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Sediment yield following severe volcanic disturbance—A two-decade perspective from Mount St. Helens

¹ U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, Washington 98661, USA
² U.S. Geological Survey, University of California, Sacramento, California 95819, USA
³ U.S. Geological Survey, Portland, Oregon 97216, USA

Explosive volcanic eruptions perturb water and sediment fluxes in watersheds; consequently, posteruption sediment yields can exceed pre-eruption yields by several orders of magnitude. Annual suspended-sediment yields following the catastrophic 1980 Mount St. Helens eruption were as much as 500 times greater than typical background level, and they generally declined nonlinearly for more than a decade. Although sediment yields responded primarily to type and degree of disturbance, streamflow fluctuations significantly affected sediment-yield trends. Consecutive years (1995–1999) of above-average discharge reversed the nonlinear decline and rejuvenated yields to average values measured within a few years of the eruption. After 20 yr, the average annual suspended-sediment yield from the 1980 debris-avalanche deposit remains 100 times (10⁴ Mg [megagrams]/km²) above typical background level (10² Mg/km²). Within five years of the eruption, annual yields from valleys coated by lahar deposits roughly plateaued, and average yields remain about 10 times (10³ Mg/km²) above background level. Yield from a basin devastated solely by a blast pyroclastic current diminished to background level within five years. These data demonstrate long-term instability of eruption-generated detritus, and show that effective mitigation measures must remain functional for decades.

Key Words: sediment yield • volcano • Mount St. Helens • geomorphology

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