Downriver from the thundering torrent known as Great Falls, and upstream from the placid pool we call the Potomac River, there is a snakelike slice of river known simply as the S-Turn.

This quarter-mile stretch of whitewater is the setting for this weekend's annual Great Falls Invitational Slalom Canoe and Kayak Race.

The S-Turn is neither spine-tingling whitewater or sluggish flat water: The river moves swiftly through the channel, and although it does provide some pleasurable paddling, the Potomac River whitewater is better almost everywhere else.

But lace the sky with wires, suspend red and green slalom poles from them line the banks with enthusiastic spectators and invite only nationally ranked paddlers: You'll have the most exciting whitewater race ever held in the area.

The upstream and reverse manuvering required in slalom kayak and canoe racing transforms a moderately difficult bit of river into a championship course.

This weekend's races offer area residents their first chance to see nine-time national kayaking champion and former Olympic paddler Eric Evans in action. He's not coming here just for fun: Local kayakers will be offering him stiff competition for a place on the U.S. Whitewater Team, which is to be selected next month.

All the competitors have been nationally ranked by computer and were invited to compete.

To the uninitiated, a slalom river race may appear complicated, but even someone who hasn't even set foot in the Tidal Basin paddle boats can get caught up in the exhilarating competition.

The course itself is composed of some 25 to 30 slalom gates; the paddlers must pass through them in succession, keeping the green pole on their right and the red pole on their left. Some gates are marked for upstream manuvering - the racer must pass the gate and then paddle against the current to clear it. Other gates are reverse gates, which the paddlers must negotiate stern-first.

The racers begin the course at one-minute intervals, racing the clock rather than one another.

The Great Falls Invitational is a two-day affair.

On Saturday, the major racing day, the races begin at 9:30 and end when each paddler has run the course twice.

Sunday, at noon, there'll be team races, in which three paddlers compete together, leaving at the same time and clearing the middle gates within 10 seconds of one another.

ON COURSE

To find the S-Turn, park at the Great Falls parking lot on the Virginia side.Walk downriver about a quarter of a mile and follow one of several trails along the river bank. Park rangers will be there to help you find the race. Be sure to wear sturdy shoes, as the terrain is a little rough.

Water levels may alter the course, making it visible from the Maryland side; call the Park Service at 759-2915 for last-minute changes. The race goes on, rain or shine. CAPTION: Picture, NEGOTIATING THE S-TURN IN THE POTOMAC. By James M. Thresher