Despite warnings of interstate conflict over shared water resources, states are reaching hundreds of treaties and agreements over their international rivers. We have extensive knowledge about the negotiations process of individual treaties, but there is a paucity of systematic analysis of the forces influencing treaty formation. In addition, the few quantitative studies examining the formation of agreements fail to consider the different factors influencing the rise of bilateral versus multilateral agreements on multilateral basins. Correcting this omission is important because scholars have discovered that states frequently sign bilateral agreements over multilateral rivers, which contradicts the integrated river basin management approach advocated by environmentalists, engineers, and water experts. This study seeks to fill this vacuum within the existing literature by distinguishing between the formation of bilateral treaties on bilateral and multilateral basins and comparing these bilateral forms of cooperation to the formation of multilateral treaties on multilateral basins. Through quantitative analysis, we argue that treaty type is a by-product of state interest, transaction costs, and distribution of power.