About 120 kayakers and rafters intend to turn out on the Mokihinui River in Buller to highlight the importance of wild rivers on Sunday.
Helicopters will ferry people and boats to the head of the river.
Meridian Energy plans to build an 85 metre high dam on the river for a hydro scheme.
Whitewater New Zealand patron Hugh Canard says the Mokihinui is one of the few remaining wild rivers worldwide which is ecologically intact, from source to sea.
