This article reviews three bilateral agreements made between Jordan and Syria over the last 50 years regarding use of the shared waters of the Yarmouk River. Jordan began constructing the long-planned Unity Dam and the permanent Adaseya diversion weir only after concluding the most recent agreement, in 2001. The article presents land-area and economic calculations to suggest why Jordan agreed to narrower and less favourable terms in the 2001 agreement—including a lowered dam height, a smaller allocation of Yarmouk waters, and a larger share of dam construction costs. It also identifies several environmental and political issues that the 2001 agreement does not address. The analysis shows that changing politics, hydrology, land use, and economics have motivated multiple past Yarmouk water agreements and may resurface to require further negotiations in the future.