Way back in 1962 a couple of John Lentz' friends asked him if he wanted to go on a canoeing trip down the Back River in the Northwest Territories of Canada.

Lentz had just landed a decent job at the Export-Import Bank in Washington and was inclined to say no, since he hadn't been at the bank long enough for anyone of consequence to even recognize him yet.

"Come on," his friends pressed him. "You're not going to stay in that job for while year , are you?"

Lentz, a circumspect fellow, went to the Library of Congress to do a little research on this river. He found there the journal of George Back, after whom the river was named.

Back had made the voyage in 1836 and his journal indicated that it had been no picnic in the park.

"He went through hell," Lentz said. "He was not a white water paddler. He was a ship's captain. He lacked any knowledge at all about fast water and it was clear in the journal that he paid for that."

Further research disclosed that there had been only one other recorded navigation of the Back since. One Factor Anderson of the Hudson's Bay Company had run the river in 1855.

Lentz decided not to be among the third party to try.

"When I mentioned the trip to my mother she said I ought to see a psychiatrist," he said.

Lentz went back to his partners hoping to convince them that they, too, would be nuts to try. Instead, they convinced him he would be nuts not to.

"I asked them a lot of questions and on every one they seemed to have a pretty good answer," he said.

So lentz went.

This is 1979. Lentz is still at the Ex-Inn Bank. And last weekend he was on the Rappahannock River at Kelly's Ford, whipping together the crew for his next assault on the rivers of the Canadian Northwest Territories.

It will be his 10th such adventure. Lentz was taken by the strak beauty of the far north and the challenge of paddling where no one could bail him out if he fouled up. So taken that he organized new trips every second year since 1962.

Lentz has paddled in the barren lands, where rivers and lakes are around avery flat, arid corner, and in the streambeds of the Mackenzie Mountains, which are tall extensions of the mighty U.S. Rockies.He has tried so many rivers that now, for the first time, he is going back for a second look at one. This year he will duplicate an earlier run on the South Nahanni. Nahanni is the Slavey Indian word for "people who live way out." Way out in a territory where the population averages out to .01 per square mile.

Lentz, four neighbours and a photographer from National Geographic will set out July 4. They will fly to a pair of beaver ponds in uninhabited land near Mount Sir James McBrien, no molehill at 9,500 feet.

From there it is a 300-mile downstream shot to the nearest town-Nahanni Butte, population 200.

Lentz is going back to the South Nahanni because the first time there in 1965 the trip went too fast.

One member of the party was startled at camp one day when he went off into the woods for a rest stop. Halfway through the mission he was interrupted by the arrival of a grizzly bear, which headed straight for him. The adventurer raced back to the tent site with his trousers about his knees, a roll of toilet paper locked under his arm in a death grip, Lentz reported.

From that point on the voyage took on the look of a white water canoe race, with the offended paddler running at times as far as two miles ahead of the pack.

"I can't say I blame him after a shock like that," Lentz said.

This time the crew hopes to take advantage of the full 30 days, stopping to climb Mount McBrien, to explore some caves that extend as deep as a mile off the river and to play in the best of the rapids.

Along the way they will cross some colorfully named landmarks, including Broken Skull Creek, Hell Roaring Creek and Dead Man's Valley (also known as Headless Valley).

The adventurers expect to see more bears, hopefully in better circumstances, as well as wolves, moose, dall sheep, mountain goats, woodland caribou, lynx and other furbearers along the river banks, and occasional passing geese and ducks, which will benesting in the north.

As they practiced on the brown waters of the Rappahannock on Sunday the paddlers frightened a nesting wood duck with her brood of justhatched young.

The mother flailed off in a frenzy, battering her wings against the water as if she were injured and emitting cries to distract the intruders from the brood.

"The old injured-duck trick," said Lents. A snapshot from the wilderness. He expects to see many more in July. CAPTION: Picture, John Lentz prepares for white water canoe trip.By James M. Tresher-The Washington Post