Sediment yield following severe volcanic disturbance - A two-decade perspective from Mount St. Helens

Explosive volcanic eruptions perturb water and sediment fluxes in watershed s; consequently, posteruption sediment yields can exceed preeruption yields by several orders of magnitude. Annual suspended-sediment yields following the catastrophic 1980 Mount St. Helens eruption were as much as 500 times greater than typical background level, and they generally declined nonlinea rly for more than a decade. Although sediment yields responded primarily to type and degree of disturbance, streamflow fluctuations significantly affe cted sediment-yield trends. Consecutive years (1995-1999) of above-average discharge reversed the nonlinear decline and rejuvenated yields to average values measured within a few years of the eruption. After 20 yr, the averag e annual suspended-sediment yield from the 1980 debris-avalanche deposit re mains 100 times (10(4) Mg [megagrams]/km(2)) above typical background level (similar to 10(2) Mg/km(2)). Within five years of the eruption, annual yie lds from valleys coated by lahar deposits roughly plateaued, and average yi elds remain about 10 times (10(3) Mg/km(2)) above background level. Yield f rom a basin devastated solely by a blast pyroclastic current diminished to background level within five years. These data demonstrate long-term instab ility of eruption-generated detritus, and show that effective mitigation me asures must remain functional for decades.