It rises in the Cascades in eastern Linn County in the Willamette National Forest, about half a mile west of Iron Mountain. It flows briefly north, then generally west and southwest through the mountains and Middle Santiam Wilderness. In central Linn County it is impounded at the Green Peter Dam to form the Green Peter Reservoir. Approximately downstream from the dam it joins the South Santiam from the northeast as an arm of Foster Reservoir.