Freshwater disputes between States are increasing, due in part to shrinking supplies of water relative to the human population. Pollution can reduce available freshwater supplies even further and can harm aquatic ecosystems on which humans depend. Much of the world’s fresh water is shared by two or more States. Over ninety per cent of the fresh water potentially available for human use is stored in underground aquifers. While much of this groundwater interacts with surface water, it moves slowly and can be contaminated if not carefully managed. Two global treaties regulate the problem of transboundary water pollution and State practice has developed general rules of customary international law on the subject. Governments are bringing freshwater disputes to international courts and tribunals at an increasing rate and decisions in these cases hold valuable lessons for governments as regards the use, protection and management of international watercourses.

This chapter surveys the basic principles of the law of international watercourses and shows how they are interrelated. For the purposes of the chapter, the basic principles of the law in this field are considered to be equitable and reasonable utilization, prevention of significant harm, cooperation in good faith, and notification of planned measures. These principles are demonstrated to be not only interrelated, but also mutually supportive. This discussion is set against the background of a survey of the four principle theoretical bases of the law of international watercourses.

This chapter considers the rules of customary international law that have developed, and are continuing to develop, concerning the non-navigational uses of international watercourses. In identifying these rules, the chapter relies to some extent upon statements by the International Court and the relevant work of the International Law Commission as the best evidence of customary international law. The chapter will also refer to State practice and statements countries have made concerning legal standards applicable to non-navigational uses of international watercourses. Space limitations dictate that the chapter focus on the principal rules that are based in customary international law. There is broad agreement on two of these obligations: equitable and reasonable utilization; and prevention of significant harm. Also important are the general obligation to cooperate, and prior notification of planned measures.