Mount St. Helens

Frequently Asked Questions

1. What happened during the big eruption of 1980?

2. What was the landscape like after the eruption?

3. Did any life survive the 1980 eruption?

4. Hasn't Mount St. Helens erupted again since 1980?

5. How were birds initially affected by the eruption?

6. How were small and midsize mammals affected by the eruption?

7. How were large mammals affected by the eruption?

8. How were amphibians affected by the eruption?

9. How were reptiles affected by the eruption?

10. How were fish affected by the eruption?

What happened during the big eruption of 1980?
On the morning of May 18, 1980, after weeks of small tremors, a magnitude 5.1 earthquake shook beneath Mount St. Helens and triggered an enormous eruption. The eruption involved a complex series of events that unfolded over the next 12 hours, with many events going on simultaneously. These volcanic events buried some areas in debris avalanches and mudflows, scoured other areas with hot gases, blew down or scorched forests on slopes several miles away, and dusted forests farther away with volcanic ash.

The entire northern side of the volcano collapsed in a gigantic debris avalanche. One lobe of the debris avalanche smashed into Spirit Lake, pushed the lake water up the surrounding slopes, and raised the lakebed 200 feet. As the water flowed back downhill, it dragged thousands of trees into the lake where the trees covered much of the surface. The second lobe of the debris avalanche surged over a 1,300-foot ridge and spilled into the South Coldwater Creek drainage. The third and largest lobe traveled 14 miles down the Toutle River valley, filling the valley to an average depth of 150 feet and leaving mounds of sediment in a bumpy pattern of hummocks.

With the north side of the mountain gone, pressure was released on hot water within the volcano. The hot water burst into steam and blasted out the new opening in a powerful lateral blast. A hot stone-filled wind surged north at speeds over 300 miles per hour and temperatures of 660 °F. This lateral blast toppled or snapped off trees over a 230-square-mile area north of the volcano, which later became known as the blowdown zone. On the outer fringes of the blowdown zone, the force of the lateral blast had diminished and trees remained standing but were seared by the hot air, leaving a band of standing dead trees referred to as the scorch zone.

Beginning about noon and lasting for several hours, super-hot (at least 1,300 ºF), fast-moving, pumice-rich pyroclastic flows poured from the crater and covered 6 square miles north of the volcano with pumice many feet deep. This sterile, desolate terrain was later called the Pumice Plain.

Heat from the eruption melted snow and glaciers on the volcano’s slopes. The meltwater picked up soil, rocks, and logs, forming mudflows that traveled for tens of miles down river channels.

The towering column of ash rose for more than 9 hours and reached a height of about 80,000 feet. Wind carried ash mostly to the northeast where it darkened skies and covered the ground with gray, volcanic ash. Some ash remained aloft and this part of the plume circled the earth in 15 days. For more information on the 1980 eruption, go to http://vulcan.wr.usgs.gov/.

What was the landscape like after the eruption?
The May 18, 1980, eruption left a seared and smoldering landscape around Mount St. Helens. Entire forests were toppled by the hot blast. Most plants and animals perished, meadows had been destroyed, and numerous new ponds and lakes had been created.

Scientists were on the ash-covered ground within days after the eruption and found a complex mosaic of disturbance zones. The eruption included many types of physical forces, such as heat, burial, scour, and so forth, and the intensity of these forces varied substantially over the blast area (for example, thin versus thick deposits, warm versus searing hot temperatures).

Generally these physical forces were most intense in areas closest to the volcano’s north side and less severe farther away, but the mountainous terrain shielded some spots from heat and funneled mudflows into stream valleys. Also, multiple forces affected many places. So, although the whole landscape looked gray and ashen, scientists found complicated patterns of disturbance and tremendous variation, or heterogeneity, in the effects on the ecosystems.

The main categories of disturbance zones after the eruption are described below.

• Debris avalanche. Rock that used to be the north side of the volcano covered about 23 square miles, primarily in the North Fork Toutle River valley, leaving hummocks (mounds) and basins. The former forest was obliterated and buried under sand and rock from 33 to 640 feet thick.
• Pyroclastic flow. The pyroclastic flow spread over 6 square miles immediately north of the volcano, which was part of the area already buried by the debris avalanche deposit. Gravel and cobble-size pumice spread out in a fan-shaped flow up to 131 feet thick, creating a barren plain of pumice. No remnants of the former forest remained.
• Mudflows. Glacier ice and snow meltwater carrying boulders, stones, and grit scoured the stream channels. Where the mudflows slowed down and eventually stopped, they buried streams and their flood plains. Large mudflows killed most vegetation in their paths, although plants survived along the flow margins. Small, shallow mudflows on the mountain slopes were less destructive and left many plant survivors.
• Tree-removal zone. Two to three feet of blast material stripped away all trees and covered the ground. A few small patches of understory vegetation survived in places shielded by ridges or other natural features or protected by late-lying snow.
• Blowdown zone. The lateral blast knocked down trees on about 143 square miles. Fragmented rock and ash blanketed the ground in a layer 4 to 78 inches thick.
• Scorch zone. Hot volcanic gases killed the trees but left them standing in a 42-square-mile scorch zone that extended along the fringes of the blowdown zone. From 4 to 16 inches of fragmented rock and ash covered the ground.
• Tephra-fall zone. Beyond the most heavily disturbed zones near the volcano, the wind dropped cool pumice and ash (tephra) over an area of several thousand square miles. Heavier tephra dropped first, and tephra deposits were deepest near the volcano, gradually diminishing farther away. At 25 miles northeast of the volcano, tephra piled up about 8 inches deep and buried tree seedlings, small shrubs, herbs, and mosses. Areas several hundred miles away received only a dusting of ash.
  

Did any life survive the 1980 eruption?
Although the ash-covered ground appeared lifeless, scientists found that not everything died. In fact, much to scientists’ surprise, thousands of plants, animals, and fungi survived in much of the disturbed area. These survivors ranged from single individuals to entire biological communities and ecosystems. Scientists discovered that the survivors, along with the thousands of dead trees and other dead organisms, played vital roles during ecological recovery.

Legacies. Living and dead organisms, termed “legacies,” were present throughout much of the disturbed area. Based on the types, amounts, and distribution of legacies, three distinct zones were apparent: zones where nearly all life was eliminated, zones of intermediate survival, and zones of widespread survival. Survivors produced seeds, spores, and offspring—these survivors initiated populations in adjacent areas where species did not survive. Dead trees, as well as surviving plants, provided food and habitat for colonizing animals, and played many important ecological roles.

Survival mechanisms. Scientists learned that four factors were critical for survival.

• Timing. The time of day as well as season of the year helped some organisms survive. Nocturnal animals were below ground, for example, when the eruption began. At higher elevations, May is still late winter, and plant buds had not yet opened. Patches of snow and ice shielded some organisms from the searing heat and abrasion of the blast.
• Location. Rock outcroppings, cliffs, and ridges protected some areas from the brunt of the blast. Plants and animals in these sheltered sites had better chances of survival, whereas valley floors and terraces collected thick deposits of ash that smothered life.
• Life histories. Animals away at the time of the eruption (some salmon and migratory birds), in daytime retreats (bats, mice, voles), below ground (pocket gophers), or in water (trout and some amphibians) were protected and survived. Plants with dormant belowground buds had high survival rates.
• Size. Small animals such as deer mice, yellow-pine chipmunk, and Trowbridge’s shrew tended to be in protected places, whereas large animals such as deer, elk, and bear were exposed. The size factor affected tree survival also. Saplings buried in late-winter snowbanks survived, but large trees were toppled by the blast or scorched by the hot gasses.
  

Hasn’t Mount St. Helens erupted again since 1980?
Over the past 4,000 years, Mount St. Helens has been the most active Cascade Range volcano, with about 20 eruptive periods. Over the millennia, debris avalanches, pyroclastic flows, lava flows, and mudflows have built, torn apart, and rebuilt the volcano.

From the May 18, 1980, eruption to 1986, the volcano erupted an additional 21 times. These were mostly dome-building eruptions, although small pyroclastic flows and mudflows occurred also. During these years, lava formed an 876-foot-high dome inside the crater, with the dome’s volume estimated at 97 million cubic yards.

The volcano was quiet from 1986 until September 2004, when swarms of small earthquakes began. Plumes of steam and ash rose from new vents, ballistic explosions hurled boulders across the crater, and small mudflows traveled down stream channels close to the mountain. A large new lava dome has grown at an impressive rate within the crater. By spring 2005, the newest dome was already taller than the dome formed from 1980 to 1986.

Mount St. Helens has a rich eruptive history, and geologists think that the volcano will likely be active off and on in years to come. The repeated episodes of eruption followed by ecological recovery make Mount St. Helens a fascinating place to learn about the forces of nature and resiliency of life. For up-to-date information on new eruptions, go to http://vulcan.wr.usgs.gov/.

How were birds initially affected by the eruption?
Bird survival during the 1980 eruption depended on the distance of birds from the volcano and disturbance zone. All birds died throughout the entire 230-square-mile blast area and in areas crushed by the debris avalanche. In contrast, many birds outside the blast area but in the path of mudflows likely fled to safety, and birds in tephra-fall areas were temporarily displaced.

The power of flight gives birds tremendous ability to move freely, and scientists observed some birds flying into the blast area within days after the eruption. These early immigrants stayed and nested wherever habitat and food were available.

After the eruption, the pattern of bird colonization was strongly influenced by habitat structure and complexity, which differed substantially across the disturbance zones. These differences determined which bird species, and how many bird species, were found in each zone. Birders can print a complete bird checklist for the Mount St. Helens area.

• Pyroclastic flow zone. The pre-eruption forest was completely destroyed and covered with rocky rubble, leaving a barren habitat. Only bird species that nested and foraged on the ground, such as the American pipit (Anthus rubescens) and horned lark (Eremophila alpestris), were able to live in the pyroclastic flow zone.
• Debris avalanche zone. The debris avalanche scoured parts of the Toutle River valley and buried the rest, replacing the former valley with an unusual topography of rock-and-sand hummocks interspersed with natural hollows where small ponds and seeps formed. The Toutle River carved a canyon through the avalanche deposit, creating terraces and a dynamic flood plain. Initially, these diverse habitats offered habitat only for ground-nesting birds like the common nighthawk (Chordeiles minor) and killdeer (Charadrius vociferus), and a few species of waterfowl. But the ponds and seeps developed into biological hotspots, filled with algae, reeds, and cattails and surrounded with thickets of willows, alder, and herbs. Plants also grew along the margins of the avalanche deposit. A spectacularly diverse assemblage of birds colonized all these habitats as they developed.
• Blowdown and scorch zones. The trees toppled by the blast and standing dead trees (snags) created an abundant supply of large dead wood, used by several bird species. Tree saplings and shrubs buried in late-winter snowbanks survived, as did many dormant plants, creating habitats with some complexity. Many bird species colonized these small patches of surviving vegetation. Common species included those that nest and forage on the ground like the dark-eyed junco (Junco hyemalis) and white-crowned sparrow (Zonotrichia leucophrys), species that use snags for nesting or foraging, such as the mountain bluebird (Sialia currucoides), northern flicker (Colaptes auratus), and Vaux’s swift (Chaetura vauxi), and species that use shrubs, like the song sparrow (Melospiza melodia).
• Tephra-fall zone. In the tephra-fall zone, the eruption’s only effect was the burial of understory herbs, mosses, tree seedlings, and very small shrubs. Birds most likely abandoned these areas temporarily, but scientists found many bird species had returned to the tephra-fall zone within a few weeks. Over the next few years, scientists found the same species of birds in the tephra-fall zone that they found in nearby undisturbed sites, although the total number of individual birds using the ash-covered forest floor was likely reduced for the first few years after the eruption.

More bird species colonized the disturbance zones as habitat complexity increased
In the years since the eruption, habitat complexity increased in all disturbance zones as surviving plants grew and spread and other plant species became established. Scientists found that additional bird species colonized the blast area as habitat complexity increased, and that the appearance of new species was closely connected to the developing vegetation.

The colonization of additional bird species occurred in stages, with species colonizing an area when the vegetation reached a threshold of development that met the birds’ habitat requirements.

• Streamside vegetation. The most dramatic threshold since the 1980 eruption occurred about 10 years after the eruption. Willow (Salix spp.) and alder (Alnus spp.) shrubs created verdant riparian vegetation along most streams in the blowdown zone, and several Neotropical songbirds such as the yellow warbler (Dendroica petechia) and willow flycatcher (Empidonax traillii) colonized the woody thickets along the streamsides. These species nest or forage in the woody riparian vegetation.
• Cottonwood trees. As cottonwood trees (Populus trichocarpa) grew in moist areas, two additional bird species, warbling vireo (Vireo gilvus) and black-headed grosbeak (Pheucticus melanocephalus) colonized. The vireo and grosbeak used the extensive canopies of 50-foot-tall cottonwood trees for nesting and foraging.
• Stands of conifers. Conifer saplings that survived the 1980 eruption beneath snowbanks had grown into small, dense stands by the mid-1990s and ushered in the arrival of hermit thrush (Catharus guttatus), which forage on the ground under dense cover, and seed-eaters like pine siskins (Carduelis pinus).
• Ponds, wetlands, and lakes. Two large new lakes, more than 130 ponds, and dozens of wetlands formed on the debris avalanche deposit. These water bodies provide habitat for many aquatic bird species, including puddle ducks such as the mallard (Anas platyrhynchos), diving ducks such as the ring-necked (Aythya collaris), and grazers like the Canada goose (Branta canadensis). Spotted sandpipers (Actitis macularia) feed and nest along the lake shores. Great blue herons (Ardea herodias), which stand up to 4 feet tall, hunt the wetlands and shallow pond waters, and hard-to-spot soras (Porzana carolina), small birds in the rail family, live in the wetlands.

Birds of prey have come back to the volcanic landscape as their food sources increased. For example, osprey (Pandion haliaetus) and bald eagles (Haliaeetus leucocephalus) forage for fish in the lakes, red-tailed hawks (Buteo jamaicensis) soar the skies hunting for rodents on the ground, and American kestrels (Falco sparverius) hover, then pounce on grasshoppers and other prey. The short-eared owl (Asio flammeus), which lives in open country and sometimes hunts by day, has also returned. Several other raptors use the area during the summer or while traveling through on their migration routes.

Scientists found that in the early years after the 1980 eruption, the bird assemblages, or groups of species, in the disturbance zones were quite different, corresponding to the dramatically different habitats offered. As plant communities developed, however, creating habitats with more structure and complexity, more bird species colonized each zone and the assemblages for the zones developed some similarities. Even so, in 2005 significant differences still remained among the bird assemblages for the different disturbance zones. The bird checklist includes information on which species are typically found in the different habitats.

Only the initial stages of succession have occurred among birds in the Mount St. Helens blast area

In the first quarter century after the 1980 eruption, scientists found that more and more bird species colonized the blast area, with the total number of bird species steadily increasing. Further stages of succession—or the replacement of one species by another—have not occurred yet among birds.

If no large eruptions or other large disturbances such as wildfire occur in the near future, scientists expect that bird species replacements will occur as forests become widespread across the Mount St. Helens landscape, and birds of open habitats are replaced by forest bird species. (The eruption in late 2004 and early 2005 has been confined for the most part to the crater.) Scientists expect that as succession occurs, the bird assemblages in the disturbance zones will become more and more similar, eventually converging into an assemblage of bird species similar to those of other Pacific Northwest forests.

How were small and midsize mammals affected by the eruption?
Before the 1980 eruption, the Mount St. Helens area supported about 35 small to midsize mammal species, not including bats. Although the volcano dramatically altered a vast terrain, scientists found that a surprisingly large number of these mammal species had survived in many locations.

Survival was related to the type and severity of volcanic disturbance and differed considerably across the volcanic disturbance zones.

• In zones where the forest was entirely blasted or scoured away and zones where the forest was buried, all mammals perished.
• In zones where trees were toppled and volcanic deposits were deep, many small and midsize mammals were killed—but for most species, at least some individuals survived. In these zones, only a few small and midsize mammal species were destroyed completely.
• In zones that received only 6 inches or less of cool ash and pumice, survival was widespread and the small and midsize mammals present were typical of undisturbed sites beyond the volcanic eruption.

Several key factors affected the survival of small to midsize mammals.

• Location. Small mammals that lived underground had higher survival rates than species that lived in tree canopies or on the surface of the ground.
• Timing. Mammals active at night had generally returned to the safety of their daytime burrows by the time the early morning eruption occurred.
• Season. Because the eruption was in May, with conditions still like late winter in the Cascade Range, patches of snow lingered in places, protecting animals beneath it.
• Landscape features. Ridges, rock outcroppings, and cliffs blocked or deflected the powerful volcanic forces in some places, protecting some animals.

Most small and midsize mammals returned within 10 years

Within 10 years of the 1980 eruption, nearly all the mammal species found in the southern Washington Cascade Range had returned to the blast area. The mammal assemblages, or groupings of species, were quite different in the various disturbance zones. Scientists attribute many of these differences to first, the amounts and types of the pre-eruption forest components that remained after the eruption, such as fallen trees, standing dead trees, and surviving patches of vegetation; and second, the rate at which new vegetation developed.

• Blowdown zone. The down trees, surviving plants, and colonizing vegetation provided a complex ground layer that offered abundant cover and hiding places and also produced diverse food items including seeds, insects, green plants, and roots. This area thus provided all habitat needs for a variety of ground-dwelling rodents such as the Cascade golden-mantled ground squirrel (Spermophilus saturatus), yellow-pine chipmunk (Tamias amoenus), deer mouse (Peromyscus maniculatus), and insectivores such as the montane shrew (Sorex monticolus).

  These small mammals were the prey for predators like the coyote (Canis latrans), short-tailed weasel or ermine (Mustela erminea), and longtail weasel (Mustela frenata). Two midsize aquatic predators, the American mink (Mustela vison) and northern river otter (Lutra canadensis), inhabit the blowdown, debris avalanche, and pyroclastic flow zones, where the mink and otters eat crayfish, amphibians, and fish, in addition to other aquatic and terrestrial prey.

  Midsize herbivorous mammals, such as the American beaver (Castor canadensis) and common porcupine (Erethizon dorsatum), colonized the blowdown zone once their forage base of bark, leaves, twigs, and buds was established.

• Pyroclastic flow zone. On the pumice plain in the severely altered pyroclastic flow zone, none of the former forest remained after the 1980 eruption. Vegetation that developed since the 1980 eruption was generally sparse and close to the ground (except for the springs and seeps, discussed below), providing little habitat for most mammals. Here, the dominant mammal was the deer mouse. About 12 years after 1980, the northern pocket gopher (Thomomys talpoides) reached the pumice plain and became established. Gophers move primarily through their tunnel-digging, which explains why the species took so long to reach the plain.

• Springs and seeps within the pyroclastic flow zone. A handful of cool springs and seeps emerged on the pumice plain, and small patches of dense, lush willow and herb plant communities developed around these wet spots. Scientists found that a diverse assemblage of 10 small mammal species, including forest insectivores and riparian habitat specialists, colonized these ecological hotspots. In fact, these small mammals, some weighing a mere two-tenths of an ounce, traversed up to 2 miles or more over barren terrain among these small, isolated patches of suitable habitat, reaching and colonizing all of them. Small mammals were several times more abundant in these oasis-like habitats than in the larger pumice plain area.

Two small mammal species remained conspicuously absent even after 25 years: the northern flying squirrel (Glaucomys sabrinus), a forest canopy species, and the southern red-backed vole (Cletherionomys gapperi), a forest understory species. Given that forests have not developed yet in the blast area, it is not surprising that these species have been absent. Scientists expect that, barring any major eruptions from the very active Mount St. Helens, trees will likely continue their spread across the landscape, filling in gaps, growing taller, and developing into a mosaic of forest types. Scientists expect that as the vegetation changes, they will see shifts in the groupings of mammal species found in different zones, and in the relative dominance of species within these groupings.

Many large mammals are highly mobile and within days of the eruption traveled into the disturbed areas in search of food, which was initially in very low supply. Because these animals have large energy reserves and can travel long distances, they could afford to roam the disturbed areas, searching for food in the isolated vegetation patches.

• Elk and deer. Herbivores such as elk and deer returned to the blast area the first summer. They influenced the growth and spread of plants in several ways, some positive and some negative. First, these hoofed animals broke up the ash surface, which promoted erosion on steep slopes, thus allowing buried plants to sprout. Second, elk and deer tracks collected wind-blown seeds that later sprouted. Third, elk and deer carried seeds and spores in their gut track and later deposited the seeds and spores with their fecal material in the disturbed area. Negative effects included grazing, which severely affected developing vegetation on the pyroclastic flow, debris avalanche deposit, and blowdown zones. Elk spread the seeds of exotic plant species as well as native plants, and some exotic plants have spread quickly and displaced native species.

Elk herds flourished in the years after the eruption because highly nutritious leafy plants became increasingly abundant, the area was closed to hunting, and a string of mild winters favored high survival and good calving success. Elk populations reached several hundred animals within 5 years of the 1980 eruption and continued to increase until heavy snows in 1999 caused a substantial winter die-off. Since 1999, the number of elk has increased. Much of the area immediately north and west of the volcano remained closed to hunting through 2005.

Outside Mount St. Helens National Volcanic Monument, adjacent lands were salvage logged and planted with conifer seedlings after the 1980 eruption. As these conifers grow into young forests, shading out the forage that elk and deer prefer, these animals are likely to spend more time grazing in the monument, which still has many open areas because no trees were planted and natural succession is allowed to take place.

• Mountain goats. In 1980, mountain goats may have survived on the south side of the volcano where the eruption had a minimal impact. The first reliable sighting of mountain goats on Mount St. Helens, however, occurred 7 years after the eruption. Since 1987, scientists have seen mountain goats on the volcanic cone. In 2000, scientists observed mountain goat tracks and fur in the volcano’s crater, and during summer 2003, scientists routinely saw a mountain goat at the base of Forsyth Glacier on the volcano’s north side.

• Black bears. Black bears had little reason to venture into the blast area for several years after the eruption as little food was available for them. However, as young conifers grew and berry-producing plants became more abundant, black bears were routinely observed in the blowdown zone foraging on conifer bark and berries.

• Cougars. Cougars, also known as mountain lions, are elusive animals that are highly secretive and difficult to observe. Scientists have spotted the remains of cougar kills and have seen cougar tracks in mud along lakes and streams in the blast area, leaving little doubt that these large cats are hunting deer, elk, and other animals in the blast area.
  

However, an important question remained. Would the amphibians continue to survive in this dramatically altered land? Over the next few years scientists learned that amphibian survival depended strongly on the habitat.

• Lakes and ponds. Amphibians using lakes and ponds were present and/or breeding in most study sites only a year after the eruption. Their eggs and larvae developed completely and they successfully metamorphosed. These species continued to do well in the ensuing years and some actually flourished. Some species, such as the northwestern salamander (Ambystoma gracile), have flourished in the posteruption landscape. Surveys conducted 15 to 20 years after the eruption showed that lake-dwelling amphibian species and the percentage of sites they occupy at Mount St. Helens were comparable to nearby undisturbed areas, such as Mount Rainier National Park in Washington and the Three Sisters Wilderness Area in Oregon.

• Streams. Amphibians associated with streams initially survived the eruption, but they died off rapidly as the streams were clogged with tremendous inputs of volcanic sediment, smothering the amphibians’ food sources. The changed streams also left the amphibians little protection from floods. Within a few years, however, the steep, swift, mountain streams flushed much of the sediment from their channels, and stream amphibians began to recover. With streamside trees and other shade plants gone, sunlight fueled exceptional growth of algae, the primary food of some amphibian larvae, and tailed frogs (Ascaphus truei) multiplied rapidly.

• Seeps. Amphibians associated with seeps survived and persisted after the eruption because their habitats were either protected from the eruption by topography or were minimally impacted because the volcanic sediment deposited on these steep habitats was rapidly removed by gravity and water. These species are among the most sensitive amphibian species in North America to environmental changes such as increased temperatures, making their survival in seeps around Mount St. Helens particularly surprising.

• Ponds in the debris avalanche zone. All amphibians perished in the debris avalanche and pyroclastic flow zones. The debris avalanche left a landscape of hummocks, and small ponds formed in the low spots. Because of this new topography, the number of lakes and ponds in the area increased fivefold, from 33 before the eruption to 163 afterward.

Scientists found this event an outstanding opportunity to study the pace and pattern at which amphibians colonize newly created habitat. Amphibians began to colonize the new ponds within 1 year after the eruption. Four species of frogs and toads colonized first, followed by one salamander species and one newt species. Some of these early colonizers traversed impressive distances (sometimes over 2 miles) across barren, ash-covered ground to reach the ponds. By 1990, six amphibian species lived in the ponds, and these species continue to live in the ponds (as of 2005).

• Debris avalanche and pyroclastic flow zones, other than the ponds. Scientists found that even by 2005, a full 25 years after the 1980 eruption, amphibian species associated with streams, seeps, and land had still not colonized the debris avalanche deposit outside the ponds and the pyroclastic flow zone. These species probably remain absent because of dispersal barriers (either distance or harsh terrain) or lack of habitat. These amphibian species may not be able to colonize these zones until forests develop.
  

In the first few years after the 1980 eruption, scientists observed only one snake species, the common garter snake (Thamnophis sirtalis), and it appeared to be uncommon. Because the volcanic blast leveled the forest and left the former forest floor flooded with sunlight and buried by well-drained volcanic material, creating a somewhat warmer and drier environment, scientists predicted that reptiles would become more abundant in a few years. In fact, about 10 years after the eruption the number of common garter snakes increased dramatically, concentrated around lakes and streams at Mount St. Helens where the garter snakes prey heavily on amphibians. No other snake species has been documented in the area since the eruption, but several confirmed sightings of the northern alligator lizard (Elgaria coerulea) have been made.

As forest cover reclaims the landscape, keeping the ground cool and moist, scientists expect reptiles to become less abundant. This change will likely take several decades.

The 1980 eruption devastated some water bodies with fish and hardly changed others. Thus fish survival and recovery had very different patterns in the various bodies of water across the disturbance zones.

• Small lakes. The lakes received differing amounts of volcanic debris and organic matter such as fine material from the shattered and burned forest. Lake water was turbid, and fish food sources such as tiny animals called zooplankton and aquatic insect larvae declined. Even so, just weeks after the 1980 eruption, biologists found that fish had survived in most lakes in the blowdown zone where fish had been previously stocked, owing to ice on the lakes at the time of the eruption. Brook trout (Salvelinus fontinalis) was the most frequently found species in the summer of 1980.

Water transparency improved greatly in the blowdown-zone lakes after a few months, and zooplankton and aquatic insect larvae began to recover. Brook trout as well as other trout species were successfully spawning within a few years after the 1980 eruption and continued to survive through 2005 without any stocking.

• Spirit Lake. So much volcanic debris slid into Spirit Lake that the lake bottom was raised 200 feet, and trees dragged into the lake covered the surface. All fish in Spirit Lake perished. After about 6 years, the lake’s grossly changed conditions had improved substantially and it appeared that the lake could once again support fish. In the early 1990s, scientists detected the first fish in Spirit Lake, a rainbow trout. Since then the lake has been sampled many times and results have shown a burgeoning trout population. Scientists have found that most of the fish are less than 4 years old and that the fish grow rapidly, an indication of abundant food. In fact, they found most 3-year-old fish to be about 23 inches long and weigh nearly 5 pounds. Spirit Lake continues to be highly productive, largely owing to the nutrients and minerals deposited during the eruption, and also the formation of shoals during the 1980 eruption. These shoal habitats support dense and complex aquatic vegetation that in turn support a diverse assemblage of plankton and insects.

• Coldwater and Castle Lakes. The 1980 eruption created two new large lakes, Coldwater and Castle, which had no fish for several years. In the early 1990s, the Washington Department of Fish and Wildlife stocked rainbow trout in Coldwater Lake. Two years later fish appeared in Castle Lake, presumably originating from fish that swam from Coldwater Lake through the river connecting the two lakes. Once established, fish populations in both Coldwater and Castle Lakes have grown well. The size of individual fish decreased from 1990 to 2004, suggesting that either the lake’s production of prey sources for fish decreased, or that the very abundant fish are competing for available food. Biologists have confirmed that rainbow trout have successfully spawned in streams associated with each lake and the populations appear to be self-perpetuating. Thus, no additional stocking has occurred.

• Rivers and streams. Most river fish were killed during or shortly after the 1980 eruption from suffocation in ash-choked waters or indirectly from the loss of habitat. Eventually the flowing water flushed the fine sediment from the larger, less movable gravels and boulders in the streambeds, improving water quality and other habitat conditions. Fish prey began to recover. Once habitat conditions had improved in blowdown-zone streams, fish that had survived in headwater lakes swam downstream, and fish from tributary streams in the less disturbed tephra-fall zone also colonized the recovering stream reaches.

With streamside forests knocked down by the 1980 eruption, blowdown-zone streams were completely open to sunlight, dramatically increasing algae growth which in turn fueled a highly productive food web. Abundant food created very good conditions for fish growth. However, within 10 to 15 years after the 1980 eruption, deciduous trees had taken root along streamsides and were growing tall enough to shade the streams, presumably reducing food for fish, which caused fish populations to decline.

In the pyroclastic flow and debris avalanche zones, most of the streams had chronic high levels of fine sediment and shifting channels. By 2005 these streams had not developed conditions suitable for fish, and decades will likely pass before these streams can support fish.

Hazard management in streams affected natural stream processes
After the 1980 eruption, the Army Corps of Engineers built sediment dams on the Toutle River to block mudflows, a serious hazard to human life and property downstream. The sediment dams block the natural migrations of fish, including salmon migrating to the ocean and returning to freshwater to spawn, and also block resident fish which spend their entire lives in freshwater but normally move up and down the river. The sediment dams protect downstream areas but hamper natural processes that would improve the habitat; the impounded water drops fine sediments, which clog the river and its tributary channels. As mitigation, the Corps of Engineers built a fish collection facility and trucks captured salmon and steelhead upstream above the dams where the fish are released to spawn.

In some areas outside the national volcanic monument, salvage logging has affected streams and lakes and thus the natural responses of native fish. Hatchery fish and nonnative fish species have been stocked in some places, influencing the natural recovery, but the exact effects of stocking are unknown.

Many fish species thrived on abundant food, even though habitat quality was poor

Overall, fish populations have rebounded remarkably since the 1980 eruption. Habitat conditions were often less than ideal, with water warmer than usual for native fish species or fine sediment levels high. But the food supply was often very rich, owing to the amount of sunlight reaching streams and fueling the food web, and fish are thriving in many bodies of water.

USDA Forest Service - Pacific Northwest Research Station - Mount St. Helens
Last Modified: Friday, 20 May 2005 at 15:20:24 EDT