Research About erosion

Posted On: Filed Under: Biological Research

Studies of hillslope erosion in the eastern part of the blast zone.

We have been measuring tephra/soil erosion rates in the Smith, Bean, Clearwater, Upper Green River areas of the eastern part of the blast zone. Debris slides and debris flows have been inventoried based on field observations and interpretation of aerial photographs for the 1967 to 1984 period. Sheet and rill erosion was measured with arrays of erosion pins. Repeat photography of hillslope, channel, and revegetation changes has been done at a variety of locations. The intensity of measurements has been reduced through time, but all sites could be revisited and longer-term trends in erosion rates estimated.

 

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Posted On: Filed Under: Biological Research

Sedimentation and geomorphic changes following the 1980-1983 eruptions of Mount St. Helens

Reduced infiltration and burial of surface roughness elements, together with the loss of root strength and reduced evapotranspiration associated with blast-toppled vegetation have dramatically accelerated erosion of blast-affected hill slopes. Other investigators have found that rill erosion rates were initially higher than the yearly average, but then declined sharply so that rill erosion rates during the second year were only 5 percent of the average first year rate. Initial rates of sheet erosion were substantially higher than the average rate for the year, but the rate of decline was not as pronounced as in the case of rill erosion (Swanson, et al., 1983b).

 

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Posted On: Filed Under: Biological Research

Stream channel adjustments in the Smith – Muddy River drainage after the 1980 Mount St. Helens eruptions

Stream channel response to the 1980 eruption of Mount St. Helens has varied widely. Stream channels that received only airfall deposits have shown no significant sedimentation following the eruption. In drainage basins that received both blast and airfall deposits, localized channel modification has been observed. However, as the blast/airfall deposits are predominately sand-sized, they have been quickly removed from the steep low-order channels surrounding the mountain with little net effect on pre-eruption channel morphology. Some subsequent channel modification has resulted in lowered gradient reaches downstream as sediment eroded from upstream hill slopes and stream channels has been redeposited in these reaches. Long-term effects in blast/airfall affected streams appear limited to channels subjected to debris torrents resulting from shallow-seated landsliding and breakup of in-channel debris jams.

 

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Surface modification of Muddy River lahar deposit, Mount St. Helens National Volcanic Monument, Skamania County, Washington

Purpose of the project is to note and describe the extent of modification, since 1980, of the surface of the upper Muddy River lahar deposit. Several traverses will involve measurements concerning the density, width, and depth of drainage channels, and changes to interfluve areas. These will be compared with features recorded on aerial photographs and related to longitudinal gradients of channels and surface. The information is to be summarized in a written report.

 

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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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