The Stillwater River is an 11.5-mile-long (18.5 km) side channel of the Penobscot River in Maine. From its source (44°57′59″N68°38′45″W) in Old Town, the Stillwater runs 2.3 miles (3.7 km) northwest along the northeast side of Orson Island, the site of the Penobscot Indian Reservation. It then runs 9.2 miles (14.8 km) south along the west sides of Orson and Marsh islands, over three dammed falls.