Introduction
On August 9, 2026, Serbian President Aleksandar Vučić stated that Serbia was examining the possibility of altering the course of the Ibër River.[1] His statement came in the context of Serbia’s political grievances concerning Kosovo, and he suggested that Serbia might alter the river’s course depending on how Kosovo behaves.[2] The precise nature, location, and consequences of the proposed intervention remain unclear. Yet the announcement raises a question that international watercourse law is well equipped to answer: Can a riparian state unilaterally alter a transboundary watercourse in pursuit of a new use when another riparian state has developed extensive and socially indispensable uses of that watercourse?
This Insight explains the historical uses of the Ibër River, summarizes the legal framework that would govern a potential dispute between Kosovo and Serbia, and highlights one proposed mechanism that could provide an effective solution.
Background on the Ibër River
The Ibër River originates in Montenegro, flows through Kosovo, is impounded by the Ujman/Gazivoda (Ujman) reservoir, and subsequently enters Serbia. The Ujman reservoir was constructed between 1979 and 1985, when Kosovo and Serbia were parts of Yugoslavia. Since then, the reservoir system has become deeply embedded in Kosovo’s public infrastructure. A 2021 study by the US Department of Energy’s Pacific Northwest National Laboratory (PNLL) found that it has approximately 375 million m³ of storage and receives mean annual inflows of approximately 13.1 m³/s—roughly 413 million m³ annually. According to the study, Kosovo obtains approximately one-third of its drinking water from the system, and its waters provide cooling for the two coal-fired power plants responsible for approximately 95 per cent of Kosovo’s electricity production. The study described Kosovo as ‘profoundly dependent’ on the lake’s waters.[3]
To date, Serbia’s use of the Ibër has principally been downstream, including municipal and other local uses in the Raska and Kraljevo areas, where the river is monitored as part of the Western Morava basin. Serbia has, however, increasingly envisaged developing the Upper Ibër as a source for additional water and energy infrastructure, most notably through the proposed Upper Ibër–Raska hydrosystem and Ribarice reservoir, which would capture part of the river’s high flows and transfer them towards the water-deficient Raska/Novi Pazar area.[4]
The political context is important. Serbia, during the last summer 2026, experienced a period of extreme heat and persistent dry conditions, with the government warning of risks to water resources and water supply and reporting historically low water levels in parts of the country’s hydrological system.[5] Yet Vucic did not frame the proposal in terms of a specific Serbian water-management need. Rather, he raised it in the context of Serbia’s grievances concerning the treatment of Kosovo Serbs, linking the possibility of altering the Ibër’s course to the conduct of the authorities in Pristina.[6] This was not the first time Vučić had raised the possibility of altering the Ibër’s flow. Following the November 29, 2024 explosion that damaged the Ibër-Lepenc canal, he stated that there had previously been a plan to divert the Ibër in central Serbia in a manner that would prevent water from accumulating in Ujman, adding that he had opposed the proposal.[7] This context does not, by itself, determine the legality of the proposed measure. Serbia may have legitimate water-management interests. But international watercourse law recognizes social and economic needs as factors in equitable utilization. It does not permit control over a shared natural resource to be transformed into an instrument of political leverage.
The Treaty Question and the Customary Law Framework
Although Serbia does not formally recognize Kosovo’s independence, the two sides have, since 2013, pursued a process aimed at the normalization of their relations, including economic normalization. This process was complemented by the 2020 Washington commitments, under which Kosovo and Serbia undertook, inter alia, to cooperate on the management and development of the Ujman Lake and to establish a joint commission for the Ibër River.[8] Although Serbia is a party to the 1992 UNECE Water Convention,[9] neither Serbia nor Kosovo is party to the 1997 UN Watercourses Convention.[10] The legal framework is therefore shaped by Serbia’s obligations under the UNECE Water Convention, alongside the customary rules of international watercourse law, which apply independently of treaty participation. It’s important therefore to proceed from the customary rules of international watercourse law independently applicable between riparian states such as the requirements of equitable and reasonable utilization, prevention of significant transboundary harm, cooperation, notification, and consultation.[11]
The International Court of Justice (ICJ) has expressly recognized equitable and reasonable utilization as a rule of customary international law, holding that every riparian state has a basic right to an equitable and reasonable sharing of the resources of an international watercourse, coupled with an obligation not to deprive other riparian states of their equivalent right.[12] The ICJ has likewise recognized procedural obligations arising where planned activities may entail significant transboundary effects. In Pulp Mills, the Court held that, under general international law, an environmental impact assessment is required where there is a risk that a proposed activity may have a significant adverse transboundary impact. Therefore, states are protected from a significant transboundary harm.[13] In Silala, the Court confirmed that the customary obligations of cooperation, notification, and consultation complement the substantive obligation to prevent significant transboundary harm.[14]
Existing Uses and the Upper Ibër
Equitable utilization does not establish a simple “first in time, first in right” rule. Historical and existing uses, such as in the case of Kosovo, are relevant to the equitable balance, but they do not create an immutable entitlement to a particular quantity or flow of an international watercourse. In this reciprocal relationship equitable utilization is conditioned by its obligation to respect the corresponding entitlement of the other.[15] Serbia, likewise, has established downstream uses, including municipal water supply and other local uses in the Raska and Kraljevo areas. Furthermore, Serbia has also contemplated legitimate future uses. Its draft Spatial Plan for 2021–2035 identifies a proposed Hydrosystem Upper Ibër–Raska, centered on a Ribarice reservoir on the Upper Ibër.[16] The proposal illustrates what might well be a genuine Serbian social and economic interest in developing the river. But it also illustrates the transboundary problem. The proposed Ribarice reservoir is envisaged immediately upstream of the backwater of Ujman. It would therefore introduce a new point of human regulation into an upper basin that has historically been relatively undeveloped and whose inflows into Ujman have remained largely in a natural state. The 2021 US Department of Energy study noted the limited development of the upper Ibër basin and treated its inflows as largely natural.[17] Addressing water scarcity is a legitimate factor in an equitable-utilization assessment. Yet if an upstream intervention affects the quantity or timing of water reaching Ujman, Kosovo’s existing uses must equally be considered. The same applies with greater force to the broader proposal to “divert the Ibër” publicly raised by Vucic.
The most constructive response is therefore institutional rather than rhetorical. The 2021 PNNL study recommended establishing a mechanism for coordinated management of Ujman and contemplated the creation of a joint commission for the Ibër River basin.[18] Such an institution would address precisely the problem now exposed by the current controversy: the absence of a permanent mechanism through which Kosovo and Serbia can exchange hydrological data, notify one another of proposed projects, assess their effects, coordinate reservoir operations, and consult before undertaking measures capable of affecting the other side. This approach is consistent with the principles reflected in the 1997 UN Watercourses Convention and the 1992 UNECE Water Convention.
A permanent Ibër River Commission would not require either side to surrender its legal position concerning Kosovo’s status. Nor would it give either side a veto over legitimate development. Its function could instead be practical and technical such as regular data exchange, monitoring of flows and reservoir levels, notification of planned measures, joint hydrological modelling, environmental assessment, emergency coordination, and consultation over new infrastructure. Such a mechanism would also transform the customary obligation of cooperation from an abstract legal principle into an institutional practice. It could provide a forum for precisely the type of balancing that equitable utilization requires.
Conclusion: The Legal Issue Is Not Who Owns the River, but Who Gets to Change It
The Ibër River illustrates a broader point about contemporary international watercourse law. Equitable utilization is not simply an allocation rule determining how much water one state may take. It also regulates changes to the conditions under which riparian states use a shared watercourse. Kosovo does not possess an absolute right to preserve every aspect of the existing hydrological regime merely because it has historically depended upon it. Serbia, however, does not possess an unlimited right to transform the river merely because works are undertaken within its territory. Kosovo’s established and highly concentrated dependence upon Ujman deserves proper consideration and legal protection. Serbia’s existing uses likewise deserve the same treatment and its legitimate future water needs must also be taken seriously. Where these interests intersect or conflict, customary international law provides the framework, requiring equitable and reasonable utilization, prevention of significant transboundary harm, cooperation, information exchange, notification, and consultation.
Creating the Ibër River Commission envisaged in the PNNL study would offer a cooperative platform for the parties to disclose proposed interventions, exchange hydrological data, conduct appropriate impact assessments, examine alternatives and develop agreed rules for future projects and emergency situations. International watercourse law does not freeze the Ibër in its historical configuration. But neither does it permit one riparian state to unilaterally reorder a transboundary river whose existing uses have become essential to another state, or to turn control over that river into an instrument of political leverage. The better answer is not to decide who owns the Ibër. It is to ensure that neither state can change the river without the other having a meaningful opportunity to understand, assess, and respond to the consequences.
About the Author: Besfort Rrecaj works as an Associate Professor of Law at the University of Prishtina Faculty of Law focusing on international law and relations. His recent publications appeared in the European Journal of International Security, UC Law San Francisco Journal of International Law Review, Utrecht Journal of International and European Law and International Journal of Human Rights and Constitutional Studies. Other than teaching, he currently serves as a member of the Scientific Council of Kosovo in charge of promoting and supporting the scientific community in Kosovo. Before that, he held senior positions with the governmental sector such as the Ministry of Foreign Affairs and the Ministry for European Integration. He also worked as a Senior Legal Advisor with the Constitutional Court of Kosovo.
[1] Vučić Says Serbia Is Considering Diverting the Ibër River, Raising Questions over Gazivode/Ujman, KoSSev (Aug. 9, 2026), https://kossev.info/en/vucic-says-serbia-is-considering-diverting-the-ibar-river-raising-questions-over-gazivode-ujman/.
[2] See generally Eur. External Action Serv., Belgrade-Pristina Dialogue, https://www.eeas.europa.eu/eeas/belgrade-pristina-dialogue_en.
[3] Nathalie Voisin et al., Water Resource Opportunities at Lake Gazivode/Ujmani (Pac. Nw. Nat’l Lab’y, PNNL-31059, June 2021), https://www.pnnl.gov/publications/water-resource-opportunities-lake-gazivodeujmani.
[4] Ministry of Constr., Transp. & Infrastructure (Serb.), Spatial Plan of the Republic of Serbia 2021–2035: Draft (2021), https://www.mgsi.gov.rs/sites/default/files/PPRS%20Nacrt.pdf; see also Strategic Environmental Assessment for the Spatial Plan of the Republic of Serbia 2021–2035 (2021).
[5] Gov’t of the Republic of Serb., Coordinated State Response to Adverse Weather Conditions (July 30, 2026); Republic Hydrometeorological Serv. of Serb., Moisture Conditions—Drought Monitoring (June 30, 2026).
[6] Kosovo Protests as Serbia Mulls Changing River’s Course, Balkan Insight (Aug. 10, 2026), https://balkaninsight.com/2026/08/10/kosovo-protests-as-serbia-mulls-changing-rivers-course/bi/.
[7] Vučić: Attempt at a Major and Fierce Hybrid Attack on Serbia, Kosovo Online (Dec. 1, 2024), https://www.kosovo-online.com/en/news/politics/vucic-attempt-major-and-fierce-hybrid-attack-serbia-1-12-2024.
[8] See Voisin et al., supra note 3.
[9] Convention on the Protection and Use of Transboundary Watercourses and International Lakes, Mar. 17, 1992, 1936 U.N.T.S. 269 [hereinafter UNECE Water Convention].
[10] Convention on the Law of the Non-Navigational Uses of International Watercourses, May 21, 1997, 2999 U.N.T.S. 3 [hereinafter UN Watercourses Convention].
[11] UN Watercourses Convention, supra note 10, arts. 5, 7–8; UNECE Water Convention, supra note 9, art. 2(1), (2)(c), (6).
[12] Dispute over the Status and Use of the Waters of the Silala (Chile v. Bol.), Judgment, 2022 I.C.J. 614, ¶¶ 95–97 (Dec. 1).
[13] Pulp Mills on the River Uruguay (Arg. v. Uru.), Judgment, 2010 I.C.J. 14, ¶¶ 101–05, 204 (Apr. 20).
[14] Silala, 2022 I.C.J. ¶¶ 114–21, 131–32.
[15] See Int’l L. Ass’n, The Helsinki Rules on the Uses of the Waters of International Rivers arts. IV–VIII, in Report of the Fifty-Second Conference 484 (1966).
[16] See Ministry of Constr., Transp. & Infrastructure (Serb.), supra note 4.
[17] Voisin et al., supra note 3.
[18] Id.