From the NYDEC website:
"One may not modify any listed waterway by impoundment, diversion, rip-rap, bulkheads or other structures or improvements that impede or alter the natural flow of water or the free flowing condition of the river."

Full text: [url=http://www.dec.state.ny.us/website/dcs/wsrr/wsrr02.html:a956e]http://www.dec.state.ny.us/website/dcs/wsrr/wsrr02.html[/url:a956e]

I assume the same is true for any river in the US with wild and scenic designation. The logic behind it is that we should let rivers do what they want to. Probably a good thing.