The poisson inconnu mentioned by Mackenzie is found here.

It is a species of the Genus Salmo, and is said by the Indians to ascend from the Arctic Sea but, being unable to pass the cascade of the Slave River, is not found higher than this place.

In the evening a violent thunderstorm came on with heavy rain, thermometer 70 degrees.
At a very early hour on the following morning we embarked and continued to paddle against a very strong wind and high waves under the shelter of the bank of the rivers until two P.M.when, having arrived at a more exposed part of the stream, the canoes took in so much water that we were obliged to disembark on a small island.

The river here is from one mile and a quarter to one mile and three-quarters wide.

Its banks are of moderate height, sandy, and well wooded.
SLAVE LAKE AND FORT PROVIDENCE.
July 24.
We made more progress notwithstanding the continuance of the wind.