils of Mandatory Cooperation

As the twenty-first century rolls onward, fresh water might be insufficient in some regions to meet the needs of swelling populations during an era of climate change and rapid economic development. Experts anticipate that by mid-century, 3.1 billion people will live in urban areas with seasonal water shortages.Footnote ¹²³ International cooperation will be essential in this perilous new world of freshwater scarcity to respect, protect, and fulfill human rights, while safeguarding international peace and security.

Regrettably, international cooperation can also aggravate some of the very problems it aspires to solve. Consider the majestic Mekong River, which rises in China and traverses Laos, Myanmar, Thailand, and Cambodia before draining into the South China Sea through Vietnam’s sprawling “Nine Dragon” river delta. Since the early 1990s, a spirit of robust regional cooperationFootnote ¹²⁴ and an influx of foreign investment have enabled states to construct over three hundred hydroelectric dams along the Mekong.Footnote ¹²⁵ This cooperative ethos has quickened the pace of development and enriched elites throughout the river basin, but at a terrible human and environmental cost. Dams have cut off the natural flow of nutrient-rich silt from China, endangering rice production and inland fisheries along the river’s course. Eighty percent of the inhabitants of the Mekong River Basin – over sixty million people – depend upon these dietary staples for survival. Meanwhile, the basin’s vibrant biodiversity, with over 20,000 types of plant and nearly 2,500 animal species, including freshwater dolphins and giant catfish, is tilting rapidly toward ecological collapse. Thus, national authorities in Mekong states have harnessed international cooperation to achieve economic development at the expense of fragile ecosystems and the most vulnerable members of their societies.Footnote ¹²⁶

Given that international cooperation has caused the Mekong’s ecological crisis, it might seem counterintuitive to recommend more international cooperation as a solution. But multilateral engagement is the only plausible and potentially legitimate path to save the Mekong. The environmental harm wrought by proliferating dams cannot be reversed by any single state unilaterally; it requires collective action by the entire community of riparian states – including China, which has invested heavily in dam construction throughout the region.Footnote ¹²⁷ Mekong states will have to share the costs and benefits of environmental conservation. To determine what equity and environmental stewardship require in this context, they must collect and share information regarding relevant factors such as their respective dependence on hydropower, the ecological health of the river, and the impact of dams and other works on their domestic agriculture and fishing. They will have to negotiate in good faith or agree to resolve their differences through third-party dispute resolution. In short, they will have to jointly embrace mandatory cooperation.