Storm Induced Mass-Wasting in the Oregon Coast Range, U.S.A.

Abstract

The rugged slopes of the Oregon Coast Range rise abruptly from the Pacific Ocean, where their folded sediments and volcanic intrusives interrupt the flow of the moisture laden subtropical jet stream yielding orographic precipitation exceeding 2000 millimeters annually. The dense conifer forest cover and deep soils allow copious rates of infiltration and throughflow, leaving sheetwash and rainsplash as relatively ineffective sediment transport processes. The steep slopes show ample evidence of deep-seated earthflow in areas of weaker bedrock, and frequent debris avalanche activity stripping soils from the surfaces covering the more resistant volcanics. The stream channels impacted by this debris quickly winnow away the fines, leaving behind gravels and large organic debris, the materials which form the gravel spawning beds and rearing pools for the region’s anadromous fish.