International river basins are mostly characterized by upstream-downstream externalities that involve asymmetric incentives to cooperate and, therefore, suggest a high conflict potential between riparian states. However, with more than 400 river basin treaties, cooperation along international rivers by far outweighs water-related conflicts. The abundance of international water cooperation despite the odds is puzzling and has so far received little systematic attention. Against this background, I develop a research framework that draws on international regime theory and combines power, interest, knowledge and contextbased approaches to water regime formation. In a second step, I probe the plausibility of my framework in two case studies on international water cooperation in the Rhine and Elbe river basins. The empirical findings suggest that there is no "one-answer-fits-all" in trying to explain water regime formation. While power-based approaches are of limited explanatory value, a thorough understanding of cooperation along the two international rivers requires the combination of interest, knowledge and context- based arguments.