Bayou Manchac is an 18-mile-long (29 km) bayou in southeast Louisiana, USA. First called the Iberville River ("rivière d'Iberville") by its French discoverers, the bayou was once a very important waterway linking the Mississippi River (west end) to the Amite River (east end). East Baton Rouge Parish lies on its northern side, while its southern side is divided between Ascension Parish (to the east) and Iberville Parish (to the west). The large unincorporated community of Prairieville and the city of St. Gabriel both lie on its southern side.