Clackamas River (KLAK-uh-muhs), c.85 mi/137 km long, NW Oregon; rises in E. Marion co., 55 mi/89 km ESE of Salem, 10 mi/16 km N of Mt. Jefferson, in Cascade Range; flows NW, through North Fork Reservoir, where it receives North Fork from E (c.15 mi/24 km long), past Estacada, to Willamette R. near Oregon City. Used for hydroelectric power.