Slowly but surely, the world’s reservoirs are getting gunked up with sediment. In an unblocked river, the flowing water carries bits of sediment along—picked up from river banks or swept into the river from rain. However, rivers whose flow has been interrupted by a dam deposit some of that sediment right behind the dam itself, in the reservoir. “Gradually, [over] years and years, it will be accumulating,” Duminda Perera, a researcher with the United Nations University Institute for Water, Environment, and Health in Hamilton, Ontario, told Ars.

According to Perera, the increased sedimentation in these reservoirs, and the resulting loss of volume, are rarely considered. However, he and some of his fellow researchers recently penned a new study, suggesting that nearly 50,000 large dams—defined as being 15 m tall or more or above 5 meters high and blocking more than 3 million cubic meters of water—are being robbed of their capacity.

This slowly accumulating sediment takes up volume in the reservoir, occupying cubic meters that would otherwise be filled with water that would ultimately flow through hydroelectric turbines or be diverted to agriculture. “If you fill a cup with water, and then you put soil… the water volume is reduced,” he said.