The watershed encompasses approximately in portions of La Porte County, Indiana, and Berrien County, Michigan. The main branch of the Galien rises from the outflow of Dayton Lake in the northwest corner of Bertrand Township and flows west to the village of Galien, Michigan. It then flows northwest into Weesaw Township and north to near New Troy, where it is joined by the East Branch and begins to flow west and then southwest to New Buffalo.
