The Cedar River originates in the Cascade Range near Abiel Peak, Meadow Mountain, and Yakima Pass, along the King and Kittitas countyline. Several headwater streams join in the high mountains fed from glacial run-off, then the Cedar River flows generally west. It is impounded in Chester Morse Lake, a natural lake that was dammed in 1900 for use as a water storage reservoir. The Rex River joins the Cedar in Chester Morse Lake, as do the two forks of the Cedar River, the north and south forks.