Debris Avalanche
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- Prior to the eruption, majestic old-growth forests cloaked the valley north of the volcano.
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- A forest of 150 foot (45 m) fir and hemlock trees covered the valley before the eruption.
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- In the spring of 1980, rising magma formed a massive bulge on the north flank of Mount St. Helens.
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- A magnitude 5.1 earthquake triggered the largest landslide in recorded history.
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- It took only 10 minutes for the landslide to scour and bury 14-miles (22 km) of river to an average depth of 150 feet (45 m).
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- Erosion cut deep canyons and actively shifting floodplains.
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- Chunks of fragmented mountain top cover the deposit surface.
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- The debris avalanche created complex habitat and a unique natural laboratory [Four months after eruption]
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- The landslide swept away trees and other vegetation depositing them in a massive debris pile downstream.
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- Some plants survived when rare root fragments on the deposit surface sprouted.
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- The massive debris avalanche filled the valley north of the volcano creating numerous small ponds and wetlands.
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- Plants established quickly on moist pond shorelines.
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- Groundwater springs fed new wetland communities. [Five years after eruption]
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- Wetlands formed important oases for colonizing life on the largely barren deposit.
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- The landslide formed more than 130 new ponds. Stable wetlands were important sites for plant colonization.
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- The springs acted as oases of habitat in the barren valley [Nine years after eruption]
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- Wind blown seeds colonized ponds and depressions.
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- Plants also established between small hills formed by chunks of the collapsing volcano.
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- The debris avalanche created complex habitat that supports an amazing diversity of plant and animal life.
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- Hardy mosses and well adapted plants like prairie lupine colonized drier, upland areas.
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- Eventually slower growing conifer trees like this Douglas-fir will outlive and overtop the faster growing alder forest.
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- Red alder, a nitrogen-fixing, broadleaf tree took root around ponds and wetlands.
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- Trees and shrubs also established, forming the beginnings of a future forest.
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- Debris avalanche deposit. [1 year after eruption]
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- Note rapid erosion of deposit surface. [3 years after eruption]
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- Stream erosion widens floodplain. [4 years after eruption]
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- A fast growing red alder forest has established. [24 years after eruption]
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- The alder forest expands. [29 years after eruption]
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- Erosion and sediment transport was at record levels as the river cut new channels in the deposit.
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- Helicopters were used to spread grass seed and fertilizer for erosion control.
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- Commercially available grass seed was spread over thousands of acres outside the monument.
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- Uneven distribution of seed and fertilizer is visible in the photo.
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- On flat areas and gentle slopes the seeding produced dense stands of grass.
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- A wildlife area downstream of the monument was heavily seeded.
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- Planted grass and clover provide winter range for elk
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- Dense stands of planted grass inhibit tree establishment and browsing elk clip trees.
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- Trees like this noble fir are continually clipped back.
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- Planted areas contrast greatly with unseeded areas upstream.
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- Plant diversity is much higher in unseeded areas.
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- Eventually trees grow above the reach of browsing elk and their height growth accelerates.
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- The planting of non-native grass and clover has greatly influencing plant succession around the volcano.
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- Emergency seeding for erosion control has forever alterred natural plant communities around the volcano.
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- Transport of sediment downstream was a mjor concern.
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- Aerial view of North Toutle River, two years after eruption.
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- Plants gradually spread to adjacent upland areas.
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- Slower growing fir, hemlock and pine trees are also taking root.
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- Fast growing alders are gradually spreading across the valley.
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- Patches of developing alder forest are transforming the valley.
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- Forest plants and animals are moving into the alder forests.
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- River floodplains remain largely unvegetated due to constant disturbance.
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- North Fork Toutle flood plain [30 years after the eruption]
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- Today, a diverse mosaic of developing forest and meadow covers the valley
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- Red alder trees were important early colonizers.
