Luther Carter, a stern judge of character, on John Seabury Thomson: $1"All you have to do is see John and you feel better for it all day."

Indeed Thomson, stalwart of the Canoe Cruisers Association, paddler par excellence, friend of the Potomac and of the people who use the river gently, is a luminary among Washington athletes.

He's retired from the spy service, a great strong bear of a man who hikes the Appalachian Trail, hoists a 17-foot Grumman on his shoulders as if it were a tot with sore feet and has never been observed in a bad mood in public.

"It's funny. Although you think of him as garrulous he's actually a quiet man," said Ellen Broderick, a grandmother who shared Thomson's canoe for a CCA-sponsored whitewater canoeing course. "But he's always cheerful and he's always helpful."

And so it was that a smile lit my face as Thomson's rattletrap brown Jeep, held together with duct tape, appeared in the rear-view mirror Sunday heading up River Road. He and I were on the way north that sunny morning, canoes strapped to the roof racks, for the season's first day of paddling the Potomac.

My first, anyway. Thomson and his almost-always partner, John Heideman, keep at it all year, warmed by rubber wetsuits that turn what otherwise would be death-defying adventures into harmless outings. This day Heideman had commitments and I had his wetsuit.

"It's really unlikely that you'll need it," said Thomson at the put-in at Violet's Lock, 20 miles from downtown Washington. "This stretch is not difficult. Still, a spill can happen and it only has to happen once."

With water temperatures just barely above freezing and ice still on the quiet pools, a spill could immobilize a paddler in minutes. I squeezed into the suit.

"You see why I wear these," Thomson chuckled, pointing to the shorts and shirt that covered his wetsuit. "To avoid the sausage look."

Then we were off, paddling across to the Virginia shore in the quiet water just above Seneca Breaks, a two-mile-long series of whitewater riffles and rapids. The course for the day was the old Patowmack Canal, a sluiceway built in the late 1700s so commercial barges could bypass the rapids. s

"You'll see the old stonework along the way," said Thomson. The theory behind the construction, which predated the C&O Canal, was to provide safe passage around whitewater stretches of the river. Commercial traffic could then use the main river where it was low and deep and slip off into the man-made alternatives at the shallow stretches.

When they got to Washington the tradesmen sold their cargo, dismantled the barges, sold the wood for lumber and walked home to Frederick, Hagerstown or Harpers Ferry. Unfortunately hte plan, backed by George Washington himself, was a financial disaster because the river rarely cooperated. It was too often in flood or drought, making it impassable.

The unexpected side benefit for modern paddlers is the Patowmack Canal, which by design diverts water enough for canoeing from the main river even in drought conditions.

In his book "Potomac White Water, a guide to safe canoeing above Washington," Thomson describes the run as "a charming tree-lined channel of fastmoving, light water with riffles and small drops for the paddler to play in."

So it was. For 90 minutes he led the way through the narrow slough, pausing to point out ancient, crumbling stonework or interesting riffles. Flocks of mallards and black ducks erupted from resting places and a huge vee of Canada geese harronked their way downriver.

At the bottom a yellow arrow, nailed on a tree by outfitter Lou Mattacia, pointed the way to the takeout. We paddled back to the Maryland side, hauled the boats up the bank and dumped them in the C&O Canal for the run back to Violet's Lock.

Great plan, except that 100 yards upstream the canal turned to ice and we wound up towing the boats home with a rope like a couple of mules. Smiling mules.

The Potomac from Seneca to Little Falls (with a mandatory portage around Great Falls) is one of the great underused resources for Washingtonians, a splendid wilderness in our own backyard. It is not appropriate water for beginning paddlers, however. They need lessons, which is where the Canoe Cruisers Association, the largest canoe club in the nation, comes in.

CCA runs beginners', Red Cross (novice) and whitewater canoeing classes. Beinners can get free instruction two nights a week starting May 15 at Fletchers Boathouse and Swains Lock. The Red Cross and whitewater classes cost $5 and run in June, July and September. For information write CCA Classes, P.O. Box 4116, Colesville, Md. 20904.

CCA also runs free guided trips on area waterways throughout the year. Information on these voyages is available on the CCA telephone hotline, 656-2586, or by writing to CCA Cruising, P.O. Box 572, Arlington, Va. 22216.