The role of customary rules and principles of international environmental law in the protection of shared international freshwater resources

Notwithstanding the recent elaboration of a number of global and regional conventional instruments expressly concerned with the environmental protection of international watercourses, certain rules and principles of customary international law have developed in recent decades that continue to have a significant role to play in this regard. In recent years, debate has raged over the precise legal status and normative content of many international environmental norms and principles, some of which are often assumed to enjoy binding force in customary international law. While some commentators characterise these norms as "declarative" rather than customary law, suggesting that their usefulness may be limited in relation to third-party dispute settlement by courts and arbitral tribunals, this characterisation possibly ignores the fact that such norms have an important role to play in terms of voluntary compliance and in terms of bilateral and multilateral negotiations. Further, certain international environmental norms contained in treaty instruments, though declaratory in nature, can be expected to play a significant role in informing the rules and principles, in particular those relating to the equitable and reasonable utilisation of watercourses and the prevention of significant harm to other watercourse States. More specifically, trends identified both in the treaty practice of States and in soft law guidelines defined by international institutions can be taken into consideration to define more concretely the material contents of "due diligence." Of course, the consistent inclusion of normative rules and principles in the declarations and resolutions of international organisations contributes significantly to the process of custom generation. This process might be expected to have made a particularly significant contribution to the development of international environmental law where the use of soft law declaratory instruments has been so widespread. The single most important source of rules and principles that may have crystallised into generally binding norms of customary international environmental law is the accumulated corpus of relevant multilateral and bilateral treaty provisions, many of which contain elaborate environmental rules. In turn, the inclusion of certain rules and principles in treaties must greatly enhance their status as established or emerging rules of general customary law. This article outlines the results of an extensive survey of international conventions and soft law instruments, State, judicial and arbitral practice, and academic commentary relating to the normative development and substantive content of each of these purported customary environmental rules and principles and to their application in the area of shared freshwater resources. Ultimately, it aims to highlight the role of such rules and principles in ensuring, by means of their detailed elaboration and normative sophistication, that an appropriate weighting is allocated to considerations of environmental protection in determining an equitable regime for the utilisation of shared international water resources under well-established rules of international water law.