Time, it was clear yesterday, has not diminished the tonic powers of the commonplace. All along the Potomac River, from Hains Point to the leafy backwoods of Montgomery County, there were scenes this Memorial Day holiday that have played the same over the years, but have not grown old.

There was meat frying, Frisbees flying, games of free-for-all tag, families planted under trees, children babbling and parents napping, grandparents smiling wistfully at the young. There were the tart tastes of soft drinks and whole watermelons hulking in the grass like lurking lizards.

Up the river from Washington, in Maryland, 10-year-old Robert Oshinsky, dubbed Little Running Mouth by his Cub Scout troop, shook a Coke and handed it to a visitor, not the first prankster to attempt the ploy. His sister Sara, who once played a rat in the Nutcracker, smacked him on his sunburn and the two of them were off roughhousing at the family picnic. Twenty miles down river from there, 59-year-old Fanny Farrar sat under an oak tree in East Potomac Park, looking out over the water.

"Howya feeling grandma?" a man said as he passed.

"Grandma's feeling fine!" she cried back.

So it went Monday as thousands upon thousands of people spread blankets, opened picnic baskets, shoved canoes into the water and broke into sweats on the hottest day of the year. The temperature hit 90 degrees at National Airport, missing the 101-year-old record by 4 degrees.

Along the Potomac, summer was unofficially here, christened with bottles of Bain de Soleil and the Beach Boy's song, Endless Summer, playing on the radio.

At Violets Lock, just east of Seneca State Park, Carol Bapst stood on the muddy bank of the Potomac in her $1 red plastic K-Mart shoes, bought just for the occasion, and prepared to shove off in her kayak with a group of friends. They were headed for Lunch Island with water-tight bags full of ham sandwiches, apples, cookies, aspirin, and Pepto Bismol tablets, in case lunch didn't sit well with the easy rapids at that section of the river.

Out of the water, paddles flashed in the sun, and canoes bobbed in the riffles.

"This is our life," Carol said, sliding into her seat and pulling at the water with her paddle. "You talk about holiday recreation, you talk about paddling."

You talk about holiday recreation to Penny Burriss, standing nearby, and you're talking fish, specifically blue gills. Fishing from under a Dutch elm tree, she hooked three of the fish using home-grown worms, which she raises in a mixture of corn meal, peat and coffee grounds and keeps refrigerated in her Silver Spring, Md., house.

"I had my grandson believing I had names for all of them," she said, reeling in her line. "I'd take the worms out of the refrigerator and say, 'Hello Pete. . . .'" Whoops, the bait was missing from the hook -- those darned fish.

"Sometimes you think they's educated," she sighed. "They just take your worm and go."

One C&O canal lock up the river, two Potomac families, the Oshinskys and the Rutbergs, were wishing they brought a scythe to trim the long grass at the river's edge where they'd spread blankets. Wake from motor boats slapped the shore, but mostly the sound of birds filled the air. And, of course, Little Running Mouth, a scampish towhead of a child who knew enough to talk "on-the-record" or "off-the-record" to a reporter, and wanted to open a restaurant called "Eggs-Zackly."

"We'll stay here till the kids drive us crazy," said Carol Oshinsky, mother of Little Running Mouth. Robert made a shrill whistle with a blade of grass.

"I had to teach him that," rued Mrs. Oshinsky.

Mellowed by pina coladas, her husband, Ed, was playing catch with his 30-year-old mitt. Then Robert saw a snake swimming in the water past a plastic lemon, and everybody stared at the river. Over by the blankets, a dragon fly landed on the potato salad.

"My mothr used to call them darning needles," Carol Oshinsky said. "She said, 'Watch our or they'll sew your mouth shut.'" She paused, then said, with a mischievous melody in her voice, "Oh Robert. . . ."

Joggers went down the tow path toward Washington. At Fletcher's Boat House there was a line to get across the canal on the hand-towed ferry. The highway was full of cars with canoes lashed on their roofs.

In mid-afternoon the police had barricaded East Potomac Park, surfeited with family life, but marketly unlike the suburban convocations in the woods. Car doors were flung open, radios, blaring, and you could circumnavigate the road and not miss a measure of the disco tunes playing on radio station, KYS.

Unlike every tree sat a grill and a family, usually with more than two generations present.

Judy Bediako of Takoma Park had set up a picnic with her husband and her kids and her mother from Panama City who was blowing bubbles and chucking the cheeks of Emanual Bediako Jr., 9 months old. Piles of crab, hamburgers, hot dogs, steaks, and pots of peas and rice were waiting for the Bediakos to finish their Scrabble game, the words of which reflected both the imminence of lunch and the gloriousness of the afternoon: Meat, Mood, Ado.