Few international water-sharing agreements have shown the flexibility to adjust to extended drought; fewer still provide safeguards for adaptation to modern climate variability. Yet, current conflicts over the development and use of transboundary rivers continue to motivate the search for negotiated water-sharing arrangements that can provide flexibility in the face of change. To avoid future conflicts, an agreement must include measures that allow for adaptation to changes in water supplies, population, climate, technology, infrastructure, and economic activity while also guiding water use patterns. The benefits of a flexible agreement can be a more predictable water supply for all riparians, greater incentives to develop needed water infrastructure and more open, transparent and accountable water institutions. Other benefits include increased food production, water security, environmental protection, reduced flood damage, better adaptation to the costs of extreme weather and variable climate, and a reduced need for complex legal, administrative and enforcement activities. This paper investigates ways to achieve sustainable transboundary water-sharing agreements. It investigates barriers to forging water-sharing agreements, describes errors that could undermine settlements and presents takeaway lessons from two North American agreements and one in south Asia. Finally, the paper proposes an approach by which information on headwater flows and historical use patterns could be used to allocate supplies between riparians that adapt to changes in water supply and demand. Outcomes from the implementation of structured, but flexible agreements could help guide future negotiated settlements for the worlds international rivers.