Volcano Hazards in the Mount Jefferson Region, Oregon · Volcano Hazards in the Mount Jefferson Region, Oregon By Joseph S. Walder, Cynthia A. Gardner, Richard M. Conrey, Bruce J

Volcano Hazards in the Mount JeffersonRegion, Oregon

Open-File Report 99-24

U.S. Department of the InteriorU.S. Geological Survey

U.S. Department of the InteriorU.S. Geological Survey

Volcano Hazards in the Mount JeffersonRegion, Oregon

By Joseph S. Walder1, Cynthia A. Gardner1, Richard M. Conrey3, Bruce J. Fisher2, and

Steven P. Schilling1

1. U.S. Geological Survey, Vancouver, Washington 986612. U.S. Geological Survey, Portland, Oregon 972163. Department of Geology, Washington State University, Pullman, Washington 99164

Open-file Report 99-24

U.S. Department of the InteriorBruce Babbitt, Secretary

U.S. Geological SurveyCharles G. Groat, Director

This report is preliminary and has not been reviewed for conformitywith U.S. Geological Survey editorial standards or with the NorthAmerican Stratigraphic Code. Any use of trade, �rm, or productnames is for descriptive purposes only and does not implyendorsement by the U.S. Government.

To purchase USGS publications contact:

U.S. Geological SurveyInformation ServicesP.O. Box 25286Denver, CO 80225(303) 202-4210

Contents

Introduction............................................................................................................................. 1Hazardous phenomena in the Mount Jefferson region............................................................ 1

Two types of volcanoes....................................................................................................... 1Hazardous phenomena at composite volcanoes.................................................................. 2

Hazards related to explosive eruptions............................................................................ 2Lava flows and domes..................................................................................................... 4Debris avalanches and lahars........................................................................................... 4

Hazardous phenomena at monogenetic volcanoes.............................................................. 5Past events at Mount Jefferson map........................................................................................ 5Volcano-hazard-zonation map................................................................................................. 6

Proximal hazard zone........................................................................................................... 6Distal hazard zone................................................................................................................ 7

Breitenbush River Valley................................................................................................. 9North Santiam River Valley.............................................................................................. 9Shitike Creek Valley........................................................................................................ 10Whitewater River Valley................................................................................................. 10Metolius River Valley..................................................................................................... 11

Regional lava-flow hazard zone.......................................................................................... 11Tephra hazard zone............................................................................................................. 11

Hazard forecasts and warnings............................................................................................... 12Protecting our communities and ourselves from volcano hazards......................................... 12References and additional suggested reading......................................................................... 13End notes................................................................................................................................. 13

Illustrations

Plate 1. Volcano hazards in the Mount Jefferson Region, Oregon ........................... In pocketProximal and distal hazard zones from an event at Mount Jefferson;regional hazard zones that could be affected by eruptions ofmonogenetic volcanoes.

Plate 2. Volcano hazards in the Mount Jefferson Region, Oregon ............................ In pocket

Map A. Lahar-inundation zone for areas downstream of Detroit Damfor the case in which the dam is breached.

Map B. Lahar-inundation zones for areas downstream of Pelton Damfor the case in which the Round Butte and Pelton dams are breached.

Map C. Regional tephra-fall maps.

Figure 1. Simplified sketch showing hazardous events at a volcano like Mount Jefferson... 3

Cover photo: Mount Jefferson from the east.Lyn Topinka, U.S. Geological Survey

Volcano Hazards in the Mount Jefferson Region, Oregon

By Joseph S. Walder, Cynthia A. Gardner, Richard M. Conrey, Bruce J. Fisher, and Steven P. Schilling

Introduction

Mount Jefferson is a prominent feature of thelandscape seen from highways east and west ofthe Cascades. Mount Jefferson (one of thirteenmajor volcanic centers in the Cascade Range) haserupted repeatedly for hundreds of thousands ofyears, with its last eruptive episode during thelast major glaciation which culminated about15,000 years ago. Geologic evidence shows thatMount Jefferson is capable of large explosiveeruptions. The largest such eruption occurredbetween 35,000 and 100,000 years ago, andcaused ash to fall as far away as the present-daytown of Arco in southeast Idaho. Although therehas not been an eruption at Mount Jefferson forsome time, experience at explosive volcanoeselsewhere suggests that Mount Jefferson cannotbe regarded as extinct. If Mount Jefferson eruptsagain, areas close to the eruptive vent will beseverely affected, and even areas tens ofkilometers (tens of miles) downstream alongriver valleys or hundreds of kilometers (hundredsof miles) downwind may be at risk.

Numerous small volcanoes occupy the areabetween Mount Jefferson and Mount Hood to thenorth, and between Mount Jefferson and theThree Sisters region to the south. These smallvolcanoes tend not to pose the far-reachinghazards associated with Mount Jefferson, but arenonetheless locally important.

A concern at Mount Jefferson, but not at thesmaller volcanoes, is the possibility that small-to-moderate sized landslides could occur evenduring periods of no volcanic activity. Suchlandslides may transform as they move intolahars (watery flows of rock, mud, and debris)that can inundate areas far downstream. Thepopulation at immediate risk in the MountJefferson region is small, but these residents aswell as other people who visit the area forrecreation and work purposes should be aware of

the potential hazards. Probably the greatestconcern in the Mount Jefferson region is thepossibility that large lahars might enter reservoirson either side of the volcano, namely, Detroit Laketo the west and Lake Billy Chinook to the east.Lahars entering these lakes could set up largewaves that could overtop dams and possibly causedam failure, with catastrophic effects downstream.Such events have very low probabilities but greatpotential consequences.

This report describes the kinds of hazardousgeologic events that have occurred in the MountJefferson area in the past and shows, in theaccompanying volcano-hazards-zonation maps,which areas will likely be at risk when hazardousevents occur again in the future.

Hazardous phenomena in the MountJefferson region

The last eruption in the Mount Jefferson regionoccurred long before human habitation of the area.Moreover, the geologic history of Mount Jeffersonis not well known [1]. Therefore, we rely on datafrom similar volcanoes around the world to gain ageneral idea of possible eruption scenarios andhazards. This is a reasonable method becausesimilar types of events occur at many volcanoes,even though exactly which types of events occurand their relative frequencies and magnitudes varyfrom volcanic centers to volcanic center.

Two types of volcanoes

Two types of volcanoes are found in the MountJefferson region: composite and monogenetic.Composite volcanoes erupt episodically over tensto hundreds of thousand of years and can display awide range of eruption styles. Mount Jefferson isa composite volcano that has been activeepisodically for about 300,000 years.

Introduction 1

Monogenetic volcanoes typically erupt for onlybrief time intervals—weeks to perhapscenturies—and generally display a narrowerrange in eruptive behavior. Most monogeneticvolcanoes are basaltic in composition, but justnorth of Mount Jefferson a few are of andesiteand dacite composition—that is, with a relativelyhigher silica content (see inset figure 1). Over atime span of hundreds of thousands of years,these monogenetic volcanoes have built a broadupland areas (hundreds to thousands of squarekilometers (miles)) of mostly basaltic lava flowsand small volcanoes. Prominent basalticvolcanoes in the Mount Jefferson region includeOlallie Butte, Potato Butte, Sisi Butte, and Northand South Cinder Peaks. Fresh-looking basaltlava flows can be seen along the Cabot Creek,Jefferson Creek, and upper Puzzle Creekdrainages. Hundreds more basaltic volcanoesform the High Cascades of central Oregon to thesouth of Mount Jefferson, as far as Crater Lake,180 kilometers (110 miles) away.

Hazardous phenomena at composite volcanoes

All of the hazardous events depicted in theaccompanying schematic illustration of acomposite volcano (figure 1) have occurred atMount Jefferson in the past and could occur inthe future. Most are caused by eruption ofmolten rock, or magma, but some, like debrisavalanches and lahars, can occur without eruptiveactivity.

Hazards related to explosive eruptions

As magma nears the surface, gases dissolvedin the magma are released. Rapid release canmake the magma frothy and propel it upwardfrom the vent in a rush of expanding hot gas.The resulting solidified rock fragments, calledtephra, range in size from large bombs (6 cm to1 meter (2 inches to 3 feet) or more in diameter)to ash (sand size or finer). Large tephra particlesfall back to the ground within a few kilometers(miles) of the vent. Particles that move on arcslike artillery shells are called ballisticprojectiles. Their range rarely exceeds5 kilometers (3 miles) from the eruptive vent,

and most projectiles are less than one meter(3 feet) across. The chief hazard from ballisticprojectiles is from direct impact. Large projectilesmay still be quite hot when they land, and can startfires if they land near combustible materials.

Relatively small tephra particles can rise morethan ten kilometers (30,000 feet) upward and becarried downwind (figure 1). The particlesgradually fall from the ash cloud and can blanketareas for tens to hundreds of kilometers (tens tohundreds of miles). Tephra fall seldom directlythreatens life except within a few kilometers of theeruptive vent, but ash suspended in the air canirritate the eyes and respiratory system, especiallyin infants, the elderly, and the infirm, andprolonged inhalation of certain kinds of tephra cancause chronic lung disease. Perhaps morehazardous are the indirect effects. Tephra plumescan create tens of minutes to hours of darkness,even on sunny days, as they pass overhead, andtephra fall can reduce visibility on highways. Inaddition, deposits of tephra can short-circuit orbreak electric transformers and power lines,especially if the tephra is wet, as well as causeroofs of buildings to collapse. (In severalhistorical examples, accumulation of more than10 centimeters (4 inches) of wet tephra causedroofs to collapse, although the precise criticalthickness depends upon the structural soundness ofthe roof. In the Pacific Northwest, most houses arebuilt to sustain heavier snow loads.) Tephra canclog filters and increase wear on vehicle engines.Tephra clouds commonly generate lightning thatcan interfere with electrical and communicationsystems and start fires. Finally, and perhaps mostimportantly, even small, dilute tephra clouds posegreat hazards to aircraft that fly into them.

Lessons learned during the 1980 eruption ofMount St. Helens in downwind Washingtoncommunities such as Yakima, Ritzville, andSpokane are now applied throughout the PacificNorthwest and elsewhere to prepare governments,businesses, and citizens for future tephra falls.These three communities experienced significantdisruptions in transportation, business activities,and community services as a result of fallout offrom 5 to 80 millimeters (1/4 to 3 inches) oftephra. The greater the amount of tephra that fell,the longer a community took to recover. As

2 Volcano Hazards in the Mount Jefferson Region, Oregon

perceived by residents, tephra falls of less than5 mm (1/4 inch) were a major inconvenience,whereas falls of more than 15 mm (2/3 inch)constituted a disaster. Nonetheless, all threecommunities recovered to nearly normalactivities within two weeks.

Sometimes the mixture of hot gases andvolcanic rock particles produced by an explosiveeruption is denser than air. Instead of rising abovethe vent to produce tephra, the mixture behaveslike a fluid and flows downslope over the groundsurface. If the mixture is made up mostly of rock

Hazardous phenomena in the Mount Jefferson region 3

Pyroclastic Flow

Pyroclastic Flow

Eruption Column

Lahar (Debris Flow)

Lava Flow

Prevailing Wind

MagmaReservoir

Dome Collapse

Fumaroles

Eruption Cloud

Ash (Tephra) Fall

Acid RainBallistic Projectiles

Lava Dome

Conduit

Crack

Ground Water

Landslide(Debris Avalanche)

Magma Silica (SiO2)Types Content

Rhyolite >68%Dacite 63-68%Andesite 53-63%Basalt <53%

Magma

(HydrothermalSystem)

Figure 1. Simplified sketch showing hazardous events at a volcano like Mount Jefferson. Events such as lahars andlandslides (debris avalanches) can occur even when the volcano is not erupting. Inset box shows classification ofmagma types on the basis of silica content. Illustration by Bobbie Myers, modified from USGS Fact Sheet 002-97.

particles, it has a high density and topographycontrols its path, just as topography controls theflow of water. Such a gas-poor mixture is calleda pyroclastic flow. If the mixture is made upmostly of gas, with only a small proportion ofrock fragments, it has a lower density and its pathis weakly influenced by topography. Such agas-rich mixture is called a pyroclastic surge.

Pyroclastic flows and surges often occursimultaneously, and both are exceedinglyhazardous. They move at such high speeds thatescape from them is difficult or impossible.Pyroclastic flows typically sweep down theflanks of volcanoes at speeds of 50 to 150kilometers per hour (30 to 90 miles per hour).Temperatures in pyroclastic flows are usuallymore than 300 degrees Celsius (570 degreesFahrenheit). Owing to their high density, highvelocity, and high temperature, pyroclastic flowscan destroy all structures and kill all living thingsin their paths by impact, burial, and incineration.The effects of pyroclastic surges may be lesssevere, because of lower densities andtemperatures, but are still usually destructive andlethal. People and animals caught in pyroclasticsurges may be killed by direct impact by rocks,severe burns, or suffocation.

Lava flows and domes

Lava is magma that reaches the Earth’ssurface nonexplosively. Depending on itsviscosity and rate of discharge, lava will form abulbous lava dome over the vent or a lava flowthat extends several to more than 10 kilometers(6 miles) downslope. Observations of lava flowsat similar volcanoes elsewhere suggest that lavaflows at Mount Jefferson would move downvalleys as tongues of fluid lava a few to tens ofmeters (10-30 feet) thick encased in a thick coverof hardened lava rubble. Such lava flows candestroy all structures in their paths and becauseof their intense heat may start forest fires.However, because lava flows advance so slowlythey seldom endanger people. Lava domes, onthe other hand, may pose a far greater hazard.Lava domes that grow on steep slopes aretypically unstable and collapse repeatedly as theygrow higher and steeper, forming fast-movingpyroclastic flows.

Debris avalanches and lahars

The slopes of a volcano may become unstableand fail, generating a rapidly moving landslidecalled a debris avalanche. This instability may becaused by magma forcing its way into a volcano,pushing aside older rocks. The best-knownhistoric example of this type of magmaticallyinduced debris avalanche occurred in 1980 atMount St. Helens. A volcano’s slopes can also failwithout the direct involvement of magma.Stability slowly declines as slopes becomeover-steepened by glacial erosion or as the strengthof the rock is reduced by hot, acidic waters of ahydrothermal system. The acidic waterschemically alter rock to clay and other minerals.Like a house infested with termites, the alteredrock gradually weakens and may collapse under itsown weight, thereby generating a debrisavalanche. Non-magmatically induced debrisavalanches occur frequently at volcanoes and canbe especially dangerous because some large onesmay occur without any warning. Earthquakes,steam explosions, and intense rainstorms can alsotrigger debris avalanches from parts of a volcanothat have already been weakened by glacialerosion or hydrothermal activity.

A debris avalanche can travel tens ofkilometers (tens of miles) and can attain speeds inexcess of 160 kilometers per hour (100 miles perhour); the larger the avalanche, the farther it canmove. Avalanche path is strongly controlled bytopography. Structures and objects in theavalanche path will likely be destroyed by impactand incorporated into the avalanche. The resultingdeposit is usually a few meters (yards) to hundredsof meters (hundreds of yards) thick, with ahummocky surface. Deposits of a large debrisavalanche can block the mouths of tributaryvalleys and cause lakes to form. When impoundedwater spills over the blockage, it can quickly cut achannel and cause catastrophic lake drainage,generating a flood or lahar ( a moving slurry ofwater and sediment much like flowing concrete).This may occur hours to months after formation ofthe impoundment.

Whatever their origin, debris avalanchescommonly contain enough water or incorporateenough water, snow or ice to transform into lahars.Lahars usually move at speeds of tens of


kilometers (tens of miles) per hour, and destroyor damage everything in their paths by impact orburial. Lahars follow river valleys and may fillthe valleys to depths of 30 meters (100 feet) ormore. Reservoirs in valleys downstream from avolcano may be partly or wholly filled by laharsmoving down valley, so if the water level of areservoir is not lowered in time, water displacedby a lahar could cause floods farther downstreamand may endanger the dam. Lahars can alsoform by other mechanisms. Pyroclastic flowscan incorporate and melt enough snow and ice togenerate lahars. Relatively small lahars can alsobe generated by outburst floods releasedsuddenly from glaciers, by failure ofglacial-moraine dams that impound small lakes,or by heavy rain remobilizing loose fragmentalmaterial on the flanks of volcanoes.

Lahars may cause problems long after theoriginal event has ceased. Once lahars fill streamchannels with sediment, channels begin to erodenew paths. In such situations, channels canbecome unstable and shift rapidly as sediment ismoved farther downstream. Also, becausechannels become filled with sediment, they canconvey less water and susceptibility to floodingincreases. Examples from many volcanoes,including Mount St. Helens, show that sucheffects can persist for years to decades.

Hazardous phenomena at monogenetic

volcanoes

Monogenetic volcanoes are usually ofbasaltic composition (figure 1) and typicallyerupt less explosively than do compositevolcanoes, so their eruption effects are spatiallylimited. A few monogenetic volcanoes in theMount Jefferson area southwest of Olallie Butteare small dacite domes. Emplacement of futuredacite domes may involve a small explosivephase, and collapse of dome margins could formsmall-volume pyroclastic flows and lahars.Nonetheless, lava flows constitute the primaryhazard associated with monogenetic volcanoes inthe Mount Jefferson area. Lava flows candestroy everything in their paths by burial orincineration, but generally travel slowly enoughthat they are not life-threatening to most humans

or wildlife. Hazards associated with tephra fall aregenerally restricted to areas within a fewkilometers (1-2 miles) downwind of an eruptivevent. Floods or lahars may be caused by lavaflows interacting with winter snowpack, or bypartial collapse of lava flow fronts or domemargins. Perhaps the most dangerous events atbasaltic volcanoes occur when rising magmainteracts explosively with surface water or shallowground water, producing steam explosions andpyroclastic flows that sweep rapidly outwardseveral kilometers (several miles) from the vent.However, deposits associated with such eventshave not been recognized in the Mount Jeffersonregion.

Past events at Mount Jefferson

Details regarding eruptive events at MountJefferson are poorly known for several reasons.The last major eruption occurred a long time ago,and since that time the volcano has been coveredby large glaciers that eroded away many deposits,including much of the lahar record. Furthermore,we know the dates of only a few of the eruptiveproducts that have been preserved. Nonetheless,although many details are unknown, we do knowthat Mount Jefferson has a history that extendsback for several hundred thousand years and that ithas exhibited a wide range of eruptivestyles—from highly explosive events to lava flowsand lava domes.

What we do know of the history of MountJefferson can be summarized as follows. Theoldest rocks at Mount Jefferson are about 300,000years old and crop out on the west-southwest sideof the volcano. The next oldest rocks are found inthe Park Butte area and are about 150,000 yearsold. Sometime between 35,000 and 100,000 yearsago, a very large explosive eruption occurred.Tephra from this eruption has been found as faraway as southeast Idaho, and within 20 kilometers(12 miles) of volcano is locally 2 meters (6 feet)thick. During this same broad period of time,pyroclastic flows moved down two drainages onMount Jefferson (Whitewater River on the eastside and Whitewater Creek on the west side).

Most of the cone (upper 1000 meters) ofMount Jefferson is less than 100,000 years old,

Past events at Mount Jefferson 5

with much of it younger than the explosive eventdescribed above. The upper cone is composedlargely of dacite lava flows and domes, many ofwhich appear to have been emplaced whenglaciers on the volcano were much larger than atpresent. It is likely that during growth of thedomes, material was shed off to form pyroclasticflows and lahars, but if so, that record has beenlargely removed by glacial erosion.

During the last few centuries, several smalllakes were formed on the flanks of MountJefferson when small tributary valleys becamedammed by glacial moraines (ridges of sedimentleft behind by glaciers). Several of thesemoraines have breached during the 20th century,producing local floods and small lahars.

The youngest lava flows in the MountJefferson area are basaltic lava flows fromForked Butte and an unnamed butte south ofBear Butte. Both of these flows postdate thelarge eruption of Mount Mazama (Crater Lake)of about 7,600 years ago.

Volcano-hazard-zonation map

The accompanying volcano-hazard-zonationmaps show areas most likely to be affected byfuture hazardous geologic events in the MountJefferson region. Individual events typicallyaffect only part of a hazard zone. The locationand size of an affected area will depend on thelocation of the erupting vent or landslide, thevolume of material involved, the snow and iceconditions around and down slope from the vent,and the character of an eruption, especially itsexplosivity.

Potentially hazardous areas around MountJefferson are divided into proximal and distalhazard zones depending on distance from thevolcano. Some zones are subdivided further onthe basis of their relative degree of hazard.Hazard-zone boundaries are drawn on the basisof: (1) the magnitude of past events at thevolcano, as inferred from deposits; (2)mathematical models that use calibrations fromother volcanoes to forecast the probable extent offuture pyroclastic flows, debris avalanches, and

lahars; and (3) our experience and judgmentderived from observations and understanding ofevents at other similar volcanoes. A regionalhazard zone for lava flows from volcanoes is alsoshown on plate 1. Maps of the Pacific Northwestdepicting hazard zones for tephra falls from allCascade composite volcanoes are shown onplate 2.

Although the maps show sharp boundaries forhazard zones, the degree of hazard does not changeabruptly at these boundaries. Rather, the hazarddecreases gradually as distance from the volcanoincreases and, for lahars, decreases more rapidly aselevation above valley floors increases. Areasimmediately beyond outer hazard zones should notbe regarded as hazard-free, because the boundariescan only be located approximately, especially inareas of low relief. Too many uncertainties existabout the source, size, and mobility of futureevents to locate the boundaries of zero-hazardzones precisely.

It is almost impossible to estimate theprobability of future eruptions at Mount Jeffersonowing to the lack of eruptive activity in the last15,000-20,000 years, the imprecise dating of olderdeposits, and the fact that the geologic recordgenerally preserves only the largest eruptiveevents. At other Cascade volcanoes where there isa better record of the last 12,000 to 15,000 years,long periods of dormancy have occurred betweeneruptive episodes. For example, Mount St. Helenswas dormant from 10,000 years ago until 4,000years ago—a period of 6,000 years—and MountHood was dormant from about 15,000 years agountil 1,500 years ago—a period of nearly 14,000years. Our knowledge of volcanoes is too limitedto know how long a dormant period at any givenvolcano can last. Because the consequences ofrenewed activity could be severe, we continue tomonitor Mount Jefferson for signs of restlessness.

Proximal hazard zone

The proximal hazard zone includes areasimmediately surrounding Mount Jefferson. Thiszone, which, depending on local topography,extends 8 to 16 kilometers (about 5-10 miles)outward from the summit, delineates the areasubject to several types of rapidly moving,


devastating phenomena including pyroclasticflows and surges, lava flows, and ballistics [2].During periods of volcanic unrest or during aneruption, these are areas to be avoided becauseevents often unfold too quickly for humans toescape harm. Debris avalanches and laharsoriginate in the proximal area and small eventsmay be restricted to this zone. However, largerdebris avalanches and lahars will move off thevolcano and far down river valleys. The extentof these larger lahars constitute the distal hazardzones.

Failure of glacial moraine dams that impoundhigh-altitude lakes on Mount Jefferson couldrelease floods of water and debris. The majorimpact of such events would be restricted to theproximal hazard zone, but parts of distal hazardzones adjacent to streams might also beinundated. Several moraine-dam failures haveoccurred during the 20th century and inundatedparts of Jefferson Park and part of the JeffersonCreek drainage below Waldo Glacier [3].

Distal hazard zones

Explosive eruptions or large debrisavalanches in the proximal hazard zone cangenerate lahars of sufficient volume to travel tensof kilometers (tens of miles) from source areas.The hazard-zonation map shows that distalhazards from such events are concentrated in thevalleys of the South Fork Breitenbush River andthe North Santiam River on the west side ofMount Jefferson, and in the valleys of theWhitewater River, Metolius River, and ShitikeCreek on the east side. Debris avalanches andlahars will tend to funnel into these valleys asthey leave the slopes of Mount Jefferson withinthe proximal hazard zone.

Geologic evidence of lahar-producing eventsat Mount Jefferson is scant and provides littleguidance with which to forecast the extent oflahars that may be produced by future eruptionsor debris avalanches. We therefore use amathematical technique calibrated with data fromother volcanoes [4] to estimate the extent ofdistal inundation by future lahars of variousvolumes. For each of the major valleys draining

Mount Jefferson, we computed three nested distalhazard zones that depict anticipated inundation byhypothetical “design” lahars of three differentvolumes. The largest design lahar reflects ourestimate of the largest debris avalanche(500 million cubic meters, or about 650 millioncubic yards) that might descend suddenly fromMount Jefferson [4]. A debris avalanche of thissize would entail wholesale failure of a large partof the volcano’s upper flanks and would requireeither complete transformation of the debrisavalanche into a lahar or considerableincorporation of down-valley material to achieve alahar volume of 500 million cubic meters(650 million cubic yards). Such an event wouldprobably occur in conjunction with volcanicactivity that would be detected by monitoring;however, we cannot rule out the low probabilitythat some large flank failures could be triggered bymechanisms other than magmatic intrusion (forexample, gravitational failure due to hydrothermalalteration slowly weakening summit rocks).

The intermediate and smallest design lahars aremore typical lahar volumes for a moderateeruption or for a debris avalanche withoutwarning. The intermediate-sized design lahar hasa volume of 100 million cubic meters (about130 million cubic yards), and the smallest designlahar has a volume of 20 million cubic meters(about 25 million cubic yards). The distal hazardzone for the smallest design lahar depictsanticipated inundation areas from events that couldoccur during small eruptive events or steamexplosions; it also depicts inundation areas fromevents that occur without eruptive activity at thevolcano and for which there would be noprecursory warning. These include rain-on-snowevents remobilizing loose sediment on the slopes,outburst floods, and small debris avalanches orrockslides.

A special case exists for the valleys of ShitikeCreek (north and east of Mount Jefferson) andMinto Creek (south and west of Mount Jefferson).For both cases we show lahar-inundation zones foronly the smallest design lahar, even though weanticipate these areas would be affected onlyduring the largest magnitude events. NeitherShitike Creek nor Minto Creek heads directly onMount Jefferson. Debris avalanches or pyroclastic

Volcano-hazard-zonation map 7

flows entering these valleys would have toovertop divides about 200 to 250 meters (660 to820 feet) high. Based on experience at Mount St.Helens during the 1980 eruptions, only extremelylarge events would be able to overtop such highdivides, and only a small percentage of the totaldebris volume would actually make it over. Thuswe suggest that the 20 million cubic meter(25 million cubic yard) design lahar representsthe largest probable lahar for Shitike and MintoCreeks. Because only extremely large debrisavalanches at Mount Jefferson are likely to affectthese drainages, the probability of a 20 millioncubic meter (25 million cubic yard) laharaffecting either drainages is equally as small asthe probability of the largest debris avalanchehappening at Mount Jefferson.

Because large lahars are less likely to occurthan small lahars, the nested distal hazard zonesshow that the likelihood of lahar inundationdecreases as distance from the volcano andelevation above the valley floors increase. Nolahar as voluminous as any of the design laharshas occurred in the past 15,000 years [5], solahars of such magnitudes have an annualprobability of less than 1 in 15,000. However,this probability would increase immediately ifrestless activity were detected at the volcano,with the inundation area of the smallest designlahar having the highest probability. Small laharsthat result from phenomena such as moraine-damfailures or rock slides are much more likely tooccur (annual probability about 1 in 1000 to 1 in100 in potentially affected valleys), but are apt toinundate only parts of the smallest design hazardzone immediately adjacent to streams (in otherwords, an area smaller than that represented bythe smallest design lahar). In such instances,some debris may travel farther downstream butstay mostly confined within stream banks.

The distal hazard zones for lahars and debrisavalanches have been depicted on the map in twodifferent ways, depending upon assumptionsused in considering the effects of the largereservoirs on the west and east sides of thevolcano. In truth, we only have a rough idea howthe reservoirs would actually be affected by laharentry. Based on field and experimental data [6],we suggest the following scenarios will apply:

1. If lahar volume is much less than bothreservoir volume and available storage (thedifference between maximum capacity and actualvolume at any given time), entry of the lahar intothe reservoir will probably generate fairlysmall-amplitude waves that propagate rapidlyacross the lake, then a slow rise in lake level, withthe lahar being completely contained within thelake basin. This is what happened when a laharentered Swift Reservoir near Mount St. Helens in1980.

2. If lahar volume exceeds maximum availablestorage, then the dam will certainly be overtoppedowing to rising water level if not by wave action.Whether the dam subsequently fails is uncertain.An overtopped earthen dam, such as Round ButteDam, which impounds Lake Billy Chinook, wouldlikely be eroded by overflowing water and wouldprobably be breached if the overflow weresufficiently sustained. A concrete dam, such asDetroit Dam, which impounds Detroit Lake,should be more resistant, but would still be at riskof failure.

3. Lahars with volumes less than maximumavailable storage may still cause overtopping andpossible failure of dams owing to large waves thattravel across the lake and run up against the damface. The amount of runup depends in acomplicated way on wave shape and amplitude,lake depth, slope of the dam face, lahar volume,lahar speed, and perhaps other factors. However,this scenario also strongly depends upon the waterlevel in the reservoirs at the time of lahar entry.

If a dam fails, flood magnitude would stronglydepend on the amount of water in the reservoir,and would be greatest if no drawdown hadpreceded lahar emplacement. In the case ofDetroit Lake, the amount of available water wouldbe seasonally dependent as available storage isstrongly linked to flood control.

The map on plate 1 depicts distal-hazard zones,determined by a semi-empirical mathematicalmodeling technique [4], assuming that lahars arecompletely contained behind Detroit Dam andRound Butte Dam. This is almost certainly correctfor the smallest design lahar, but almost certainlyincorrect for the larger design lahars unless thereservoirs have been significantly drawn downbeforehand. Maps A and B on plate 2 depict distal


hazard zones downstream of these dams forlahars not contained behind Detroit Dam andRound Butte Dam. The inundation areas forthese large events were also determined using thesame mathematical modeling technique as above,but implicitly assuming that the dams andreservoirs provide no resistance to laharmovement. Maps A and B provide an admittedlyrough estimate of inundation hazard zones,because the calculations [4] account for thevolume of the lahar but not that of the waterimpounded behind the dams. Dam operators (theUS Army Corps of Engineers for Detroit Dam,and Portland General Electric for Round ButteDam) have constructed maps of probable floodinundation areas in the event of dam failure, butthese maps account for only the water impoundedbehind the dams and not the volume of potentiallahars.

Breitenbush River Valley

The South Fork Breitenbush River drains partof the north and northwest slopes of MountJefferson, and flows into the north side of DetroitLake (a reservoir with a storage capacity of about560 million cubic meters, or 455 thousandacre-feet, impounded behind Detroit Dam, aconcrete structure.) Debris avalanches fromMount Jefferson directed a bit west of north, intoJefferson Park, would enter this drainage basin.Calculations using a mathematical technique forestimating lahar inundation area [4] show thatlahars in this drainage of volume less than about4 million cubic meters (5.2 million cubic yards)would stop short of Detroit Lake. Larger laharswould enter Detroit Lake, which is used for floodcontrol and whose level varies greatly during theyear [7]. The typical operating level (so-calledmaximum conservation pool) of the lake fromMay through August, when the lake is heavilyused for recreational purposes, leaves a freeboardof about 1.7 m (5.5 feet) and an available storageof about 23 million cubic meters (19,000acre-feet). The lake is drawn down fromSeptember through November to a level at whichthe freeboard is about 36 meters (110 feet) andthe available storage is about 370 million cubic

meters (300,000 acre-feet), and held at this leveluntil February, when it is allowed to start refilling.Even our smallest design lahar, at 20 million cubicmeters, might generate waves that would overtopthe dam during summer if the lake were notlowered in advance. In contrast, the same laharentering Detroit Lake during winter, with the lakeat its lowest level, would almost certainly becontained. Indeed, even a 100-million-cubicmeter-lahar would probably be contained duringwinter. A 500-million-cubic-meter lahar—thelargest modeled lahar—would cause the dam to beovertopped and perhaps breached unless the lakehad been significantly drawn down. Townsdownstream of Detroit Lake that could beinundated by large lahars or dam-failure floodsinclude Niagara, Mill City, and perhaps Stayton.

North Santiam River Valley

Whitewater Creek, Russell Creek, and MilkCreek drain the west slopes of Mount Jeffersonand enter the North Santiam River, which flowsinto the east side of Detroit Lake. The NorthSantiam River continues below Detroit Dam.Lahars in this drainage of volume less than about3 million cubic meters (4 million cubic yards) willstop short of Detroit Lake. Larger lahars will enterDetroit Lake and behave essentially the same asthose entering the lake via the South ForkBreitenbush River, as described above.

A lahar could also enter the North SantiamRiver via Minto Creek, southwest of MountJefferson. Although it does not head on MountJefferson proper, the headwaters of Minto Creekare within the proximal hazard zone such that verylarge pyroclastic flows or very large debrisavalanche could overtop the ridge southeast ofGrizzly Peak. Any debris avalanche enteringMinto Creek drainage would have to overtop aroughly 200 meter (660 ft) high divide. Based onexperience at Mount St. Helens during the 1980eruptions, this is possible but very unlikely exceptfor the largest of debris avalanches. The amount ofdebris entering Minto Creek would probably be asmall fraction of the total. Thus we have shownthe inundation area for a 20 million cubic meterlahar, and suggest this is a reasonable estimate of


the maximum lahar that could move along thispath.

Evidence has recently been discovered ofprehistoric lahars that reached the site ofpresent-day Salem about 100,000 years ago [5].We presume these lahars originated at MountJefferson during an eruption and traveled alongthe valley of the North Fork River. However,we know little about the condition of thedrainage basin at that time (for example, whetherriver level was near or at present river level) ormuch about the volume of glacier ice on themountain other than that it was probably muchgreater than now. Our models suggest that underpresent conditions, it would take a lahar with avolume in excess of 1 cubic kilometer (1.3 billioncubic yards) to reach Salem. In our opinion, itwould be extremely difficult to generate a laharof such volume from a single instantaneous event(a single avalanche event or single explosiveevent). In the case of long-lived explosive eventsor multiple dome collapse events, it may bepossible to shed so much debris into the riverdrainage to eventually cause flooding far beyondthe limits shown for the largest design lahar. Itmay also be possible to inundate much largerareas than those shown if dam failure occurred asthe result of a large lahar entering Detroit Lake ata time when water level was high.

Shitike Creek Valley

Shitike Creek heads in the area betweenMount Jefferson and Olallie Butte to the north,then flows to the east toward the town of WarmSprings, beyond which it flows into theDeschutes River. Although it does not head onMount Jefferson proper, Shitike Creek, likeMinto Creek on the west side, could besusceptible to inundation by very large debrisavalanches that could overtop a 250 meter(800 feet) ridge north of Jefferson Park.However, like Minto Creek, only a small fractionof the total avalanche volume would probablyimpact Shitike Creek. Thus, we have shown theinundation area for a 20-million-cubic-meter-lahar(25 million cubic yards), and suggest this is areasonable estimate of the largest lahar that could

enter Shitike Creek owing to an extremely largenorthward-directed debris avalanche from MountJefferson. Such an event could affect the town ofWarm Springs, but we stress that an event of thismagnitude has very low probability (annualprobability much less than 1 in 15,000).

Whitewater River Valley

The Whitewater River drains the WhitewaterGlacier and northeast slopes of Mount Jefferson,and is also fed by Milk Creek (not the same MilkCreek as on the west side of the mountain). Allthree hypothetical lahars considered in ourassessment would move along the WhitewaterRiver valley to the confluence with the MetoliusRiver. Any lahar in the Whitewater River valleyof volume greater than about 4 million cubicmeters (5 million cubic yards) would reach LakeBilly Chinook, a reservoir with a capacity of about660 million cubic meters (535,000 acre-feet)impounded behind Round Butte Dam, anearth-and-rock fill structure which is operated byPortland General Electric for hydropowergeneration [7]. The dam is operated with afreeboard of about 8 m (25 feet) and an availablestorage of about 165 million cubic meters(135,000 acre-feet). The smallest volume designlahar (20 million cubic meters, or 26 million cubicyards) would certainly generate waves in thereservoir, but it seems unlikely that the dam wouldbe overtopped. The intermediate volume lahar(100 million cubic meters, or 130 million cubicyards) may cause an overtopping wave and thelargest lahar (500 million cubic meters, or650 million cubic yards) would almost certainlycause the dam to be overtopped and breached,generating large floods downstream, if lake levelswere not lowered beforehand. Immediatelydownstream of Round Butte Dam, on theDeschutes River, is Simtustus Lake, a reservoirimpounded behind Pelton Dam and also operatedfor hydropower. Floods caused by breaching ofRound Butte Dam would probably overtop PeltonDam and cause this dam to fail as well, sendinglarge flood waves down the Deschutes River.Warm Springs, a city at the confluence of ShitikeCreek and the Deschutes River, would be affectedby such a flood wave.


Metolius River Valley

Jefferson Creek and Parker Creek drain thesoutheast flank of Mount Jefferson and enter theMetolius River, which flows into Lake BillyChinook. Lahars in this drainage of volume lessthan about 20 million cubic meters (26 millioncubic yards) will stop short of Lake BillyChinook. Larger lahars will enter the lake andbehave essentially the same as those originatingin the Whitewater River valley, as describedabove.

Regional lava-flow hazard zone

The regional lava-flow hazard zone outlinedon the hazard map is the area affected by lavaflows and near-vent explosive material fromrelatively short-lived eruptions frommonogenetic volcanoes. The zone was definedby first determining the distribution of vents andlava flows less than 2 million years old. (Thereare a few exceptions to the rule. For example,Battle Axe Mountain, located about 15 kilometers(9 miles) north of the town of Detroit, is thoughtto be less than two million years old, yet issurrounded by terrain older than two millionyears and is thus excluded from this zone.) Thevast majority of vents in this zone are basaltic; afew are dacite lava domes. We assume thatfuture eruptions will occur within this zone Asbasaltic lava flows generally travel no fartherthan about 15 kilometers (9 miles) from theirsource vents, we locate the hazard boundaryeither 15 kilometers (9 miles) downslope fromwhere vents may open, or where topographicfeatures would probably stop or divert lavaflows. Thus the zone juts out slightly in rivervalleys not already occupied by hazard zonesfrom lahars. The regional lava-flow hazard zoneencompasses land that is largely unpopulated, butwhich contains important infrastructure, mostnotably power lines.

The area immediately north of MountJefferson contains several domes younger thantwo million years of age of dacitic rather thanbasaltic composition. If new dacite domes shouldform in this area, it is possible that smallexplosive eruptions may accompany their growthand that pyroclastic flows might be generated by

rock avalanches from the margins of the growingdomes. However, domes formed here would beingextruded on very gentle slopes, such that anypyroclastic flows or lahars formed by collapse ofthe dome margins would probably be restricted tothe regional lava-flow hazards zone.

It is impossible to determine annualprobabilities from vent distributions over the last2 million years, because many of the older ventshave since been buried by younger lava flows.Therefore, annual probabilities are based on therecord since the last major ice age, about 12,000years ago. Since that time there have been foureruptive episodes. Two of these involved ventsvery near each other that erupted at about the sametime. Such a frequency suggests an averageannual probability of about 1 in 4,000 to 1 in3,000. However, because only a relatively smallarea (generally less than 1 percent) of the regionallava-flow hazard zone is affected during any giveneruptive episode, and because large areas withinthe zone have not been affected by renewedvolcanism for about 1 million years, the annualprobability of any point in the zone being affectedis much less than the average annual probabilityand is more on the order of 1 in 1,000,000 to 1 in300,000.

Tephra hazard zone

Even small thicknesses of tephra canprofoundly disrupt social and economic activityover broad areas. The thickness of tephranecessary to cause buildings to collapse dependson construction practices and whether the tephra iswet or dry. Eruptions of Mount Jefferson as wellas of more distant volcanoes in the Cascade Range,are all sources of potential tephra fall in localcommunities. In fact, since the last ice age, thethickest tephra fall in the Mount Jefferson area,about 10 to 15 centimeters (4 to 6 inches),occurred from the eruption of ancient MountMazama that created Crater Lake about 7,600years ago.

The accompanying maps (plate 2) indicate theannual probability of tephra fall affecting thecentral Oregon region from all major Cascadevolcanic centers. The maps are based on thecombined likelihood of tephra-producing eruptions


occurring at Cascade centers, the relationshipbetween thickness of a tephra-fall deposit anddistance from its source vent, and regional windpatterns [8]. Probability zones extend farthereast of the range because winds blow fromwesterly directions most of the time. One mapshows annual probabilities for a fall of onecentimeter (about 0.4 inch) or more and the otherfor a fall of 10 centimeters (about 4 inches) ormore. The map pattern illustrates clearly thedominating influence of Mount St. Helens as atephra producer. The area around MountJefferson has an annual probability of tephra fallof 1 centimeter (0.4 inches) or more of about 1 in5,000 and an annual probability of a fall of10 centimeters or more of about 1 in 10,000 orless.

Hazard forecasts and warnings

Scientists recognize several signs ofimpending volcanic eruptions. The upwardmovement of magma into a volcano prior to aneruption causes changes that can usually bedetected by geophysical instruments and visualobservation. Swarms of small earthquakes aregenerated as rocks break to make room for risingmagma or as heating of fluids causesunderground pressures to increase. Heat fromthe magma can increase the temperature ofground water and boost temperatures andsteaming from fumaroles; it can also generatesmall steam explosions. The composition ofgases emitted by fumaroles can change as magmanears the surface. Injection of magma into thevolcano can cause swelling or other types ofsurface deformation.

A regional seismic network operated jointlyby the U.S. Geological Survey and theGeophysics Program at the University ofWashington can detect and locate earthquakes inthe Mount Jefferson region. However, the regionhas experienced almost no detectable earthquakesduring the two decades since the network wasinstalled. The onset of earthquake activity wouldquickly gain scientists’ attention. At monitoredvolcanoes similar to those in the Mount Jeffersonregion, a notable increase in seismicity has

occurred weeks to months before the onset oferuptions.

An increase in seismicity would promptdeployment of additional seismometers to betterlocate earthquakes, establishment of a deformationnetwork to detect slight ground movements, andmonitoring of volcanic gases. Changes in some orall of these parameters might indicate intrusion ofnew magma into the volcano.

Periods of unrest at volcanoes are usually timesof great uncertainty. Although outstandingadvances have been made in volcano monitoringand eruption forecasting over the past few decades,scientists are often able to make only very generalstatements about the probability, type, and scale ofan impending eruption. Precursory activity can gothrough accelerating and decelerating phases, andsometimes die out without leading to eruption.Government officials and the public must realizethe limitations in forecasting eruptions and beprepared for such uncertainty.

Protecting our communities andourselves from volcano hazards

Communities, businesses, and citizens need toplan ahead to mitigate the effects of futureeruptions, debris avalanches, and lahars.Long-term mitigation includes using informationabout volcano hazards when making decisionsabout land use and siting of critical facilities.Development should avoid areas judged to have anunacceptably high risk or be planned to reduce thelevel of risk.

When volcanoes erupt or threaten to erupt,emergency responses are needed. Such responseswill be most effective if citizens and publicofficials have an understanding of volcano hazardsand have planned the actions needed to protectcommunities.

Because an eruption can occur within weeks tomonths of the first precursory activity and becausesome hazardous events can occur without warning,suitable emergency plans should be madebeforehand. Although it has been a long timesince Mount Jefferson has erupted and it isunknown when it may erupt again, public officialsneed to consider issues such as public education,


communications, and evacuations. Emergencyplans already developed for floods may apply,with modifications, to hazards from lahars. Amap showing the shortest route to high groundwill also be helpful.

The most important item is knowledge aboutvolcano hazards and, especially, a plan of actionbased on the relative safety of areas aroundhome, school, and work. Lahars pose the biggestsudden threat to people living or recreating invalleys that drain Mount Jefferson. The beststrategy for avoiding a lahar is to move to thehighest possible ground. A safe height aboveriver channels depends on many factors includingsize of the lahar, distance from the volcano, andshape of the valley. For areas beyond theproximal hazard zone, all but the largest laharswill rise less than 30 meters (100 feet) aboveriver level.

References and additionalsuggested reading

Blong, R.J., 1984, Volcanic hazards: Academic Press,Orlando, 424 p.

Conrey, R.M., 1991, Geology and petrology of the Mt.Jefferson area, High Cascade Range, Oregon: Ph.D.dissertation, Washington State University, Pullman,357 p.

Francis, Peter, 1976, Volcanoes: Penguin Books,Middlesex, England, 368 p.

Hoblitt, R.M., Miller, C.D., and Scott, WE, 1987, Volcanichazards with regard to siting nuclear-power plants inthe Pacific Northwest: U.S. Geological SurveyOpen-File Report 87-297, 196 p.

Myers, B., Brantley, S.R., Stauffer, P., and Hendley, J.W.,1997 (revised 1998), What are volcano hazards?: U.S.Geological Survey Fact Sheet 002-97, 2 p.

Saarinern, T.F. and Sell, J.L., 1985, Warning and responseto the Mount St. Helens eruption: State University ofNew York Press, Albany, 240 p.

Tilling, R.I., ed., 1989, Volcanic hazards: Short course ingeology, vol. 1, American Geophysical Union,Washington, D.C., 123 p.

End notes

[1] The geologic data used in this report come largely fromConrey, R.M., 1991, Geology and petrology of the Mt.Jefferson area, High Cascade Range, Oregon: Ph.D.dissertation, Washington State University, 357 p. Otherreferences used include Scott, WE, 1977, Quaternaryglaciation and volcanism, Metolius River area, Oregon:Geological Society of America Bulletin, v. 88, p. 113-124;Beget, J.E., 1981, Evidence of Pleistocene explosiveeruptions of Mount Jefferson, Oregon: EOS, v. 62, no. 45, p.1089; O’Connor, J.E., Hardison III, J.H., and Costa, J.E.,Debris flows from moraine-dammed lakes in the ThreeSisters and Mt. Jefferson Wilderness Areas, Oregon: U.S.Geological Survey Open-File Report, in press.

[2] The proximal hazard zone was created by projectingenergy cones described by H/L = 0.2 from the summit ofMount Jefferson on a digital topographic base map of theMount Jefferson area (see, for example, Malin, M.C., andSheridan, M.F., 1982, Computer-assisted mapping ofpyroclastic surges: Science, vol. 217, p. 637-640; Hayashi,J.N., and Self, S., 1992, A comparison of pyroclastic flowand debris avalanche mobility: Journal of GeophysicalResearch, vol. 97, p. 9063-9071; Iverson, R.M., Schilling,S.P. and Vallance, J.W., 1998, Objective delineation oflahar-hazard zones downstream from volcanoes: GeologicalSociety of America Bulletin, vol. 110, p. 972-984). Here His the vertical distance of descent from the summit, and L isthe horizontal distance of travel from the summit.Pyroclastic flows typically exhibit energy-cone slopes ofabout H/L = 0.2. Debris avalanches of large volume (morethan 100 million cubic meters) can exhibit lowerenergy-cone slopes than do pyroclastic flows (travel fartherfor a given drop), and most such far-traveled avalanches atMount Jefferson would funnel from the proximal hazardzone into valleys encompassed within distal hazard zones.

[3] Descriptions of floods and lahars associated withemptying of moraine-dammed lakes at Mount Jefferson havebeen described elsewhere (O’Connor, J.E., Hardison III, J.H.,and Costa, J.E., Debris flows from moraine-dammed lakes inthe Three Sisters and Mt. Jefferson Wilderness Areas:Oregon, U.S. Geological Survey Open-File Report, in press).These descriptions indicate that moraine-dammed lakes onMount Jefferson contain less than 1 million cubic meters ofwater each. On the basis of this water volume, lakegeometry, and the past behavior of dam-breach floodselsewhere (Walder, J.S., and O’Connor, J.E., 1997, Methodsfor predicting peak discharge of floods caused by failure ofnatural and constructed earthen dams: Water ResourcesResearch, vol. 33, p. 2337-2348), we estimate that waterdischarges resulting from future failures of moraine dams onMount Jefferson will probably not exceed 300 cubic metersper second (10,000 cubic feet per second). Flood magnitudemight increase where substantial volumes of sediment areentrained to form lahars, but the resulting lahar volume isunlikely to exceed several million cubic meters. The

References and additional suggested reading 13

resulting areas of downstream innundation will probably besmaller than those shown for the smallest (innermost) distalhazard zone shown on the map.

[4] Distal lahar hazard zones were constructed byassuming hypothetical lahar volumes of 20 million, 100million, and 500 million cubic meters. Using publishedmathematical and digital cartographic techniques (Iverson,R.M., Schilling, S.P., and Vallance, J.W., 1998, Objectivedelineation of lahar-hazard zones downstream fromvolcanoes: Geological Society of America Bulletin, vol.110, p. 972-984), these three volumes were used tocompute the probable extent of inundation downstreamfrom the proximal hazard zone. A volume of 500 millioncubic meters is considered to represent the largest likelydebris avalanche from Mount Jefferson, based on thefollowing analogy to the 1980 debris avalanche at MountSt. Helens. The 1980 avalanche removed about 2,300million cubic meters from an area on the north flank of thevolcano with an average surface slope of 0.5 (about 30degrees). The avalanche removed about 25% of the cone’svolume above the altitude at which the failure planeintersected the lower north flank. In contrast to Mount St.Helens, Mount Jefferson is deeply eroded with veryprominent buttresses, which seem unlikely to fail, andslopes greater than 0.5 are chiefly confined to the upperpart of the volcano. Thus, there is less volume available(as compared to Mount St. Helens) to generate large debrisavalanches. Assuming that the 25% value from Mount St.Helens applies to Mount Jefferson, we estimate that thevolume of a large debris avalanche is likely to be no morethan 500 million cubic meters. If the debris avalanchecompletely transformed into a lahar, the volume would beabout 500 million cubic meters, which we feel is the largestvolume that could be generated from a single landslidefailure or explosive event. The smaller design volumes of100 million cubic meters and 20 million cubic metersproduce less extensive hazard zones.

[5] Personal communication, J.E. O’Connor, U.S.Geological Survey, Portland, Oregon.

[6] There are few data pertaining to the entry of lahars intolakes (see, for example, Pierson, T.C., 1985, Initiation andflow behavior of the 1980 Pine Creek and Muddy Riverlahars, Mount St. Helens, Washington: Geological Society ofAmerica Bulletin, vol. 96, p. 1056-1069.) A substantial bodyof work does exist regarding wave generation in water bodiesby landslides and seismic ground displacement. The mostpertinent work for present purposes is probably that done bySwiss investigators on landslide-generated waves (Huber, A.,1980, in Seen als Folge von Felsstürzen: Mitteilung Nr. 47der VAW-ETH, Zürich, 222 p.; Huber, A., 1982, Impulsewaves in Swiss lakes as a result of rock avalanches and bankslides: Proceedings of 14th International Congress on LargeDams, Q.54, R. 29, p. 455-476; Huber, A., 1997, Quantifyingimpulse wave effects in reservoirs: Proceedings of 19thInternational Congress on Large Dams, Q. 74, R.35, p.563-581; Huber, A., and Hager, W.H., 1997, Forecastingimpulse waves in reservoirs: Proceedings of 19thInternational Congress on Large Dams, C. 31, p. 993-1005;Müller, D.R., 1995, Auflaufen und Überschwappen vonImpulswellen an Talsperren: Mitteilung Nr. 137 derVAW-ETH, Zürich, 297 pp.). Nonetheless, differences in thedynamic behavior of landslides and lahars make it difficult toapply the Swiss results with a high degree of confidencewhen we consider lahar entry into lakes. We have thereforeapplied the concepts developed in the Swiss work in alargely qualitative fashion.

[7] Information about Detroit Lake and Lake Billy Chinook,and about reservoir operations, comes from Johnson, D.M.,Petersen, R.M., Lycan, D.R., Sweet, J.W., Neuhaus, M.E.,and Schaedel, A.L., 1985, Atlas of Oregon lakes: OregonState University Press, Corvallis, 317 pp.; and from personalcommunications with personnel of the U.S. Army Corps ofEngineers and Portland General Electric in Portland, Oregon.

[8] Tephra-hazard maps were generated by a computerprogram developed by R.M. Hoblitt (U.S. GeologicalSurvey, Cascades Volcano Observatory, 1996).


Documents

Volcano Hazards in the Mount Jefferson Region, Oregon · Volcano Hazards in the Mount Jefferson Region, Oregon By Joseph S. Walder, Cynthia A. Gardner, Richard M. Conrey, Bruce J