GEOMORPHIC RESPONSE TO WIDESPREAD, CATASTROPHIC DISTURBANCE BY VOLCANIC ERUPTION

Explosive volcanic eruptions can catastrophically disturb hundreds to thousands of km² of landscape, profoundly affecting fluxes of water and sediment. Vegetation can be defoliated, razed, buried, or removed, and the landscape can be mantled with tephra. Channels can be partly or completely filled with laharic or pyroclastic debris, and valleys can be overwhelmed by massive deposits of debris avalanches and pyroclastic flows. Valley fill can truncate tributary channels, impound transient lakes, alter drainage networks, and modify watershed boundaries. Tephra deposits commonly reduce landscape infiltration capacity, which promotes overland flow. Such hydrologic changes typically increase posteruption flood and lahar discharge and frequency, as well as basin sediment yield.

Recent studies show that streamflow perturbations caused by some explosive eruptions are relatively transient, whereas impacts on sediment delivery systems are much more persistent. At Mount St. Helens, posteruption stormflow discharges increased as much as 70% for 5 years after the eruption, then the perturbation signal diminished. On a time scale of tens of months, overland flow, bioturbation, freeze-thaw cycles, and erosional sorting disrupted low-permeability pyroclastic surfaces, increased landscape infiltration capacities and reduced the quantity and rate of water delivery to channels. In addition, sediment sorting, soil compaction, and revegetation stabilized rill networks and diminished hillslope sediment supply.

Annual sediment yields (Mg per m³/km² runoff) following volcanic eruptions can transiently exceed preeruption yields by more than 100 times. But in contrast to streamflow responses, sediment yields up to 100 times above preeruption levels can be sustained for years to decades. At Mount St. Helens, persistent channel instability and bank erosion have sustained abundant sediment supply even after terrace and floodplain surfaces have been revegetated.

Recovery of volcanically disturbed landscapes involves two major geomorphic systems that respond on different time scales. Effective management of volcanically disturbed landscapes requires recognition of this reality.