THREE-QUARTERS of a mile below Maryland's Great Falls National Park, 15 of the nation's best whitewater canoeists and kayakers are practicing on the Potomac every day.

These Washington area athletes are among 46 Americans who will fly to Austria next month to vie for the world whitewater canoe and kayak championships.

Just a short distance beyond the rush of the falls, the 13 young men and two women put into the treacherous swirls for sweeping runs through slalom gates.

They use the force of the currents where they can, but often they fight the white water with mind and muscle as they slip their craft along the course.

Boats often disappear in the foaming river and all that is visible are the torsos of the paddlers, arm muscles straining to control paddle and craft.

Their rigorous training schedule will continue until the Washington contingent departs July 10 for the week-long competition in Spittal, Austria.

All but three of the local paddlers are under 21 years old. In most international amateur sports teenage competitors are common, but paddling was the exception until recently. Why the switch?

Jack Brosius, a member of the 1972 Olympic team and coach of area paddlers for the Canoe Cruisers Association, attributes it to the CCA's seven-year-old C'cat program, which teaches, paddling to 12- to 14-year-olds.

"Now these same kids are coming of age," Brosius said.

And while most paddlers must endure harsh weather to keep in training over the winter, Brosius and his students have been enjoying the Navy's indoor David Taylor Model Basin in Carderock.

The basin has two tanks, one a quarter-mile and the other a half-mile long, and is the only facility of its kind in the world.

"When we're doing stamina and technique training, the basin is great because we're able to push the kids to a fatigue point that wouldn't be safe in whitewater," Brosius said. "We can cencentrate on grinding away in mileage workouts while those in other parts of the country have to literally get into swimming pools to work out.

"We'd put in six to 10 miles of paddling in a two-hour workout during the winter."

While Brosius tested the youngsters' endurance, U.S. national team coach Bill Endicott worked with them on slalom technique. In whitewater slalom, both in canoes and kayaks, competitors must go through 30 gates and are penalized seconds for touching the poles or missing gates.

In the canoe competition, where one-bladed paddles are used and the competitor kneels in the cockpit of his craft, Washingtonians made a particularly good showing in trials.

In the C-1s (one-man canoes), Kent Ford, Ron Lugbill and Bob Robison took three of the top four qualifying times. Only Jamie McEwan, Olympic bronze medalist in this class, was able to keep Washington from a complete sweep at the trials.

"Washington has always been a good area for canoeists," McEwan said by telephone from his home in New Haven, Conn. He should know - he was a Silver Spring resident during his Olympic years.

The differences between kayaks and canoes is that the kayaker uses a double-bladed paddle and sits, rather than kneels, in the boat.

Cathy Hearn 19, the only area resident to qualify in the solo K-1W women's class, took a year off between high school and college to train for the world championships, which are held every two years.

Then, two weeks after the national trails, the two boats she had designed specifically for whitewater racing and built herself were stolen from outside her home in Kensington.

She had put in 30 or more hours building each boat. She got more fiber-glass but had to arrange for a commercial company to build a new boat for the Spittal competition.

How well will the Americans do against the Europeans, particularly those from the Communist countries, who have traditionally dominated the sport? Mcewan, a veteran of overseas competition, thinks there are chances of success.

"In C-1, all four of us on the U.S. team have the potential to place. In Europe there are about one or two paddlers better than us, and 10 others who are about the same level. I'd say in the end it will depend on who adapts well to the Spittal course."

Coach Endicott also is "very optimitic," even though the Leiser River is familiar water to the Europeans.

Seven-time national K-1 champion and Olympic medalist Eric Evans also has high hopes for American prospects.

The CCA kids have a little bit different approach than Americans have had in the past," he said.

"There is so much competition among themselves that they go all out for speed, and that's how the Europeans paddle. The way they figure it, they've got to take chances with accuracy, because if they don't someone else will and they might lose the race because they were too cautious."

Evans says he's "just thrilled" to see Washington younsters coming up into world class. He expects them to figure prominently in whitewater competition for years to come.

The area competitors: Carl Gutschick, John Hefti, Yuri Kusuda, Steve and Mike Garvis, Steve Draper, Don Morin, Ron and Jon Lugbill, Jeff Huey, David and Cathy Hearn, Tom McEwan, Kent Ford and Bob Robison.