This paper assesses the impact of water supply variability on treaty cooperation between international bilateral river basin riparian states. Climate change is anticipated to change the variability of water supply, as well as its expected magnitude. Previous studies have focused mainly on water scarcity, measured in terms of mean precipitation or per capita water availability in the country, as a trigger for conflict or cooperation. The water variability measure used here captures both annual runoff variability and precipitation variability over periods of 30 and 100 years. The analysis used economic and international relations data to identify incentives for international cooperation in addressing water supply variability. The authors find that small-to-moderate increases in variability create an impetus for cooperation, although large increases in variability would reduce incentives for treaty cooperation. Stronger diplomatic and trade relations support cooperation, while uneven economic power inhibits cooperation. Various measures of democracy/governance suggest different impacts on cooperation across the basin riparians. The findings have policy implications in the context of preparedness for impacts of climate change on the water sector.