Another six people were injured and up to six more were believed to be missing, according to rescue workers. There was no immediate word on the nationality or ages of the victims.
The accident happened in a gorge of the river Saxeten, near Interlaken. Ramblers raised the alarm after they found a dozen bodies floating in Lake Brienze.
Efforts were continuing last night under emergency lights to find survivors or bodies. Up to 30 people took part in the expedition, which was organised by a company called Adventure World, used mostly by American and Japanese tourists.
At a press conference in a school in Boenigen last night, Andreas Haesler, who was out running along the river, said he saw bodies in the water downstream from where the accident occurred. "When I saw the first body I immediately knew that they must be dead. I saw seven or eight in the water before the river goes into the lake," he said. "The river was black with mud and there were large pieces of wood and large stones in the water." He said he thought the victims he saw were not drowned but smashed to death.
A police spokesman said nothing of this scale had happened in Switzerland before; one person taking part in the sport two years ago had drowned.
Swiss police said the canyoners were washed away when the water in the river Saxeten turned into a torrent after a rainstorm soon after 6pm local time yesterday. The wave of water was measured at one metre high at the other end of the gorge where the river meets another before going into the lake.
Martin Trapp, the examining magistrate in Interlaken, who is leading the investigation, said that 44 people had entered the Saxeten, led by a guide from Adventure World. "There was thunder and lightning in the afternoon and it will be part of the investigation whether thunder and lightning caused the deaths.
"As far as I know there aren't any regulations concerning these companies and canyoning, but the companies have lots of clients and to my knowledge there has never been such a case in this area before."
Canyoning, which began in the United States, is a new craze for those seeking adventure and extreme physical danger. It involves exploring narrow valleys and underground caverns. Canyoners are roped together and helmeted. Wearing wetsuits, they clamber down fissures and steep gorges to crawl and swim through mountain streams.
Rescue services in France and Switzerland put out warnings last month that the sport might be especially hazardous this year. The heavy snowfalls in the Alps earlier this year are still melting. If combined with sudden downpours, the melting snow can turn mountain streams into raging torrents in minutes.
Adventure World, based in the town of Wilderswil, declined to comment on the accident last night.