It drains an area of the high Cascade Range in the watershed of Puget Sound north of Seattle.

The river flows generally northwest through Seventysix Gulch to Monte Cristo, where it is joined by Glacier Creek.

The South Fork continues to flow northwest until it is joined by Weden Creek, where it turns north.

A number of camping sites are located along the South Fork Sauk River north of Monte Cristo Lake.

The difficulty of access from the south led to the 1891 construction of a narrow wagon road along the Sauk River.

[11] Today the Mountain Loop Highway follows part of the Sauk River more or less along the route of the old wagon road.