Sediment Yield Following Severe Volcanic Disturbance— a Two-Decade Perspective from Mount St
Downloaded from geology.gsapubs.org on April 4, 2012 Sediment yield following severe volcanic disturbance— A two-decade perspective from Mount St. Helens J.J. Major* U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, Washington 98661, USA T.C. Pierson R.L. Dinehart U.S. Geological Survey, University of California, Sacramento, California 95819, USA J.E. Costa U.S. Geological Survey, Portland, Oregon 97216, USA ABSTRACT lowing the eruption, and establish a conceptual model for posteruption sedi- Explosive volcanic eruptions perturb water and sediment fluxes in mentary response to a severe explosive eruption. We restrict our discussion watersheds; consequently, posteruption sediment yields can exceed pre- to suspended-sediment yield because bedload data are limited and sus- eruption yields by several orders of magnitude. Annual suspended-sed- pended sediment averaged ≥80% of the total sediment yield (Lehre et al., iment yields following the catastrophic 1980 Mount St. Helens eruption 1983; Simon, 1999). were as much as 500 times greater than typical background level, and they generally declined nonlinearly for more than a decade. Although 1980 MOUNT ST. HELENS ERUPTION sediment yields responded primarily to type and degree of disturbance, The catastrophic 1980 Mount St. Helens eruption affected some wa- streamflow fluctuations significantly affected sediment-yield trends. tersheds severely, others mildly. Watersheds north of the volcano underwent Consecutive years (1995–1999) of above-average discharge reversed the the most severe disturbance and the greatest accumulation of new, loose nonlinear decline and rejuvenated yields to average values measured sediment from deposition by a 2.5 km3 debris avalanche (Glicken, 1998) within a few years of the eruption.
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