The world’s dams have allowed cities to sprout in dry lands—but at a steep cost to hundreds of millions of already impoverished people, according to a new report.

Lead author Brian Richter, co-director of The Nature Conservancy’s Global Freshwater Program, knew from previous estimates that 40 to 80 million people have been directly displaced over the past decade by dam and reservoir construction.

But he wanted to know how many people living farther downstream had been harmed.

Richter and his coauthors used published studies, population estimates and geographic information system (GIS) data to take a look.

“Our conservative estimate of 472 million suggests that the number of people . . . exceeds by six to twelve times the number directly displaced by