It flows down the east side of the Cascade Range, through Wenatchee National Forest and the William O. Douglas Wilderness.

From its source at American Lake, it flows north for several miles, picking up its tributary the Rainier Fork American River, which flows east down from the vicinity of Chinook Pass.

State Route 410 crossing Chinook Pass from Mount Rainier National Park, then follows the Rainier Fork and the American River valleys.

After its confluence with Rainier Fork, the American River flows through Pleasant Valley.

It joins the Bumping River within the American Forks Campground.