POST-ERUPTION SUSPENDED-SEDIMENT TRANSPORT AT MOUNT ST. HELENS--DECADAL SCALE RELATIONSHIPS WITH LANDSCAPE ADJUSTMENTS AND RIVER DISCHARGES

Widespread landscape disturbance by the 1980 eruption of Mount St. Helens abruptly increased sediment supply in multiple watersheds. The magnitude and duration of the redistribution of sediment deposited by the eruption, as well as decades- to centuries-old sediment remobilized from storage, has varied chiefly with the nature of volcanic impact. Post-eruption sediment transport has been greater and more persistent from zones of channel disturbance than from zones of hillslope disturbance. Small- and large-magnitude discharges have locally and episodically transported substantial amounts of suspended sediment. Discharges less than mean annual flows generally have transported <5%, and locally more than 10%, of the annual sediment loads, and large, infrequent (p<0.01) floods have transported as much as 50% of the annual suspended-sediment loads in a single day. Moderate-magnitude discharges greater than mean annual flows but smaller than 2-year floods, however, have transported the greatest amount of sediment from all disturbance zones. Such discharges have transported, on average, 60% to more than 90% of the annual suspended-sediment loads, usually within cumulative periods of 1 to 3 weeks each year. Although discharges less than mean annual flows may have transported great amounts of sediment during the first few years after the eruption, the overall nature of the sediment-transporting flows has not changed substantially; moderate-magnitude flows have been the predominant discharges responsible for transporting the majority of suspended sediment during 20 years of post-eruption landscape adjustment.