Alexander Mackenzie travelled from his winter quarters near Finlay Forks to the Pacific coast in 1793. On his return to the east, travelling through the same unnamed pass through the Rocky Mountains that he traversed on the trip west, he wrote in his journal:

Friday, 16 August, 1793. The weather continued to be the same as yesterday, and at two in the afternoon we came to the carrying-place which leads to the first small lake; but it was so filled with drift wood, that a considerable portion of time was employed in making our way through it. We now reached the high land which separates the source of the Tacoutche Tesse, or Columbia River, and Unjigah, or Peace River: the latter of which, after receiving many tributary streams, passes through the great Slave Lake, and disembogues itself in the Frozen Ocean, in latitude 69-1/2 North, longitude 135. West from Greenwich; while the former, confined by the immense mountains that run nearly parallel with the Pacific Ocean, and keep it in a Southern course, empties itself in 46. 20. North latitude and longitude 124. West from Greenwich.

Mackenzie believed it to be the Columbia, or a major tributary thereof, and Lewis, and Clark, in 1805 the next non-Indigenous party to cross the northern part of the continent of North America, shared the assumption. Mackenzie’s coordinates are accurate for the mount of the Mackenzie River, to which the Unjigah or Peace is a tributary. His coordinates for the mouth of the Columbia River are also accurate, but he was mistaken in thinking that Tacoutche Tess was the Columbia; it is the Fraser River, neither the Columbia nor a tributary, but this was not discovered until 1807 by Simon Fraser.