It was Lac du Delasse on the Montmorency River.
Chuck tied one end of the rope to the aircraft and let it drift downstream. When he got to the top of the Montmorency Falls (which are higher than Niagara) he jumped off and tied the other end of the rope to a tree. He then set himself up, engines primed and ready to go. Once he was ready he untied the rope and let the aircraft drift over the falls. He pointed the nose down and once he'd picked up airspeed he airstarted the engines - simple really.
