The man lost his footing while jumping to a rock and fell into the fast-moving water near Hydro-Quebec’s La Gabelle dam, according to others who were there.
The incident was witnessed by the man's friends and family.
Provincial police are now searching the river with boats but cold water and a fast current are complicating their efforts to find the man.
Hydro-Quebec kept the dam gates shut Friday night to facilitate the search.
Police divers were also at the scene.