Situated near Mount St. Helens in the Cascade Range in the southern part of the state, it flows generally west through Mount St. Helens National Volcanic Monument and industrial timberlands for 37.4 miles (60.2 km).

The river drains more than 130 square miles (340 km2) in parts of three Washington counties: Skamania, Lewis, and Cowlitz.

It rises from snowmelt on the opposite side of a ridge from Spirit Lake, in the Gifford Pinchot National Forest in Skamania County.

The river initially flows north, but turns northwest after the confluence with Grizzly Creek.

Turning west, it crosses into Lewis County, then back south into Skamania, and almost immediately afterwards flows into Cowlitz County.