Long Creek is a river in central North America that begins in Saskatchewan, flows south-east into North Dakota, and then flows back north into Saskatchewan. It is a tributary of the Souris River. The Souris River drains into the Assiniboine River, which is part of the Red River drainage basin in a region called the Prairie Pothole Region of North America, which extends throughout three Canadian provinces and five U.S. states. It is also within Palliser's Triangle and the Great Plains ecoregion. In 1957, a dam was built on Long Creek near where it meets the Souris River to create Boundary Dam Reservoir.