The Big South Fork of the Cumberland River is a major drainage feature of the Cumberland Plateau, a major tributary of the Cumberland River system, a world-class whitewater canoeing and kayaking stream, and the major feature of a National Recreation Area bearing its name.

The Big South Fork is formed by the confluence of the New River and the Clear Fork River at the southern end of the Big South Fork National Recreation Area near an oil field developed by petroleum interests. It is downstream from extensive coal deposits. This confluence occurs approximately four miles (six km) line distance northwest of the U.S. Highway 27 bridge over the New River, in Scott County, Tennessee. From here, the river runs roughly north. This area is extremely remote. The river flows through a deep gorge which has been eroded through sandstone of the Pennsylvanian Period. A large island, unsurprisingly called "Big Island", is located fairly near the Kentucky state line. Many rapids features have names by which they are well known in the whitewater community. The stream enters McCreary County, Kentucky shortly below this point.

One of the minor tributaries of the Big South Fork in McCreary County is Troublesome Creek, which was known at one time for a small population of people whose appearance was bluish. This was thought to be due to inbreeding, and could be treated by methylene blue, seemingly validating the concept of homeopathy. This area was depopulated when the National River and Recreation Area was formed. The balance of the stream course prior to its confluence with the Cumberland River downstream of the National Recreation Area is in Daniel Boone National Forest. It is bridged by Kentucky Highway 92 near the community of Hilltop.