Postglacial Influence of Volcanism on the Landscape and Environmental
Pringle and Scott: Postglacial Influence of Volcanism on the Puget Lowland Postglacial Influence of Volcanism on the Landscape and Environmental History of the Puget Lowland, Washington: A Review of Geologic Literature and Recent Discoveries, with Emphasis on the Landscape Disturbances Associated with Lahars, Lahar Runouts, and Associated Flooding Patrick Pringle Washington DNR, Division of Geology and Earth Resources Kevin Scott US Geological Survey, Cascades Volcano Observatory Abstract Geologic and geotechnical studies during the last several decades have shown that the environmental history of several major river systems of the Puget Lowland includes periodic severe geomorphic and environmental disturbances from lahars (volcanic debris flows), lahar runouts, and associated flooding. Episodic lahars and post-laharic sedimentation have significantly aggraded the Nisqually, Puyallup, White, Skagit, Duwamish, Stilliquamish, and Nooksack Rivers, caused delta progradation that has dramatically altered the coastline of the Puget Lowland, and, for extremely large events, triggered stream piracy, as exemplified by that in the Stillaguamish/Skagit River, Fraser/Nooksack, and White/Puyallup River systems respectively. These volcanic disturbances buried extensive landscapes including mature, old-growth forests and destroyed and/or severely disrupted pre-Euro-American-settlement human communities. Geologists have identified two main types of lahars, cohesive (>3% matrix clay) and noncohesive (<3% matrix clay), each of which has distinctive flowage and depositional characteristics. The recognition of flow transformations in noncohesive lahars now allows geologists to better identify lahar runout and laharic flood deposits in downstream areas. Collectively, the recent and ongoing studies of volcanic history in the Puget Sound drainage basin have greatly improved our understanding of the magnitude and frequency of the associated landscape disturbances.
[Show full text]