Book excerpt: In Preservation of Ecosystems of International Watercourses and the Integration of Relevant Rules: An Interpretative Mechanism to Address the Fragmentation of International Law, Lee Jing takes an innovative approach to developing an international legal framework for preserving ecosystems. Deploying Article 31(3)(c) of the 1969 Vienna Convention an analytical framework is devised that examines ‘the ecosystem approach’ under international law through the prism of Article 20 of the UN Watercourses Convention. The analysis provides an enhanced normative scope and content for the UN Watercourses Convention’s approach to the obligation to preserve, taking into account contemporary developments in international law. The work demonstrates the full potential of the Vienna Convention’s Article 31(3)(c) as an integration tool in addressing the fragmentation of international law.