Posteruption suspended sediment transport at Mount St. Helens: Decadal‐scale relationships with landscape adjustments and river discharges

Abstract

Widespread landscape disturbance by the cataclysmic 1980 eruption of Mount St. Helens abruptly increased sediment supply in surrounding 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 have varied chiefly with the style of disturbance. Posteruption suspended sediment transport has been greater and more persistent from zones of channel disturbance than from zones of hillslope disturbance. Despite the severe landscape disturbances caused by the eruption, relationships between discharge magnitudes and frequencies and suspended sediment transport have been remarkably consistent. Discharges smaller than mean annual flows generally have transported <5%, but locally ∼15%, of the annual suspended sediment loads, and infrequent (p < 0.01), large floods have transported as much as 50% of the annual suspended sediment loads in a single day. However, moderate‐magnitude discharges (those greater than mean annual flows but less than 2‐year floods) have transported the greatest amounts of sediment from all disturbance zones. Such discharges have transported, on average, 60% to ∼95% of the annual suspended sediment loads, usually within cumulative periods of 1–3 weeks each year. Although small‐magnitude and large‐magnitude discharges have locally and episodically transported considerable amounts of suspended sediment, there has not been any notable change in the overall nature of the effective discharges; moderate‐magnitude flows have been the predominant discharges responsible for transporting the majority of suspended sediment during 20 years of posteruption landscape adjustment.