It shall be unlawful for any person, State, or municipality, for the purpose of developing electric power, to construct, operate, or maintain any dam, water conduit, reservoir, power house, or other works incidental thereto across, along, or in any of the navigable waters of the United States, or upon any part of the public lands or reservations of the United States (including the Territories), or utilize the surplus water or water power from any Government dam, except under and in a

ccordance with the terms of a permit or valid existing right-of-way granted prior to

No person may commence any significant modification of any project licensed under, or exempted from, this chapter unless such modification is authorized in accordance with terms and conditions of such license or exemption and the applicable requirements of this subchapter. As used in this paragraph, the term “commence” refers to the beginning of physical on-site activity other than surveys or testing.