Instead of rowing on the Potomac River, with its tides and currents, flotsam and jetsam and effluent from the Blue Plains Sewage Treatment Plant, Northern Virginia's high school rowing teams soon will have an Olympic-class race course - on their own clear drinking water in Occoquan Reservoir.

Trials for the junior boys world championships are expected to be held on the new Occoquan course this summer, if the reservoir doesn't run dry as it almost did last summer, and the Northern Virginia championships may be held there later this spring.

To house their long, thin, glistening shells, local crews will have a $156,000 boat house as soon as fellow Fairfax County students, who are practing their building skills in vocational education courses, finish mortaring up the brick walls.

On Occuquan the high school rowers, among the nation's top scholastic rowing teams, can have their lake and drink it too since the reservoir is the main public water supply for most of Northern Virginia. The boat house is being built thanks to gung-ho friends, student construction workers, the Northern Virginia Regional Park Authority and a $78,250 federal matching grant.

The crews' friends, organized as the Northern Virginia Boat House Committee, are trying to raise $25,000 - they've raised $17,000 so far - and already are dreaming of yet another boat house, one on the Potomac River by Key Bridge. The National Park Service met last week with the committee and the Northern Virginia Regional Park Authority to discuss a possible boat house on or near Roosevelt Island.

Local high school and college crews and private rowing clubs presently are jammed into two boat houses by Key Bridge and one on the dilapidated docks of Alexandria's Torpedo Factory. While they practice and race on the Potomac, the river has not been ideal since there are strong currents and tides, much debris, including treated sewage from Blue Plains, and the wide tidal estuary is windy, according to Charles Butt, coach of the Potomac Boat Club and the "father" of high school rowing in the Washington area.

Butt began coaching rowing at Arlington's Washington-Lee High School in the early 1950s, and his crews won national championships for eight straight year until 1964. Four other Northern Virginia high schools now have crews - no District or suburban Maryland schools have rowing teams - "and they provide the toughest level of scholastic rowing in the United State for both boys and girls . . . though that could start a fist fight in Philadelphia," said Butt. "This year Fort Hunt and T.C. Williams are the power on the river . . . or reservoir."

The new 1,500 meter, six-lane race course at the reservoir will be just above the dam and the town of Occoquan. The boat house, on a lovely spit of land called Sandy Point about a mile up from the dam, would cost at least $250,000 if it were built commercially, according to Darrell Winslow, executive director of the Park Authority. Like all regional park facilities, the boat house will be open to the public for the storage of canoes and other non-motorized boats.

The regional park agency, which owns 25 miles of parkland on the northern side of Occoquan, plans to build a small natural amphitheater on the high banks overlooking the reservoir, as well as docks, boat ramps and a parking lot.

Construction of the boat house, begun last winter, has proceeded slowly because student workers also must attend classes - in temporary cabins they built beside the reservoir - and because the last two winters have been severe.

The students also have other obligations. They's still building an environmental studies center for the Park Authority, a few miles further up the reservoir. To be called Hemlock Overlook Regional Park, the center will have six dormitories, a dining hall and meeting rooms and will spend as long as a week there studying, first-hand, subjects like botany, biology and earth sciences. Its cost will be less than $200,000, although it would cost about $500,000 to build commercially, park officials estimate.

Fairfax County's ambitious student building program, began in 1972, already has produced two suburban homes, which originally sold for $70,000 and since have resold for much more.

The program is popular with students and credited with helping start electricial, plumbing and carpentry careers for hundreds of students who otherwise might have dropped out of high school.