Here the Frenchmen gathered from the natives that the sea was only ten days distant, and this sea they knew (for Jolliet was able to take astronomical observations and to make a rough survey) could only be the Gulf of Mexico.

Jolliet feared if he prosecuted his journey any farther, he and his people would fall into the hands of the Spaniards and be imprisoned, if not killed.
Therefore, at this point on the Lower Mississippi, the expedition turned back.

Its return journey was a weary business, for the current was against the canoes as they were propelled northwards up the Great River.

But Jolliet learnt from the natives of a better homeward route, that of following the Illinois River upstream until the expedition came within a very short distance of Lake Michigan, near where Chicago now stands.

The canoes were carried over a low ridge of ground, launched again in the Chicago River, and so passed into Lake Michigan.
(There is, in fact, at this point the remains of an ancient water connection between Lake Michigan and the Illinois River, and a canal now connects the two systems.) Jolliet, in describing this region, realized that by cutting a canal through two miles of prairie it would be possible to go "in a small ship" from Lake Erie or Lake Superior "to Florida".
Father Marquette remained at his new mission on the Fox River (he died two years afterwards on the shores of the Straits of Michili-makinak).
Jolliet, on returning by way of the Ottawa River to Quebec, was nearly drowned in the La Chine Rapids (Montreal), and all his papers and maps were lost.