It was summer of the year 1806 when Thompson returned from the Muskrat country to headquarters at Fort William. There he found a new excitement and a new enthusiasm in the air. During the preceding year the American officers, Lewis and Clark, had crossed from the upper Missouri to the valley of the Snake river, and followed that stream to its junction with the lower Columbia, down which they had made their way to the Pacific ocean. John Jacob Astor, the great American merchant, was exerting all his strength to build up a fur trading empire on the Pacific slope under the flag of the Republic; and the full extent of his ambitions was now disclosed. For some years the Hudson's Bay Company had been knocking at the barrier of the Rockies, and, spurred by competition, they might at any moment burst through. Fraser for the North-Westers had already advanced from the Forks of the Peace, and accomplished the difficult and dangerous feat of descending the Fraser river. But between the Fraser river on the north and the Snake on the south was a vast region on which the feet of white men had never trod; and the North West Company, no longer crippled by their civil war with the XY traders, were resolved to claim this region for themselves. Thompson was at once despatched to Rocky Mountain House, with definite instructions to cross the mountains, this time as senior officer at the post and in full charge of the operations. The winter of 1806-7 was spent in preparation; and John McDonald of Garth, trader of the Company, together with Quesnel and Finan McDonald, clerks, lent a hand in the work. The route selected was by way of the North Saskatchewan, which Thompson knew must be practicable, because it was along this road that the Kootenay Indians had returned, when he sent them back to their homes some years before. He therefore despatched one of his men, a half-breed called Jaco Finlay, up this road into the mountains, with instructions to spy out the land. Everything was done with the utmost quietness. The Piegan Indians suspected nothing, although they were at all times visiting the fort. Even the Hudson's Bay people, encamped on the river just below the North-Westers, had not an inkling of Thompson's plans. In the spring Jaco Finlay returned to the post with his report. He had penetrated through the gap in the mountains to a large valley near the head of the river, where he had built a small outpost and got in touch with the Kootenays. From this valley he had crossed the watershed to a small stream which flowed south-westward and emptied into a mighty river. At the mouth of this stream he had built a canoe, which he had carefully "cached" for Thompson's use.