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How to estimate the number of displaced people

In order to estimate the number of people displaced, you may want to find the populations of towns in the area. Wikipedia can be useful for this, though it doesn't have information for many small places. The dam's environmental impact study should include this data.

Read an article about indigenous people struggling against large dams.
Read a Bank Information Center article about relocating entire villages in Peru for mines and dams.
Watch a video from the New York Times/UBC School of Journalism profiling displacement issues due to planned Brazilian hydroelectric projects.