Nature has no boundaries. This statement is particularly palpable in the case of freshwater resources, which require close cooperation among neighbouring countries for sound management.
At the international level, principles of customary law constitute the main source of freshwater management. There are four main principles governing the management of transboundary international watercourses: cooperation, avoidance of transboundary harm, equitable and reasonable use and peaceful settlement of disputes. The two main instruments that codify international custom, despite being non-binding, are the UN Convention on the Law of Non-Navigational Uses of International Watercourses (UN Watercourses Convention, not yet entered into force) and the Helsinki Rules on the Use of Waters of International Rivers developed by the International Law Association (ILA). The latter instrument has played an essential role in the development of international law related to watercourses. In 2004, the ILA adopted the Berlin Rules on Water Resources (Berlin Rules).
As sources of freshwater are depleting, common management approaches are imperative. Thus far, progress has only been made regionally. The European Water Charter was adopted by the Council of Europe in 1967 and common approaches towards freshwater management have also been developed within the UNECE. The harmonization of freshwater management measures is a major concern within the Common Environmental Policy of the European Union, which includes a series of Directives dealing with freshwater management. The adoption of the European Union Water Framework Directive in 2000 undoubtedly marked a milestone in freshwater management 
 Gündling, L. (2005, 2nd Revised Edition), International Environmental Law: Atmosphere, Freshwater and Soil, UNITAR, Geneva, Switzerland