Transport, Storage, and Geomorphic Effects of Large Wood Delivered to Clearwater Creek During and After the 1980 Eruption of Mt. St. Helens, 1980-2005

The pyroclastic surge that occurred in the first phase of the May 18, 1980 eruption of Mount St. Helens, Washington, flattened advanced-growth coniferous forests in a 600-km2 area, including 44% of the drainage area (100 km2) of Clearwater Creek, which drains to the Muddy/Lewis River system east and south of the volcano. Streamside trees were toppled by the surge directly into the channel, where they remained for years after the eruption. This event set up a unique case study of wood transport and interaction with fluvial processes punctuated by post-eruption floods. Wood volume and distribution and channel morphology were surveyed periodically from 1980 to 2005 in five study reaches distributed along the main stem of Clearwater Creek; aerial photographs and stream gages record floods and landslides. Wood volumes (250-990 m3ha-1 are among the highest measured in streams draining old-growth conifer forests in the Pacific Northwest. Subsequent wood routing was controlled by post-eruption hydrologic events and coupling to hillslope, riparian, and channel sources. Large quantities that were contributed by post-eruption landslides and debris torrents choked channels in expansions downstream of confined valleys. In a broad, mid-basin valley, wood remained in place until the flood of February, 1996, which entrained much of the wood in some reaches and concentrated it in jams. The tendency for displacement increased downstream where channels are wider. Channel segments that were swept clean of wood developed full-scale bars. Large volumes of wood and sediment were transported into the reach furthest downstream in 1980 to 1982 and again in 1996. The entire valley bottom was buried and a new channel course was set by interactions between transported wood and residual snags of riparian trees killed by inundation. This study tracks the fate of unusually large inputs of wood and sediment in a varied channel system and provides a large, long-term data set with which to test models of wood transport and its interaction with fluvial processes.