Two years ago when kayakers crossed the finish line near Smoke Hole Carverns in the Peterburg whitewater races they were coated with ice. Yet they were grinning from ear to ear, saying things like "Wasn't that awful!" and "Glorious!"

The Petersburg races the last weekend in March and the first in April open the canoeing season for paddlers throughout the Potomac basin. The North Fork of the South Branch of the Potamac teems with winter-weary canoeists and kayakers, even if the weather is less than balmy.

The slalom races, run through gates strung across the river at Harmon's Rapids, are held first, folowed the next weekend by the wildwater races, point-to-point runs.

They attract paddlers from as far as Maine, Minnesota, and Texas and as close as Cabins, W. Va.

The paddlers compete in more than 30 classes and for the experience of the clear cold water, the icicles along the shore, the first hint of spring green in the fields and trees, the powdering of snow on the hilltops.

The Petersburg races sometimes co-incide with a blinding snowstorm in the mountains above the still-chill valley. They are possibly the only ones with the hazard of cows as well as rocks and fallen trees in the stream bed - do you pass the nose of the cow on her stern?

The Petersburg weekends kick off a series of races that ride the spring run-off from the ice and snow of winter, the time when all streams of the Potomac basin are at their most challenging.

Next come the Virginia Championships April 23-24, on the Shenandoah at Front Royal, probably the largest gathering of canoeists in the country. On the first of May comes the Potomac River White Water Race, then the Seneca Slalom, May 21-22. The last two are Washington specials, attracting primarily local paddlers.

The Potomac race, now in its 22d year, is one of the oldest organized whitewater races in the country and it has one of the longest and nastiest carries. The canoe is off-loaded in the Great Falls parking area on the Maryland side and portaged a half-mile down the towpath, another quarter-mile along the Billygoat Trail then down a treacherous gully to the starting point. At the finish at Sycamore Island it has to be carried back up a cliff to MacArthur Boulevard.

In the meantime, the paddlers have raced through a 7 1/2-mile stretch of a beautiful and unspoiled American wilderness right in Washington's backyard. They have run down the Mather Gorge known in the 19th Century as "The Grand Canyon of the East," through Wet Bottom Chute, Difficult Run and Offutt Rapids, through Yellow Falls and Stubblefield Falls, and three miles beyond. All that's seen of man's handiwork is the Beltway bridge and one home.

After these four races, which comprise the formal rises of spring, river canoeists fan out all over the watershed to their favorite rivers and streams. Famous and fearsome - to be run by experienced experts only - are such stretches as Little Falls Rapids above Chain Bridge, and the Youghiogheny (flowing westward) in Pennsylvania. But all along the Potomac, the Shenandoah and their tributaries there are gentler runs and small riffles that less experienced paddlers can travel, guided by practiced friends.

Whitewater or river canoeing differs from flatwater or lake canoeing just as sailing does from rowing. It is the art of using the river currents, rocks and eddies - not fighting them - to take paddlers where they want to go, just as sailors use the winds.

Skilled paddlers are constantly "reading" the water, looking for the best current, the telltale signs of underwater rocks, of crosscurrents. They are constantly adjusting their paddle strokes as they go. In whitewater, contrary to flat water, the most important person in time of stress is the bow paddler who sees an emergency soonest and takes action to avoid it. The stern paddler must follow the bow's lead.

Another difference between take paddling and whitewater is the location of the life jacket. While government regulations now require lifejackets in all boats, they are usually kept in the bottom of the canoe. In the rapids, however, where even the most expert peddlers expect the unexpected, life jackets are worn. When the unexpected does happen, whitewater paddlers haul the boat ashore, swearing a bit, empty out the water, then set off again with a smiling comment like. "Wasn't that awful!"