POTOMAC GORGE put an end to Capt. John Smith's explorations in this part of the New World, hamstrung George Potomac for commerce, and has thwarted a thousand schemes for "developing" the rocky palisades and the river that runs past them. Praise the Lord.

Which leaves Washington with a natural wonder almost unrivaled in large American cities. It is possible to leave Capitol Hill and 30 minutes later be riding the whitewater below Great Falls. For 7 1/2 miles the wild scenery stretches unmarred save for a bridge, some beer cans and occasional pieces of aluminum canoes left behind by paddlers who misjudged the rapids.

"Isn't this incredible?" exulted John Seabury Thomson, who probably knows this stretch of the Potomac better than anyone else, as he set off on a rare summer morning to lead a party of four canoes down the river. "I'ye done this river hundreds of times, but it's always fresh."

Thomson and longtime friend John Heidemann commuted by canoe from the Maryland shore several miles to their offices at the Central Intelligence Agency, summer and winter, nearly every morning for the last seven years before they retired. Only whenice choked the rapids or floors filled the river with debris did they settle for driving in from their home in Chevy Chase.

And most weekends, then and now, have found Thomson back on the river in his capacity as chairman of the Canoe Cruisers Association of Greater Washington, giving lessons in advanced canoeing techniques or over seeing races and outings.

"I can't remember the last time I saw John with his bottom dry," a friend said. "He looks a little odd standing on his feet, like a loon on the shore."

He looks impressive on shore, actually, at least when he is balancing his 17-foot Grumman canoe on his shoulders while waltzing down Billy Goat Trail from the Great Falls Tavern Museum to the put-in at Rocky Cove. Thomson is 56 and not slender, but he finished the precipitous carry less winded than a canoe-less companion 20 years his junior.

"I always tell people to take a long break here before launching," he said. "Pushing off while you're still trembling from the trail is a sure way to get yourself in trouble."

After paddling around in Rocky Islands Rapid long enough to satisfy himself that the stranger in the bow was as inexpert as he claimed. Thomson led off down Wet Bottom Chute at the head of Mather Gorge. Since the bow man was even heavier than Thomson, both got their bottoms good and wet in the choppy standing waves.

"It's a good idea to get wet right away, because that keeps you from worrying about shipping water," he said. "The natural thing, if you get into a little difficulty, is to lean upstream, away from the problem, and that is almost guaranteed to make the river roll the canoe right out from under you."

After watching too many people get in trouble along the river Thomson wrote a booklet (A Guide to Safe Canoeing above Washington, Appalachian Books, $2), detailing the rapids and access points and giving a short course in river running. It is available at Appalachian Outfitters and many other local outdoors equipment suppliers and is the Bible on the gorge and the area from Seneca down to River Bend.

Thomson is a frequent and fairly successful competitor in whitewater events, but the best testimony to his skill is his elderly but unbattered aluminum canoe. Most Grummans that go on rocky rivers are lucky to survive a season or two, as the boat-renting Fletcher family will testify (there is an affable hermit on one of the islands in the gorge who makes part of his living selling canoes pieced together from fragments that drift past his dock).

The party had the river to itself except for a pair of rock-climbers on the Virginia cliffs and a family of hikers on the Maryland side with whom Thomson exchanged echoing halloos.

"The river is as low as it's been in 10 years," he said. "Some of these rapids are Class III or IV (on a scale of VI) when the water's high, but at this stage they are no more than IIs. It looks easier when the water gets really high, because the rocks disappear and you just see slopes of water, but that means if you capsize the current can suck the boat right down and you don't have any place to climb out on the banks.

"A lot of people have had to walk home from this gorge, and quite a few of them weren't even that lucky. This is not a particularly dangerous stretch of whitewater, but it demands respect."

At Sherwin Island the gorge opens out after passing through three chutes that draw crowds of kayakers summer and winter because there is an easy put-in from the Old Angler's Inn parking lot. The chutes are too easy for decked boats, so they run them upstream.

"From here on down to Sycamore Island (the last, and imperative, takeout above the deadly Brookmont Dam near Chain Bridge) there are side-channels that are really worth the time," Thomson said. "The water is too low now for most of them, which is a shame. You can pull up on one of these little islands and be in wilderness, with nothing to indicate you are anywhere near a city . . ." He broke off with a grimace as a jet screamed low overhead. "Except of course those things."

The intrusion of the airliner was wiped away minutes later when two rough-shouldered hawks wheeled over the river, beating their wings with unusual vigor because the coolness of the morning weakened the thermal updrafts they normally ride.

Photographer James Thresher of The Washington Post still was watching the birds when a three-pound, 17 1/2-inch smallmouth bass jumped into his canoe. He managed simultaneously to subdue the frantic bass, take its picture and successfully negotiate the rapid. Fishermen take note: Thresher's was the only yellow boat of the four in the party, and a bass of similar size had jumped over his canoe a quarter-mile upstream.

"I hope we don't run into a game warden." Thresher said, "I don't have a license and I'd hate to have to try to sell him this fish story."

Halfway, through the gorge on the Virginia side is Scott Run Falls, a god-send to overheated canoeists (and lovers), who take a break to disport in its tumbling chill. "Nicest spot in the river," said Thomson as the party lunched on home-grown tomatoes, home-baked (by Thomson) sourdough bread, cheese and white Spanish wine from a goatskin bota. On the rocks above were a boy and girl who wished the men would go away, which they finally did after collecting a sack of beer cans.

"Hey, here's a full one," Thomson said, stashing a can of Budweiser in his pack. "Second one I've found this week." He shook his head. "These people go to all the trouble of hauling this beer down here, you'd think they wouldn't find it too difficult to take away the empties."

Just below Scott Run is Stubblefield Falls, the longest run of fast water and standing waves in the gorge. Thomson ran the rapid along several times for the photographer's benefit and then took his bowman through. Consequently he suffered the indignity not only of being upset in a Class II rapid but of having the whole thing recorded on film, from the long, slow tipover to the scramble for paddles and loose gear.

If it bothered him. Thomson didn't show it. He is much in demand as a race chairman because of his affable unflappability, and is not known ever to have lost his cool since he was six years old, when a Chinese looter refused to take his teddy bear during the 1927 Rape of Nanking.

("My father was a missionary who refused to take the family out of the city when the Nationalist rebels approached; he felt responsible for our Chinese friends and was sympathetic to the revolution. Our parents knew there would be looting, so they had each of the children choose a favorite possession and offer it to the first looter. It was good psychology, for us and for the rebels, but when they broke in the first young soldier was too kind to take my teddy bear and I burst into tears. Another soldier saw this, and understood, and he took it and said he would give it to his little boy. It made me feel much better."

Thomson has a thousand other stories of his career as an Old China Hand in the intelligence and diplomacy games, and now that he is retired - for the second or third time - he promises to put them in writing, "as soon as I get organized."

"He's been saying that for years," the abovementioned friend said. "I don't think he'll ever get it down on paper unless he can find a bow man who can take dictation and paddle at the same time."