USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Sediment yield following severe volcanic disturbance --
a two decade perspective from Mount St. Helens
--
Major, J.J., Pierson, T.C., Dinehart, R.L., and Costa, J.E., 2000,
Sediment yield following severe volcanic disturbance --
a two decade perspective from Mount St. Helens:
IN: Geology, v. 28, p. 819-822.
Abstract
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-sed-iment
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 years,
the average annual sus-pended-
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.
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06/06/02, Lyn Topinka