Thirty years of tephra erosion following the 1980 eruption of Mount St. Helens

Abstract

AbstractRepeated measurement of tephra erosion near Mount St. Helens over a 30‐year period at steel stakes, installed on 10 hillslopes in the months following the 1980 eruption, provides a unique long‐term record of changing processes, controls and rates of erosion. Intensive monitoring in the first three post‐eruption years showed erosion declined rapidly as processes shifted from sheetwash and rilling to rainsplash. To test predictions about changes to long‐term rates and processes made based on the 3‐year record, we remeasured sites in 1992, 2000 and 2010. Average annual erosion from 1983 to 1992 averaged 3.1 mm year−1 and ranged from 1.4 to 5.9 mm year−1, with the highest rate on moderately steep slopes. Stakes in rills in 1983 generally recorded deposition as the rills became rounded, filled and indistinct by 1992, indicating a continued shift in process dominance to rainsplash, frost action and bioturbation. Recovering plants, where present, also slowed erosion. However, in the second and third decades even unvegetated hillslopes ceased recording net measurable erosion; physical processes had stabilized surfaces from sheetwash and rill erosion in the first few years, and they appear to have later stabilized surfaces against rainsplash erosion in the following few decades. Comparison of erosion rates with suspended sediment flux indicates that within about 6 years post‐eruption, suspended sediment yield from tephra‐covered slopes was indistinguishable from that in forested basins. Thirty years after its deposition, on moderate and gentle hillslopes, most tephra remained; in well‐vegetated areas, plant litter accumulated and soil developed, and where the surface remained barren, bioturbation and rainsplash redistributed and mixed tephra. These findings extend our understanding from shorter‐term studies of the evolution of erosion processes on freshly created substrate, confirm earlier predictions about temporal changes to tephra erosion following eruptions, and provide insight into the conditions under which tephra layers are preserved. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.