I think you can throw a small hydro electric generator in a river without anybody noticing, and maybe generate a couple 1000 watts.  If you want to start building a hydro electric dam that would generate the type of power you are talking about you would 100% need local approval, state approval, and federal approval.  Think of a community board, the state assembly, and the EPA.  It would take millions of dollars and probably about 5 -10 years to get all of the approvals needed.