KIRK BRYAN AWARD: Posteruption Suspended Sediment Transport at Mount St. Helens—the Significance of Disturbance Style and Streamflow Hydrology

Widespread landscape disturbance by the major 1980 eruption of Mount St. Helens greatly altered the hydrology and geomorphology of several watersheds and abruptly increased sediment supply in affected basins. The profound hydrogeomorphic perturbations triggered massive redistribution of newly deposited tephra and coarse volcaniclastic sediment deposited by avalanches and lahars as well as decades- to centuries-old fluvial and laharic sediment remobilized from storage. Although detectable perturbations to basin hydrology were relatively transient (<5-10 years), enhanced sediment yields from some basins have persisted for decades. Nearly three decades of suspended-sediment measurements show that magnitudes, timing, and durations of sediment redistribution are related chiefly to the type of disturbance, which affects the type, thickness and distribution of sediment, but they also bear a strong hydrological imprint. Suspended-sediment transport has been greater and more persistent from basins with sediment-laden channels than from basins with thinly veneered hillslopes. Overall, discharges smaller than mean flows have transported <5% (but locally ~15%) and infrequent (p<0.01), large floods have transported &le30% of the annual suspended load. Moderate-magnitude discharges (those greater than mean flows but less than 2-year floods) have transported the greatest amounts of sediment from all disturbance zones. Such discharges have transported 60-95% of the annual suspended-sediment loads, usually within cumulative periods of 1-3 weeks each year. Temporal patterns of posteruption transport largely reflect depletion and isolation of primary sediment sources. Rapid stabilization of rills and gullies before significant vegetation recovery swiftly diminished supplies of hillslope sediment to channels. Development of wide, shallow channels, coarsening of channel beds, and transition from multi-thread to single-thread thalwegs gradually isolated banks and terraces and made them less vulnerable to erosion. Nevertheless, persistently high yields from severely disturbed channels show that mobile sediment supplies remain accessible, and those supplies likely will not be exhausted for many more years to possibly decades.