The river draws water from a drainage basin of Carboniferous rock in the Cumberland Plateau.
Taken together, the Big South Fork and its tributaries drain between 1,123 square miles (2,910 km²) and 1,382 square miles (3,580 km²), of which about 17% is covered by the associated National Recreation Area.

The maximum recorded water flow on the Big South Fork was 93,200 cu ft/s (2,640 m³/s), the minimum recorded was 11 cu ft/s (0.31 m³/s), and the river averages a flow of 1,760 cu ft/s (50 m³/s).
Around half of the Big South Fork's annual streamflow occurs during the months of January through March, and the lowest streamflow typically occurs during September and October.

Along the river basin, water drops from an maximum original elevation of 1,250 feet (380 m) above sea level at Peter's Bridge on the Clear Fork, to 723 feet (220 m) where the waters meet the Cumberland.
Between the confluence of the New River and Clear Fork, and Leatherwood Ford the river has an average gradient across 6.2 miles (10.0 km) of 20 feet per mile (fpm), and a maximum gradient of 40 fpm.

Major tributaries of the Big South Fork include:
New River
Clear Fork River
North Whiteoak Creek
Pine Creek
Station Camp Creek
Parch Corn Creek
No Business Creek
Williams Creek
Troublesome Creek
Bear Creek
Roaring Paunch Creek
Rock Creek
Wolf Creek
Kogar Creek
Water quality is generally good, but may be negatively affected by mine drainage, logging, poor road quality, and ground disturbances.
The river contains about two times as much suspended and dissolved solids at a comparable river basin which does not have a history of mining.
Both Tennessee and Kentucky have designated their respective portions of the Big South Fork as Outstanding National Resource Waters, and cleanup and reclamation efforts by a number of organizations have been made.
The river draws water from a drainage basin of Carboniferous rock in the Cumberland Plateau. Taken together, the Big South Fork and its tributaries drain between 1,123 square miles (2,910 km²) and 1,382 square miles (3,580 km²), of which about 17% is covered by the associated National Recreation Area.
The maximum recorded water flow on the Big South Fork was 93,200 cu ft/s (2,640 m³/s), the minimum recorded was 11 cu ft/s (0.31 m³/s), and the river averages a flow of 1,760 cu ft/s (50 m³/s). Around half of the Big South Fork's annual streamflow occurs during the months of January through March, and the lowest streamflow typically occurs during September and October.
Along the river basin, water drops from an maximum original elevation of 1,250 feet (380 m) above sea level at Peter's Bridge on the Clear Fork, to 723 feet (220 m) where the waters meet the Cumberland. Between the confluence of the New River and Clear Fork, and Leatherwood Ford the river has an average gradient across 6.2 miles (10.0 km) of 20 feet per mile (fpm), and a maximum gradient of 40 fpm.
Major tributaries of the Big South Fork include:
New River
Clear Fork River
North Whiteoak Creek
Pine Creek
Station Camp Creek
Parch Corn Creek
No Business Creek
Williams Creek
Troublesome Creek
Bear Creek
Roaring Paunch Creek
Rock Creek
Wolf Creek
Kogar Creek
Water quality is generally good, but may be negatively affected by mine drainage, logging, poor road quality, and ground disturbances. The river contains about two times as much suspended and dissolved solids at a comparable river basin which does not have a history of mining. Both Tennessee and Kentucky have designated their respective portions of the Big South Fork as Outstanding National Resource Waters, and cleanup and reclamation efforts by a number of organizations have been made.
Big South Fork of the Cumberland River