Effects of hillslope erosion on revegetation and sediment influx to the Toutle River

The 1980 eruption of Mount St. Helens covered soils with a tephra blanket and killed the forest tree cover in a 550 km2 area. After the eruption, rates of erosion and plant cover were measured on tephra-covered hill slopes north of the volcano, some of which had been subject to one of three land-management practices. Rill erosion was initially greater than sheetwash, but its importance decreased comparatively quickly. On hill slopes left undisturbed since the eruption, rill erosion and sheetwash underwent a rapid and early decline due to the development of a stable rill network and the exposure and creation of more permeable and less erodible substrates. This decline was independent of plant recovery. Logging of trees downed by the eruption and scarification of the surface in preparation for reforestation slowed erosion, although the effect was small because erosion rates had already slowed substantially by the time these two practices were implemented. An experimental grass seeding program gave rise to a plant cover only after erosion had slowed, and then only in a limited range of environments.

 

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