Is the 1821 book you are referring to Alexander Henry's Travels and Adventures 1760-1776?
A link to the passage in question would be helpful!

Your description of the general location implies that it is located somewhere between La Petite Faucille and Recollet Falls. That would mean anywhere from what is Robinson Bay on today's maps up to Ox Bay and then up the Main Channel to Recollet Falls.
Eric Morse and Toni Harting in their modern-day books on the voyageur route make no mention of such a stop between those two points. They do mention the place that the fur traders would use at the end of their one-day run
down the French on their way to Fort William. It is located at the west end of what is now called the Voyageur Channel. The place they would stop for the night was known back then as La Prairie des Francais.
Here is Alexander Henry's description of this stop -
Near the mouth of the river a meadow, called La Prairie des Français, varies for a short space the rocky surface which so generally prevails; and on this spot we encamped and repaired our canoes. See here for a close-up of the flat area at la Prairie and some pix.
https://albinger.me/2020/01/21/canoeing ... ay-marina/Re: pictograph sites on the French. What exactly does the writer say?
There are maybe as many as eight sites on the river; most are very humble and are made up of one faded image or mark. The single-most impressive site is the one at the top of the French River at Kennedy Island. Other sites reported in historical journals were not pictographs but rather lichenographs scratched out of the lichen-covered rock face; those have disappeared over the past 150 years! The Ox Bay area apparently had a very large one in the 1790s.
See this post of a recent trip down the French for more info and pix on the three pictograph sites on the upper French -
https://albinger.me/2019/07/28/canoeing ... annel-dam/