Complex mass-movement terrains in the western Cascade Range, Oregon

A variety of mass-movement processes interact to form complex mass-movement terrains in the western Cascade Range in Oregon. Slow, deep-seated (>5 m depth) processes of creep, slump, and earthflow operate simultaneously and sequentially, resulting in unstable conditions that may initiate rapid, shallow (<5 m) soil mass movements on hillslopes and debris torrents in stream channels. This combination of massmovement processes supplies large volumes of sediment to streams and determines the geometry of the channel and valley floor. Creep movement in western Oregon has been monitored at rates as high as 15 mm/yr. Relative movement between discrete blocks in the Lookout Creek earth flow ranges up to nearly 10 times as fast. Movement rate accelerates during periods of high moisture availability. Geomorphic observations and tree-ring analysis indicate that mass-movement terrains may have histories spanning centuries and possibly millennia.