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CHAPTER 3
Lahar Deposits
What is a lahar?
Lahars, from a Javanese (Indonesia) designation, are pre- ferred to the inelegant English term “mudflow” when re-
ferring to events on volcanoes. Lahars include any rapidly
flowing masses of earth saturated by water flowing under
the force of gravity. They can be triggered when natural
rubble dams collapses to unleash a pent-up lake trapped
behind it. Lahars are mudflows formed in several ways by
volcanism. When hot tephra falls onto a cone laden with
snow and ice, the rapid melting causes lahars that flow
down canyons. Slurries entrain soil, rocks and anything
caught in the path, and severely erodes the canyon mar-
gins. Similar events can occur by heating from within, as
hot magma moves into the cone. As a lahar ebbs, it usually
leaves a deposit of sediments sorted by distance from its origin. A lahar can also form as a debris avalanche hurtles from
a volcano entraining everything in its path. As it becomes
increasingly liquid, leaving larger materials behind, the de- bris avalanche becomes a less turbulent and continues to
flow as a lahar. While lahars usually stay confined to the
river channel, they can overflow their constraints and spread
L. C. Bliss leans against a surviving Douglas fir on the edge of the Muddy River Lahar. The scour marks are 8 m above the deposit (July 1980).
out. Glaciers and snow fields melt rapidly and small block-
ing dams (often glacial moraines) collapse to produce mas-
sive surges that swallow everything in its path. While sweep-
ing down steep canyons, the lahar scours margins before spreading out and coming to rest on nearly level terrain to
form lahar deposits. On its journey, a lahar can surge
through lakes, fill deep canyons, block streams (Fig. 3.1) and
wreak havoc on villages and fields sitting on flood plains.
Lahars often fill former river valleys with loose rubble that is soon eroded to form very steep-sided, unstable channels
(Fig. 3.2). Lahar deposits s are more fertile than tephra,
pumice or decomposed lava because new deposits come from older, reworked volcanic materials, plus a bit of top-
soil, plants and even a little animal remains. Lahar deposits,
particularly when bounded by undisturbed vegetation, are recolonized more quickly than large, isolated habitats.
La hars and debris avalanches in history
Lahars have always threatened populations living in valleys
associated with volcanoes. Casualties from lahars are com-
mon, but only rarely are they on the massive scale of pyro- clastic flows (Chapter 5) or deep tephra deposits (Chapter
1). Occasionally lahars strike from a distance to produce
massive casualties. Lahars are usually associated with other
volcanic terrors, each threatening a different population. As
human populations grow, more people are at risk; as global warming continues, ice masses on mountains shrink to shrink the volume of future lahars.
An infamous documented lahar happens to have de-
scended from another Cascades volcano. About 5,600 years
ago, the flanks of Mt. Rainier collapsed producing a cata-
clysmic event. There was no eruption, but over the millen-
nia, the summit rock of the then much higher cone was re- peatedly heated and cooled, becoming “rotten” (or as geol-
ogists say, hydrothermally altered). Triggered by an earth-
quake or possibly by magma movement in the volcano’s throat, the summit buckled to form what today we know as
the Osceola Lahar. This lahar formed deposits at least 150
m thick and covered an astonishing 500 km 2 of Puget
Sound. Several Washington towns remain at risk to future
lahars (e.g., Orting, Buckley, Puyallup, Auburn and Kent).
Such lahars cannot be outrun (lowland speeds exceeded 70
km per hr). Current residents of Puget Sound might have
from 45 minutes to about 3 hours to reach the safety of high ground. Lahars continue to inflict immense damage, and have re-
shaped the modern landscape. Their potential for destruc-
tion increases as people increasingly build towns and farms
on valleys below active volcanoes and their flanks ( del
Moral and Walker 2007). In densely populated central Hon-
shu, Japan, the notorious Bandai volcano experienced a
huge phreatic eruption in the summer of 1888. Phreatic
eruptions occur when magma contacts water within the
pressurized confines of the cone. Vaporization explodes the
cone. Steam expands, entraining everything in the way. A
prodigious coughing fit called a Plinian eruption soon fol-
lows. So it was with Bandai. The eruption also generated a
debris avalanche to the north, followed by pyroclastic flows.
The warm, torrential rain caused by the eruption plume then
Fig. 3.1. Castle Lake was created when Castle Creek was blocked by North Fork Toutle River debris avalanche (July 8, 1980).also prominent (July 30, 1980.
Fig. 3.2. Toutle River valley showing deeply incised canyons in the debris avalanche deposit produced in one year (June 8, 1981).
Lahars devastated valleys and colonial towns such as Lat-
acunga in 1744, 1768, 1877 and 1903. Amazingly, the 1977
eruption generated lahars that reached both the Pacific Ocean (over 100 km distant) and descended far down the
Amazon River. The Pan American highway, which skirts
the western flank of Cotopaxi, is studded with serious lahar
warnings. Skeena et al. (2010) studied high elevation suc-
cession on lahars of Cotopaxi. On this seasonally wet and
perennially cool mountain, they found that succession fol- lowed a traditional path: lichens and mosses dominated
early succession. Alpine herbs and prostrate shrubs com-
mon at higher elevation dominated lahars that several cen-
turies old. Shrubs, including fuchsias, were common on la-
hars from 1534, the year that also saw the last pitched clash
between Spaniards and Incas. Even after 475 years, succes-
sion in these paramo habitats was far from complete.
The lahars of Mount St. Helens.
Two distinct mechanisms formed lahars on Mount St.
Helens (Foxworthy and Hill 1982). By far the largest and
most terrifying was caused by the debris avalanche that an-
nounced the start of the May 18 eruption. The trigger was
an earthquake that caused a landslide. As the avalanche
raced down the slope, it entrained everything in its path, mixing huge amounts of the cone, giant boulders, large trees
and huge chunks of ice and much of Spirit Lake. The di-
rected blast followed immediately and overtook the ava- lanche so all of these components were joined by steam and
molten materials from the throat of the cone. This blast re-
sulted because the weight of the cone kept the superheated water in liquid form; when the avalanche released the pres-
sure, water flashed to steam. The force of the blast forced
much of the debris avalanche towards the slopes of Coldwa- ter Ridge (Fig. 3.4) and then it flowed west to rampage down
the North Fork of the Toutle River. As it roared down the
drainage, it impounded creeks to form Castle Lake, Coldwa- ter Lake and Green Lake, among others, and left deep de-
posits in its wake. It scoured valley walls from 10 to 20 m
above the original stream level. The surging debris ava-
lanche exceeded 40 m, and new deposits average about 45
m deep. Channels were quickly cut into this mass to form
steep-sided canyon walls. About 20 km from the crater, the
massive conglomeration slowed and spread out on more gentle terrain, becoming a lahar that continued far down stream (Fig. 3.5), eventually disrupting navigation when it emptied into the Columbia River (Major et al. 2009).
Small lahars can be formed when glaciers and snowfields melt rapidly under the combined effects of magma heating the cone from within and from pyroclastic flows descending
from the crater. Magma rises in the throat of the volcano
and when the heat pulse surfaces, a predictable result is that glaciers and snowfields began to melt…slowly at first, then
with frightening speed. Episodic pyroclastic flows acceler-
ated this effect to create lahars entrained soil, trees and boul- ders, removed soil and vegetation along its margins and cut
deep canyons. They produced scouring along existing land-
forms, but then, as the topography flattened, substantial de-
posits formed. This mechanism spawned lahars on the
south and east side of the volcano, including those on the Muddy River, Kalama Creek and the South Fork of the Toutle River (Fig. 3.6). Far less dramatic, but of considerable interest, were two small lahars that swept the southwest flank of the cone
above Butte Camp. One terminated near a gentle bench,
burying existing vegetation. The other left a thick deposit
on a broad ridge, and then carried on to the Kalama River,
washing out trails and roads (Fig. 3.7) as it went. This pair
of lahars has allowed me to study the effects of proximity of colonists on vegetation recovery rates and species com-
position (see del Moral 1998).
Vegetation recovery on lahars at Mount St. Helens Several groups of ecologists studied aspects of vegetation
recovery on lahars at Mount St. Helens. Refreshingly, each
group has developed similar insights into the factors that
drive succession. However, each location has its own story.
Debris avalanche. Virginia Dale and her collaborators
(Dale et al. 2005) studied several sites at lower elevations on
the debris avalanche of the North Fork Toutle River. Recall
that the debris avalanche that started on the north face of Mount St. Helens turned westward; it became a lahar about
Fig. 3.6. Lahar , South Fork Toutle River (July 20, 1984).
Lahars have volumes that are limited by the size of the drain- age from which they are spawned.
18 km from the volcano, then continued on to the Colum-
bia River. The debris avalanche covers about 60 km^2. As
would be expected, there has been a gradual increase in spe- cies numbers and overall cover since 1980, but some sur-
prising patterns emerged. Study plots were located near the
newly formed Castle Lake and lower on the deposit near
Jackson Lake. The debris avalanche deposit initially lacked
seeds, but it was not purely a primary succession because some plants survived as rhizomes (e.g., fireweed, Canada
thistle and Cascade lupine). Species richness increased
slowly at first, then accelerated. The number of species de-
clined from 1994 to 2000, and may have had to do with ero-
sion during the exceptional rains and runoff of 1996-1997.
The mean number of species in 2000 was about 19 per 250
m 2 plot. Vegetation cover increased steadily to reach about
66% of the surface, and the increase in some species may
have excluded others. By 2000, the mixture of species types
Sidebar 3.1. Life never really left the mountain
On July 26, 1980, I found myself slogging up Pine Creek Ridge, a dry, barren ridge that only two months before was an inferno that had suffered a lethal nightmare of hot mud and boulders. My mis- sion was to find plants…any would do. Below, on the Muddy la- har, plants were exceedingly scarce and no large animals had yet to be observed. I thought that perhaps on the ridge, where im- pacts may have been less severe, something would have survived. Suddenly, my attention was drawn to a solitary ant ( Formica sub- nuda ) bravely scouting this now alien landscape. I was amazed. Ants normally are predators, of course. It may take some imagi- nation, but perhaps a deeply buried, dormant nest could have sur- vived those awful, recent, events. But, I asked myself, what could it be looking for? I soon found a large group of foragers, focused above what I assumed to be their nest. The ants appeared to have been transformed from predators to carrion feeders. This specu- lation heralded two major discoveries. Survivors (i.e., legacies) are crucial to the pace and direction of recovery. Novel food chains, in which predators like ants and spiders become cannibals and scavengers, are likely to develop, if only ephemerally. These ants were hastening succession by incorporating nutrients into the sub- surface, breaking up the impervious silt surface and creating mi- crosites where seeds might safely germinate. Despite an intensive search, I found no plants, birds or mammals on this ridge until the following year. These less deadly lahars produced intense lo-
cal effects, each with compelling ecological stories. The
Muddy River Lahar was generated by numerous melting ice
fields and the Shoestring glacier. The slurry swept across the
Plains of Abraham and the upper Muddy River floodplain.
The lahar then squeezed through a gorge at Lava Canyon
and formed two deltas before reaching the eastern edge of
the Swift Reservoir. Along the margins of this lahar, many
stately Douglas firs survived even though their bark was re- moved on one side up to 8 m above the new surface.
Fig. S3.1. An agitation of ant workers (Formica subnuda)
searching for any food. The rills had exposed the original surface
through about 12 to 15 cm of lahar deposit (July 26, 1980).
Fig. 3.7. Butte Camp lahars from above (September 10, 1980).
Lahar 1 is at the extreme left, Lahar 2 is on the right.
Fig. 3.8. The upper Muddy River Lahar, looking north, with scoured large trees and remnants of the forest that once clothed
this drainage. Note the start of an incision that reached over 12
m within 10 years (July 26, 1980).
burial, rather than succession, and were relatively predicta- ble. Where mortality was moderate and sites were protected by standing trees that had died over several years the suc- cession rate was rapid. Primary succession on deep deposits was dominated by red alder. Intermediate deposit depths in- vited stochastic establishment by lowland conifers and some surviving trees. Shallow deposits recovered quickly as red alder joined conifers and soon established roots in old sur- faces.
Frenzen et al. (2005) also reported on vegetation at Ce- dar Flats in detail. The lahar was relatively narrow (200 to 350 m wide) and there were refugia in areas with little dep- osition. Thus, understory vegetation could develop quickly, at least in the shallow deposits. Nurse logs and root mounds provided habitats for key understory species and standing trees assisted in regeneration by providing shade and reduc- ing wind. Trees that survived despite thin deposits pro- moted recovery of the understory species. On thicker de- posits, tree seedling establishment was needed. Shallow de- posits are reverting to a conifer canopy with an understory of salal. Thicker deposits are developing conifers with dense red alder in the overstory and trailing blackberry and sword fern in the understory. The several studies of lahar effects at Cedar Flats all emphasized that survivors and nearby seed sources were prime drivers of succession.
The lower Muddy River (360 to 520 m), near its conflu- ence with Smith Creek (550 m) and the upper Muddy River alluvial fan (900 to 1350 m) were also studied by Frenzen et al. (2005). They concentrated on comparisons of site stabil- ity. As expected, the number of plant species and their cover increased from 1981 to 1991. Stable surfaces always had more species than unstable sites, some of which received repeated erosion events that restarted succession. Vegeta- tion on stable primary surfaces was sparse at low elevation and became progressively sparser at higher elevations. There was a pronounced reduction of species diversity with elevation. Species that occurred consistently were all wind- dispersed. These species include pearly everlasting, white- flowered hawkweed and cat’s ear. Wood groundsel was common at lower elevations, and willows were scattered throughout the study area. These studies provided further evidence for the importance of biological legacies and dis- persal limitations in directing succession.
The upper Muddy River Lahar has been the site of sev- eral studies. Larson and Bliss (1998) explored conifer inva- sion patterns across the lahar deposits where they are over 1 km wide. They found that age and size of saplings were not correlated. Instead, development was related to the thickness of the lahar deposit and old seedlings could be small or large. Species composition changed with distance
as a function of local pools of colonists and the dispersal ability of the seeds, demonstrating that distance by itself is a significant filter. In subsequent years, I studied dispersal patterns in this region, with an emphasis on all plant species (del Moral and Ellis 2004) and described patterns of vegetation (Fig. 3.10). Along the creek drainages, little vegetation has developed and the surface remains a jumble of cobbles. In places, re- current lahars prevent vegetation establishment (Fig. 3.11). However, much of this lahar deposit has experienced dra- matic transformations since 1980. As the surface stabilized, dust that had been deposited on the surface during erup- tions began to fill in between the cobbles. Eventually rock moss and prairie lupine invaded and A moss-lupine meadow with beardtongue and scattered conifers covered much of the lower lahar deposit. Along its margins, there were more conifers and persistent woody species, particularly bird-dis- persed ones. Much of the lower lahar had a varied mix of species in a matrix of prairie lupine and rock moss. Moist sites near the surviving forest included surviving conifers with Cascade lupine and beardtongue and species adapted to shadier conditions. A few plots, dominated by pinemat
manzanita, occurred sporadically over the range of eleva- tions and formed a well-vegetated surface. Hiking up this lahar became progressively easier. What once required a tricky balancing act, hopping from rock to rock, became a relatively simply, though sweaty, hike on terrain with only a few challenging segments. Higher on the lahar, vegetation remains relatively scarce In 2007, we conducted a survey of the vegetation of the upper Muddy River Lahar (del Moral et al. 2009). We
Fig. 3.10. The upper Muddy River Lahar, looking south. The
surface is dominated by logs and boulders (August 26, 1980).Fig. 3.10. The upper Muddy River Lahar, looking south. The surface is dominated by logs and boulders (August 26, 1980).
sampled 151 plots on the 1980 surface. This vegetation was classified into nine communities using standard methods. The typical and common species communities are shown in Table 3.1. The values in bold designate characteristic spe- cies. Most communities had significant concentrations of prairie lupine and rock moss, but there were different amounts of conifers and of persistent forbs.
The presence of these mats reduced local diversity and demonstrate how priority effects (competition from an early colonizer) can arrest succession (see Chapter 9). In some places, tall shrubs and black cottonwood were set among various herbs, but this vegetation appeared to establish in a capricious way. Upper lahar sites had low dominance and heterogeneous composition. The vegetation of this lahar de- posit remains in early succession, still demonstrating the ef- fects of random dispersal. Species composition remains poorly predicted by environmental factors.
Together these studies of the Muddy River Lahar de- posit revealed several general principles of recovery. Recov- ery will be accelerated if there is any biological legacy and if surfaces are stable. Trees that were smothered, but remained upright provided some shade and their leaves dropped to provide an enhanced seed bed. Thin deposits permitted spe- cies to emerge to start the recovery process rapidly. Inter- mediate sediment deposits kill selectively. When with sur- face heterogeneity, nurse logs and root wads combine het- erogeneous vegetation results. The width of a lahar affects species composition and succession rates simply through distance effects.
Lahars at Butte Camp. Vegetation on the small lahars at Butte Camp has been followed since 1982. Lahar 1 termi- nated north of the Butte (an old lava dome) on a gentle slope. It is next to a young intact forest dominated by sub- alpine fir and lodgepole pine (Fig. 3.12). The deposit thick- ness was at least 1 m except for the tongue of the lahar. This lahar smothered conifers along the margin, but it took sev- eral years for these stress-tolerant trees to die (Fig. 3.13).
Lahar 2 was larger and continued down the slope wreaking destruction to forest roads and campsites many kilometers from the cone. It spread over a broad ridge and was isolated from forests by several hundred meters (Fig. 3.14). These lahars shared an initiation date and were of sim- ilar materials, yet the rate of plant community development differed significantly and species composition diverged over time. Unfortunately, torrents from vicious storms in the winter of 2006 cut so deeply into the canyon separating the
Fig. 3.11. Secondary erosion; the small lahar barely suggests the
power of full scale events (July 26, 1981).
Fig. 3.12. Lahar 1 viewed from above and to the west (July 9,
2008). Adjacent forest vegetation strongly influenced the de-
velopment of this vegetation.
lahars that access was interdicted, so comparisons monitor- ing data ceased in 2005.
Permanent plots were established in 1982 (del Moral 2010). Two were established on Lahar 1 and five on Lahar
- The number of species increased relatively quickly (Fig. 3.15A), but plots on Lahar 1 had more species than did those on Lahar 2. A subsequent decline occurred in all plots when the developing dominant species excluded rare ones. On Lahar 1, subalpine fir and lodge pole pine became dom- inant. On Lahar 2, lupines and grasses achieved dominance. Due to the proximity of dense vegetation, cover percentage on Lahar 1 increased rapidly as conifer seedlings began to develop. By 2009, plots on Lahar 1 were dense and difficult to walk through, while those on Lahar 2 remained easily traversed (Fig. 3.15B). These changes are documented by photographs taken from the same point in representative plots over the years (Fig. 3.16A-D).
Changes in species composition are reflected in time- course vectors. They indicate moderate changes (Fig. 3.17) compared to other habitats; each arrow represents vegeta- tion dynamics determined from DCA and thus directly compare degree of change in time. The two plots on Lahar 1 move away from the others, a result of the conifer inva- sion. By the end of the study, plots on Lahar 1 were similar to each other, and those on Lahar 2 were also relatively sim- ilar to each other. However, floristic differences between la- hars were four times greater than those among Lahar 2
Fig. 3.13. Large subalpine fir survived burial by the lahar, but
its roots were denied oxygen and the tree slowly perished (Au- gust 22, 1982).
Fig. 3.14. Lahar 2 viewed from above and to the east (July 9,
2008). All portions of this lahar were equally isolated from
sources of colonists.
Fig. 3.15. Vegetation structure on permanent plots estab-
lished on lahars near Butte Camp: A. richness; B. percent cover.
plots. Thus, proximity to the intact forest made a huge dif- ference how vegetation developed on these lahars. course vectors. They indicate moderate changes (Fig. 3.17) com- pared to other habitats; each arrow represents vegetation dynamics determined from DCA and thus directly compare degree of change in time. The two plots on Lahar 1 move away from the others, a result of the conifer invasion. By the end of the study, plots on Lahar 1 were similar to each other, and those on Lahar 2 were also relatively similar to each other. However, floristic differences between lahars were four times greater than those among Lahar 2 plots. Thus, proximity to the intact forest made a huge difference how vegetation developed on these lahars.
Both lahars were also sampled using a grid system start- ing in 1987 in order to develop a detailed idea of recovery. The grids used contiguous square 100m 2 plots. Each species was recorded in each plot using an index of cover, from which the number of species (richness) and cover percent- age were determined (see del Moral and Wood 2012 for de- tails). Both increased during the study (Fig. 3.18A, B). Veg- etation on both lahars was initially sparse. By the end of monitoring, plots on Lahar 1 were dominated by subalpine fir and lodge pole pine. Richness on the two lahars was sim- ilar throughout the study. The ground layer of Lahar 1 be- came sparser as conifers matured. In contrast, Lahar 2 sup- ported a diverse ground layer assemblage that included pussypaws, alpine buckwheat, prairie lupine, Cardwell’s beardtongue, Davis’ fleeceflower, hawkweed and red heather. Total cover was lower due to the limited conifer cover. When trees were excluded, the cover of species on Lahar 2 was twice that of that on Lahar 1, suggesting that conifers reduce understory cover. The species were grouped
Fig. 3.17. Trajectories of seven permanent plots on lahars at
Butte Camp (1982-2005). Numbers in parentheses indicate
Euclidean distance traveled by plots in floristic space deter- mined using DCA.
Fig. 3.16. Lahars at Butte Camp: A. Lahar 1, 1982; B. La- har 1, 2005; C. Lahar 2, 1982; D. Lahar 2, 2005.
Fig. 3.18. Vegetation structure comparisons on grids on Butte Camp lahars: A. Richness, with and without tree species; B. Cover percent, with and without tree species.
steep, recovery has been slow. Established plants were fre- quently removed during years with excessive precipitation. Scoured sites on gentler terrain have recovered substantially, and by 2008 they differed little from surrounding sites that had only received tephra fall (Fig. 3.21). The scour of Pine
Creek Ridge showed how disturbance intensity affects com- munity structure. In addition to the blast, which killed the conifers clinging to this slope above 1400 m, rapid melting of the Shoestring Glacier unleashed a torrent that over- whelmed the upper canyon of Pine Creek and swept away most soil and vegetation on the ridges. As this lahar receded, a coating of fine mud clung to the scoured surface. The depth of this infertile material diminished as the elevation dropped because the ridge got wider and Pine Creek canyon got larger. When I first set foot on this ridge on July 26, 1980, it was unbelievable that anything could have survived (Fig. 3.22). The landscape was bleak, dusty, hot and dry. However, as I trudged up slope, I came upon clear evidence that life had indeed survived. A small company of ants was busily attending to its business on a barren surface above 1500 m (Fig. S3.1; see Sidebar 3.1). If ants could survive, perhaps plants had as well.
When I returned to this site on August 20, I found that the meager summer rains and relentless wind had started to erode the mantle of mud (Fig. 3.23). In gullies and in rills, a few plants, mostly bentgrass, struggled to persist (Fig. 3.24). When I returned on September 10 to establish permanent plots, five of nine plots had a few plants, all in sites from which the mud had been removed. Permanent plots estab- lished at 1370 m suffered less damage than those at 1525 m. Later, I found that no plants had survived where the mud had persisted to the following year.
Floristic trajectories of scoured plots show moderate
change compared to other permanent plots (Fig. 3.25).
Fig. 3.21 .This nearby scoured site was nearly barren in 1980, but
recovered substantially (August 6, 2008). Dominant species included aster, pussypaws, buckwheat and sedges.
Fig. 3.22. Devastated landscape along Pine Creek Ridge (July 26,
Fig. 3.2 3. Pine Creek Ridge showing wind erosion and effects of
water erosion. Plants in canyons and rills have been seared by
eruptions in July and August 1980 (August 20, 1980).
Fig. 3.2 4. Rills due to water erosion developed quickly on Pine
Creek Ridge. This allowed rhizomatous plants such as bent-
grass and aster to survive (September 9, 1980).
BCC-1, perched on a gentle slope, changed little and merely increased in cover; it and converged in composition to the nearby BCD-3. BCC-2, in contrast, changed dramatically. It occupies a steeper slope and the new conditions led to its composition becoming distinct from the others. BCD- also changed greatly. The Pine Creek scours became in- creasingly distinct from the Butte Camp ones, generally moving away in composition due to strong bentgrass dom- inance. The trajectories of plots on gentle terrain moved in parallel, reflecting the development of similar dominant spe- cies. PCA-3 and PCB-4 changed little as they represent veg- etation on steep slopes that retain snow. Along each transect, plots appear to be converging.
Scour sites show the combined importance of species
survival and habitat stability. Consistently unstable sites have not developed, while stable ones have become similar to unscathed plots. Where persistent species survived under stable conditions, there has been little change in species composition, only recovery of the survivors.
Impact of lahars on plants
Lahars are devastating. In contrast to lava flows, lahars move swiftly and they often move across a broad front. Few animals can avoid the lahars, so the immediate vicinity be- comes devoid of terrestrial animals. Recolonization depends on the nature of disturbances in the surrounding sites. For- ests are scooped up to become part of the lahar. Lahars gradually diminish and deposit variable, often coarse and in- fertile, materials to form the substrate for primary succes- sion. Few plants can survive except on the margins and
where the lahar spreads out and slows on gentler terrain. Some trees survive the initial impact of the lahar, only to succumb gradually over several years of by being deprived of oxygen. However, delayed mortality can produce a dense litter of dropped needles and serve to ameliorate conditions on the lahar and hasten the establishment of the first wave in colonists. A few rhizomatous species do occasionally sur- vive if they happen to land near the surface. Lahar deposits, in contrast to pumice, are composed mostly of reworked materials and may include some organic matter. They are more fertile than pumice or other tephra types, and thus re- covery is expected to be more rapid than on such substrates. Species that have strong vegetative growth became domi- nant on lahar deposits and scoured areas. Importance of lahar deposits Disturbances associated with lahars demonstrated that once a deposit is deep enough to kill any buried plants, recovery rate is related to distance from sources of propagules. Fur- ther, the rate of recovery is a function of the growing season length, so recovery proceeds rapidly at lower elevations (e.g., Cedar Flats) and slowly on high elevation lahars (e.g., Lahar 2). Scours showed that survivors enhance the rate of recovery because they provide an abundance of local seeds and because they help to temper initially stressful condi- tions. Where to see lahar and debris avalanche deposits Lahars on Mount St. Helens provide long vistas and com- pelling landscapes that enhance the experience of visiting this volcano (Fig. 3.26; see Fig. I.1). SR-504 : North Fork Toutle River debris avalanche can be viewed from several vantages. If you travel east along SR- 504, you will have ample chances to see recovering vegeta- tion on the floodplain. These include the Hoffstadt Bluff Visitor Center, the Hoffstadt Bridge, the Forest Learning Center, Elk Rock View Point, the Coldwater Ridge Visitor Center and Johnston Ridge Observatory. You can hike onto the debris avalanche from the Hummocks trailhead, South Coldwater and Johnson Ridge Observatory. FR-83: The Muddy River lahar may be seen above the Lava Canyon Trail and on your left as you walk up the Ape Canyon Trail #234. If you keep to the left of the lahar, you follow the deeply incised Fire Creek, and up, past the Loo- wit Trail #216, and further up to Pine Creek Ridge. The views of the lahar are worth the strain of slogging the 6 to 7 km uphill.
Fig. 3.25 Trajectories of 10 scoured permanent plots based on
DCA scores. Butte Camp trajectories determined from 1980
to 2008; Pine Creek trajectories determined from 1980 to
2009. Numbers in parentheses indicate Euclidean distance
traveled by plots in floristic space.
