INTERLAKEN, Switzerland (AP) _ The first wave was crystal clear, sweeping up adventure seekers who were enjoying an afternoon of plunging feet first through rapids and down waterfalls in the Saxeten River.

But the second wave was a towering black wall of water, and it brought logs and boulders crashing down on the 45 young tourists and their guides who were bodysurfing in mountain streams of the Swiss Alps.

By the time Tuesday's raging torrent _ caused by a flash storm _ had settled, some 19 people were dead and two were missing near this central Alpine resort.

The victims' nationalities weren't immediately known, but Swiss television reported that as many as 13 of the dead were Australians. No victims were American.

The deaths were the worst tragedy to affect young Australians overseas ``outside the theater of war,'' Prime Minister John Howard said today.

``Like all Australians I'm deeply distressed by this terrible tragedy,'' Howard said.

The deaths had officials wondering how far the growing number of people drawn to a widening range of high-risk sports should be allowed to go in search of a thrill.

The adventurers _ many counted rock-climbing and parachuting among their hobbies _ had signed up for the 1 1/2-hour trip with Interlaken-based Adventure World, as one of the highlights of their stay in Switzerland.

The company was well known among backpackers here for its extreme sports activities, such as bungee jumping and glacier climbing, aimed at those bored with simply hiking or biking.

The ``canyoning'' trip had started normally enough, for a sport that includes jumping off cliffs into tiny mountain pools and shooting through rapids and over waterfalls, with just a life jacket, wet suit and helmet as protection.

The tourists, from South Africa, Australia, New Zealand, the United States, Canada and Britain, were split into four groups with guides who were trained to watch for potential dangers including sudden changes in the weather.

And yet, according to some locals, when they set off on the tour at about 4:30 p.m., the guides ignored the heavy clouds gathering over the Alpine peaks that unleashed the fatal 15-minute downpour. Previous experience showed that when a storm hit the upper reaches of the stream, it would take about 20 minutes for the flash flood to reach the mouth of the brook.

``Something absolutely extraordinary, incomprehensible and unforeseeable must have happened, because our guides know every centimeter of the river,'' said Adventure World manager Georg Hoedle.

But local fisherman Fritz Goetz questioned the guides' judgment. ``What I cannot understand is how the people that know this region and have experience in canyoning could allow something like this to take place in these weather conditions,'' he said.

Survivors all said they were caught completely unawares.

``They heard no noise,'' said Dr. Manfred Studer, who treated six injured Australians and New Zealanders at Interlaken's main hospital. ``All of a sudden they saw a wall of black water coming on them and that's it.''

The accident was discovered around 6 p.m. by a local jogger who saw seven bodies clad in their life jackets floating into Lake Brienz, near Interlaken, a popular tourist town 35 miles southeast of the Swiss capital, Bern.

As the evening progressed, the scale of the tragedy became clear. Eighteen bodies _ many badly battered _ were recovered during the night and one more was found Wednesday. More than 100 rescue workers continued the hunt for two tourists missing and presumed dead.

Authorities said identification of the bodies may take days, if not weeks, because most of the tourists weren't carrying identification and dental records might be needed.

Swiss official Samuel Bhend said authorities would be examining whether there should be tighter limits on such sports.

``There are questions about these adventure holidays and how far we should go in search of an adrenaline rush,'' he said. ``Maybe we should learn again to respect the limits set by nature.''