USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Evolution and timing of suspended-sediment transport
following the 1980 Mount St. Helens eruption
--
Major, J.J., 2001,
Evolution and timing of suspended-sediment transport
following the 1980 Mount St. Helens eruption:
IN: Sediment: Monitoring, Modeling, and Managing: Proceedings of 7th Federal Interagency Sedimentation Conference, Reno, NV,
March 25-29, 2001, p. I-137 to I-144.
Abstract
Continuous monitoring of streamflow and suspended-sediment discharges from
disturbed basins following the catastrophic 1980 eruption of Mount St.
Helens reveals when, and under what conditions, sediment redistribution
occurs following a major landscape disturbance. The redistributed
sediment includes material deposited by the eruption as well as
centuries-old sediment that has been remobilized from storage.
Suspended-sediment yields, as much as 104 Mg/km2/yr
shortly after the eruption, declined nonlinearly in all basins for more
than a decade. Yet, after 20 years, suspended-sediment yields from some
basins remain 10-100 times greater than typical background values.
Suspended sediment is transported dominantly by stormflows; more than 50%
of the suspended-sediment load is transported in 1 to 4 weeks each year.
Very large floods (p<0.01) have transported as much as 50% of the
annual suspended-sediment load in a single day from some basins. Although
large stormflows can transport quantitatively significant volumes of
sediment, the majority of the annual suspended-sediment load is
transported by common stormflows. On average, about half of the annual
suspended-sediment load is transported by stormflows that have return
intervals of less than 1.5 years. Discharges smaller than mean annual
flows transport #10% of the annual suspended-sediment load. Two decades
of monitoring suspended-sediment discharges and channel geometry changes
in the aftermath of the catastrophic Mount St. Helens eruption demonstrate
the long-term instability of eruption-generated detritus and show that
geomorphically significant evolution of disturbed watersheds generally
proceeds under commonplace hydrologic conditions.
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06/06/02, Lyn Topinka