Bob Robison was the fourth-best slalom canoer in the world at age 15. Great things were predicted for the phenom from Fairfax County. But four years later, in 1981, he didn't even make the U.S. team.

Yesterday, Robison served notice that he is back on the river to stay with an upset victory in the slalom event at the Pan American Cup trials on the Potomac.

As hundreds watched from cliffs just below Great Falls, Robison put on a display of strength, speed and balance as he played the waves and white water through a 30-gate course to beat four-time world champion Jon Lugbill of Fairfax County and Dave Hearn, Garrett Park, rated second best in the world.

"This is the biggest race I've won in a couple of years," said Robison, who is only 20 but until today was being called by some in the sport a has-been. Robison won his event despite a broken right foot suffered two weeks ago when he performed an inelegant back flip on a dance floor at a fraternity party.

If Robison's victory was unexpected, the first place finish of three-time world champion Cathy Hearn in the women's single kayak slalom event was not. The 23-year-old from Garrett Park, sister of Dave, finished 41 seconds ahead of Elizabeth Hayman of Washington.

"I grew up out here," said Hearn, who has as much gold, silver and bronze as a precious metals dealer. "I know this water."

Many of the competitors in the two day trials grew up beside the Potomac. For that reason the competition, though small-time compared with the international circuit of events, was an important showcase for the home town crowd. It was also the last event for many of the best kayakers and canoers who will begin a two-month tour of Europe in two weeks.

Not all of America's best white water racers will make that trip. For those who can scrape together the money it will be eight weeks of camping out, cadging rides from European competitors and gladly accepting leftovers from teams that are funded by their governments.

"We are the best team in the world and the only one that isn't funded," said Cathy Hearn, who has been traveling the international circuit the last six years.

America's white water canoe and kayak team is unique in the world both because of its rapid ascendancy to international dominance and the hand-to-mouth existence of its members.

"There is no payoff in this sport," said the U.S. coach, 36-year-old Bill Endicott. This weekend's competition was the exception that proved the rule. For the first time, the organizers were able to persuade a sponsor, the Bell System Yellow Pages, to provide T-shirts, visors, trophies and a Saturday night party. It was a major accomplishment that netted only a few thousand dollars.

"Our fund raising is a joke," said Cathy Hearn. "We have brownie and T-shirts sales."

Hearn has been successful at some minor league fund raising of her own, but being a world champion and having golden orange hair and an all-American face full of freckles made her job easier. She looks out at the world of corporate advertising and sees a vast, untapped market.

"White water has been discovered by the general public. Watches, liquor, McDonalds, all kinds of things use white water in their advertisements," said Hearn. "There is so much money out there. We just don't have anybody to hustle for us."

But Hearn and her fellow paddlers only talk money when asked. Whether they are subsidized or not, they say they will continue to compete on the international circuit where they now ride high. River riding, they say, is just too much fun to let reality interfere.

"We train most of the time like kids playing in snow," said Hearn. "You never know you're tired until you stop."

Lugbill shrugs off the vicissitudes of his sport. "It's not like we were misled. We knew there was no money in it when we started."