The Mekong River, largely undeveloped prior to 1990, is undergoing rapid dam construction. Seven dams are under construction on the upper mainstem in China and 133 are planned for the Lower Mekong River and tributaries. These dams will trap sediment that formerly was transported downstream to the lower river and delta. The delta is dependent on sediment delivered from upstream to balance losses to subsidence and coastal erosion, and if sediment load is reduced, the future of the delta becomes increasingly in doubt. To estimate the cumulative reduction in sediment supply to the delta, we delineated distinct geomorphic regions and estimated sediment yields based on geomorphic characteristics, tectonic history, and available sediment transport data available. We then applied a network model to calculate cumulative sediment trapping by dams, accounting for changing trap efficiency over time and multiple dams on a single river system. Under full build-out of all dams as currently planned, cumulative sediment trapping will be 96%. That is, once in-channel stored sediment is exhausted, only 4% of the pre-dam sediment load would reach the Delta.