Lahars
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- Prior to 1980, Mount St. Helens’ south flank featured a narrow glacier and stunted forests growing on ancient lahar deposits.
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- The eruption melted the Shoestring Glacier and the resulting cement-like slurry stripped off the forest.
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- Melting ice and snow mixed with rock and ash producing debris-laden floods called lahars (mudflows).
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- Flood waters from melting ice and snow produced lahars that flowed miles from the volcano.
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- The boulder-filled mudlfows scoured stream channels uprooting and depositing trees downstream.
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- Lahars scoured and buried miles of river channel. Note flow marks on trees and geologist for scale. (circle person for scale)
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- Trees were swept away and washed downstream.
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- Homes close to the river were washed away or partially buried by ash and rock.
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- Aerial view of bridge washed out by Toutle River lahar.
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- Bridge buried in Toutle River lahar deposit.
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- The dense high-energy slurry floated and transported huge boulders.
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- The cement-like slurries carried tremendous energy transporting even large boulders downstream.
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- Lahar depth is shown by the high flow marks on the trees. Note helicopter and geologists for scale.
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- Sprouting of fragmented roots was an important early mechanism of survival.
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- Erosion of upstream deposits transported additional sediment and debris downstream.
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- Damage was limited to the valleys in which the lahars were channeled downstream.
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- The lahar stripped away the forest [1 year after eruption]
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- The valley was covered with ash and boulders. Note pickup truck [2 years after eruption]
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- Seeds from adjacent forests planted the deposit [19 years after eruption]
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- A young forest covers much of the valley [29 years after eruption]
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- Tree establishment has been rapid due to countless seeds blown in from the adjacent undisturbed forest.
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- Larger river channels shifted back and forth across the deposit.
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- Woody debris provided an importance source of stability and shade for early plant establishment.
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- Floodplains were repeatedly scoured and buried and were slow to revegetate.
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- Small tributary streams were a key place for early establishment.
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- Willow and alder seedlings established in moist sediments.
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- Red alder seedlings took root on stable terraces and debris piles. [two years after eruption]
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- Entire terraces and alder stands were uprooted and washed away by a massive flood 16 years after the eruption.
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- Lahars are an ongoing hazard. This small lahar was triggered by a small crater explosion in 1983.
Mycorrhizae Studies
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- View of beneficial mycorrhizal fungi on lupine roots.
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- Studies revealed that plants without the benefit of mycorrhizae faired poorly. [3 years after the eruption]
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- Scattered tree seedlings developed mycorrhizal associations from wind-dispersed spores. [5 years after eruption]
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- Scientists wanted to determine how colonizing plants aquire important fungal and bacterial associations.
