For most of a century taking a dam down was unthinkable, then four forces converged: the 1974 Boldt decision giving treaty tribes up to half the harvestable salmon, the Endangered Species Act, aging-dam economics, and the 1999 breaching of Maine's Edwards Dam, the first federal order to remove a hydroelectric dam against its owner's wishes. The East that killed its salmon first showed how to bring them back.
Historians weigh the four forces differently, crediting the Endangered Species Act, tribal treaty standing or relicensing economics. Edwards is not the first dam ever removed; it was the first federal order to remove an operating licensed dam over the owner's objection on environmental grounds.