VOLCANICLASTIC SEDIMENTATION 

IN THE LEWIS RIVER VALLEY, 

MOUNT ST. HELENS, WASHINGTONPROCESSES, EXTENT, AND HAZARDS

iUEill
COVER

Aerial view of Mount St. Helens and Spirit Lake from the northeast (compare with similar 

view on covers of Bulletins 1383A-C). Catastrophic debris avalanche and ensuing eruption on 18 May 1980 removed a vast portion of the volcano and created the large horseshoeshaped crater. A widespread lateral blast removed almost all vegetation on nearby slopes 

north of the mountain and knocked down trees on slopes farther out (lower left). Large 

'lestructive lahars that formed within minutes of the onset of the eruption swept down 

! he east and west flanks of the volcano. Photograph taken on 24 July 1980 by Austin Post.
Volcaniclastic Sedimentation 

in the Lewis River Valley, 

Mount St. Helens, Washington 

Processes, Extent, and Hazards

By JON J. MAJOR and KEVIN M. SCOTT

GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

U.S. GEOLOGICAL SURVEY BULLETIN 1383-D

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1988
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging-in-Publication Data

Major, Jon J.

Volcaniclastic sedimentation in the Lewis River valley, Mount St. Helens, 

Washington.

(Geology of Mount St. Helens Volcano, Washington)

U.S. Geological Survey Bulletin 1383-D

Supt. of Docs. No.: 119.3:1383-D

Bibliography.

1. Volcanic ash, tuff, etc. Washington (State) Lewis River Watershed. 2.

Sedimentation and deposition Washington (State) Lewis River Watershed. 3.

Saint Helens, Mount (Wash.) I. Scott, Kevin M., 1935 . H. Series. IE.

Series: U.S. Geological Survey Bulletin 1383-D.

QE75.B9 No. 1383-D 557.3s 87-600008 

[QE461] [552'.23'0979784]

For sale by the Books and Open-File Reports Section, U.S. Geological Survey, Federal Center, 

Box 25425, Denver, CO 80225
CONTENTS

Abstract- ----------------------------------- Dl

Introduction ---------------------------------- 2

Terminology- --------------------------------- 4

Acknowledgments ------------------------------- 5

Eruptive history of Mount St. Helens --------------------- 6

Description of deposits ---------------------------- 6

Swift Reservoir ------------------------------- 6

Swift Dam to Yale Lake- -------------------------- 8

Older Swift Creek assemblage ----------------------- 8

Younger Swift Creek assemblage --------------------- 10

Yale Lake area ------------------------------- 11

Yale Dam to Lake Merwin ------------------------- 13

Speelyai Valley and Lake Merwin ---------------------- 19

Downstream from Merwin Dam ----------------------- 23

Flood-plain age and development in the lower Lewis River valley ------- 24

Lahar and debris-avalanche hazards in the Lewis River valley- -------- 28

Present eruptive period --------------------------- 28

Future eruptive periods --------------------------- 30

Debris avalanches ----------------------------- 30

Lahars- ---------------------------------- 33

Future eruptive periodicity ------------------------- 35

References cited- ------------------------------- 35

ILLUSTRATIONS

Page

FIGURE 1. Map showing location of river systems draining Mount St.

Helens ----------------------------- D3

2-10. Photographs showing:

2. Aerial view of Lewis River valley looking upstream near

Swift Dam ------------------------- 8

3. Granular lahar overlain by runout-flow deposits along the

eastern shore of Yale Lake ---------------- 12

in
IV CONTENTS

FIGURE 4. Megaclast of stratified alluvium incorporated in lakebreakout-type lahar near Yale Dam and megaclast of 

poorly sorted debris-avalanche deposit incorporated in 

lake-breakout-type lahar near Yale Dam ---------- D14

5. Lahar overlain by transition-facies deposits in borrow pit

north of State Highway 503 bridge over Lewis River - - - 17

6. Younger slope-mantling lahars unconformably overlying

older section of lahars in roadcut near State Highway 503

bridge over Lewis River ------------------ 18

7. Runout-flow, lahar, and alluvial deposits in a gravel pit in

the bay of Speelyai Creek ----------------- 20

8. Cobble-rich flood-surge-type lahar exposed near the mouth of

Jim Creek ------------------------- 22

9. Slope-mantling lahar deposit exposed below Merwin Dam - - 23 

10. Transition-facies deposit exposed about 1 kilometer

downstream from Merwin Dam -------------- 25

TABLES

TABLE 1. Summary of eruptive periods of Mount St. Helens -------- D7

2. Summary of radiocarbon dates of volcaniclastic deposits in the

Lewis River valley ----------------------- 9

CONVERSION FACTORS

For the convenience of readers who prefer to use inch-pound units rather than the metric 

(International System) units used in this report, the following conversion factors are 

provided:

Multiply metric unit By To obtain inch-pound unit

centimeter (cm)

meter (m)

cubic meter (m3)

kilometer (km)

cubic kilometer (km3)

cubic meter per second (m3/s)

0.394

3.281

35.287

0.621

0.239

35.287

inch (in.)

foot (ft)

cubic foot (ft 3)

mile (mi)

cubic mile (mi 3)

cubic foot per second (ft 3/s)

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 

1929 (NGVD of 1929) a geodetic datum derived from a general adjustment of the firstorder level nets of both the United States and Canada, formerly called "Mean Sea Level 

of 1929."
GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

VOLCANICLASTIC SEDIMENTATION

IN THE LEWIS RIVER VALLEY, 

MOUNT ST. HELENS, WASHINGTONPROCESSES, EXTENT, AND HAZARDS

By JON J. MAJOR and KEVIN M. SCOTT

ABSTRACT

During its approximately 50,000 year history, Mount St. Helens episodically has contributed large volumes of granular volcaniclastic sediment into the Lewis River valley 

and its tributaries. Although much of this sediment resulted in thick valley fill near the 

volcano, lahars (volcanic debris flows) did travel long distances down the Lewis River 

valley. Deposits of large lahars coat valley walls at least 50 km (kilometers) downstream 

from the volcano to heights of more than 30 m (meters) above the present valley floor. 

Similar slope-mantling deposits (dated at 3,920 ±365 radiocarbon years) have been identified at a distance of 35 km from the volcano. These deposits clearly indicate that valleyfilling lahars from relatively recent eruptive periods traveled long distances down the 

Lewis River valley. Paleohydrologic analyses suggest that these flows had discharges of 

more than 100,000 m3/s (cubic meters per second).

Flow transformation has been an important sedimentologic process in the Lewis River 

valley. Downstream from Mount St. Helens, lahars generally transformed from dense, 

high-strength debris flows to less-sediment-rich runout flows (hyperconcentrated streamflow evolved from a distal lahar). Beyond 50 km, the transformations of lahars from 

Mount St. Helens accelerated in response to valley widening. Consequently, sedimentation at the mouth of the Lewis River valley was dominated by fluvial processes. The 

process of flow transformation in the downstream decay of lahars in the Lewis River 

valley also was influenced by the noncohesive nature of the lahars (less than 2 percent 

clay). Many lahars in the valley probably did not originate as fully developed mass 

flowage on the slopes of the mountain, but rather as water surges or underdeveloped 

lahars that bulked by erosion of stream alluvium from the tributary valleys draining the 

mountain.

Because the crater of Mount St. Helens now faces north, lahars are unlikely to affect 

the southerly draining Lewis River valley during the present eruptive period, unless 

large-scale explosive activity resumes or the location of the vent changes. If lahars 

generated as a direct result of an eruption threaten Swift Reservoir, they probably will 

not exceed a volume of 170 million m3 (cubic meters).

Although lahars constitute a potential hazard to the Lewis River valley, the constraining design event during a future eruptive period is a debris avalanche. Studies conducted 

since 1980 indicate that debris avalanches have occurred several times in the volcano's 

brief history. A debris avalanche of even one-half the size of that of 1980 would probably 

have devastating consequences if it were to occur in the Lewis River drainage basin.

Dl
D2 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

INTRODUCTION

Volcanogeomorphic processes observed and inferred at Mount St. 

Helens since the paroxysmal 1980 eruption have provided considerable 

insight into the modes of origin and transport of volcaniclastic debris. 

Lahars (volcanic debris flows) precipitated by the eruptions of 1980-82 

were generated in several ways: (1) by slumping and flowage of watersaturated portions of the debris-avalanche deposit, which caused the 

volumetrically largest and most devastating flow in the North Fork 

Toutle River (Janda and others, 1981); (2) by catastrophic ejection of 

wet debris that was transformed (Fisher, 1983) into dense, flowing 

masses (Janda and others, 1981; Scott, 1985a,b; Major and Voight, 

1986; Scott and Janda, 1987); and (3) by pyroclastic material melting 

and mixing with snow and ice (Janda and others, 1981; Pierson, 1985; 

Fairchild, 1987; Major and Newhall, 1987; Brantley and Waitt, in 

press; R.B. Waitt, oral commun., 1987). Small eruptions since 1980 

have generated floods of water that have traveled beyond the crater, 

eroded large amounts of loose sediment, and transformed into lahars 

that flowed down the Toutle River valley. For example, an explosive 

dome-building event in March 1982 generated a lahar that traveled 

down the Toutle River valley (Waitt and others, 1983) and became 

progressively diluted with streamflow (Scott, 1985b; Pierson and Scott, 

1985). As a result, coarse particles settled out and this flow underwent 

a downstream transformation from a coarse, high-strength lahar to a 

less-sediment-rich, finer grained hyperconcentrated streamflow known 

as runout flow (Scott, 1985b; Pierson and Scott, 1985).

Recognition of the process of flow transformation allowed Scott 

(1985b, 1986) to identify and date more than 30 prehistoric Mount St. 

Helens lahar deposits in the Toutle River valley (fig. 1). One of Scott's 

most significant findings was the recognition of several lahar deposits 

that resulted not directly from eruptions of Mount St. Helens, but from 

erosion of massive amounts of unconsolidated deposits by flood surges 

that resulted from the breaching of naturally dammed lakes during the 

Pine Creek eruptive period (3,000 to 2,500 radiocarbon years ago). One 

such deposit represents the largest lahar known from Mount St. 

Helens; the flow that produced this deposit had an estimated instantaneous peak discharge of 200,000 to 300,000 m3/s as far away as 

50 km from the volcano (Scott, 1986, in press) and was equivalent to 

the Amazon River at flood stage, or 10 to 15 times the mean annual 

discharge at the mouth of the Mississippi River.

An understanding of the magnitude and frequency of past lahars in 

the Toutle River system permitted an improved estimate over past 

studies (Crandell and Mullineaux, 1978) of the potential hazards 

associated with lahars from future eruptions of Mount St. Helens. Scott
0246 MILES 

02468 KILOMETERS

FIGURE 1. Location of river systems draining Mount St. Helens. Letters refer 
D4 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

and others (1986) have concluded that the Toutle River is an example 

of a volcaniclastic fluvial system in which potential hazards can be 

better assessed from a design lahar rather than from a design flood.

In comparison with the Toutle River watershed, the magnitude and 

frequency of lahars in the Lewis River system, which drains the south 

and east flanks of Mount St. Helens (fig. 1), are not well known. In a 

study of volcaniclastic deposits on the south flank of Mount St. Helens, 

Hyde (1975) recognized two stratigraphic sequences of lahars, pyroclastic flows, and associated alluvium, which he called the older and 

younger parts of the Swift Creek assemblage. The older part of the 

Swift Creek assemblage is bracketed by radiocarbon dates of about 

36,000 and 18,000 years, and the younger part of the Swift Creek 

assemblage is about 13,000 radiocarbon years old (Hyde, 1975). Subsequent to the 1980 eruptions, Mullineaux and Crandell (1981) and 

Newhall (1982) examined several of these deposits and concluded that 

a debris avalanche may have occurred during the later stages of 

emplacement of the older part of the Swift Creek assemblage. Newhall 

concluded that the debris avalanche probably dammed the Lewis River, 

and that later breaching of the dam generated a large lahar similar to 

the largest lahars recognized in the Toutle River system by Scott 

(1985b, 1986, in press).

This report presents the results of a stratigraphic and sedimentologic 

study conducted in 1985-86 in the Lewis River valley, mostly downstream from Swift Reservoir. The aim of this study was to reinterpret 

and extend the stratigraphic record of lahars and to reassess potential 

hazards from future lahars.

Three lakes impounded for hydroelectric power on the Lewis River 

(fig. 1) limit most exposures to areas between the lakes and the dams. 

However, while lake levels were drawn down to accommodate seasonal 

filling, shorelines were examined to look for additional exposures, as 

were streambank exposures along the lower Lewis River downstream 

from Merwin Dam.

TERMINOLOGY

The term lahar is used in this report to refer to the mass flowage of 

water-saturated volcanic debris, as well as the subsequent deposits. 

This definition mainly follows previous usage of the term (Crandell, 

1971; Hyde, 1975; Janda and others, 1981; Pierson, 1985; Scott, 1985b, 

1986; Major and Voight, 1986). This type of mass flow generally consists of not less than 80 percent sediment by weight (Beverage and 

Culbertson, 1964; Pierson and Scott, 1985), although some investigators have used threshold concentrations as low as 70 percent
VOLCANICLASTIC SEDIMENTATION IN LEWIS RIVER VALLEY D5

(Osterkamp and others, 1986). The debris incorporated in these flows 

may be either hot or cold, but generally it is cold.

Debris avalanche refers to the process of mass wastage of parts of 

the volcano by large landslides that quickly develop into rock-fragment 

flows. Commonly, the failed rock mass develops into a rockslide the 

rapid downward and outward movement of the rock mass along one or 

several surfaces or relatively narrow zones (Voight and others, 1981, 

1983). Disaggregation of the rock mass can then cause the movement 

to take on the character of a flow. At Mount St. Helens, this process 

has been referred to as rockslide-avalanche (Voight and others, 1981, 

1983) and rockslide-debris avalanche (Glicken, 1986). We use the term 

debris avalanche for brevity.

Pyroclastic flow refers to the flow of hot, dry, fragmental volcanic 

debris. Unlike a lahar, whose fluid medium is water, the fluid medium 

of a pyroclastic flow generally is heated air and other gases. Pyroclastic 

flows may be composed of either pumiceous or lithic material, and they 

may be generated in several different ways (see Chapin and Elston, 

1979, and references therein).

The term runout flow refers to a sediment-rich flow, generally devoid 

of coarse gravel, that is characterized by hyperconcentrated streamflow containing 40 to 80 percent sediment by weight (Beverage and 

Culbertson, 1964; Scott, 1985b; Pierson and Scott, 1985). This type of 

flow results from the progressive dilution of a lahar with overrun 

streamflow and the concomitant loss of coarse sediment. Runout flows 

thus evolve from lahars, and the presence of the deposit produced by 

such a flow generally implies an associated lahar somewhere upstream. 

All traceable deposits of hyperconcentrated streamflows in the Toutle 

and Lewis Rivers are the distal equivalents of lahars, but this need not 

be uniformly the case, and exceptions probably occur at other volcanoes 

less productive of volcaniclastic debris.

Tephra refers to explosively ejected fragmental volcanic debris transported through the air and deposited as fallout. Tephra deposits may 

be composed of pumiceous fragments (magmatic material), lithic 

fragments (pieces of older rock), or some combination. For a more 

complete discussion of tephra, the reader is referred to Fisher and 

Schmincke (1984).

ACKNOWLEDGMENTS

We thank many of our Geological Survey colleagues for stimulating 

discussion and helpful reviews during the course of our field work and 

report preparation. In particular, we thank D.R. Crandell, W.R. 

Osterkamp, C.D. Miller, R.J. Janda, and J.H. Hyde for their careful,
D6 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

thought-provoking critiques of this report, and D.R. Mullineaux for his 

assistance in identifying pumice fragments in the field. We also thank 

D.F. Meyer, P.T. Pringle, and Jack Kleinman for their boating skills 

and assistance during the course of the field work.

ERUPTIVE HISTORY OF MOUNT ST. HELENS

Since Verhoogen (1937) demonstrated that Mount St. Helens is a 

youthful cone overlying an older volcanic complex, the history of this 

volcano has been studied in detail (Mullineaux and Crandell, 1962, 

1981; Hopson, 1971; Crandell and Mullineaux, 1973,1978; Hyde, 1975; 

Hoblitt and others, 1980; Mullineaux, 1986; D.R. Crandell, written 

commun., 1986) and has been divided into a series of eruptive periods 

(Hoblitt and others, 1980; Crandell and others, 1981; Mullineaux and 

Crandell, 1981). Each period is defined by eruptive activity that was 

associated in time, and the periods are separated by dormant intervals 

inferred chiefly from buried soils and the lack of identifiable eruptive 

products (Mullineaux and Crandell, 1981; Mullineaux, 1986). Until 

about 2,500 years ago, Mount St. Helens apparently erupted only 

dacite and silicic andesite, and these rocks have come to be known as 

the old Mount St. Helens lithologies (Verhoogen, 1937). After about 

2,500 years ago, the mountain began to erupt mafic andesite and basalt 

in addition to silicic andesite and dacite (Hoblitt and others, 1980; 

Mullineaux and Crandell, 1981). Known eruptive periods of Mount St. 

Helens, their approximate ages, and the major tephra units associated 

with each period are summarized in table 1.

DESCRIPTION OF DEPOSITS 

SWIFT RESERVOIR

Eruptions from the old Mount St. Helens volcanic center sent flows 

of volcaniclastic debris into the Lewis River valley via Pine Creek, 

Muddy River, and Swift Creek (fig. 1). Near the upper end of Swift 

Reservoir, Crandell and Mullineaux (1973) identified three sequences of 

deposits: (1) The oldest sequence consists of the deposits of lithic 

pyroclastic flows and lahars that are younger than the S tephra set and 

older than the J tephra set. These deposits belong to the Swift Creek 

eruptive period (table 1; Mullineaux and Crandell, 1981; Mullineaux, 

1986). (2) A sequence of pyroclastic-flow, lahar, and alluvial deposits 

younger than tephra set J but older than tephra set Y has been 

recognized near the mouth of Pine Creek. Based on soil development
VOLCANICLASTIC SEDIMENTATION IN LEWIS RIVER VALLEY D7

at the top of this sequence, Crandell and Mullineaux (1973) suggested 

that it was probably closer in age to set J than to set Y. (3) The most 

extensive and best preserved deposits are younger than tephra set Y 

but older than tephra set B. Close to the mountain, this sequence consists primarily of pyroclastic-flow deposits having individual units as 

much as 15 m thick. At a distance of about 11 km from the base of the 

cone, lahars begin to predominate, and they compose most of the fill 

observed near the head of Swift Reservoir (Crandell and Mullineaux, 

1973).

TABLE I. Summary of eruptive periods of Mount St. Helens

[Modified from Mullineaux and Crandell, 1981; Mullineaux, 1986]

Eruptive period Approximate age Associated tephra unit

Goat Rocks

Kalama

Sugar Bowl

Castle Creek

Pine Creek

Smith Creek

Swift Creek

Cougar

Ape Canyon

180

500

1 1,200

2,200

3,000

4,000

13,700

20,400

40,000 to 50,000

- 123 yr before 1980

- 350 yr before 1980

- 1,700

- 2,500

- 3,300

- 9,200

- 18,200

- < 36, 000

T

X 

W

D

B

P

Y

J 

S

K 

M

C

Age of this and older eruptive periods in radiocarbon years before 1950.

Few deposits are exposed along the shores of Swift Reservoir, except 

at its east end and near Swift Dam. Some deposits are preserved along 

the southeastern shore of the reservoir. Most of those deposits are fineand coarse-grained lahars containing older Mount St. Helens dacites 

and andesites. Individual flow units range in thickness from about 1 to 

7 m and are interbedded with alluvium and sandy runout-flow deposits. 

All of the deposits observed are capped by well-developed soil zones as 

much as 2 m thick, indicating that they are at least as old as the Swift 

Creek eruptive period, and probably older (table 1).
D8 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON 

SWIFT DAM TO YALE LAKE

Near Swift Dam at the mouth of Swift Creek, volcaniclastic deposits 

form a fan more than 150 m thick (fig. 2). The deposits composing this 

fan were ascribed by Hyde (1975) to the Swift Creek assemblage, which 

he divided into older and younger parts.

FIGURE 2. Aerial view of Lewis River valley looking upstream near Swift Dam. The 

thick Swift Creek assemblage (Hyde, 1975) forms the valley fill on the left side of the 

photograph (traversed by road). Swift Reservoir is in the background (compare with 

figure 3 of Hyde, 1975).

OLDER SWIFT CREEK ASSEMBLAGE

The oldest exposed part of the assemblage crops out on the valley 

bottom between Swift Dam and Yale Lake (location A, fig. 1). In the 

SEV4 sec. 30, T. 7 N., R. 5 E., a lahar is exposed in the right bank of 

the Lewis River. This deposit, about 1 m of which is exposed, is 

weathered and indurated. It has an average clast size of about 1 to 2 

cm but contains clasts as large as 30 cm. Although the composition of 

the clasts is predominantly older Mount St. Helens dacites and silicic 

andesites, a few are of a white, biotite-bearing pumice. Noncarbonized 

wood fragments are abundant in this deposit. Hyde (1975) reported a
VOLCANICLASTIC SEDIMENTATION IN LEWIS RIVER VALLEY D9

radiocarbon date of about 36,000 years, and wood that we collected has 

been dated as greater than 35,800 radiocarbon years (table 2). This 

deposit thus represents a lahar from the Ape Canyon eruptive period, 

the earliest known period of activity of Mount St. Helens. The base of 

the deposit is not exposed; however, drilling logs (cited in Hyde, 1975) 

indicate angular rock fragments, pumice, and wood down to a depth of

49 m below the present river level. Although it is not known whether 

these holes penetrated one or several texturally similar deposits, these 

data indicate that the Lewis River was flowing at a level locally about

50 m lower in altitude when Mount St. Helens came into existence 

(Hyde, 1975).

TABLE 2. Summary of radiocarbon dates, in years, of volcaniclastic deposits in the

Lewis River valley

Date

440+70

695+340

3,920+365

Material

Wood from 

Woodland

Wood from 

Woodland

Composite

alluvial

alluvial

section near

section near

wood sample from lahar

Reference

This

This

This

study

study

study

veneer deposit near State Highway 

503 bridge

13,130+^350 Charcoal from pyroclastic flow near mouth Hyde (1975) 

of Swift Creek

13,415+310 Charcoal from top of lahar in pit near This study 

State Highway 503 bridge

18,560+550 Charcoal from pyroclastic flow on south Hyde (1975) 

side of Lewis River

20,350+500 Charcoal from pyroclastic flow on south Hyde (1975) 

side of Lewis River

22,720+1400 Charcoal from alluvial sediments below This study 

lahar

>35,800 Wood from valley-bottom lahar near Swift This study 

Dam

36,000+2,000 Wood from valley-bottom lahar near Swift Hyde (1975) 

Dam
DIG GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

The section of fluvial gravels, pyroclastic flows, and lahars overlying 

the valley-bottom fill is described by Hyde (1975). Of particular note is 

a borrow-pit exposure on the south side of the Lewis River valley about 

1.5 km west of Swift Dam. In this borrow pit, which exposes a section 

near the top of a huge volcaniclastic fan, lies a coarse, clast-rich flowage 

deposit of unknown origin overlain by pyroclastic flow deposits, fluvial 

gravels, and a tephra couplet (Hyde, 1975). Radiocarbon dates of charcoal samples from the pyroclastic flow deposits, which bracket the 

tephra couplet, yielded ages of 20,350 + 500 and 18,560 + 550 radiocarbon years. The radiocarbon dates indicate that those events occurred 

during the Cougar eruptive period (table 1).

Since the 18 May 1980 eruption of Mount St. Helens, the coarse 

flowage deposit described by Hyde (1975) has generated particular 

interest. On the north side of the Lewis River valley near Swift Dam, 

this deposit, which is as much as 20 m thick, contains angular biotitebearing clasts in a compact silty-sand matrix. Locally, this deposit contains texturally similar megaclasts, several meters in diameter, of pink, 

hydrothermally altered dacite similar to that exposed in the modern 

crater. Mullineaux and Crandell (1981) and Newhall (1982) have proposed that this deposit may represent an ancient analog of the 18 May 

1980 debris avalanche. It is possible, however, that the megaclasts 

were eroded from a debris avalanche closer to the volcano and that the 

deposit enclosing these megaclasts in the Lewis River valley represents 

a lahar generated from the debris avalanche. The biotite-bearing clasts, 

presumed to be products of the Ape Canyon eruptive period because 

similar lithologies have not been identified in products of the Cougar 

eruptive period, indicate that the avalanche probably was derived from 

the oldest parts of the ancestral cone (Mullineaux and Crandell, 1981; 

Newhall, 1982). Hyde (1975), Mullineaux and Crandell (1981), and 

Newhall (1982) have correlated the flowage deposit underneath the 

pyroclastic-flow deposits on the south side of the Lewis River valley 

with the possible debris-avalanche deposit on the north side of the 

valley. Whether the coarse flowage deposit is a debris avalanche or a 

lahar containing megaclasts of a debris avalanche, it nonetheless 

represents an extremely large flow that reached the Lewis River valley 

and may have dammed the Lewis River during an early part of the 

Cougar eruptive period (Newhall, 1982).

YOUNGER SWIFT CREEK ASSEMBLAGE

In the Swift Creek valley about 4 km north of Swift Reservoir, 

deposits of the younger part of the Swift Creek assemblage overlie till 

of the Eraser glaciation (Hyde, 1975; D.R. Crandell, written commun.,
VOLCANICLASTIC SEDIMENTATION IN LEWIS RIVER VALLEY Dll

1986). On the east valley wall near the mouth of Swift Creek, Hyde 

(1975) observed that a younger section of pyroclastic flow and lahar 

deposits, overlain by tephra set S, rests upon units of the older part of 

the Swift Creek assemblage. Charcoal from a pyroclastic flow deposit 

in that younger section yielded a date of 13,130 ± 350 radiocarbon years 

(Hyde, 1975).

On the south side of the Lewis River valley about 1 km downstream 

from Swift Dam, a terrace composed of multiple flowage deposits is 

probably inset against the older part of the Swift Creek assemblage. The 

oldest deposit exposed in this terrace is a lahar containing angular, 

pebble-sized clasts of older Mount St. Helens lithologies. This deposit 

exhibits normal grading of only the coarser particles, from pebbles to 

coarse sand and granules, over a thickness of 4 m. Downsection, the 

deposit is covered, but a small window in the vegetation suggests that 

its lower part may contain subangular clasts several centimeters in 

diameter, and that the deposit may be more than 10 m thick.

The basal lahar is separated from an overlying lahar by about 0.5 m 

of coarse, pebbly alluvium and by a runout-flow deposit. The younger 

lahar, which is about 3 m thick, contains subangular to subrounded 

clasts as large as 20 cm and is inversely graded over its basal 20 to 25 

cm. Again, clast composition is exclusively older Mount St. Helens dacites 

and andesites.

Capping this suite of deposits is a third, very coarse-grained lahar 

that is separated from the older lahars by a 2-m-thick section of pumicerich gravelly sand alluvium. Clasts in this youngest lahar are also composed of the older Mount St. Helens lithologies. The clasts generally 

are subangular to subrounded and are as large as 1.5 m across. On top 

of this unit there is a well developed soil zone.

The top of this suite of deposits forms a flat topographic surface at 

an altitude of 225 m. Although the mineralogic composition of the 

pumiceous material in these deposits is similar to that of tephra older 

than tephra set S (D.R. Mullineaux, oral commun., 1986), lack of evidence of erosion, such as fluvial reworking at the top of this sequence, 

suggests that the terrace is a depositional, rather than an erosional, 

feature. Although the deposits apparently are younger than the older 

part of the Swift Creek assemblage, their precise age is uncertain.

YALE LAKE AREA

Along the shores of Yale Lake, lahar and runout-flow deposits are 

exposed at lake level at several localities, commonly underlying 

alluvium as thick as 10 m. The runout-flow deposits are typically less 

than 1 m thick and consist of a poorly sorted mix of coarse lithic sand
D12 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

and granules of older Mount St. Helens lithic lithologies (fig. 3). The 

lahars typically are very poorly sorted, and most clasts in them are less 

than 10 cm in diameter. The clasts are generally angular to subangular 

and are composed of older Mount St. Helens lithologies. Exposures of 

lahars generally are less than 2 m thick (fig. 3).

FIGURE 3. Granular lahar (A) overlain by runout-flow deposits (B, C, D) along the 

eastern shore of Yale Lake. Contacts between runout-flow deposits are shown as solid 

lines, or dashed where contact is inferred. Exposure is about 4 m thick.

One lahar contains subangular to subrounded clasts as large as 20 cm. 

This deposit, exposed at lake level along the western shore of Yale 

Lake (location B, fig. 1) in the SEV4 sec. 4, T. 6 N., R. 4 E., is about 

2 m thick and overlies fractured Tertiary volcanic bedrock. Clast composition of this deposit differs from that of other lahar deposits in that 

about 10 percent of the clasts are composed of Tertiary rock.

Most of the sediment exposed along the shores of Yale Lake consists 

of alluvium with minor interbedded lahars. In places, the alluvium is 

10 m or more thick, ranges in size from sand to boulders, and primarily 

consists of older Mount St. Helens lithologies. Biotite-bearing and 

pumice-rich lithologies were not observed, indicating that this fill 

postdates the Ape Canyon eruptive period. In the bluffs near the head
D13

of the Speelyai Creek valley, along the western shore of Yale Lake, 

both modern and buried soil zones are exposed, each between 1 and 2 m 

thick, in a 10-m-thick alluvial fill. This fill is almost exclusively composed of older Mount St. Helens lithologies. Hyde (1975) correlated this 

fill with his older part of the Swift Creek assemblage.

The best exposed section of lahars in the Yale Lake area is in a gravel 

pit just west of the town of Cougar. Hyde (1975) considered these 

deposits to belong to the younger part of his Swift Creek assemblage. 

These lahars are relatively fine grained, consisting chiefly of sand, 

granules, and pebbles to small cobbles. The clasts in these deposits, 

generally smaller than 10 cm, are predominantly angular or subangular 

and are composed of older Mount St. Helens lithic debris.

YALE DAM TO LAKE MERWIN

Several lahar and lahar-related deposits are exposed about 35 km from 

Mount St. Helens between Yale Dam and Lake Merwin. One such 

deposit, unique in the Lewis River valley, is exposed in a gravel pit about 

1.3 km west of Yale Dam (location C, fig. 1). This deposit is at least 10 m 

thick (the base is not exposed) and contains megaclasts incorporated in 

a very poorly sorted laharic matrix of subrounded to well-rounded, sandto cobble-sized fragments. The megaclasts, as much as 3 to 5 m in maximum exposed diameter, consist of at least three distinct types: (1) One 

megaclast is composed of well-stratified fluvial deposits (fig. 4A) in which 

deformed sedimentary structures such as ripple marks are preserved. 

(2) Several megaclasts are massive and consist of pink, poorly sorted 

breccia composed of hydrothermally altered Mount St. Helens rocks from 

the ancestral volcanic complex (fig. 4B). These masses probably represent pieces of a debris avalanche that were eroded by the lahar. (3) A 

third type of megaclast occurs as pods of grayish, poorly sorted sand 

and granules with some small pebbles. This type represents masses of 

runout-flow deposits. Megaclasts derived from a pumiceous pyroclasticflow deposit also have been observed by D.R. Crandell (written commun., 

1986).

In addition to these unconsolidated megaclasts, the deposit also contains rounded clasts, generally less than 1 m in exposed diameter, of 

black andesites and basalts, weathered Tertiary volcanic rocks, and other 

non-Mount St. Helens lithologies derived from the Lewis River valley. 

The dominant clast type, however, is older Mount St. Helens dacites and 

andesites. Pre-Mount St. Helens lithologies make up about 15 percent 

of the clasts. In addition, the deposit contains a small percentage of 

prismatically jointed dacite clasts from Mount St. Helens that are 

generally less than 20 cm in diameter.
D14 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

w,.

FIGURE 4. Megaclasts of (A) stratified alluvium incorporated in lake-breakout-type lahar 

near Yale Dam (location C, fig. 1), and of (B) poorly sorted debris-avalanche deposit 

incorporated in lake-breakout-type lahar near Yale Dam. In each photograph shovel is 

1 m long.
VOLCANICLASTIC SEDIMENTATION IN LEWIS RIVER VALLEY D15

The significant rounding of the fragments in the deposit matrix and 

of many of the larger clasts, and the relatively large proportion of nonMount St. Helens lithologies, indicate that alluvium was the primary 

source of material. A significant amount of incorporated alluvium in a 

lahar is characteristic of a flood-surge origin, as documented in the Toutle 

River valley by Scott (1985b). Newhall (1982) has proposed that the large 

lahar deposit resulted from the breaching of a lake impounded in the 

Lewis River valley by a debris avalanche during the Cougar eruptive 

period. The megaclasts of stratified alluvium and runout-flow deposits 

represent material plucked from the valley floor and incorporated into 

the flow, whereas the megaclasts of poorly sorted ancestral cone breccia 

are interpreted to be fragments of a debris avalanche. Given the lack 

of unequivocal debris-avalanche deposits upstream, however, it is not 

known if the megaclasts originally were eroded from a yet older debris 

avalanche, whether a debris avalanche formed a natural dam, or where 

the dam was located. The hypothesis of Newhall (1982) is a reasonable 

scenario, however. Although the prismatically jointed clasts in the deposit 

were clearly hot at some time, they may have been associated with a 

debris avalanche, or they may have been simply eroded from another 

deposit during emplacement of the lahar. Generally, lahar deposits known 

to be the distal equivalents of lithic pyroclastic flows contain a much 

larger percentage of prismatically jointed clasts than is observed in this 

deposit (Scott, 1985b).

Although a possible debris-avalanche deposit near Swift Dam is thought 

to be of Cougar age, the age of the lahar deposit exposed in the gravel 

pit is uncertain. D.R. Crandell (written commun., 1986) has assigned the 

lahar to the Cougar eruptive period. However, Hyde (1975) has ascribed 

the deposit to his younger part of the Swift Creek assemblage, which 

is now interpreted as originating during the Swift Creek eruptive period 

(table 1). Lack of a significant degree of soil development at the top of 

this lahar suggests to us that perhaps it may be even younger than Swift 

Creek age. The surface of the deposit, however, has been disturbed by 

logging and by the construction of Yale Dam, so that some of the 

apparent lack of soil development may be the result of soil removal. No 

deposits have been found upstream from Swift Dam to indicate that a 

lake was present for any significant length of time, leading Hyde (1975) 

to conclude that the Lewis River has been blocked only temporarily during the eruptive history of Mount St. Helens. This conclusion corresponds 

with the generally brief histories of lakes dammed by debris avalanches 

and other mass movements (Meyer and others, 1985; Costa and Schuster, 

1987). Unfortunately, no suitable material has been found to precisely 

date the lahar deposit.

Lahars are well exposed in roadcuts about 1.5 km farther west, 

immediately north of the State Highway 503 bridge over the Lewis River
D16 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

(location D, fig. 1) In a small borrow pit about 300 m north of the bridge, 

a lahar at least 2 m thick contains subrounded clasts of older Mount St. 

Helens rocks as large as 10 cm in a silty sand matrix (fig. 5). Several 

large pieces of fragmental charcoal were found in the top of this deposit 

that yielded a radiocarbon date of 13,415 ±310 years. Overlying this 

deposit are two poorly sorted deposits of coarse sand to small pebbles. 

These deposits exhibit inversely graded bases and normally graded tups 

and represent a transition facies, characteristic of the interval of flow 

during which a lahar transforms to a runout flow. These deposits are 

1.5-4.5 m thick and are capped by a locally developed soil approximately 0.5 m thick. A thin tephra unit separates the basal lahar from the 

overlying deposits. Hyde (1975) attributed the deposits to the younger 

part of the Swift Creek assemblage and speculated that the tephra 

deposit represents a layer of tephra set S. The radiocarbon age of the 

basal lahar indicates that it is indeed a product of the Swift Creek eruptive period (Mullineaux, 1986); an age of about 13,000 years corresponds 

with the degree of soil development.

In a roadcut about 100 m south of and downslope from this borrow 

pit, the Swift Creek age lahars are overlain, apparently unconformably, 

by two slope-mantling lahars (fig. 6). Each of these younger deposits is 

less than 1 m thick, massive, and composed of poorly sorted sand, 

granules, and small pebbles. On the slope above this roadcut, a veneer 

of the largest of these slope-mantling deposits is preserved. Small 

fragments of wood extracted from this veneer yielded a composite 

radiocarbon date of 3,920 + 365 years. This radiocarbon date indicates 

that at least one, and probably both, of the flows that produced these 

slope-mantling deposits occurred during the Smith Creek eruptive period 

(table 1).

Near the roadcut, the Lewis River flows in a bedrock channel. If the 

reasonably certain assumption is made that the channel configuration 

of the Lewis River during post-Swift Creek time was similar to the 

present-day configuration, the depth of flow of the slope-mantling lahars 

can be estimated. The veneer deposit reaches an altitude of at least 

105 m, which is about 45 m above the floor of the Lewis River channel 

beneath the bridge. Thus the post-Swift Creek age flows were at least 

40 to 50 m deep at this site.

Paleohydrologic analyses of prehistoric lahars at Mount St. Helens suggest that lahars having depths in excess of 25 m may have mean velocities 

in excess of 20 m/s (Scott, 1986). For comparison, the 18 May 1980 North 

Fork Toutle River lahar had a peak-flow depth of about 16m and a mean 

peak-flow velocity of about 9 m/s in the constricted channel reach near 

Kid Valley (Fairchild and Wigmosta, 1983). If we assume that the 

prehistoric lahars traveling down the Lewis River had a similar depthvelocity relation to that of these flows, then the mean velocities of the
VOLCANICLASTIC SEDIMENTATION IN LEWIS RIVER VALLEY D17

FIGURE 5. Lahar (A) overlain by transition-fades deposits (B, C) in borrow pit north of 

State Highway 503 bridge over Lewis River (location D, fig. 1). Upper transition-fades 

deposit is overlain by a 0.5-m-thick soil. Lahar has a radiocarbon age of 13,415 ±310 

years. Contacts between deposits are shown as solid lines. Shovel is 1 m long.
D18 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

post-Swift Creek age lahars, 40 to 50 m deep at the Lewis River bridge, 

are conservatively estimated to have been at least 10 to 15 m/s. This 

velocity and the topographic channel configuration at the bridge suggest that the post-Swift Creek age lahars had peak discharges of 130,000 

to 200,000 m3/s at a distance of 35 km from the mountain. This 

estimated instantaneous discharge is about 10 times the mean annual 

discharge at the mouth of the Mississippi River (Oltman, 1968), but it 

is less than the peak discharge estimated for the largest prehistoric lahar 

in the Toutle River valley (Scott, 1986), which drains the north flank 

of Mount St. Helens.

FIGURE 6. Younger slope-mantling lahars (A, B) unconformably overlying older section 

of lahars in roadcut near State Highway 503 bridge over Lewis River (location D, fig. 1). 

Veneer deposit from these younger lahars coats valley wall above outcrop and has a 

radiocarbon age of 3,920 ± 365 years. Shovel is 1 m long. Solid lines delineate contacts.
VOLCANICLASTIC SEDIMENTATION IN LEWIS RIVER VALLEY D19 

SPEELYAI VALLEY AND LAKE MERWIN

Exposed in a gravel pit in the bay of Speelyai Creek near the upper 

end of Lake Merwin, 38 km from Mount St. Helens (location E, fig. 1), 

is a spectacular section of (1) a runout flow, (2) a lahar, and (3) fluvial 

deposits that locally contain thin lahars (fig. 7). The runout-flow deposit 

is at least 6 m thick and forms the basal unit in a fill that is about 20 m 

thick. This deposit, previously interpreted as a lahar by Hyde (1975), 

consists of poorly sorted coarse sand, granules, and some small pebbles 

that are mainly low-density, older Mount St. Helens rocks. It appears 

to be massive but exhibits some crude stratification; it is not dominated 

by the horizontal bedding that some investigators have indicated as being 

characteristic of hyperconcentrated streamflow deposits (for example, 

Smith, 1986). To date, this is the thickest runout-flow deposit known 

from Mount St. Helens. Overlying the basal deposit is a lahar about 10 m 

thick that contains angular clasts of older Mount St. Helens lithologies 

no larger than 10 cm. Capping these two deposits is a section of alluvium, 

interbedded with thin lahars, on which a well-developed soil has been 

formed. Near the top of the section, lenses of pumice mineralogically 

similar to tephra set J are found (table 1; Hyde, 1975). Hyde (1975) 

attributed this sedimentary section to his older part of the Swift Creek 

assemblage.

Deposits of a similar age almost certainly underlie much of the Speelyai 

Creek valley. Limited outcrops typically expose lahars containing 

relatively small clasts and runout-flow deposits generally less than 3 m 

thick, which interbed with alluvium composed of older Mount St. Helens 

lithologies. An outcrop at the intersection of State Highway 503 and 

Baker Road exposes a slightly indurated 0.8- to 1.5-m-thick lahar containing some angular pebbles of older Mount St. Helens lithologies. This 

lahar, which is capped by 1 to 1.5 m of well-developed soil, is underlain 

by alluvium from which fragmental pieces of charcoal were extracted. 

This charcoal yielded a date of 22,720 ± 1,400 radiocarbon years (table 2). 

The clast composition of these deposits indicates that the valley occupied 

by Speelyai Creek was formerly a channel of the Lewis River. Hyde 

(1975) suggested that the valley near Yale Dam in which the Lewis River 

presently flows was eroded when a pre-Fraser alpine glacier blocked the 

former main valley. Subsequent to the inception of Mount St. Helens, 

both valleys were aggraded. Deposits at the upstream end of the former 

main valley, which consist exclusively of alluvium and contain both a 

modern and buried soil, indicate that it was abandoned early in the history 

of Mount St. Helens. The presence of Cougar age tephra near the top 

of that fill (Hyde, 1975), the degree of soil development, and the lack 

of younger deposits indicate that this channel was completely abandoned 

during the later part of the Cougar eruptive period or shortly thereafter.
D20 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

y B

FIGURE 7. Runout-flow (A), lahar (B), and alluvial deposits (C) in a gravel pit in the bay 

of Speelyai Creek (location E, fig. 1). Person is pointing to contact between runout flow 

and lahar. Thickness and development of soil horizon suggest that these deposits are 

of Swift Creek age (9,200-13,700 years) or Cougar age (18,200-20,400 years).
VOLCANICLASTIC SEDIMENTATION IN LEWIS RIVER VALLEY D21

Stream piracy triggered by massive lahar-induced aggradation in the 

older channel probably caused the Lewis River to shift to its present 

course.

Along the shores of Lake Merwin, we have identified several lahar 

and runout-flow deposits. Runout-flow deposits are typically less than 

1 m thick and are generally interbedded with lahars and alluvium. These 

deposits, the best examples of which are exposed several hundred meters 

downvalley from the mouth of Jim Creek, are typically unstratified. Some 

are ungraded; others are normally graded. A few of these deposits contain rare biotite-bearing pumice clasts of Ape Canyon age (table 1) as 

large as 6 cm. Lack of biotite content in the sand matrix of these deposits, 

in conjunction with their interbedded nature with sedimentary sections 

typically capped by well-developed soil horizons, suggests that they are 

of Cougar or Swift Creek age.

Some of the lahars around Lake Merwin contain a significant proportion of clasts that are pebble sized and larger and exhibit some degree 

of rounding. Such sedimentologic characteristics, similar in part to those 

of deposits of Pine Creek age in the Toutle River system, indicate lahars 

that have been bulked to a significant degree with eroded alluvium (Scott, 

1985b). The best example of this type of deposit in the Lewis River valley 

is exposed at the mouth of Jim Creek, about 43 km from the mountain 

(location F, fig. 1). Here, a deposit about 3 m thick contains poorly sorted 

cobbles dispersed in a compact silty sand matrix (fig. 8). The basal 0.5-1 

m of this deposit exhibit inverse grading; the upper 0.5-1 m exhibit 

normal grading of the coarse clasts. This deposit is overlain by weathered 

alluvium on which a soil was developed and subsequently buried. Thus, 

this deposit is of Cougar or Swift Creek age also.

Another lahar deposit along Lake Merwin that appears to be of similar 

age to that exposed near Jim Creek is composed of older Mount St. 

Helens clasts that are subangular to angular and as large as 2 m in 

diameter (location G, fig. 1). This deposit, 2 to 3 m of which are exposed, 

represents a lahar that did not bulk with a significant quantity of 

alluvium. Rather, it probably formed on the volcano and traveled at least 

40 km while incorporating less streambed alluvium than many other 

lahars.

Alluvial deposits exposed about 50 m above the pre-lake valley floor 

along the north shore of Lake Merwin (location H, fig. 1) indicate that 

an extensive fill of Ape Canyon age extended throughout the lower Lewis 

River valley. Here, a 10- to 12-m-thick section of pumice-rich alluvial sand 

is exposed. Locally, sand beds contain rounded pumice clasts as large 

as 6 cm. Most of the pumice clasts contain biotite, and detrital biotite 

also is abundant in the sand beds. This lithologic composition is characteristic of Ape Canyon age products. The pumice content lends a 

distinct white color to the fill, which is remarkably similar to the fill of
D22 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

FIGURE 8. Cobble-rich flood-surge-type lahar of Cougar or Swift Creek age exposed near 

the mouth of Jim Creek (location F, fig. 1). Note inverse grading near the base and normal 

grading of coarse clasts near the top of the deposit. Shovel is 1 m long.
VOLCANICLASTIC SEDIMENTATION IN LEWIS RIVER VALLEY D23

similar age, thickness, and extent in the Toutle-Cowlitz River system 

on the north side of the volcano (Scott, 1985b, 1986). Previously, deposits 

of Ape Canyon age in the Lewis River valley were known only as far 

downstream as Swift Dam.

DOWNSTREAM FROM MERWIN DAM

Immediately downstream from Merwin Dam, lahar and runout-flow 

deposits coat the upper valley wall in a narrow bedrock gorge to an elevation of about 30 m above the valley floor (location I, fig. 1). The sequence 

here includes, from oldest to youngest, two massive, sand and granule 

runout-flow deposits, each of which is about 1 m thick and capped by 

a clayey soil horizon; and two overlying lahars, the lower of which is less 

than 0.5 m thick, is weathered, and contains pumice mineralogically 

similar to tephra set S. The uppermost slope-mantling lahar is as much 

as 1 m thick, inversely graded, and contains clasts only as large as 5 cm 

(fig. 9). Smaller pebbles and granules generally are subangular to angular,

FIGURE 9. Slope-mantling lahar deposit from the Smith Creek or Pine Creek eruptive 

period exposed below Merwin Dam (location I, fig. 1). Solid line delineates basal contact. Scale is 10 cm long.
D24 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

whereas the larger pebbles typically are subangular to subrounded. Lack 

of weathering indicates that this uppermost deposit is significantly 

younger than the underlying units, though its clasts are mineralogically 

similar to the older Mount St. Helens lithologies. The texture and slopemantling nature of this deposit suggest that it could correlate with one 

of the slope-mantling lahars exposed near the State Highway 503 bridge 

between Lake Merwin and Yale Dam.

The farthest known downstream extent of Mount St. Helens lahars is 

about 1 km west of Merwin Dam, along a clearing for power-transmission 

lines (location J, fig. 1). Here, two exposures show that a gently sloping 

surface 40 m above the valley floor is covered by a veneer of fine-grained 

debris interpreted to represent a facies transitional from a lahar to a 

runout flow. The deposit, about 1.5 m thick, is poorly sorted and inversely 

graded from coarse sand near the base to fragments as large as 5 cm 

at the top (fig. 10). Fragments typically are subangular to angular, but 

a few show a significant degree of rounding. This deposit overlies a lahar 

that contains small clasts and exhibits soil development. The uppermost 

deposit probably correlates with the slope-mantling lahar observed at 

Merwin Dam and may be finer grained at this location because of valley 

widening.

The presence of the slope-mantling deposit downstream from Merwin 

Dam clearly shows that the lahar traveled at least 50 km downvalley 

from the volcano. Furthermore, the existence of the deposit on the gently 

sloping terrain above the narrow bedrock channel through which the river 

flows indicates that at peak flow the lahar must have been at least 40 m 

deep if it is assumed that the shape of the valley has not changed 

significantly since the lahar occurred.

The mean peak-flow velocity of this lahar may have been 10 to 15 m/s 

at this point. If so, the present channel configuration suggests that this 

lahar had a peak discharge of 90,000 to 140,000 m3/s at this point.

FLOOD-PLAIN AGE AND DEVELOPMENT IN THE 

LOWER LEWIS RIVER VALLEY

Throughout its history, the area of the lower Lewis River valley near 

where the town of Woodland (fig. 1) now sits has been dominated by 

fluvial deposition. Exposed deposits in this reach of the valley consist 

mainly of interbedded, stratified clayey silts; silty sands; and massive 

to cross-stratified beds of sand. Exposed units typically vary from less 

than 1 to about 3 m in thickness.

Sedimentation in the lower Lewis River valley has been influenced by 

several geological processes. During a Pleistocene glaciation prior to the 

inception of Mount St. Helens, ice extended to within 8 km of the present
FIGURE 10. Transition-facies deposit exposed about 1 km downstream from Merwin 

Dam (location J, fig. 1). Note inversely graded texture and lack of weathering. Hammer 

is 30 cm long.
D26 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

site of Woodland (Mundorff, 1984), deposited glacial drift, and induced 

glaciofluvial sedimentation. Since the inception of Mount St. Helens about 

50,000 years ago, the valley has been affected by glaciofluvial sedimentation from, and alteration of baselevel by, the Fraser glaciation (which 

achieved a maximum stand about 20,000 years ago; Crandell and 

Mullineaux, 1973; Hyde, 1975), by many of the colossal Lake Missoula 

floods on the Columbia River between 19,000 and 13,000 years ago 

(Waitt, 1980), and by episodic alluviation by volcaniclastic sediment produced during eruptions of the volcano. During at least postglacial time, 

alluviation in the lower valley has been dominantly influenced by the 

episodic influx of volcaniclastic sediment upstream. Such radical influx 

of sediment, and the consequent excessive stream sediment load, has 

caused episodic aggradation in the lower Lewis River valley similar to 

that occurring today in the lower Toutle and Cowlitz Rivers (fig. 1; U.S. 

Army Corps of Engineers, 1984). Drilling logs (Washington State Department of Ecology, written commun., 1986) report pumice-bearing sands 

to an average depth of about 15 m near the town of Woodland, and a 

few report pumice-bearing units to depths of as much as 35 m, indicating 

that sediment related to the volcano composes a large part of the fill 

in the lower valley.

The maximum strandline of the colossal floods on the Columbia River 

near Woodland (about 120 m in elevation) indicates that the Lewis River 

valley was affected by backwater flooding rather than through-flooding. 

In the Columbia River drainage basin, many of the tributary channels 

affected by backwater flooding were sites of deposition rather than erosion (Bretz, 1929; Waitt, 1980, 1984, 1985), even when the peak-flow 

depth was on the order of several hundred meters. We infer that the 

lower Lewis River valley was probably a site of deposition, or at worst 

a site of mild erosion, during passage of the great floods, although 

deposits from those floods have not been recognized. Therefore, the thick, 

volcanically related alluvial fill in the lower Lewis River valley probably 

represents a nearly complete stratigraphic record since the inception of 

the volcano.

Although the petrologic character of the deeply buried pumice in the 

alluvial section near Woodland is unknown, it may be from the Ape 

Canyon eruptive period. The presence of an extensive Ape Canyon age 

fill is consistent with observations made by Scott (1985b, 1986) in the 

Toutle River valley and also is consistent with the thick fill of Ape Canyon 

age observed farther upstream.

Remnants of several stages of alluviation are present in the lower 

Lewis River valley. The oldest exposed fill, of Pleistocene age, is represented by a flat surface at an altitude of about 60 m (Mundorff, 1964). 

A remnant from a younger fill was found on the south side of the valley, 

8 to 12 m above the present flood plain (SEV4 sec. 18, T. 5 N., R. 1 E.).
VOLCANICLASTIC SEDIMENTATION IN LEWIS RIVER VALLEY D27

This younger terrace is underlain primarily by fine- to medium-grained 

alluvial sands that are cross-stratified and laminated in their upper few 

meters and generally massive in the lower portions of the exposure. 

These deposits contain biotite, similar to that found in products of the 

Ape Canyon eruptive period. Small fragments of charcoal retrieved from 

the exposure of this terrace yield a composite radiocarbon age of 

695 ± 340 years. This terrace represents alluviation by material reworked 

from deposits farther upstream because products of only the Ape Canyon 

eruptive period contain biotite, and deposits of that age are not exposed 

in the lower valley.

The youngest and most prominent geomorphic feature in the lower 

Lewis River valley, which we informally refer to as the Woodland terrace, 

forms a broad flat surface at an elevation of about 6 to 8 m above the 

present Lewis River flood plain. The lower 3 m or so of deposits beneath 

this terrace are composed of stratified clayey silt and massive, mottled 

silty fine sand. These fine-grained sediments are overlain by well-sorted, 

cross-stratified to nearly massive medium-grained sand. The upper, more 

massive grayish sand deposit forms lenticular bedding and contains 

rounded pebble-sized fragments of white pumice. Crandell and 

Mullineaux (1973) observed gray pumice-bearing sands within a 7.5-mhigh terrace along the reach of river extending 8 km upstream from 

Woodland. They stated that the pumice was mineralogically similar to 

that of tephra set P and inferred that those fluvial sediments were the 

downstream correlative of the post-Y part of the assemblage of deposits 

exposed in Pine Creek.

During our investigation of this terrace, fragments of wood were 

collected from a zone near the contact between the stratified clayey silt 

and massive silty fine sand (SEV4 sec. 18, T. 5 N., R. 1 E.). This wood, 

extracted from a level at least 1 m below the pumice-bearing sands, had 

a radiocarbon date of 440 ± 70 years. The white pumice in the fill above 

this contact belongs to tephra set W rather than set P. Although both 

tephra sets have a similar mineralogic composition (Mullineaux and 

others, 1975; Mullineaux, 1986), the minimum refractive index of 

hypersthene in set W pumice is greater than 1.70, whereas that of 

hypersthene in set P pumice is less than 1.70 (Mullineaux and others, 

1975). Of 10 pieces of pumice examined from this fill, 7 contained 

hypersthene having a minimum refractive index of greater than 1.70. 

The radiocarbon date and pumice identification indicate that the upper 

part of this sedimentary fill was formed in response to aggradation during the Kalama eruptive period (table 1).

To date, flows of volcaniclastic debris bearing set W pumice are not 

known to have entered the Lewis River drainage basin. The set W pumice 

was deposited in the Lewis River drainage basin only as tephra fall east 

of Mount St. Helens during the early part of the Kalama eruptive period.
D28 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

This indicates that the W pumice in the fill at Woodland either was eroded 

from hillslopes east of the volcano or that the alluviation in the lower 

Lewis River valley during the Kalama eruptive period was related to 

heavy tephra fall directly into rivers in the Lewis River drainage basin. 

A recent study of erosion of 1980 Mount St. Helens tephra by Collins 

and Dunne (1986) indicates that the rate of erosion of tephra on hillslopes 

rapidly declined within a few years after the eruption without significant repopulation of vegetation. This result suggests that if the alluviation during the Kalama eruptive period in the lower Lewis River valley 

did not occur immediately after tephra fall in the drainage basin, then 

it probably occurred within a few years after the tephra fall.

Lahars apparently did not reach the lower Lewis River valley; flood 

plains were aggraded primarily by lahar-induced fluvial sedimentation. 

Depositional evidence suggests that most of the lahars that flowed down 

the Lewis River valley lost much of their coarse debris and had undergone 

or were undergoing a transition to runout flow by the time they reached 

the present site of Merwin Dam. These transformations were caused 

primarily by the granularity of the flows (less than 2 percent clay). Scott 

(1985b) observed that lahars having clay contents of more than 3 percent would travel great distances from their source (as much as 100 km 

or more) without losing textural integrity, but that relatively noncohesive 

(less than 3 percent clay) lahars invariably underwent transformation 

to runout flows. The granularity of the latter lahars increased their ability 

to mix with overrun streamflow, which resulted in their consequent 

transformation. Moreover, although the Lewis River flows through a 

bedrock gorge near Merwin Dam, the river valley emerges into a broad 

flood plain a few kilometers downstream from the dam. This valley widening additionally enhanced the flow transformation of lahars. By the time 

the initially sediment-rich lahars reached the lower part of the valley, 

they had become completely transformed, and alluviation dominated the 

depositional process.

LAHAR AND DEBRIS-AVALANCHE HAZARDS IN THE 

LEWIS RIVER VALLEY

PRESENT ERUPTIVE PERIOD

Changes in the configuration of Mount St. Helens as a consequence 

of the paroxysmal 1980 eruption, combined with the construction of three 

hydroelectric dams (fig. 1) during the past 50 years, have diminished the 

direct threat of lahars that may reach the lower Lewis River valley. Swift 

Reservoir, however, lies immediately downvalley from Mount St. Helens 

and could be threatened if large lahars were to occur on the south or 

southeast flank of the volcano. Removal of a vast portion of the volcano
VOLCANICLASTIC SEDIMENTATION IN LEWIS RIVER VALLEY D29

by the 1980 debris avalanche has greatly reduced the risk of another 

debris avalanche during the present eruptive period.

Rain-induced lahars generated on the south and southeast flanks of 

the volcano probably would be small and would not flow to any significant distance beyond the base of the cone. Major (1984) documented 

posteruption, rain-induced lahars that flowed no more than 6 km from 

the crater rim on the southwest flank of the volcano, and both R.P. 

Hoblitt (U.S. Geological Survey, oral commun., 1987) and T.C. Pierson 

(U.S. Geological Survey, oral commun., 1987) have documented raininduced and glacial-meltwater-induced lahars that traveled no farther 

than the Pine Creek-Muddy River debris fan at the base of the east 

flank of the cone. Rain-induced lahars pose no serious threat to Swift 

Reservoir.

Given the present configuration of the volcano and the present style 

of eruption (periodic extrusion of lava lobes onto the dome, endogenous 

dome growth, and small explosions), lahars generated by eruptions will 

affect primarily the Toutle River system, possibly for the next several 

decades or centuries. Lahars could threaten Swift Reservoir if the style 

of eruption changes, however. Such lahars may occur if large-scale 

explosive activity resumes and causes pyroclastic flows that melt snow 

and ice outside of the crater on the volcano's upper flanks, or if the location of the vent changes and causes a dome to form on the volcano's 

south-southeast flank, similar to several domes formed during past eruptions (Verhoogen, 1937; Hoblitt and others, 1980). Eruption of a lava 

flow from a vent on the south-southeast flank of the volcano could also 

melt snow and ice and produce a lahar. An obvious concern then is what 

size lahar might reach Swift Reservoir should these events occur.

Emplacement of a flank dome or eruption of a lava flow probably would 

affect a limited area of the volcano. However, a pyroclastic flow generated by collapse of an eruption column could affect a broad area of 

the volcano, and thus this is thought to represent a worst-case scenario.

Crandell and Mullineaux (1978) suggested that the largest single lahar 

that might be expected to enter Swift Reservoir possibly would have 

a volume of no more than 125 million m3 . They arrived at this value by 

assuming, as a worst case, that a pyroclastic flow, similar in size to one 

that occurred during the Pine Creek eruptive period, would melt as much 

as 5 m of snow (density = 0.25) over an area of 30 km2 to produce 35 to 

40 million m3 of water. They further assumed that an average lahar has 

a volumetric distribution of roughly 65 percent sediment and 35 percent 

water, and that this volume of water potentially could mobilize about 

75 million m3 of volcaniclastic debris to produce a lahar with a volume 

of about 110 million m3 . They then rounded their estimate conservatively 

upward. In the spring of 1980, prior to the 18 May eruption, Swift Reservoir was drawn down to accommodate a potential fill of that size (Miller
D30 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

and others, 1981). This amount of drawdown proved adequate only 

because the catastrophic explosive phase of the 1980 eruption was fortuitously directed northward. If the 1980 debris avalanche occurred on 

the volcano's south or southeast flanks, the reservoir possibly would have 

been overwhelmed and much water displaced.

Recent snow studies at Mount St. Helens (R. Denlinger, U.S. Geological 

Survey, oral commun., 1987) indicate that Crandell and Mullineaux (1978) 

underestimated typical snow density. In the Mount St. Helens crater, 

fresh snow has an average density of about 0.3, but typical snow has 

a density that ranges from 0.4 to 0.6. By late spring, snow density is 

as much as 0.75. If we assume that 4 m of snow having an average density of 0.5 is incorporated by the postulated pyroclastic flow of Crandell 

and Mullineaux (1978), and that melting is 100 percent efficient (that 

is, all melted snow is converted to water with no loss by steam), such 

an event could generate as much as 60 million m3 of water. That volume 

could possibly produce a lahar with a volume of about 170 million m3 . 

The lahar, however, could continue to incorporate sediment, so a larger 

volume may be possible.

A recent report by Cassidy and others (1980) suggests that the maximum probable lahar that could reach Swift Reservoir probably is smaller 

than the estimate of Crandell and Mullineaux (1978) and may not exceed 

62 million m3 . Although Cassidy and others (1980) assume the same 

input hydrograph (35 million m3 of water) as Crandell and Mullineaux 

(1978), they assume that the flowing sediment-water mixture is composed 

of 57 percent water by volume instead of about 35 percent. That assumption means that the flowing mixture that reaches Swift Reservoir is not 

a lahar (defined here as a debris flow generally containing 60 percent 

sediment by volume or 80 percent by weight) but rather is a moderately 

dilute hyperconcentrated streamflow (Beverage and Culbertson, 1964). 

Along the channels of Pine Creek, Swift Creek, and Muddy River, abundant sediment is available for entrainment by an evolving lahar. Although 

factors that control the bulking of lahars by sediment entrainment are 

not yet adequately quantified, the geological record of lahar behavior 

in both the Lewis and Toutle River systems indicates that the conservative approach of Crandell and Mullineaux (1978) is more technically valid 

for estimating lahar volume.

FUTURE ERUPTIVE PERIODS

DEBRIS AVALANCHES

Possibly the most catastrophic event from a future eruption of Mount 

St. Helens that could affect the Lewis River valley downstream from 

the volcano is the direct impact of a large debris avalanche (>1 km3) on
VOLCANICLASTIC SEDIMENTATION IN LEWIS RIVER VALLEY D31

Swift Reservoir. In hazard assessment, an obvious concern is the travel 

distance of such a debris avalanche.

A simple method for estimating the runout distance of a large debris 

avalanche is based upon the ratio of vertical drop (H) to travel distance 

(L) (Schuster and Crandell, 1984). If we assume that a debris avalanche 

will have a volume of at least 1 km3, and that it will have an HIL ratio 

similar to that of large volcanic debris avalanches worldwide (Voight and 

others, 1983; Siebert, 1984), the travel distance can be estimated by

L = Hlf,

where/is the equivalent coefficient of friction. The vertical drop from 

the present rim of the volcano to the Swift Creek valley, 8 km south 

of the former summit of the volcano, is about 1,700 m. The estimated 

travel distance of a debris avalanche having that vertical drop and a coefficient of friction conservatively estimated to be 0.065 (Voight and others, 

1985, fig. 5) is about 26 km, a distance similar to the travel length of 

the 1980 debris avalanche. If we assume a value of 0.09 for / (the 

estimated equivalent coefficient of friction of the 1980 debris avalanche 

(Voight and others, 1983)), the estimated travel distance is about 19 km. 

The distance between Swift Dam and the former summit of Mount St. 

Helens is 15 km, a distance well within the estimated travel length of 

a future, large debris avalanche. Furthermore, the gradient between 

Mount St. Helens and Swift Reservoir along Swift Creek is greater than 

that of the North Fork Toutle River and thus enhances avalanche mobility 

down Swift Creek.

The renewed risk of a debris avalanche could justify drawing Swift 

Reservoir down to the level of dead storage, depending on the focus of 

activity as indicated by precursor seismicity and geodetic measurements. 

A debris avalanche from the south or southeast flanks of the volcano 

similar to the 1980 event would have a devastating impact on Swift 

Reservoir. If a debris avalanche into any part of the Swift Creek, Pine 

Creek, or Muddy River drainage basins had a volume only one-half that 

of the 1980 avalanche (estimated to be 2.8 km3 by Voight and others, 

1981), and if only one-fourth of that volume reached the reservoir, more 

than 300 million m3 of debris would impact the reservoir. Deposition of 

such a vast quantity of volcaniclastic sediment, equivalent to one-third 

the maximum storage capacity of Swift Reservoir, could have devastating 

consequences.

If such a debris avalanche occurred in the Pine Creek-Muddy River 

system and passively entered a capacity-full Swift Reservoir at its eastern 

end, it would raise the level of the reservoir 17 m, which would cause 

overtopping of the dam. However, such a debris avalanche would not 

enter the reservoir passively; rather, it would enter the reservoir with
D32 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

a probable high velocity (the average velocity of the 1980 debris avalanche 

was 35 m/s (Voight and others, 1983)), which would generate a large 

translational wave on the lake.

Dimensions of such a debris avalanche can be approximated by using 

the previously estimated travel length (26 km) and by routing the 

avalanche down the shortest travel path to the east end of the reservoir 

(18 km down Pine Creek). The estimated travel length of the postulated 

debris avalanche suggests that it would travel about 8 km into the reservoir. Dividing the hypothetical volume of such a debris avalanche by this 

travel length, and then matching avalanche width and thickness with 

the cross-sectional area of the Lewis River valley at the east end of the 

reservoir, indicates that a debris avalanche with a volume of 300 million 

m3 could be about 30 m thick and 1,200 m wide when it enters the reservoir. The mean depth of Swift Reservoir at capacity is presently about 

50 m. Thus, a debris avalanche 30 m thick would approximate a wall 

moving into the reservoir.

The size of a wave generated on Swift Reservoir by the impact of such 

a large, fast-moving debris avalanche may be estimated by using a twodimensional approximation for a horizontally moving wall (Noda, 1970). 

The maximum wave amplitude (rj) at a distance x/d = 2 is given by

d

where d is the mean water depth of the reservoir, V is the velocity of 

the avalanche upon impact with the reservoir, g is the acceleration due 

to gravity, and x is the distance of maximum wave amplitude from the 

point of impact. This theory predicts that the maximum wave amplitude 

(above mean water level) is about 104 m at a distance of about 100 m 

in front of the debris avalanche.

Although such a wave is enormous, it is not improbable nor is it unprecedented. A landslide into Lituya Bay, Alaska, in 1958 generated a 

large gravity wave that left trimlines on a rock spur as much as 520 m 

above the water surface (Miller, 1960), and back calculations suggest a 

maximum wave height of about 100 to 300 m (Slingerland and Voight, 

1979). Where Lituya Bay empties into the Gulf of Alaska, wave height 

was estimated to be about 25 m above trees growing on a bay-mouth 

spit (Slingerland and Voight, 1979). It is of interest to note that the 

geometry and dimensions of Lituya Bay are similar to Swift Reservoir, 

and that the landslide that caused the giant wave in Lituya Bay had a 

volume 10 times smaller than the volume of debris avalanche entering 

Swift Reservoir postulated here. A wave of such magnitude would certainly overtop Swift Dam at water levels near capacity and might possibly 

precipitate dam failure.
VOLCANICLASTIC SEDIMENTATION IN LEWIS RIVER VALLEY D33

If such a debris avalanche traveled down Swift Creek, the most direct 

route and the approximate path of a prehistoric debris avalanche, it might 

destroy Swift Dam and displace or release much water. Even if the dam 

was not destroyed, such a debris avalanche could overwhelm the valley 

and add to the height of the dam, with the threat of later failure and 

consequent lake-breakout lahars.

Failure of Swift Dam, and the concomitant release of water, could 

possibly cause successive dam failures downstream. The effects of 

sequential dam failure in the lower Lewis River valley would be 

devastating. The final safety assessment report for the Trojan nuclear 

plant (Portland General Electric Company, 1976) models sequential dam 

failure on the Lewis River and shows that the flood wave would reach 

Woodland within 1 hour and would inundate the lower Lewis River valley 

to an altitude of about 12 m above sea level, inundating the town. Such 

a flood would not affect the nuclear plant, 28 km downstream from 

Woodland (Portland General Electric Company, 1976).

LAHARS

In addition to a possible debris avalanche, lahars may also affect Swift 

Reservoir in future eruptive periods. Although lahars large enough in 

volume to reach Swift Reservoir can be generated in many ways, the 

following discussion of the hazards posed by lahars is limited to perhaps 

the three worst scenarios, each of which is capable of producing lahars 

of very large volume.

(1) Dewatering of a debris avalanche. Although the magnitude of the 

paroxysmal 1980 eruption was small in comparison with many past eruptions of the volcano, the 1980 debris-avalanche deposit is the largest 

volcaniclastic deposit in the history of the volcano. Dewatering of parts 

of that deposit produced the largest lahar in 1980 (Janda and others, 

1981), which had an estimated volume of 140 million m3 (Fairchild and 

Wigmosta, 1983). Because of the modest size of the volcano's cone, a 

future debris avalanche from the south or southeast flanks of the volcano 

probably will not exceed the size of the 1980 avalanche, and it may likely be smaller. If such a debris avalanche occurs and does not directly 

impact Swift Reservoir, lahars formed by dewatering of the deposit are 

likely.

Steeper channel gradients on the volcano's southern sector and perhaps 

a wetter debris avalanche could combine to promote greater erosion of 

the deposit during dewatering, potentially generating a lahar with a 

volume greater than the largest 1980 lahar. Because of the unknown 

size, water content, and rate of erosion of a future debris-avalanche
D34 GEOLOGY OF MOUNT ST. HELENS VOLCANO, WASHINGTON

deposit, the 1980 lahar formed by avalanche dewatering may approximate the possible maximum size of this type of lahar for planning 

purposes.

(2) Breaching of a natural dam and consequent lake breakout. Estimation of sizes of lahars that could be produced by flood surges from 

breaching of a volcaniclastic dam is more difficult. Although the distribution of deposits in the Lewis River valley shows that repeated dams must 

have existed in the past, and indeed many deposits exhibit characteristics 

of flood-surge origin, the preservation and exposure of individual lahars 

are at best spotty, and thus volume estimates cannot be made. However, 

the filling of a natural reservoir and breaching of a dam may require 

a period of months or years, which would allow time for mitigation and 

reduce the relative hazard of this type of flow.

If a debris avalanche flowed down Swift Creek or Pine Creek, it is 

unlikely that a lake would form upstream from Swift Reservoir because 

there are no significant channels tributary to these main channels. 

However, if a debris avalanche flowed down Muddy River, the channels 

of Smith Creek and Clearwater Creek could be blocked and large lakes 

created. The size of Swift Reservoir could be increased if a debris 

avalanche flowed down Swift Creek, overwhelmed the valley, and created 

a blockage taller than Swift Dam. At the east end of the reservoir, a 

debris avalanche moving down Pine Creek or Muddy River could cause 

blockage of the Lewis River.

The largest flood-surge-type lahar in the history of Mount St. Helens 

occurred in the Toutle River system about 2,500 years ago during the 

Pine Creek eruptive period. That lahar, which could perhaps serve as 

a worst-case example for the south side of the volcano, resulted from 

a lake breakout and had a volume of about 1 billion m3 after bulking 

(Scott, 1985b, in press). Catastrophic breaching of a natural dam, similar 

to that in the Toutle River system, has occurred at least once in the Lewis 

River valley during the volcano's 50,000-year history.

(3) Meltwater generated by a large pyroclastic flow. As a worst case, 

Crandell and Mullineaux (1978) computed the maximum probable size 

of a single lahar generated by a pyroclastic flow assumed to be similar 

in size to one that occurred on the south-southeast side of the volcano 

during the Pine Creek eruptive period. As discussed earlier, this estimate 

has been refined (toward a larger volume) based on more realistic values 

of snow density. On the basis of the bulking and debulking behavior of 

lahars preserved in the stratigraphic record, the pragmatic approach of 

Crandell and Mullineaux (1978) for estimating maximum probable size 

of a meltwater-generated lahar may be more technically valid than the 

approach of Cassidy and others (1980). Thus, for planning purposes, the 

largest single lahar generated by meltwater may be assumed to have 

a volume of about 170 million m3 .
VOLCANICLASTIC SEDIMENTATION IN LEWIS RIVER VALLEY D35 

FUTURE ERUPTIVE PERIODICITY

During the past 4,500 years, the eruptive periods of Mount St. Helens 

generally have lasted less than a century and have been separated by 

dormant intervals lasting about 100 to 500 years (Crandell and 

Mullineaux, 1978, table 1). However, during the past 500 years, eruptive periods have occurred more frequently, with dormant intervals 

lasting only about 100 to 150 years (Crandell and Mullineaux, 1978, 

table 1). This time interval is well within the time span normally considered in long-range engineering and land-use decisions. Future behavior 

at similar intervals is highly probable, although the effect of the recent 

removal of such a vast part of the volcano is uncertain.

Given the near-certainty of another eruptive period, possibly within 

the lifetime of Swift Reservoir, the recurrence of a debris avalanche, 

a process that may have happened several times in the volcano's brief 

history, is the constraining design event in the Lewis River system. 

Although debris avalanches are less frequent than many other types of 

volcanic events, their enormous size and high velocity prevent them from 

being controlled by man-made structures. The only mitigation of such 

an event is evacuation of areas directly endangered (Schuster and 

Crandell, 1984).

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Waitt, R.B., Pierson, T.C., MacLeod, N.M., Janda, R.J., Voight, Barry, and Holcomb, 

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Geology of

Mount St. Helens Volcano,

Washington

U.S. GEOLOGICAL SURVEY BULLETIN 1383

This volume was published as 

separate chapters A D
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director
CONTENTS

[Letters designate the separately published chapters]

(A) Pine Creek volcanic assemblage at Mount St. Helens, Washington, by Dwight 

R. Crandell and Donal R. Mullineaux.

(B) Upper Pleistocene pyroclastic-flow deposits and lahars south of Mount St. Helens 

volcano, Washington, by Jack H. Hyde.

(C) Potential hazards from future eruptions of Mount St. Helens volcano, Washington, 

by Dwight R. Crandell and Donal R. Mullineaux.

(D) Volcaniclastic sedimentation in the Lewis River valley, Mount St. Helens, 

Washington, by Jon J. Major and Kevin M. Scott.
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