Its watershed drains a portion of the western side of Mount Rainier, part of the Cascade Range.

The South Mowich River's headwater tributaries flow around a highland called Jeanette Heights.

The river, having been formed by meltwater from the South Mowich and Edmunds Glacier, flows northwest for several miles.

The two join just inside the western boundary of Mount Rainier National Park.

Another highland, the plateau-like Sunset Park lies just southwest of the South Mowich River.

It passes Paul Peak to the north, then exits Mount Rainier National Park.

After descending from the upper slopes of Mount Rainier the Mowich River flows through a broad glacier-carved valley where thick sediment deposits in the stream bed create sand and gravel bars causing the river to become complexly braided and meandering.