For the next five years Thompson was sent trading into the Muskrat Country, the well-known, waterlogged, bug-ridden region between York Factory and Reindeer Lake. He took astronomical observations and fixed positions as he travelled, but routine company business, piled on by Colen, occupied most of his time. Finally, in 1796, Colen couldn’t stall London any longer, and he reluctantly gave Thompson permission to map a new route to the Athabasca. Colen could still influence the outcome, however, and he knew that without good men and sufficient supplies the expedition would likely fail, thus putting an end to London’s incessant demands. Colen also had Thompson’s most recent editions of the Nautical Almanac from London. For the last two years he had somehow forgotten to send them forward. What Colen didn’t count on, though, was the determination of the young surveyor.

      On June 10, Thompson headed north on the Reindeer River. He was spared no company men and given no canoe. Undeterred, he hired two inexperienced Chipewyan youths as guides and constructed a birchbark canoe himself. The meagre ration allowed him meant his small party would have to rely on a fishnet and a musket for food as they advanced toward the treeless northern barrens. Secured in the belly of his five-metre canoe were his surveying instruments and notebook, a small tent and bedding, thirty rounds of ball ammunition for caribou, two kilograms of shot for geese and ducks, two kilograms of gunpowder, and three spare flints.

      They travelled fast under the light load for the first week, but eventually the rivers lost depth and they began tracking. When the waterways shallowed further, they carried their canoe and pack for eighty kilometres over rocks and mosquito-infested marshes before finally reaching Lake Wollaston on June 23. Over the next two nights, while his guides rested, Thompson was busy with sextant and pencil completing notes for the survey.

      They paddled across windswept Lake Wollaston for several days before finally descending the treacherous rapids of the Black River to Lake Athabasca. Six hours onto the lake they came to a protected shore well suited for a canoe camp. To their surprise, the site had been used before. Here, on a blazed pine tree, Thompson discovered the survey marking of his old master. It read “Philip Turnor – 1791.” Finally, after five years of frustration, David had mapped another route to the fur rich Athabasca region. Now he faced a bigger challenge: the long journey home.

      Trail weariness had crept up on the men, depleting their strength imperceptibly each day, until they were exhausted. Such fatigue, unheeded, compounds even small problems. In this weakened state their judgment was impaired and their reactions were slowed. That is how it was when Thompson tried to line his canoe homeward up a stretch of rapids. The young Chipewyan guides, weary and unmindful, allowed the river’s current to shear the canoe crosscurrent. Thompson, slow with his paddle, corrected too late and was now faced with the threat of capsizing. He sprang forward and cut the bowline with his knife, but he had forgotten the falls behind him. By the time the sluggish crew noticed the line was slack, Thompson was plunging headlong nearly four metres over the cataract. He was battered brutally against the rocks below the falls but managed to surface, grasp the canoe, and swim to shore. He lay gasping, his body bruised and the flesh torn from the heel of his left foot, while what few provisions they had were carried off and lost in the river.

      The crew later recovered the cork-lined box containing Thompson’s instruments and notebook but they could not find the fishnet and ammunition. Once Thompson’s foot was bandaged with tent canvas and the canoe was repaired with spruce gum, they struck out again for home. The men knew that with only berries for food, they would soon die. In desperation, one of the Chipewyan stole two young eaglets from a treetop nest. The fledglings put up a fearsome struggle and sank their talons deep into the young man’s arm before he could kill them. The men ate the birds’ yellow fat and saved the meat for future rations. The fat, however, was contaminated and produced severe gastroenteritis. Thompson and his men spent the night and the next few days disabled by vomiting, diarrhea, and intestinal cramps. Weak, and with no extra clothes or fire to warm them, they could not continue. Death seemed inevitable as the cold of the night closed in.

      Fortunately, in the morning a passing band of Chipewyan discovered the near-lifeless men and fed them some nourishing broth. In a few days the Chipewyan moved on, but they left behind a small cache that included powder and ammunition. The men, now recovered, could hunt for food.

      Some weeks later Thompson and his crew arrived at Fairford House, weak and emaciated but alive. By August, a new brigade had arrived with trade goods, and over the winter, Thompson persuaded the brigade to accompany him on his new route to the Athabasca. This time he had a party of seventeen experienced men, four canoes, and plenty of supplies. Still, the outcome was almost as disastrous as the first trip. The route, although shorter than Turnor’s route, was not practical for the fur trade. The waterways were often too shallow and the eighty-kilometre portage made the movement of heavy canoe loads overwhelming.

      The expedition was forced to a standstill late in the season and had to winter in a makeshift log hut, which they named Bedford House. The winter was hard. Game was scarce, temperatures plummeted to record lows, and worse still, trade for fur was unprofitable. When word reached York Factory, Joseph Colen finally had what he wanted. Thompson’s mapping expedition had failed to produce commercially useful results, and the young surveyor was ordered to return to the business of trading full time.