The misplaced modifier in the sentence is "Built during the Great Depression." This modifier should be placed closer to the noun it is meant to describe. In this case, it seems to be modifying the Hoover Dam, but the sentence structure makes it seem like it is referring to the people who receive hydroelectric power from the dam.
To correct this, you could rephrase the sentence to clearly indicate what was built during the Great Depression. For example: "The Hoover Dam, built during the Great Depression, provides hydroelectric power to more than a million people in the southwestern United States."
By moving the modifier closer to the noun it describes, the sentence becomes clearer and eliminates any confusion about what was built during the Great Depression.
