The Mekong River is unique among the world’s great rivers in the size of the human population supported by its ecosystem. Approximately 60 million people derive their livelihoods from aquatic life in the river system. Largely unregulated through most of the 20th century, the Mekong River system is undergoing extensive dam construction throughout the basin for hydroelectric generation, with over 140 dams planned, under construction, or built. What will be the cumulative effects of these dams on the geomorphology, ecology, and human populations of the river and its delta? How will these changes interact with other changes such as deforestation in steep uplands, levees and channelization, and accelerated sea level rise?