It's spring. The rivers are wide and getting wild with runoff from melting snow and early rains. Canoeists and kayakers are rubbing their hands, planning the year's first assaults on the white water.

Those, that is, who still have boats.

There's a fellow from Brookmont who used to have a Grumman. Had it, in fact, for one whole day.

He called his friend about this time of year. They took the shiny canoe to the Potomac and mounted an attack on the fast water below Great Falls.

Great fun, until a rock rose up angry and stopped them cold. "I didn't know what to do," said the guest. "My instincts told me to lean upstream so I did. It was exactly what not to do."

He leaned upstream and the water came pouring in. It swamped the boat. The men clambered out onto a rock and watched, pitifully helpless, while the surging current first bent, then broke the handsome canoe in half.

And that was that. They walked out and left the pieces there.

A pity that David Brown lacked a cape and the ability to fly. Then he could have burst on this unhappy scene and announced:

"Have no fear, Dr. Alumo is here."

Brown is Dr. Alumo, the man with the magic hammer, emergency metallurgical technician, practitioner of the almost forgotten art of open-hull surgery.

He fixes canoes.

Brown has repaired about 600 canoes, 90 percent of them aluminum, since he took up the trade six years ago in a halting way. In that time he said he has seen only two boats so badly mangled they weren't worth repairing.

And one of those he fixed up anyway, just so he could say he did it.

"I've had them torn completely in half. I just cut out the middle and rivet the two halves together. It's a little shorter but it floats."

Brown was particularly perplexed to hear of the Brookmont man's dilemma. "I don't care how bad the damage is. A new canoe can be fixed. If it was 10 years old and all-metal fatigued it might not be worth it."

He said the average price he charges to repair an aluminum canoe that's "been wrapped" (around a boulder) is $35 to $85, and the highest repair bill he's ever handed out was $150.

If that sounds high, compare it to the $500 price of a new canoe.

Not all canoe accidents happen in white water. Brown fixed one boat last year that was wiped out when the wind blew a man's chimney off his house and into his canoe. Another fellow tried to chop his boat out of the ice during a freeze and got a little close with the ax.

All kinds of things happen to canoes, but Brown winds up mostly with mundane work. Last week he was struggling with a fleet of rental canoes; every one of which had a hole in the bow and one in the stern from renters dragging them across the beach.

Brown has a curiously appropriate background for this sort of thing. He studied auto body repairs at Fisher Body; he learned about all the terrible things that happen to aluminum as an air traffic controller; he got the beat down for his riveting work when he played drums for four years behind Mark Russell at the Shoreham.

It's a lonely trade he plies in a little garage in the suburbs, but he has all the work he needs. And to him it's something of an art.

"Gee," he moaned, "I wish you'd come when we had some really mashed boats. Straightening is pretty exciting."

An avid paddler himself, when business is slow Brown heads to the rivers. "I feel like an undertaker sometimes, sitting on the rocks at Bull Falls and handing out cards when the wrecks go by."

He recommends aluminum canoes first, and Grumman as the best of the aluminums, but he also fixes ABS, epoxy and fiberglass boats when he has to. Like all right-minded people, he hates working with fiberglass.

After all, he's Dr. Alumo. "I keep 'em riveted to their seats."

Brown is swamped with work right now, so he's not soliciting. However, he said he would take letters of inquiry from serious repair applicants through his parents; at 10318 Geranium Ave. Adelphi, Md. 20783.