China had a dam collapse in 1975 that resulted in an additional 67 other dams to fail. The Chinese government recently acknowledged the death toll at half a million people but knowing the government their and the poor statistics of the people living alone that watershed area, I suspect the number is well north of a million people.

Honestly I would far rather live next to a nuclear plant then downstream of a damn. In every nuclear accident, you could safely walk, not run, but walk away from danger. Chernobyl included. When a damn fails, there can be little warning and the destruction is complete.