This paper presents a typology of international rivers based on asymmetries in economic and political power among riparian states. This typology is then used, along with other factors such as the spatial location of riparians on the river, to explore the question of why some riparians on international rivers have been able to successfully negotiate treaties and others have not. The findings lend support to both economic and political economy explanations of cooperative action on international rivers. International rivers with riparians with countervailing economic and political power are far more likely to have negotiated treaties than other river types. Riparian states on international rivers sharing a western civilization were much more likely to have concluded treaties than riparian states on rivers in other civilizations. Somewhat surprisingly, rivers that cross civilization boundaries appear no less likely to have treaties than international rivers than run entirely through riparian states that share a single civilization. Adjacent upstream/downstream or side-by-side riparians were less likely to have concluded treaties than country pairs with other spatial relationships.