The French-Canadian fur trader, had lived for years among the Sioux tribes.  Lewis and Clark met him when he was St. Louis bound and persuaded him to turn back with them.  Pierre Dorian spoke "Sioux" fluently and acted as an interpreter when they reached "Sioux" territory (present day Yankton, South Dakota).   After Lewis and Clark counciled with the Yanktons, Dorian agreed to spend the winter among the tribe; assisting at peace keeping.