The current state of development of the no significant harm principle: How far have we come?

The duty to prevent significant transboundary harm remains a cornerstone principle of international law, and especially of international environmental and water resources law. However, this rule focuses on the conduct of a State where harm originates, rather than on the fact that harm has resulted from such conduct, and thus requires that States exercise due diligence in anticipating and in preventing or mitigating such harm. At a practical level, the due diligence standard of conduct expected of States can be uncertain and difficult to determine, as it must be deduced from the applicable primary rules of international environmental or water resources law, which have traditionally been elaborated in rather vague terms. In addition, the standard of due diligence required under the no-harm rule may be influenced by a range of variable and context-specific factors which might prove relevant in the particular circumstances of any dispute. Such uncertainty is further compounded in the field of international water law by the complex interrelationship between the no-harm rule and the other key norms of international water law, particularly the cardinal principle of equitable and reasonable utilization, which embodies a high degree of flexibility and adaptability and suffers from a corresponding degree of normative indeterminacy. Thankfully, recent developments in international water law and related practice regarding the requirement to protect riverine ecosystems and maintain related ecosystem services lend a welcome measure of clarity as regards the preventive measures expected of watercourse States under international law. Judicial recognition of obligations to maintain minimum environmental flows and to preserve or restore riverine ecosystem services, based on the proliferation of such values in treaty and declarative practice, along with the continuing development of sophisticated technical methodologies for ecosystems assessment and evaluation, do much to inform the due diligence conduct required of States. Such advances can only enhance the practical utility of the no-harm rule, and thus of the entire corpus of international water law, in addressing the challenges emerging globally for water resources management in the twenty-first century.