Magnitude and Frequency of 

Lahars and Lahgr-Runout Flows 

in the Toutle-Cowlitz River System

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1447-B
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Magnitude and Frequency of 

Lahars and Lahar-Runout Flows 

in the Toutle-Cowlitz River System

By KEVIN M. SCOTT

LAHARS AND LAHAR-RUNOUT FLOWS IN THE TOUTLE-COWLITZ RIVER 

SYSTEM, MOUNT ST. HELENS, WASHINGTON

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1447-B

Geologic evidence suggests 

large, destructive flows 

have a recurrence interval 

of less than 100 years

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1989
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging-in-Publication Data

Scott, Kevin M., 1935- 

Lahars and lahar-runout flows in the Toutle-Cowlitz River system, Mount St. Helens, Washington.

(U.S. Geological Survey professional paper ; 1447-AIncludes bibliographies.

Contents: [1] Origins, behavior, and sedimentology of lahars and lahar-runout flows in the Toutle-Cowlitz River 

system [2] Magnitude and frequency of lahars and lahar-runout flows in the Toutle-Cowlitz River system.

Supt. of Docs, no.: I 19.16:1447-A-B

1. Lahars Washington (State) Toutle River Watershed. 2. Lahars Washington (State) Cowlitz River Watershed. 

3. Sediments (Geology) Washington (State) Toutle River Watershed. 4. Sediments (Geology) Washington 

(State) Cowlitz River Watershed. I. Title. II. Series: Geological Survey professional paper ; 1447-A, etc. 

QE598.2.S38 1989 551.3 86-600198

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

Federal Center, Box 25425, Denver, CO 80225

Any use of trade names in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Geological Survey.
CONTENTS

Abstract ........................................... Bl

Introduction ....................................... 1

Purpose, scope, and location of study ............... 1

Methods of study ................................ 2

Acknowledgments ............................... 4

Lahars and lahar-runout flows of pre-1980 eruptive stages and

periods .......................................... 4

Ape Canyon eruptive stage ....................... 4

Cougar eruptive stage ............................ 5

Swift Creek eruptive stage ........................ 5

Spirit Lake eruptive stage ........................ 6

Smith Creek eruptive period ................... 6

Pine Creek eruptive period ..................... 8

Castle Creek eruptive period ................... 9

Sugar Bowl eruptive period .................... 9

Kalama eruptive period ....................... 9

Goat Rocks eruptive period .................... 10

Magnitude of lahars ................................. 10

Flow depths based on thickness of flood-plain facies . . 10

Measurements of cross sections of lahar flood waves . . 11

	Page

Magnitude of lahars Continued

Measurements and estimates of velocity and discharge Bll

Flow magnitudes for planning purposes ............. 15

The nature of lahar hazards ....................... 17

Recurrence intervals of lahars and lahar-runout flows ..... 17

Evidence of time grouping of lahars ................ 18

Character of the eruptive episodes ................. 19

Lake-breakout lahars of the Pine Creek eruptive period ... 19

Evidence of lake-breakout origin ................... 20

Comparison of magnitudes of lahar PC 1 and lahars of

other origins ............................... 20

Implications of lahar bulking factors ............ 21

Megaclasts .................................. 21

Natural dams at volcanoes ........................... 21

Lahar-margin lakes modern and ancient ............ 22

Comparison of lahar PC 1 with modeled flows ....... 23

Conclusions ........................................ 24

References cited ..................................... 24

Appendix Selected measured stratigraphic sections ..... 27

ILLUSTRATIONS

Page

FIGURE 1. Map showing localities discussed in text ................................................................ B2

2. Selected columnar sections of lahars and lahar-runout deposits in the Toutle River system ...................... 7

3. Plot of ratio of thickness of flood-plain facies to flow depth versus distance from crater, for the two largest 1980 lahars 11

4. Selected measured cross sections of flow for lahars PC 1 and PC 3 .......................................... 14

5. Plots of lahar velocity versus flow and channel properties ................................................. 15

6. Photograph of vertical tree mold penetrating flood-plain facies of lahars PC 1 and PC 3 in Green Mountain Mill section 19

7. Plot comparing peak discharges of lahar PC 1, a modeled lahar from a modern breakout of Spirit Lake, a modeled flood

	flow from a modern breakout of Spirit Lake, and the North Fork lahar of May 18, 1980 .................... 20

8. Drawings showing examples of megaclasts in lahar PC 1 .................................................. 22

TABLES

Page

TABLE 1. Occurrence and extent of known lahars and lahar-runout flows in the Tou tie-Co wlitz River system ............... B3

2. Radiocarbon dates indicating age of largest lahar in history of the Toutle-Cowlitz watershed and three contemporaneous

lahars .......................................................................................... 8

3. Thickness of flood-plain facies, flow depth, and measured and estimated peak discharges for lahars in the Toutle-Cowlitz

river system ...................... .............................................................. 12
IV CONTENTS

CONVERSION FACTORS

For use of readers who prefer to use inch-pound units, conversion factors for terms used in this 

report are listed below.

Multiply SI By To obtain inch-pound

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

square kilometer (km2) 0.3861 square mile (mi2)

cubic meter per 35.31 cubic foot per

second (m3/s) second (ft3/s)
LAHARS AND LAHAR-RUNOUT FLOWS IN THE TOUTLE-COWLITZ RIVER SYSTEM,

MOUNT ST. HELENS, WASHINGTON

MAGNITUDE AND FREQUENCY OF LAHARS AND

LAHAR-RUNOUT FLOWS IN THE 

TOUTLE-COWLITZ RIVER SYSTEM

By KEVIN M. SCOTT

ABSTRACT

The recurrence interval of a lahar or lahar-runout flow at least large 

enough to inundate flood plains 50 kilometers from Mount St. Helens 

is less than 100 years. Lahars are volcanic debris flows and their 

deposits; lahar-runout flows are the hyperconcentrated streamflow 

evolved from distal lahars. The recurrence interval is conditional on 

eruptive state and is based on the most recent 4,500 years of the 

volcano's approximately 40,000- to 50,000-year history; it applies to 

the Toutle-Cowlitz River system that drains the northwest sector of 

the mountain and the modern crater. Lahars of the last 4,500 years 

have multiple origins, any one of which may cause lahars in the future. 

For a flow of 100-year recurrence interval, the flood plain would be 

inundated to a depth of 2 meters or more in the Toutle River valley 

between 30 and 50 km from the volcano.

The 100-year recurrence interval is within a normal time frame for 

long-term planning. Therefore engineering works in the Toutle River 

system should be designed for lahars, as well as floods, of a particular frequency. Characteristics of lahars that should be considered 

include inundation levels, peak discharges, and impact force and shear 

stress on structures such as dam spillways and bridge piers.

Lahars large enough to inundate flood plains may be viewed as 

hazards like floods, but lahars differ from floods both in frequency 

distribution through time and in their effects at a particular depth 

of inundation. They are subject to separate and different statistical 

analyses. For example, lahars tend to cluster in time within eruptive 

periods, in effect increasing the present short-term potential. Unlike 

a water flood, a lahar that has a flow depth at least 1 meter on flood 

plains can cause a significant part of the maximum possible damage. 

Trees are killed, many structures are inundated and made unusable 

even if they are not crushed by timber floating in the lahar, and 

agriculture is not feasible for periods of as much as several years. On 

the positive side, the dead timber can be harvested, regrowth of some 

species is rapid, and the impacted watershed may be stimulated 

economically by reconstruction and tourism.

The largest lahar in the history of the watershed was formed by 

the bulking of sediment in a flood surge that originated from breaching 

of a natural dam of ancestral Spirit Lake. The entrained sediment is 

mainly stream alluvium, and megaclasts of an ancient debris avalanche 

are abundant in the flow unit. The flow had a peak discharge of 200,000 

to 300,000 m3/s at a distance of 30 to 50 km from the volcano, and

was the first of four lake-breakout lahars that occurred during a span 

of several years near the end of Pine Creek time. The third in the series 

is the second largest in the history of the watershed. This series of 

lahars is interpreted as an analog of the events that would have 

happened, without engineering intervention, after the 1980 eruption. 

In 1980, a debris avalanche catastrophically raised Spirit Lake more 

than 60 m and created new lakes in blocked tributaries.

The risk of a lahar with a peak discharge similar to that of the 

Amazon River at flood stage, like the huge flow of Pine Creek age, 

is reduced substantially, but only temporarily, by engineering works 

that have lowered and stabilized the level of Spirit Lake. Even with 

these works, however, a lahar is possible that is large enough to inundate flood plains and, locally, older lahar terraces throughout the 

river system. Because the stream channels and the debris avalanche 

both contain large quantities of readily entrained sediment, and 

because lahars of multiple origins are still possible in both the North 

and South Forks of the Toutle River, lahar hazards in the system warrant continuing and conservative concern.

INTRODUCTION

PURPOSE, SCOPE, AND LOCATION OF STUDY

The purpose of this report is to analyze the magnitude and frequency of lahars and lahar-runout flows in 

the Toutle-Cowlitz River system by means of paleohydraulic and paleohydrologic approaches. The study 

encompasses the entire 40,000- to 50,000-yr history of 

Mount St. Helens but is focused particularly on the 

more recent eruptive periods. A companion report 

(Scott, 1988) describes the origins and behavior of both 

modern and ancient lahars, based in part on observations and textures of the 1980 and post-1980 flows, and 

extension of those results to the many ancient lahars 

in the river system.

Bl
B2 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

The Toutle River flows into the lower Cowlitz River 

(fig. 1), the headwaters of which drain part of Mount 

Rainier, the largest Cascade Range stratovolcano and 

one presently in a quiescent state. All observed lahars 

and lahar-runout flows affecting the lower Cowlitz River 

were identified (from their contained rock types) as 

originating in the Toutle River with volcanic activity 

at Mount St. Helens.

METHODS OF STUDY

The record of flows reaching long distances from 

Mount St. Helens can be treated like a flood-frequency 

analysis to calculate the probability of similar future 

flows. Such an analysis involving lahars will be most 

applicable to relatively long-term planning (involving 

at least several tens of years), whereas shorter-term 

hazards are best assessed from monitoring of the 

volcano and comparisons with the history of similar 

volcanoes (for example, Newhall, 1983). A basic difference is between the statistics of a long-term time 

series, in which the magnitude and frequency of events 

are in part inferred, and a case-history approach in 

which the appropriate examples are not yet completely 

known. The approaches are complementary; the farther

the horizon of the planner, the more useful will be the 

probabilities based on the flow record.

In addition, the record of ancient flows is a better 

guide to the probable magnitudes, transformations, and 

rates of attenuation of future lahars and lahar-runout 

flows than any model developed to date. (See Scott, in 

press.) The most typical lahar in the river system has 

consisted of the middle segment of a flood wave that 

began and ended as streamflow (Scott, 1985,1988). The 

proximal flood surges are progressively bulked with 

sediment, and the distal flood waves progressively lose 

sediment, evolving into lahar-runout flows. Once the 

magnitudes of the input hydrographs and the observed 

rates of attenuation can be quantified in models (with 

both these aspects based on the actual behavior of large 

lahars), modeling will serve as an useful adjunct to the 

study of real-life lahars. Even then, however, a volcano's 

detailed flow history will still give the best indication 

of what lahar types and behaviors to expect.

Multiple lahars have been produced in the Toutle 

River system during virtually all of the eruptive stages 

and periods established by Mullineaux and Crandell 

(1981), Crandell (1987), and Mullineaux (1986). Their 

chronology (table 1) is used as the time scale for the frequency analysis, and their concept of the eruptive 

period is discussed in the section "Evidence of time

EXPLANATION

 Area inundated by largest lahar

of Pine Creek age (PC 1)

Debris avalanche 

of May 18, 1980

FIGURE 1. Index map of Toutle-Cowlitz River system showing localities mentioned in text. The area inundated by lahar PC 1 is also shown 

and reflects its occurrence as the medial part of a flood wave beginning and ending as a flood surge.
INTRODUCTION B3

TABLE 1. Occurrence and extent of known lahars and lahar-runout flows in the Toutle-Cowlitz River system

[Eruptive history of Mount St. Helens after Crandell (1987, and written commun., 1984) and Mullineaux (1986); tephra units from Mullineaux and Crandell (1981) and Mullineaux (1986). 

The number of flows large enough to have inundated flood plains at the confluence of the forks of the Toutle River is indicated for each eruptive period]

Eruptive stages and periods Approximate 

age

Tephra 

set

No. of

overbank

flows

Occurrence and extent

SPIRIT LAKE ERUPTIVE STAGE: 

Period beginning in 1980 Modern

Goat Rocks eruptive period 150-93

1980 2 1980 lahar in South Fork Toutle R. extended to confluence 

of forks (a second, non-overbank lahar also reached 

confluence). Runout phase reached the Cowlitz R.

1980 lahar in North Fork Toutle R. extended to Columbia R.

1982 lahar in North Fork extended to Kid Valley. Runout 

phase reached Cowlitz R.

-Dormant interval of 123 yr ----- - -

T 0 No lahars extended significant distances past base of cone.

Kalama eruptive period

Sugar Bowl eruptive period

 Dormant interval of about 200 yr 

470-320 W, X 3 Avalanches and coarse rubbly lahars buried forests in upper

South Fork, may correlate with 2 downstream overbank 

lahars that extend to confluence.

3 lahars probably of this age in North Fork; at least 1 was 

overbank and extended to confluence.

 Dormant interval of about 700 yr 

 0 No lahars extended significant distances past base of cone. 

-Dormant interval of about 500 yr ~

1,200

Castle Creek eruptive period 2,200-1,700 B 0 Lahar probably of this age in North Fork extended in paleochannel at least to Camp Baker. Lahars of this period are 

unlikely to be preserved in stratigraphic record because 

of the erosional regime that probably existed after 

emplacement of the voluminous Pine Creek lahar deposits.

 Dormant interval of about 300 yr 

Pine Creek eruptive period 3,000-2,500 P 5 At least 2 initial lahar-runout flows, 1 overbank, extended

to confluence. 1 huge lahar followed by 3 smaller flows 

in North Fork extended to confluence. At least the first 

and third lahars in this sequence reached Cowlitz R. and 

probably extended to Columbia R.

 Dormant interval of about 300 yr 

Smith Creek eruptive period 4,000-3,300 Y 53 lahars and at least 2 overbank runout flows extended at

least to confluence. Youngest 2 lahars originated in 

North Fork. Largest lahar probably reached Columbia R.

 Dormant interval of about 5,000 yr 

SWIFT CREEK ERUPTIVE STAGE 13,700->9,200 S, J 11 10 lahars and 1 overbank runout flow extended at least to

confluence.

 Dormant interval of about 5,000 yr 

,400-19,200 M, K 1 At least 1 large lahar extended to Cowlitz R. and probably

to Columbia R.

COUGAR ERUPTIVE STAGE 20

APE CANYON ERUPTIVE STAGE

 Dormant interval of about 15,000 yr 

50,000(?) to C >8 At least 5 lahars and several overbank runout flows 

36,000 extended at least to confluence. At least 2 lahars

probably extended to Columbia R.

Years before 1950. Based on radiocarbon dates, except for ages of Goat Rocks period (based on tree rings and 

historic records) and Kalama period (based on tree rings and radiocarbon dates).

grouping of lahars." The use of "stage" as a designation 

for each of the older eruptive periods and for the entire 

group of the seven most recent eruptive periods (since 

4000 B.P.) is after Crandell (1987). Of the radiocarbon 

dates from samples obtained by the writer, nearly all 

fit within the chronology of the established eruptive 

periods.

The recognition and correlation of the many lahars 

and associated flows in the river system (table 1) are 

based in part on the exposures produced by lateral erosion by the 1980 lahars and subsequent streamflow. By 

1985, exposures of the older flow deposits in the river 

system were modified by channel aggradation, bank 

slumping, dredge spoils, and vegetation regrowth; and
B4 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

several older units were obscured. Nevertheless, the 

overbank deposits of lahars and lahar-runout flows 

many tens of kilometers away continue to provide a 

detailed history of events not necessarily evident at or 

near the volcano. More importantly for the assessment 

of lahar hazards, they reveal important characteristics 

of the flows: magnitudes (as discharges, either true or 

relative), flow types, behavior, and probable origins.

The detailed analysis of the older flows focuses on the 

area around the confluence of the forks of the Toutle, 

more than 50 km from the volcano, because that area 

is the farthest upstream part of the watershed that is 

well populated. Although the following record of older 

flows should be viewed as one in a series of successive 

approximations, it certainly is complete enough to indicate the continuing, long-term hazards of lahars in the 

system. No new stratigraphic terminology is proposed.

Radiocarbon dates with conventional one-sigma statistical counting errors less than ±100 yr are by I. C. 

Yang, and uncited dates with larger statistical error 

factors are by commercial labs. Dates of individual 

samples are in radiocarbon years unless otherwise 

stated. Following Mullineaux and Crandell (1981), the 

ages of the eruptive episodes (table 1) are in radiocarbon years, except for those of the Goat Rocks and 

Kalama periods, which are in calendar years before 1950.

ACKNOWLEDGMENTS

Advice on pumice identifications by D. R. Mullineaux 

was invaluable. His identifications are cited as oral communications in the text and in the Coal Bank Bridge 

section in the appendix. Uncited identifications at that 

section and at other localities were made by the writer.

Several local residents provided access to important 

exposures. Among these was N. B. Gardner, who had 

recognized that the tree molds penetrating the thick 

lahar sequence of Pine Creek age were evidence that the 

sequence had been emplaced within a relatively short 

time span.

I thank J. C. Brice, D. R. Crandell, R. J. Janda, and 

W. R. Osterkamp, all of the U.S. Geological Survey 

(USGS), and J. H. Hubert, University of Massachusetts, for beneficial, constructive reviews of all or parts 

of the report.

LAHARS AND LAHAR-RUNOUT FLOWS

OF PRE-1980 ERUPTIVE STAGES

AND PERIODS

The first part of this report consists of lahar and 

lahar-runout-flow stratigraphy, by eruptive stage and

period. This, with the noted qualifications, is the foundation of the subsequent discussion of lahar magnitude 

and frequency. Selected examples of many detailed 

stratigraphic sections appear in the appendix, and these 

are mainly from the vicinity of the forks of the Toutle 

River. A glossary of many terms used in describing 

volcanic flow phenomena, lahars, lahar-runout flows, 

and volcaniclastic sediment is included in Scott (1988). 

That some lahars had moved long distances from 

Mount St. Helens in the Toutle River system was first 

recognized by Mullineaux and Crandell (1962). They 

defined a Silver Lake lahar assemblage near the confluence of the forks of the Toutle River, and noted that 

Silver Lake had formed behind the natural dam created 

by those lahars. The lahar sequence at that point was 

shown to include three lahars of Pine Creek age and a 

lower lahar of Smith Creek age (Crandell and others, 

1981; Crandell, 1987).

APE CANYON ERUPTIVE STAGE 

(50,000(?) TO ~36,000 YEARS)

A valley fill containing at least five lahars and many 

lahar-runout flows is exposed near the confluence of the 

forks of the Toutle River, near Tower, and at the confluence of the Toutle and Cowlitz Rivers. At these 

localities the fill surface extends to a level as much as 

30 m above the surface of the better preserved laharemplaced terrace of Pine Creek age. The older valley fill 

represents gradual valley-wide aggradation, in contrast 

to the brief series of catastrophic events that formed 

the younger terrace. Also unlike the younger terrace, 

which is underlain almost entirely by lahars and locally by lahar-runout deposits, the older fill includes a 

significant volume of pumice-rich alluvium, a part of 

which represents glacial outwash.

The fill of Ape Canyon age is exposed near Tower at 

levels indicating that drainage during the eruptive stage 

was partially out of the Toutle River watershed and 

northward into Salmon Creek (fig. 1). Salmon Creek 

joins the Cowlitz River approximately 19 river kilometers upstream from the confluence of the Toutle and 

Cowlitz Rivers.

The lahars of Ape Canyon age are as much as 3.0 m 

thick, but their correlation and detailed sequence could 

not be determined because of sporadic exposure. Accessible outcrops are in the valley-side slopes approximately 200 m upstream from the Coal Bank Bridge and 

in valley-side slopes short distances north and west of 

the Green Mountain Mill. Both forks of the Toutle River 

yielded multiple lahars during this stage. Deposits of 

Ape Canyon age occur sporadically both at channel level 

and at high levels (up to 60 m) in both forks, indicating
PRE-1980 ERUPTIVE STAGES AND PERIODS B5

the system-wide nature of the fill. These characteristically light-colored deposits form the basal parts of 

thick sections of pyroclastic flows, lahars, and volcaniclastic and glacial sediment near the base of the volcano 

in the upper South Fork.

Dacite and pumice clasts in the deposits are relatively 

highly weathered and impregnated with secondary clay. 

Their condition prevents any meaningful size analyses 

for comparison with younger flow deposits (as in table 

5 of Scott, 1988). Nevertheless, the textures of the 

lahars are similar to those of the younger flows in that 

the matrices are predominantly silty sand and probably 

have relatively small amounts of primary clay-size sediment. Most are relatively fine grained compared to the 

flows of younger eruptive periods. In sections near the 

Toutle-Cowlitz River confluence, lahar-runout flows are 

more abundant than lahars.

Soil development in the Ape Canyon deposits extends 

locally to depths of more than 3 m. Sufficient clay to 

cause a sticky consistency in the lower A and B horizons 

is characteristic, as is mottling of the B and C horizons 

in irregular clay-rich zones. Silt loams of the Gee and 

Seaquest soil series (Call, 1974) occur on the surface of 

the fill. The red color locally present in the soil developed 

on Ape Canyon deposits, and less commonly on those 

of younger periods at lower levels, reflects upslope 

pedogenesis of volcanic rocks of pre-Mount St. Helens 

age. The lateritic pedogenesis also predates Mount St. 

Helens.

The dating of the sequence is based on several lines 

of evidence. The degree of weathering is significantly 

greater than that of deposits of Cougar age, the next 

youngest eruptive stage. At the Coal Bank Bridge section (appendix) the fill disconformably underlies lahars 

of Swift Creek age that can be seen to overlap a valleyside slope cut in the older sequence. The pumice, the 

lahars, and the nonlaharic sediment of the sequence 

contain abundant biotite. Tephra set C (table 1) of the 

eruptive stage contains biotite as a characteristic ferromagnesian phenocryst (Mullineaux and Crandell, 1981, 

table 1). Pumice collected from the oldest deposits in 

the Coal Bank Bridge section is believed by Mullineaux 

(oral commun., 1982) to be of Ape Canyon age with 90 

percent probability.

The range given for the start of the eruptive period 

is based on the thick and extensive deposits present in 

both the Tbutle River and Lewis River systems. An 

unknown but probably lengthy period was necessary for 

accumulation of the sequence. Hyde (1975) determined 

a date of ~ 36,000 yr for a lahar in the upper Lewis 

River, which drains the south and east sides of the volcano. A new radiocarbon age for the same lahar (>3 5,800 

yr; Major and Scott, 1988) confirms his date. Note, however, that samples used for both determinations are

radiologically "dead," and thus yield minimum ages. The 

Ape Canyon-age fill extends to high levels on valley-side 

slopes throughout the lower reaches of the Lewis River 

system, like the system-wide deposits of the Tbutle River 

(Major and Scott, 1988).

COUGAR ERUPTIVE STAGE 

(20,400 TO 19,200 YEARS)

A thick lahar of Cougar age is exposed in a small 

quarry excavated during 1981 in the Cowlitz River 

valley west of Castle Rock (NEtt sec. 4, T. 9 N., 

R. 2 W.). The deposit is at an elevation similar to that 

of the subsequent huge flow of Pine Creek age, but the 

two lahars cannot be compared in size because of the 

unknown configuration of the valley in Cougar time. 

The deposit is at least 1.3 m thick, and the included 

clasts are dominated by rock types from Mount St. 

Helens. Abundant pumice from the uppermost part of 

the unit is probably from tephra set K or M of Cougar 

age (table 1) but may be of Smith Creek age, according 

to Mullineaux (oral commun., 1982). The original exposures showed that the flow deposit is overlain by at 

least 15 m of cyclically bedded, silt-rich sediment probably correlative with backwater deposits of major Channeled Scabland flooding of about 13,000 years ago. 

Consequently, a Cougar age for the lahar is a near 

certainty.

Although this flow was large enough to have reached 

the Columbia River, at least in its runout phase, its 

deposits were not definitely identified elsewhere in the 

Toutle-Cowlitz River system. Its deposits, and those of 

any other large lahars of the eruptive stage, are probably covered by an inset younger assemblage. A probable Cougar-age lahar is present in the South Fork 

Toutle River as far as 10-12 km from the volcano (see 

Disappointment Creek section in appendix), but is 

unlikely to represent the proximal phase of the lahar 

seen in the Cowlitz River. Other Cougar-age deposits 

are present in the Bear Creek section (appendix).

Sections near Tower may contain Cougar-age flows. 

Those deposits are definitely post-Ape Canyon and prePine Creek in age but most are more probably of Swift 

Creek age, as discussed in the following section.

SWIFT CREEK ERUPTIVE STAGE 

(13,700 TO >9,200 YEARS)

The lower 7 units of the post-Ape Canyon part of the 

Coal Bank Bridge section (appendix), 6 of which are 

lahars, are of probable Swift Creek age. Unit 3 was the 

oldest pumice-bearing unit in the section and yielded
B6 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

pumice regarded by Mullineaux (oral commun., 1982) 

as probably set J of Swift Creek age. Unit 8 is a rubbly 

lahar that is locally disconformable with older units, 

suggesting the possibility of change in the valley erosional regime. That lahar, and overlying units 9 through 

14 (including 3 lahars and a lahar-runout flow), are more 

certainly of Swift Creek age. In general, the certainty 

of a Swift Creek age increases throughout the sequence 

of 14 units.

The upper part of a soil developed on unit 14 contained tiny charcoal fragments yielding in aggregate a 

radiocarbon date of 3,760±180 yr. Based on the date 

alone it is not clear if the soil represents (1) the approximately 5,000-yr interval between the Swift Creek and 

Spirit Lake stages, (2) the approximately 300-yr interval between the Smith Creek and Pine Creek periods, 

or (3) an interval between eruptions within the Smith 

Creek period. The degree of weathering is at least as 

great as that on the unerodeid terrace of Pine Creek age, 

representing a 2,500-yr period; therefore the soil probably represents the earlier, longer dormant period. Thus 

the conformably underlying units (8-14) are of Swift 

Creek age. This conclusion is supported by the presence 

of pumice containing phenocrysts of hypersthene and 

hornblende in units 8, 11, and 12. Pumice with that 

mineralogy is characteristic of Swift Creek age, but not 

exclusively so. It is, however, definitively different from 

set Y of Smith Creek age.

At the north side of the modern flood plain upstream 

from Tower, cutbank exposures reveal marked facies 

changes within a sequence of lahars and lahar-runout 

deposits. The sequence has almost no pumice, but 

almost all the few pumice clasts that were gleaned from 

talus at the cliff base contain hypersthene and hornblende and probably represent set J. The extensive soil 

development to a depth as great as 1.0 m is highly compatible with a probable Swift Creek age for most of the 

sequence. The possibility of Cougar-age flows low in the 

section should be evident from soil development within 

the section. No evidence of a hiatus was observed, 

however. The runout flows in the sections near Tower 

are the likely downstream transformations of the Swift 

Creek-age lahars in the Coal Bank Bridge section. The 

high cutbanks on the south side of the valley near Tower 

contain flows of Pine Creek age downstream but are 

composed of deposits of probable Swift Creek age 

upstream.

Near the volcano a sequence of pyroclastic flows and 

lahars of Swift Creek age forms part of an apron of 

flowage and alluvial deposits that extends into the headwaters of the South Fork of the Toutle River and locally inundated the valley bottom (Crandell, 1987). (See 

Bear Creek and Disappointment Creek sections in appendix.) The extent (up to 20 km at the Bear Creek

locality) and number of Swift Creek-age flows extending into the upper South Fork suggest that the 

numerous lahars of that period observed at the confluence of the forks could mainly reflect flow from the 

South Fork. The largest flows of the subsequent two 

periods clearly were derived from the North Fork of the 

Toutle River.

SPIRIT LAKE ERUPTIVE STAGE 

SMITH CREEK ERUPTIVE PERIOD (4,000 TO 3,300 YEARS)

Two flow units in the Coal Bank Bridge section (appendix) represent the Smith Creek eruptive period with 

a relatively high degree of probability. A runout-flow 

deposit, formed just beyond the transition from a lahar, 

overlies the soil of the preceding dormant period, and 

the uppermost part of the bed (unit 15) contains concentrated pumice identified by Mullineaux (oral 

commun., 1983) as material from set Y of Smith Creek 

age. On a textural basis, the overlying large lahar (unit 

16) is a highly probable correlative of the lowest lahar 

of the Outlet Creek section (appendix, unit 1) and is 

probably the lahar with a relatively high clay content 

seen beneath the Pine Creek deposits at several locations ranging from the community of St. Helens on the 

North Fork to the Highway 99 Bridge on the lower 

Toutle River (fig. 2). The lowest lahar at Outlet Creek 

was interpreted by Crandell (1987) to be Smith Creek 

in age, as based on the the presence of pumice from set 

Y and wood fragments yielding a date of 2,810± 200 

radiocarbon years in the overlying fluvial sequence. This 

overlying sequence is not present in the Coal Bank 

Bridge section but is widely distributed throughout the 

North Fork and main Toutle River. What is likely the 

same lahar occurs at Alder Creek beneath a soil containing wood fragments with a radiocarbon age of 

2,900±250 years (Crandell, 1987). The extent, thickness, 

and texture of the lahar indicate that it probably 

reached the Columbia River. The main lahar at Kid 

Valley another probable correlative is overlain by a 

much smaller but similar lahar, which probably reached 

the confluence of the forks of the Toutle River but is 

not recorded in the Coal Bank Bridge section. Both 

lahars at Kid Valley have a relatively high primary clay 

content, the behavioral implications of which are discussed in Scott (1988).

The mainly fluvial sequence overlying the two lahars 

of Smith Creek age includes at least six runout-flow 

deposits, two of which are included in the Outlet Creek 

section (appendix, units 3 and 4). The sequence probably 

includes deposits of both Smith Creek and Pine Creek 

age, as well as the intervening dormant period. The
Highway 99 Tower Outlet Creek Coal Bank Bridge Harry Gardner Park Green Mounta

(channel fac

PC 3

hhPC 1

hhchSC

PC 3

PC 2

PC 1

:: £>. 

;.-O'::::

$: ? 

c#>0 

;&v 

;OP

(3) 

(2)

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hh

ch

Sequence of lahars of Pine

Creek age 

Lahar of Smith Creek age wi

distinctive texture

Unit numbers in stratigraphi

section at Coal Bank Bridg

(described in appendix)

"Ball-bearing bed"

Deposit of lahar-runout flow 

(or basal part of transition 

facies)

Pumice with hypersthene and 

hornblende (similar to set P

Pumice with cummingtonite 

hornblende (similar to set Y

FIGURE 2. Selected columnar sections of lahars and lahar-runout flow deposits in the Toutle River system. Numbers assigned to units of the Coa

respond to those in the measured stratigraphic section at that locality described in the appendix.
B8 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

upper part at the Green Mountain Mill section is probably of Pine Creek age. The lower part at the Highway 

99 locality (fig. 2) contains pumice mineralogically 

similar to set Y and thus is probably Smith Creek in 

age; the upper part contains pumice like that of set P 

and thus is probably Pine Creek in age.

Near the volcano, lahars and pyroclastic flows of 

Smith Creek age compose a significant part of the apron 

of deposits extending north from the volcano into the 

valley of the North Fork Toutle River west of Spirit 

Lake (Crandell, 1987). These units are now covered by 

the 1980 debris avalanche.

PINE CREEK ERUPTIVE PERIOD (3,000 TO 2,500 YEARS)

The Pine Creek age of the uppermost lahar-runout 

deposits in the mainly fluvial sequence beneath the first 

and largest Pine Creek lahar (PC 1) is also indicated by 

radiocarbon dates of old-growth forest locally inundated 

by the deposits along the streambanks (table 2). The 

forest grew during the preceding dormant interval and 

survived on flood plains throughout much of Pine Creek 

time until it was inundated by a series of four lahars 

late in the eruptive period (PC 1 through 4). That the 

lahars were confined to Pine Creek time is indicated 

both by the radiocarbon dates, and by the lack of rock 

types from the modern cone of Mount St. Helens in the 

youngest of the flows. Those rock types are present in 

the deposits of subsequent eruptive periods. The series 

of lahars late in the period is interpreted to be of lakebreakout origin, and it includes the two largest flows 

in the history of the watershed. (See subsequent discussion of magnitude.)

Lahar PC 1, the largest lahar from Mount St. Helens 

in the history of the Toutle River watershed, occurred

about 2,500 yr ago (table 2). The subsequent three flows 

occurred within several years thereafter, the evidence 

for which is discussed in the section on time clustering 

of lahars. The channelized parts of lahar PC 1 stripped 

the forest from areas marginal to channels; the floodplain flow surrounded standing trees and preserved 

their forms as vertical tree molds. Facies of the series 

are shown in figure 12 of Scott (1988).

Radiocarbon dates relevant to the age of lahar PC 1 

are summarized in table 2. The only aberrant date 

(3,290± 195 years) was from a cedar tree in growth position on a surface inundated by the lahar. The cedar may 

have been dead at the time of inundation. Exposed 

cedars are relatively well preserved in growth position 

at Old Maid Flat on Mount Hood about 200 years after 

they were killed by a lahar that inundated the area 

around their bases (Crandell, 1980), based on observations by P. T. Pringle and myself. Some other species 

are nearly entirely decayed at Old Maid Flat, forming 

tree molds like those in the series of Pine Creek age at 

Mount St. Helens. Another possibility is that the 

sample was from the interior of the tree, on the premise 

that the outer layers were abraded by boulders, which 

were concentrated in the basal flow as energy was reduced in the expanding reach below the gorge (See 

Scott, 1988.) These explanations assume no laboratory 

error.

Near the volcano, pyroclastic flows were much more 

common in the Pine Creek eruptive period than in the 

Smith Creek period (Crandell, 1987). Pine Creek-age 

deposits underlie large areas around the base of the 

volcano. These include deposits of several lahars of 

probable Pine Creek age in the upper South Fork Toutle 

River that apparently did not reach the confluence in 

significant volume. Lahars of the period traveled at

TABLE 2. Radiocarbon dates indicating age of largest lahar in history of the Toutle-Cowlitz watershed (PC 1) and three contemporaneous

lahars 

[Sequence of dates is from upstream to downstream]

Location

Green Mountain Mill section (See

, a *t ^ Analyst Comments (yrs BP) y

2,510± 40 I. C. Yang Outermost wood from highest part of

appendix.)

Buried forest at distal end of gorge 

beginning below the Coal Bank Bridge.

Detrital limb immediately beneath lahar 

PC 1, along Cowlitz River 3.0 km upstream from mouth of Toutle. (Lahar 

flowed upstream from confluence.)

2,550±145 

2,560±155 

3,290±195

2,440± 40 

2,790±245

Geochron, 

Inc.

I. C. Yang 

Geochron, 

Inc.

stump remaining at base of channel 

facies of lahar PC 1.

Outermost wood about 2 m above 

original ground surface. First 

two samples probably Douglas firs. 

Third is cedar. (See text.)

Two analyses of same sample, 

split by I. C. Yang.

Sample
PRE-1980 ERUPTIVE STAGES AND PERIODS B9

least as far as the Bear Creek section (appendix) in the 

South Fork, however. Crandell also reports lahars interbedded with Pine Creek-age pyroclastic flows on the 

north side of the volcano, in the drainage area of the 

North Fork Toutle River, and a thick fill of pyroclastic 

flows and lahars in the Castle Creek valley.

CASTLE CREEK ERUPTIVE PERIOD (>2,200-1,700 YEARS)

The dormant interval of less than 300 years, which 

followed emplacement of the huge volume of Pine Creek 

lahars, was marked by extensive reworking of laharemplaced sediment. A widespread terrace underlain by 

fluvial deposits as much as 2.5 m thick illustrates the 

rapid response of the river system to laharic sedimentation. These deposits have little (1 to 2 percent at most) 

of the darker, more mafic rock types of the Castle Creek 

and subsequent eruptive periods. Field work with R. J. 

Janda likewise revealed only small amounts of these 

lithologies in a fill emplaced subsequent to the Pine 

Creek lahars in the Cowlitz River below its confluence 

with the Toutle River. On the south side of the volcano, 

however, paleomagnetic data indicate that two lava 

flows with the changed, more mafic chemistry are of the 

same age as Pine Creek dacites (R. T. Holcomb, USGS, 

written commun., 1986). The lack of basic lithologies 

in the Pine Creek lahars and their low content in reworked Pine Creek deposits can be explained by the lack 

of a significant volume of basaltic lava flows on the 

north side of the volcano at that time. These observations do not rule out the possibility that the chemistry 

changed during the Pine Creek eruptive period, as suggested by the paleomagnetic data.

The sediment-transport regime in the Toutle River 

system during the dormant interval preceding the 

Castle Creek eruptive period was probably one of 

rapidly migrating braided channels, in which the vast 

influx of laharic sediment was reworked. Consequently, 

the preservation of lahar deposits of Castle Creek age, 

inset against eroding cutbanks of the lahar-emplaced 

terrace of Pine Creek age, is unlikely.

A coarse, rubbly lahar, probably from the Castle 

Creek period, was recognized in the basal part of the 

fill of a pre-Kalama-age paleochannel near Camp Baker. 

Based on the relative degrees of soil development on 

the fill deposits and on a younger lahar of probable 

Kalama age at the same locality, the fill is probably of 

Castle Creek age. A significant content of modern-cone 

rock types establishes the basal channel fill as post-Pine 

Creek in age.

In the headwaters of the Toutle River system, according to Crandell (1987), the deposits of the period are 

predominantly lava flows and pyroclastic flows accompanying a single lahar in the Castle Creek drainage.

Several lahars of this eruptive period were recognized 

near the volcano in the upper South Fork, and numerous 

flows occur in other drainages outside the Toutle River 

system.

SUGAR BOWL ERUPTIVE PERIOD (AGE: 1,200 YEARS)

Lahars representing this brief eruptive period, the 

chief product of which was the Sugar Bowl dome on the 

north flank of the mountain (Mullineaux and Crandell, 

1981), have not been seen downstream in the river 

system. Crandell (1987) recognized apparently local 

lahars of this period that were formed by interactions 

between pyroclastic flows and snowpack at the base of 

the cone near Spirit Lake. Crandell and Hoblitt (1986) 

have recognized two northeastward-directed lateral 

blasts associated with formation of the Sugar Bowl 

dome.

KALAMA ERUPTIVE PERIOD (500 TO 350 YEARS)

In both the North Fork Toutle River at Camp Baker 

and the South Fork at Weyerhaeuser Camp 12, similar 

lahars of probable Kalama age overlie soils containing 

dated wood fragments (1,250±40 years at Camp Baker 

and 1,360±215 years at Camp 12). All flow deposits are 

relatively fine-grained flood-plain facies that have 

granular matrices. Near Camp Baker, a channel facies 

that may correlate with the North Fork flow consists 

of coarse cobbles and locally has a clast-supported 

framework. Relative thicknesses of incipient soil 

development on the units in each fork are compatible 

with a Kalama age. Mature trees growing on the units 

establish a pre-Goat Rocks age for the flows.

The largest Kalama-age lahar in the North Fork 

Toutle River forms a low terrace level near Kid Valley, 

where it conformably overlies two smaller lahars that 

are also of Kalama age. Wood from an in-place stump 

overlain by the three units yields a date of 505±155 

years. Mature trees have grown on the terrace surface. 

Near the Green Mountain Mill the largest flow contains 

a hypersthene-rich pumice that is mineralogically 

similar to set W.

The upper, glaciated part of the South Fork Toutle 

River valley contains a sequence of deposits of avalanches that transformed to lahars extending at least 

20 km from the crater. Wood from the outermost part 

of a mature tree buried by the largest of the avalanche 

deposits yielded a radiocarbon date of 460±40 years. 

That date corresponds to a calendar date of about A.D. 

1440 on the tree-ring calibration curve of Suess (1970). 

Given the standard deviation of the date, and the 

presence of about 25 annual rings in the sample, that 

deposit may correlate with the eruption producing layer
BIO LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

Wn of set W, which has been dated as A.D. 1480 by 

Yamaguchi (1983).

One of the later avalanche deposits in this sequence 

is transitional with the deposit of a major pyroclastic 

surge that has many similarities to the 1980 blast 

deposit (Scott, 1988). The surge deposit contains abundant, generally surficially charred, wood and is locally 

dominated by light-gray, fine-grained dacite of the same 

type that formed the pre-1980 summit dome of the 

volcano. Mapping of the surge deposit during 1984 

showed that it includes pebble-size, nonpumiceous clasts 

at elevations in excess of 300 m on valley-side slopes as 

much as 7 km from the crater. The surge crossed divides 

into both Sheep Canyon and the drainage of the North 

Fork Tbutle River, and was strongly directed westward 

down the South Fork, against the probable prevailing 

wind direction. At greater distances from the volcano, 

the episode is marked by a fine-grained layer 1 cm or 

less thick within the grass-root zone. That unit is the 

deposit of the ash cloud associated with the surge (or 

flow; note the variety of terminology applied to the 1980 

lateral blast). The locally thick valley-fill facies of the 

surge is locally overlain by a synchronous or nearly synchronous lahar of similar composition. At other locations the surge deposits grade into avalanche deposits 

that have, in 1982 exposures, probable ground-surge 

deposits at their bases.

This pyroclastic surge is almost certainly the same 

one dated by Yamaguchi and Hoblitt (1986) within the 

range of A.D. 1647-1668. Illustrative of the spread in 

radiocarbon ages from young material (see Suess, 1970, 

and Stuiver, 1978), three wood fragments from the main 

surge deposit yielded dates of 485± 155, 435± 125, and 

145± 125 radiocarbon years. A radiocarbon date of 145 

years can correspond to various calendar year ages between approximately A.D. 1670 and A.D. 1810 (Stuiver, 

1978, fig. 1). The tree-ring dating by Yamaguchi and 

Hoblitt (1986) more accurately defines the time of this 

event, but it places the surge and its associated effects 

slightly later than the Kalama time interval recognized 

by Mullineaux and Crandell. Therefore, these obviously 

eruptive events are treated here as if they occurred 

during a brief extension of that eruptive period.

The two Kalama-age South Fork lahars that extended 

to the confluence of the forks of the Toutle River (table 

1) the only ones included in the subsequent frequency 

analysis can be correlated with this catastrophic activity, even though a long interval of bedrock channel 

(fig. 6; Scott, 1988) makes the continuous tracing of individual flow units impossible. Stratigraphic evidence 

strongly suggests that these two lahars originated as 

transformations of either the avalanches or the pyroclastic surge itself, or both.

Lahars are interbedded with other types of flows and 

eruptive products of the Kalama period near the

volcano. Lahars of almost certain Kalama age occur in 

nearly every watershed draining Mount St. Helens. 

Crandell (1987) notes a variety of lahar emplacement 

temperatures and the extension of flows at least as far 

as 11 km from the base of the volcano in the South Fork 

of the Toutle River. A large slope-mantling lahar of 

Kalama age occurs in the Disappointment Creek section (appendix).

GOAT ROCKS ERUPTIVE PERIOD (180-123 YEARS)

The Goat Rocks dome formed in the segment of the 

volcano drained by the North Fork Toutle River. The 

associated downslope deposits include material emplaced either by pyroclastic flows or lahars carrying 

both hot and cold debris (Crandell, 1987). The upper 

South Fork Toutle River valley contains one lahar of 

this eruptive period and another that dates from A.D. 

1855 (Yamaguchi and Hoblitt, 1986). No lahars large 

enough to inundate flood plains a significant distance 

from the volcano were recognized from this youngest 

of the pre-1980 eruptive periods.

MAGNITUDE OF LAHARS

FLOW DEPTHS BASED ON THICKNESS OF 

FLOOD-PLAIN FACIES

The thicknesses of modern lahar deposits that remain 

as the flood-plain facies seem to be proportional to the 

sizes of the flows that deposited them. This should also 

be true for the ancient lahars. Using similar reasoning, 

Kochel and Baker (1982) found that paleoflood discharge and slack-water sediment thickness appeared to 

be directly related at a given depositional site. Given 

the general similarity in texture of most of the lahars 

in the watershed, the thicknesses of the flood-plain 

facies at a downstream location, such as the confluence 

of the forks of the Toutle River, can indicate the relative 

sizes of the flows.

Figure 3 is a plot of the ratio of the thickness of the 

flood-plain facies to flow depth for the two largest 1980 

flows in the system. The plot indicates that, downstream and with increasing channel width, the deposits 

fill a greater proportion of the flow cross sections in the 

case of both the relatively high-clay North Fork lahar 

and the more typical granular and noncohesive South 

Fork lahar before the transformation to a lahar-runout 

flow began. That is, rates of deposition increased as 

slope decreased and valley width increased.

A clearly definable and constant relation between deposit thickness and flow depth is not to be expected, particularly because of the close time relation between some 

older flows. Overbank deposition of a sheet of lahar
MAGNITUDE OF LAHARS Bll

CL 0.9

LJ 

Q

O 0.8

(ft

LJ 

O

0- 

I

Q 

O 

O

0.7

0.6

0.5

0.4

0 0.3

0.2

LJ 

O

£

u. 0. 

o

o 

5 0.

I I

 South Fork lahar 

o North Fork lahar

Trend reflects large 

increase in valley width

I__I 0

10 20 30 40 50 6070

CHANNEL DISTANCE FROM CRATER, 

IN KILOMETERS

FIGURE 3. Ratio of thickness of flood plain facies to flow depth 

plotted against distance from 1980 crater, for the two largest 1980 

lahars described in Scott (1988).

deposits must cause a period of channel readjustment 

to the higher flood-plain levels, during which the relation between deposit thickness and flow depth for a subsequent flow will be changed temporarily. Nevertheless, 

for older flows that are textural analogs of one of the 

1980 flows, the ratio of the thickness of the flood-plain 

facies to flow depth at places that have similar channel 

slopes and configurations should in most cases yield a 

useful estimate of the overbank flow depth (table 3). 

Such estimates, however, are less satisfactory than measurements of the actual cross sections of the older flows.

MEASUREMENTS OF CROSS SECTIONS OF 

LAHAR FLOOD WAVES

A significant benefit of the study of lahar boundary 

features (Scott, 1988) is the ability to identify specific

flows in lateral deposits on slopes. Figure 4 shows how 

the flow cross sections of lahar PC 1 were measured and 

how the cross sections of lahar PC 3 were estimated. 

The lateral equivalents of the "ball-bearing bed" at the 

base of PC 1 and a sandy sole layer at the base of lahar 

PC 3 were keys in correlating the respective units. These 

two lahars are the largest in the history of the postMount St. Helens Toutle River.

The altitude of the channel thalweg in the North Fork 

Toutle River at the start of lahar deposition in the Pine 

Creek eruptive period was close to its modern altitude. 

The base of the channel facies of lahar PC 1 is nearly 

coincident with the present channel at such widely 

separated locations as Pullen Creek and the Green 

Mountain Mill on the North Fork, and Tower on the 

main Toutle River. Once the peak flow level was identified, the cross-sectional area of the flow could be determined by adding the appropriate cross-sectional areas 

of Pine Creek deposits and modern alluvium to the existing cross-sectional area. This approach yields highly 

accurate flow sections in predominantly bedrock 

reaches, where erosion of valley-side slopes has been 

minimal since Pine Creek time.

MEASUREMENTS AND ESTIMATES OF 

VELOCITY AND DISCHARGE

Velocities of recent flows can be determined from 

superelevations of deposits on steep slopes (for example, Scott, 1988), but such determinations for older flows 

are difficult because of the thinness and sporadic distribution of the depositional coating. The deposits have 

eroded from most steep slopes, which is not surprising 

considering that by 1985 the peak flow deposits of the 

1980 lahars had been eroded at many analogous locations. Peak flow deposits of lahars PC 1 and 3 are 

preserved, however, on moderate slopes where runup 

was locally significant, and are easily recognized 

because of their characteristic boundary features.

An example of a locality where peak mean velocity 

of lahar PC 1 could be determined was the island near 

the center of the flow upstream from the community 

of St. Helens. At several locations on the upstream 

flank of the island the levels of the highest clasts of 

Mount St. Helens rock types were compared with the 

same peak flow level on the downstream side, with adjustment for channel slope. The levels were also compared with upstream peak flow levels and the probable 

ratio of deposit thickness to flow depth. A distinction 

must be made between velocities determined this way, 

applying to the time of peak stage, and velocities applying to the time of peak discharge. In small flows, log 

jams or boulder fronts may act as moving dams and
B12 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

TABLE 3. Thickness of flood-plain fades, depth of flow on flood plain, and measured and estimated peak discharges for lahars in the ToutleCowlitz River system

[Leaders ( ) mean not determined or not applicable. "Confluence" denotes the confluence of the forks of the Toutle River]

Lahar

Thickness 

of 

flood-plain 

facies (m)

Overbank Discharge 

flow depth Location Remarks

. 1 m3 /s Basis 2 

Meters Basis 1

Lahars of the eruptive period beginning in 1980

March 1982 North Fork 

lahar. 

May 1980 North Fork 

lahar. 

First May 1980 South 

Fork lahar. 

Second May 1980 South 

Fork lahar.

0 

1.5-2.0 

0.5 

0

0 Kid Valley 960 P Runout phase not overbank at confluence.

1.0-4.0 Mm Confluence 6,800 F

<1.0 Mm Confluence 3,600 F

dammed by North Fork 

lahar at confluence.

Lahars probably of Kalama eruptive period

Lahar from North Fork 

Conformably underlying 

lahar. 

Lowest conformable 

lahar at Kid Valley. 

Lahar from South Fork 

Lahar from South Fork

>1.8 

0?

0?

1.3 

1.0

>4.0 E Confluence 

2.0 E Camp 12 

1.5 E Camp 12 

Lahar probably of Castle Rock eruptive period

0

paleochannel.

Lahars of Pine Creek eruptive period

Fourth lahar (PC 4) 

Third lahar (PC 3) 

First lahar (PC 1) 

0.0-1.5

2.6->4.4

U U £. £.

(mean 0.8) 

2.4-7.0

<2.5 E Confluence 

>8.0 Md Kid Valley 48,000 Me

<3.0 E Confluence 

214.0 Md 30-50 km from 200,000- Mr 

crater. 300,000 

Gorge below Coal 85,000 Me 

Bank Bridge.

Highway 99 50,000 Mr

Lahars of Smith Creek eruptive period

Smaller lahar of this 

age at Kid Valley. 

Coal Bank Bridge 

section: 

Unit 16 

Unit 15 

0.4

3.2 

1.0

0.8 E Kid Valley <1,200 Ee

6.5 E Confluence 25,000 Ee

Highway 99 20,000 A

1.5 E Confluence 2,000 Ee

Lahars of Swift Creek eruptive stage

Coal Bank Bridge 

section:

Unit 14 

Unit 12 

Unit 9 

TTni t- 8

0.5 

1.4

1.2

i <;

1.0 E Confluence 

3.5 E Confluence 

3.0 E Confluence 

0 "5 17 r1 ,-.!-! £ 1 1 1 QT-1 /- 0 
MAGNITUDE OF LAHARS B13

TABLE 3. Thickness of flood-plain fades, depth of flow on flood plain, and measured and estimated peak discharges for lahars in the ToutleCowlitz River system Continued

Lahar

Thickness 

of

facies (m)

Over bank

flow depth Location

Meters Basis*

Discharge

m3 /s 9

Basis

Remarks

Lahars probably of Swift Creek eruptive stage

Coal Bank Bridge 

section:

Unit 6 -

TTn i t- i_ _ _ _

Unit 3 -

Unit 2 

Unit 1 -

 (J. 4

- 0.3

- 0.6

- 1.2

- 3 >1.0

1«U

. o

1.5

3.0

4.2

Lahar of Cougar eruptive stage

Lahar in Cowlitz River >1.3 

Valley.

>3.2 Cowlitz River 

Lahars of Ape Canyon eruptive stage (at least five lahars in all)

Largest lahar 

Most other lahars 

3.0 ~6.0 

>2.0

Confluence 

Confluence 

Bases for determining flow depths:

E, Estimated from ratio of thickness of flood-plain facies to flow depth of appropriate 1980 lahar 

at a location with a similar slope and channel configuration.

Md, Measured from distribution of peak-flow deposits.

Mm, Measured from peak-stage marks. 

Bases for determining discharges:

A, Attenuation rate of the texturally similar 1980 North Fork lahar.

Ee, Estimated flow depth (based on thickness of flood-plain facies) and estimated velocity.

F, Fairchild and Wigmosta (1983).

Me, Measured cross sections and estimated velocities.

Mr, Measured cross sections and velocities based on measurements of runup.

P, Pierson and Scott (1985).

Thickness of flood-plain facies based on relative thicknesses of channel and flood-plain facies of 

1980 lahars.

form peak stages (mudlines) hydraulically inconsistent 

with the subsequent peak discharges. Peak stage and 

discharge for most significant lahars, and particularly 

for those the size of PC 1 and PC 3, were almost certainly coincident.

The peak mean velocity of lahar PC 1 was in the range 

of 15-22 m/s (meters per second) in the North Fork 

Toutle River between 30 and 50 km from the crater. 

This range encompasses the determined velocity of 

15-18 m/s in the broad valley containing the flow island 

near St. Helens, and other determinations based on 

measurements of runup and superelevation in bends in 

downstream reaches where flow was deeper and more 

constricted. Velocity certainly increased between the 

flow island and the narrower reaches; the amount of increase is also indicated by (1) an increase in velocity 

from 7.8 to 9.5-12.0 m/s for the 1980 North Fork flow

over the same channel interval (Fairchild and 

Wigmosta, 1983), and (2) a reduction in lahar crosssectional area that is expectable from configuration of 

the reach rather than from attenuation of the peak 

discharge. For example, the cross-sectional area of lahar 

PC 1 near Kid Valley was 13,500 m2 (square meters); 

upstream at the flow island, the flow cross section was 

a minimum of 15,600 m2. Given the certainty of no 

flow transformation between these points, and assuming hydraulic continuity and minimal deposition, the 

decrease in cross section translates into a velocity increase from 15-18 m/s at the flow island to 17-21 m/s 

at Kid Valley. This extrapolation corresponds with PC 

1 flow velocities derived from measured runups and 

superelevations near Kid Valley, which were, however, 

less certain than the velocities determined at the flow 

island.
B14 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

40

20

< 10

Pepk_stage_pf_PC_ _ _ 

Terrace underlain Terrace underlain

by PC deposits

Flow from North \

A

200

50 i40

30

10 -

B

50

40

30

20

10

c

400 600 800 

HORIZONTAL DISTANCE, IN METERS

1000

Minimum scour of PC 1

// Boundary of PC 1 

Boundary of PC 3

Boundary of PC 1 

Estimate of flood-plain 

thicknesses of PC 1, 2

200 400 600 

HORIZONTAL DISTANCE, IN METERS

800

Pe k P C

Peak staqe of PC 3 O , 

 ^ I Pre-Pme Ureek

deposits 

7 / 

Boundary of third flow

Boundary of PC 1

Estimate of flood plain 

thicknesses of PC 1, 2

Pre-Pine Creek 

deposits

200 400 600 

HORIZONTAL DISTANCE, IN METERS

800

FIGURE 4. Selected measured cross sections of flow for lahars PC 1 and PC 3 of Pine Creek age. A, Near downstream 

end of flow island near Alder Creek. B, Near Pullen Creek. C, Near Kid Valley.
MAGNITUDE OF LAHARS B15

The flow velocities of the huge lahars can also be 

determined from general relations (fig. 5) of velocity to 

depth, or velocity to the product of slope (S) and 

hydraulic radius (R) in the Manning equation. Hydraulic 

radius is the channel area divided by the wetted 

perimeter, which is twice the depth plus the width. Both 

relations are plotted in figure 5, where the velocitydepth relation appears with little scatter. The plot of 

velocity versus the product of R and S is based only 

on measurements of lahar PC 1. Figure 5 is a basis for 

estimating velocities at downstream locations where the 

flow cross sections, but not velocity, can be determined. 

These estimated velocities may then be used in estimating the discharges of large flows at such locations.

The discharges calculated for lahars PC 1 and 3 (table 

3), and based on measured cross sections and on 

calculated and estimated velocities, are good indicators 

of the truly catastrophic sizes of those flows. The instantaneous peak discharge of lahar PC 1 is equal to 

the discharge of the midcourse Amazon River at flood 

stage (Oltman and others, 1964), or to more than three 

times the largest recorded flood peak of the Mississippi 

River. Although lahar PC 3 attained an estimated 

discharge less than one-fourth that of PC 1 upstream, 

its inundation level locally approached and may have 

locally exceeded that of PC 1 in downstream reaches, 

as it flowed on top of the voluminous deposits of the 

earlier flow. However, the inundation levels mapped by 

Scott and Janda (1982), as shown in figure 1, predominantly reflect the levels attained by lahar PC 1.

The largest discharge determined for any flow at 

Mount St. Helens is over 1 million m3/s for the proximal phase of the 1980 lahar in the South Fork Toutle 

River (Scott, 1988). That discharge, however, applies to 

the dispersed phase of that flow, which is best described 

as a lithic pyroclastic surge. It is, therefore, not directly 

comparable to the discharges discussed here.

FLOW MAGNITUDES FOR PLANNING PURPOSES

The magnitude of a lahar with a recurrence interval 

of slightly less than 100 years (see section on frequency) would be sufficient to inundate nearly all flood plains 

throughout the Toutle River system to a probable depth 

of at least 2 m (mean flow depth of overbank lahars in 

table 3, excluding lahar PC 1, is 2.7 m). This depth corresponds to a peak discharge of at least 10,000 m3 on 

the broad flood plain near the confluence of the forks 

of the Toutle River. This figure is somewhat larger than 

estimates of design discharges for lahars based only on 

hypothetical interactions of pyroclastic flows with 

snowpack, as described in the section on origin of the 

lahar series of Pine Creek age. The variety of lahar

1980 North Fork Lahar 

(In confined channel)

X 1980 North Fork lahar-runout flow 

(in confined channel)

3 45678 910

FIGURE 5. Lahar flow velocity versus flow and channel properties: 

A, Velocity in relation to mean depth. B, Velocity in relation to 

p2/3gi/2 wnere R = hydraulic radius and S = slope. Position of 

regression line in B is based on lahar PC 1; slope is that shown by 

Pierson (1985) for the same relation and is also nearly the same as 

in A.

origins in both forks of the river (Scott, 1988) shows that 

large lahars can form in other ways.

The true significance of lahar hazards in the river 

system is apparent from a comparison with flood flows 

of similar recurrence interval. For example, the U.S. 

Army Corps of Engineers (1982, table B-l) determined 

that a flood with a recurrence interval of 100 years 

would have a peak discharge of 1,810 m3/s at Highway 

99, near the mouth of the Toutle River, and a peak discharge of 2,880 m3/s at Castle Rock in the Cowlitz 

River below the confluence with the Toutle River. At 

upstream stations in the Toutle River, flood discharges 

of the same recurrence interval would be less. Both flood 

discharges reflect posteruption changes in the rainfallrunoff regime.

The emphasis in this report is on lahar magnitudes 

at the forks of the Toutle River, more than 50 km 

downstream from the 1980 crater. Of the lahars and
B16 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

lahar-runout flows listed in tables 1 and 3, at least 35 

inundated flood-plain surfaces in that vicinity. At least 

six of the flows reached the Columbia River as lahars 

or lahar-runout flows large enough to cause serious inundation there, more than 100 km downstream.

In hydrologic analysis, the use of the term "worstcase" is not good practice because it is always possible 

to have a flow that is worse than those recorded. In this 

example, however, the spectrum of flow magnitudes and 

origins is known throughout the entire history of the 

volcano, as are the worst possible lahar-forming conditions. Consequently, the term is used in the following 

discussion with a connotation more valid than usual. 

Two extreme or worst-case lahars are described: (1) a 

lake-breakout lahar and (2) a smaller lahar derived from 

a slope failure. The first of these has a probability that 

will increase over the long term; the latter is more probable over the short term because of remedial engineering works that have lowered lake levels or stabilized 

their outlets.

Lahar PC 1 of Pine Creek age is clearly the worst to 

be considered for design purposes, because it very probably originated as a flood surge released by breaching 

of a natural dam impounding an ancestral Spirit Lake. 

(See Scott, in press.) Spirit Lake is the largest source 

of water available, or potentially available, to form a 

lahar. A natural dam consisting of debris-avalanche 

deposits has blocked the natural outlet of the lake since 

May 18, 1980. In 1985 a drainage tunnel lowered and 

stabilized the level of the lake, and thereby eliminated 

the immediate risk of a lahar like PC 1 being caused by 

overtopping of the dam by precipitation runoff. With 

time, however, the potential for future volcanic activity will increase the possibility of a PC 1 analog. A large 

flow of volcanic origin entering the lake could displace 

sufficient water to overtop the dam or cause a piping 

failure of the avalanche deposit. Extrapolations of 

catastrophic phenomena based on rate of dome growth 

and the behavior of other volcanoes are made by Glicken 

(1986). Given the history of the volcano and the lengths 

of past eruptive periods, the probability of renewed explosive activity within a 100-yr time span is significant.

If the risk of lake-breakout lahars is reduced by the 

present engineering works, however, the magnitude of 

the extreme or worst-case lahar is also reduced. In the 

last 4,500 years, the largest lahar thought to have 

originated through slope failure is the widely distributed lahar of Smith Creek age (unit 17 of Coal Bank 

Bridge section; appendix). That lahar is linked to slope 

failure because it was texturally similar to the 1980 

North Fork lahar: it had a relatively low lahar-bulking 

factor and relatively high clay content (Scott, 1988). The 

flow of Smith Creek age had a peak discharge estimated 

at 25,000 m3/s (table 3). Although much smaller than

lahar PC 1, it was approximately 4 times the size of the 

1980 North Fork flow. A flow of this size would inundate flood plains throughout the Toutle-Cowlitz River 

system. The inundation would occur even if the Corps 

of Engineers were to greatly enlarge the channel capacity in the Cowlitz River.

A lahar having the size and texture of the Smith 

Creek flow would behave much like the 1980 North Fork 

lahar, which had a lower rate of attenuation than the 

more typical flows with a lower clay content. Peak 

discharge of the 1980 North Fork lahar, about 

7,000-8,500 m3/s near Camp Baker, only declined to 

6,000-6,500 m3/s on the lower Toutle River (Fairchild 

and Wigmosta, 1983). The older flow, like its 1980 

textural analog, did not transform to a lahar-runout 

flow. Even the huge lahar PC 1 began to transform in 

the lower Toutle River; it was granular and formed 

mainly of stream alluvium bulked into a flood surge.

With time, however, the extreme or worst-case lahar 

will again become a PC 1 analog. A lahar the size of PC 

1 would overtop the terrace of the lahar-emplaced fill 

of Pine Creek age; this fill locally occupies much of the 

valley. The terrace surface is well populated throughout 

the river system; on it are built significant parts of the 

communities of Toutle, Silver Lake, Tower, and Castle 

Rock. Lahars equivalent to the smaller Smith Creek 

flow would only locally inundate the terrace, by an 

amount dependent on the amount of Pine Creek-age fill 

locally remaining. The terrace was overtopped in some 

places by the 1980 North Fork lahar downstream from 

Camp Baker and, downstream from the Green Mountain Mill, the terrace surface was in places less than 

2.5 m above the peak stage of that flow.

Two factors presently influence a risk analysis of 

lahars in the river system. One is the unknown stability of the blockage of Castle Lake (fig. 1), one of the 

avalanche-impounded lakes (Meyer and others, 1985). 

A combination of elevated ground-water levels and an 

earthquake with a seismic coefficient one-half that of 

the maximum credible earthquake could cause substantial deformation of the blockage. Even under those circumstances, however, failure cannot be assumed. 

Should the blockage fail, though, whether the result 

would be debris flow or hyperconcentrated streamflow 

depends on the size of the initial flood wave, the degree 

of saturation of the downstream debris avalanche, and 

the amounts of downstream bulking and debulking. The 

size of any such flood wave would depend strongly on 

the width of breaching and the time of development. 

Preliminary modeling of a flood wave that could result 

from a failure indicates a rapidly attenuating hyperconcentrated flow that would inundate flood plains 

throughout at least the Toutle River system (A. Laenen, 

USGS, oral commun., 1986).
RECURRENCE INTERVALS OF LAHARS AND LAHAR-RUNOUT FLOWS B17

The second factor influencing short-term decisions is 

a large sediment-retention structure being constructed 

at a site near Pullen Creek on the North Fork Toutle 

River. The structure also will function to impound 

lahars and lahar-runout flows for an unknown period. 

Reservoir capacity will not be maintained under present 

plans, and the reservoir will fill progressively. No 

impoundment structure is planned for the South Fork 

Toutle River, a prolific source of ancient lahars as well 

as two lahars in 1980.

THE NATURE OF LAHAR HAZARDS

This study's principal conclusions regarding lahar 

magnitude are (1) that a lahar of 100-yr frequency would 

inundate modern flood plains and low terraces throughout at least the Toutle River system and (2) that either 

extreme or worst-case lahar described above would inundate all flood plains and most terraces throughout 

the Toutle-Cowlitz River system. Both the highfrequency lahar and the larger worst-case flow would 

evolve distally into lahar-runout flows. The smaller 

worst-case lahar, derived from a slope failure like the 

lahar of Smith Creek age and the 1980 North Fork lahar, 

would not.

Extensive observations of damage from the 1980 

lahars indicate that most of the damage from a lahar 

occurs upon initial inundation of the flood-plain surface. 

Further increase in depth does not necessarily increase 

damage proportionally. Where the 1980 North Fork 

lahar inundated flood plains, trees were killed (but later 

salvaged), structures were destroyed, and most agricultural uses of the flood plain were lost at least temporarily. Most houses inundated by the lahar were 

salvaged for materials, even those that weren't damaged structurally (such as the mobile homes that floated 

in place to the surface of the deposits in areas of passive 

flow on the flood plain).

Timber floating in the lahar caused extensive structural damage, and the lahar deposits permeated all 

parts of inundated structures, including interior walls. 

The main source of the timber in the flow was flood 

plains adjacent to active channels, where trees were 

removed by the high-velocity channel flow. (See fig. 32, 

Scott, 1988). Felled trees stored at Camp Baker were 

also incorporated.

Even if the possibility of lake-breakout lahars were 

eliminated by the stabilization of the avalancheimpounded lakes, damage from the smaller worst-case 

lahar would approach that caused by the larger. The 

smaller of the two would still inundate flood plains and 

parts of the main terrace surface, and the reduction in 

damage would not be proportional to the reduction in 

size.

The risk of lahars is only partly reduced by engineering stabilization of the modern avalanche-impounded 

lakes. The debris avalanche remains as a potential 

source of sediment for any meltwater surge from the 

volcano. The lahar bulking factors of the lake-breakout 

lahars (Scott, 1988) show that bulking of stream 

alluvium is sufficient to transform a flood surge to a 

lahar, probably without other sediment sources. 

Moreover, rocks forming the flanks of the volcano 

within the South Fork Toutle River drainage could be 

mobilized into a catastrophic flow by explosive activity less violent than the eruption of May 1980.

RECURRENCE INTERVALS OF 

LAHARS AND LAHAR-RUNOUT FLOWS

As shown in the preceding section, lahars large 

enough to inundate flood plains are of critical magnitude and are recorded by a significant thickness of 

deposits of the lahar flood-plain f acies. Such lahars are 

thus the object of frequency analysis (tables 1 and 3). 

To be most useful, a frequency analysis need consider 

two levels of probability, one based on the total distribution of flows and the other on the frequency of flows 

within eruptive periods. The latter thus is conditional 

on an active eruptive state, like the present. An analysis 

based on only the total distribution of flows would obviously underestimate the existing potential, but would 

be appropriate for the volcano in a quiescent state.

Evidence presented below indicates that, after one 

lahar has occurred, the probability of others is significantly increased. The occurrence of lahars is thus less 

random than the occurrence of precipitation-induced 

floods. In a normal flood frequency study, the occurrence of one major flood does not necessarily increase 

the probability of a second large flood beyond the 

relatively small statistical effect of adding a large flow 

to the total time series.

The number of flows inferred from the stratigraphic 

record provides a minimal estimate of the actual 

number. For example, flows of Castle Creek age are less 

likely to be recorded because of the dominantly erosional regime that followed emplacement of the Pine 

Creek-age deposits. Although the succession of flows 

away from the volcano mainly coincided with the eruptive chronology as determined by Mullineaux and 

Crandell (1981), the durations of the episodes cannot be 

considered exact.

The recurrence interval of an overbank lahar or laharrunout flow varies with the length of the record that 

is analyzed. The recurrence interval of a lahar or laharrunout flow that extends overbank at the confluence 

of the forks of the Toutle River, within the eruptive
B18 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

episodes of the last 13,700 years, is 247 years (based 

on 26 overbank flows during 6,414 years considered to 

be within eruptive episodes; table 1). For the eruptive 

periods of the last 4,500 years, the comparable recurrence interval is 128 years. If the Castle Creek period 

is excluded, for the reason previously mentioned, the 

recurrence interval within eruptive periods of the Spirit 

Lake eruptive stage is 94 years.

Thus, given the present eruptive state of Mount St. 

Helens, the indicated recurrence interval of an overbank 

flow at the confluence of the forks of the Toutle River 

would be about 100 years (an exceedence probability of 

about 1 percent). However, a shorter interval seems 

more probable, based on the likely occurrence of lahars 

not preserved or exposed and on the evidence of grouping of lahars discussed in the following section. The 

shortening of the recurrence interval, as only recent 

eruptive periods are considered, is more likely due to 

a lack of exposure of older flows than to an increasing 

frequency of flows. It is also due to a less precise definition of the older eruptive episodes.

Because of the change in composition of the eruptive 

products of Mount St. Helens at the close of Pine Creek 

time, the frequency or type of lahars may also have 

changed. Factors that support a uniformitarian extrapolation of past events to the future are (1) the essential comparability of each 1980 flow to one or more 

pre-modern-cone lahars and (2) the present potential for 

a sequence of lake-breakout lahars analogous to that of 

Pine Creek age. Crandell and Mullineaux (1978) 

presented a map of volcanic deposits of the last 4,500 

years the period for which design recurrence intervals 

were calculated above as a guide to hazard potential; 

they selected this time period partly because they 

believed it likely to represent future volcanic activity.

Long-term planning (tens of years) can employ a 

100-year lahar recurrence interval in the same way that 

a design-flood recurrence interval is used for the purposes of zoning on flood plains or for benefit-cost appraisals of engineering works. Similarly, decisions for 

the terrace surface underlain by the Pine Creek-age sequence can reflect the magnitudes of the worst-case 

lahars discussed above. Planning for some structures 

well above the Pine Creek terrace surface might also be 

affected by the projected worst-case lahar; for example, 

bridges and highways along major evacuation routes 

would need to remain open during the highest possible 

flow. As in most analyses of extreme events, a meaningful recurrence interval cannot be assigned to the 

worst-case flows.

EVIDENCE OF TIME GROUPING OF LAHARS

The conservative approach to lahar hazards advocated here is warranted by evidence that lahars do

not occur randomly within the established eruptive 

periods. The occurrence of one lahar increases the odds 

of a recurrence above those based on the overall recurrence intervals in eruptive periods. This change in probability cannot be calculated because the time intervals 

involved are too short for accurate definition by 

radiometric dating.

One line of evidence, based on tree molds, shows that 

a runout flow and the four lahars of Pine Creek age occurred within a few decades, rather than having been 

distributed throughout the 500-year span of the eruptive period. Tree molds (fig. 6) penetrated all four lahars, 

and occur as vertical cylindrical shafts open at the surface. They are known locally as tree "wells." They are 

especially common in the triangle formed by the forks 

of the Toutle and a north-south line transecting the 

Green Mountain Mill. Depths reportedly exceed 25 m, 

but I measured none in excess of 10 m, a depth close 

to the probable maximum total thickness of the floodplain facies of the Pine Creek lahar series. Wood within 

the lahar sequence has been totally removed by 

weathering, but the underlying stumps are preserved 

within the basal runout deposit and reworked runout 

sands.

The interior surfaces of some freshly exposed tree 

molds have a pattern indicating that bark remained intact throughout emplacement of the Pine Creek lahar 

series. Based on the normal decompositional evolution 

of a standing old-growth Douglas fir (Franklin and 

others, 1981, fig. 16), this indicates that the entire sequence was emplaced in less than 20 years.

A second line of evidence indicates an even shorter 

time interval for emplacement of the Pine Creek lahars. 

The stratigraphic contacts are sharp and nearly planar 

where there was no primary relief. Locally the units are 

separated by thin intercalations of sand, deposited by 

dewatering of the flow or by the first runoff after 

emplacement, but no traces of vegetation are found 

along the contacts. By 1985, however, the flood-plain 

facies of the 1980 lahars were covered with a nearly impenetrable growth of alders. This evidence suggests 

that the entire sequence of Pine Creek age was emplaced 

within several years or less.

This second type of evidence applies to the lahars of 

Swift Creek and Kalama age as well, but generally in 

lesser degree. The sequence of Swift Creek units in the 

Coal Bank Bridge section (appendix) contains lahars 

with sharp contacts indicating a very close time association, although the channeled base of unit 8 is a possible exception. The original exposures at that locality 

revealed contacts over distances of 100 to 200 m. Some 

of the sharp contacts may result from the overlapping 

of approximately synchronous flows from each fork of 

the Toutle River. The original exposures of the Kalama
LAKE-BREAKOUT LAHARS OF THE PINE CREEK ERUPTIVE PERIOD B19

FIGURE 6. Vertical tree mold penetrating flood-plain fades of lahars 

PC 1 and PC 3, of Pine Creek age, near the Green Mountain Mill 

section. Shovel is inserted at the contact between those lahars. Lahar 

PC 2 is missing at this point. Horizontal trunk mold at upper left.

sequence at Kid Valley in the North Fork Toutle River 

were not as extensive as those of the Swift Creek 

sequence, but they showed the same stratigraphic 

relations.

CHARACTER OF THE ERUPTIVE EPISODES

All the lines of evidence described above indicate that 

large lahars formed during relatively small parts of the 

eruptive periods and stages. This in turn strongly suggests that large eruptions were likewise concentrated, 

and that the actual eruptive periods could be both more 

numerous and shorter than the periods (table 1) used 

to calculate the lahar recurrence intervals. This interpretation implies that the probability of a lahar within

an eruptive episode like the modern period is higher (the 

recurrence interval is lower) than that calculated on the 

basis of the durations of the periods and stages as 

shown in table 1. That is, once a lahar has occurred, 

more are likely to follow.

Apparent concentrations of eruptive activity were one 

reason leading Crandell (1987) to designate the three 

oldest eruptive periods Ape Canyon, Cougar, and 

Swift Creek as eruptive stages, and to group the 

younger periods in a single eruptive stage, the Spirit 

Lake stage. The evidence from the lahar sequences supports this approach, so Crandell's terminology is used 

here. Implicit in this terminology is the probability that 

the three older eruptive stages incorporate eruptive 

periods yet to be recognized.

The lengths of the eruptive stages and periods (table 

1) are in part a function of the largely unavoidable 

spread in radiocarbon ages (as illustrated by table 2) due 

to sample and sampling variations, as well as to variations from changes in atmospheric radiocarbon. Such 

changes were particularly pronounced during the Pine 

Creek and Kalama eruptive periods. Another factor is 

the removal by weathering of wood from virtually all 

lahars of pre-Kalama age beyond the base of Mount St. 

Helens; by necessity, then, radiocarbon dates are 

established for wood from adjacent strata that are older 

or younger than the lahars (albeit only slightly). This 

factor adds further uncertainty to the ages of the lahars.

LAKE-BREAKOUT LAHARS OF THE 

PINE CREEK ERUPTIVE PERIOD

The four lahars of Pine Creek age, including the two 

largest lahars in the history of the watershed, apparently originated through lake breakouts resulting from the 

failure of natural dams (Scott, in press). Large lahars 

also formed this way in the Lewis River, south of Mount 

St. Helens (Newhall, 1982; Major and Scott, 1988). The 

engineering works that stabilize the natural dams 

created by the 1980 debris avalanche in the North Fork 

Toutle River thus reduce the chances of an analogous 

modern sequence of flows. Although failure as a result 

of precipitation inflow to the lakes has been eliminated, 

the injection of a large volcanic flow into Spirit Lake 

could cause it to overtop and destroy its natural dam. 

If the risk of lake-breakout lahars were eliminated completely, the lahars of Pine Creek age could be removed 

from the time series from which the recurrence intervals were calculated. This would, in effect, increase the 

recurrence interval (thereby decreasing the probability) 

of a lahar or lahar-runout flow large enough to inundate 

flood plains 50 km away during the last 4,500 years 

from about 100 years, to approximately 130 years. This
B20 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

decrease in probability, however, may not compensate 

for the increase required by the close time association 

of lahars.

EVIDENCE OF LAKE-BREAKOUT ORIGIN

Evidence that the sequence of Pine Creek lahars was 

derived from lake-breakout flood surges includes (I) the 

great magnitude of the initial lahar relative the lesser 

magnitudes attributable to other origins; (2) a high content of alluvium introduced into the flood surge beyond 

the base of the volcano; (3) abundant megaclasts derived 

from a debris avalanche that may have blocked the 

drainage of the North Fork Toutle River; and (4) occurrence in a time-clustered sequence.

COMPARISON OF MAGNITUDES OF 

LAHAR PC 1 AND LAHARS OF OTHER ORIGINS

A common cause of volcanically induced flood surges 

is the melting of a thick snowpack by pyroclastic flows, 

tephra, and other hot, explosively distributed eruptive 

products. The high pumice content of the flows of Ape 

Canyon age indicates that this was probably a common 

mode of origin during that time. Subsequently, snowmelt resulting from lithic pyroclastic flows has probably 

accounted for many lahars of intermediate and small 

size in the river system (Scott, 1988).

As part of a preliminary analysis of hazards in the 

river system, Dunne and Leopold (1981) calculated the 

potential size and rate of attenuation of mudflows 

resulting from various sizes of pyroclastic flows over 

snow. The most extreme conditions of snow accumulation and width of pyroclastic flow yielded a peak 

discharge of 3,900 m3/s at Kid Valley. The calculation 

reflected the present topography of the volcano, representing optimal conditions for discharge into the North 

Fork Toutle River. Peak discharge of the 1982 lahar, 

which resulted from eruption-induced snowpack melting 

in the crater, was 960 m3/s (Pierson and Scott, 1985) at 

the same location.

The U.S. Army Corps of Engineers (written commun., 

1984) used as a design criterion an "operating basis 

mudflow" with a "most likely" peak discharge of 

7,100 m3/s upstream from Camp Baker. The likelihood 

of the event was considered "very small." The calculation assumed a pyroclastic flow affecting an area of 

13 km2 and melting of the entire underlying snowpack 

within "minutes." Also considered was the idea of a 

"maximum mudflow," defined as the "most severe 

mudflow believed to be possible * * * in the foreseeable 

future on the basis of geologic and volcanic evidence."

This flow had a peak discharge upstream from Camp 

Baker of 42,500 to 113,000 m3/s, and also would have 

attenuated downstream. It assumed a pyroclastic flow 

that would melt all the snow over an area of 45 km2 

and occur at the time of maximum snow accumulation. 

The above hypothetical flows are small compared to 

the peak discharge of lahar PC 1 (table 3 and fig. 7). 

Catastrophic ejection could have formed a large lahar, 

but the flow would have a low lahar bulking factor 

(LBF), unlike lahar PC 1 (Scott, 1988). By far the most 

likely source of the flood surge that transformed to that 

lahar was ancestral Spirit Lake, impounded by a natural 

dam at or downstream of the pre-1980 outlet. The most 

probable cause of the surge is failure of the natural dam, 

but lake water could have been displaced by a volcanic

o 

o

OQ;

pt- LL.

300,000

250,000

200,000

O

m 150,000

O

100,000

50,000

BULKING 

TO LAHAR LAHAR DEBULKING TO LAHAR-RUNOUT FLOW

Modeled lahar from

modern breakout of

Spirit Lake

LAHAR PC 1 

(Spirit Lake breakout)

Modeled flood from

modern breakout of

Spirit Lake

/1980 North Fork lahar 

9 I________I

10 20 40 60 80 100 200 

DISTANCE FROM CRATER, IN KILOMETERS

FIGURE 7. Peak discharges of lahar PC 1, a modeled lahar from a 

modern breakout of Spirit Lake, and a modeled flood flow from a 

modern breakout of Spirit Lake. Peak discharge of the relatively 

high-clay (3-5 percent) North Fork lahar is shown for comparison.
NATURAL DAMS AT VOLCANOES B21

flow into the lake. The existence of a summit crater lake 

in Pine Creek time cannot be entirely ruled out, but such 

lakes are rare at modern Cascade Range stratovolcanoes. Craters now filled or partly filled with significant amounts of glacial ice are present at Mount 

Rainier, Wash., and South Sister, Oreg. Neither of these 

craters, however, contains more than a tiny fraction of 

the water needed to create a lahar comparable in size 

to PC 1.

IMPLICATIONS OF LAHAR BULKING FACTORS

The two largest lahars in the Pine Creek sequence, 

PC 1 and PC 3, have high LBF's (Scott, 1988). The two 

smaller flows also have high LBF's, although this conclusion is based on field comparisons of clast roundness 

with that of the larger flows. Thus the sequence was 

formed by transformation of flood surges to lahars 

beyond the base of the volcano. In the case of lahar PC 

1, the deposits record this transformation only after 

almost 20 km of flow from the north side of the volcano. 

Comparable LBF's are rare in the entire record of lahars 

in the Toutle-Cowlitz River system, indicating the 

uniqueness of the Pine Creek lahars in this respect.

MEGACLASTS

Masses of hydrothermally altered dacite breccia (fig. 

8.4 and B) are common in lahar PC 1, and locally are 

sufficiently abundant that they form a diamicton in 

which the laharic diamicton acts as the matrix. Locally the clasts form surface mounds. The megaclasts are, 

with the exception of the brecciation, like the rocks of 

pre-Castle Creek age exposed in the core of the modern 

crater. The dacite megaclasts are as much as 8 m in exposed intermediate diameter, and originated as blocks 

from an ancient debris avalanche. The evidence for this 

conclusion is described in Scott (in press). The abundance of the dacite megaclasts suggests that the natural 

dam that failed, giving rise to the PC 1 flood wave, had 

that origin.

The only flow of pre-Pine Creek age in the river 

system with any similar megaclasts is the rubbly lahar 

of Swift Creek age (unit 8 of the Coal Bank Bridge section; appendix), in which two megaclasts of hydrothermally altered, brecciated dacite were observed. This 

flow is notable for the angularity of its clasts, so it cannot be demonstrated to have begun as a flood surge.

A second, much rarer variety of megaclast consists 

wholly or partly of alluvium. The megaclast of figure 

8C, for instance, was more than 6 by approximately 12 

m in original exposure and contains five stratigraphic 

units, as listed in the figure caption. Units 1 through 

4 represent strata from the flood plain in the broad

valley upstream from the community of St. Helens that 

predate lahar PC 1. The flood-plain surface, represented 

by the upper surfaces of units 3 and 4, was inundated 

by lahar PC 1, whose deposits form unit 5 in the 

megaclast. As lahar depth increased, the block was 

detached, probably by lateral erosion of a channel cutbank. As indicated by their textural identity, unit 5 is 

probably an early deposit of lahar PC 1. The alluviumbearing megaclast "grounded" on numerous dacite 

megaclasts in the flow, tilting the delicate mass upstream at an angle of 7 degrees. The rafted deposits of 

the enveloping flow, which form unit 5, may have been 

deposited before detachment, or later as the mass was 

riding in the lahar. They clearly were part of the megaclast as seen in original exposures, and deposition before 

detachment is considered more likely.

The sequence of events recorded by the megaclast illustrates how the large volume of valley-fill alluvium 

was incorporated into the Pine Creek flows. That the 

mass probably was not transported far is shown by the 

fragile nature of the alluvium (it crumbles to the touch). 

Most masses of eroded alluvium probably disintegrated 

immediately, dispersing their components into the flow. 

The alluvium in the megaclast was preserved only 

because it was "sandwiched" between the two lahars 

(units 1 and 3). Estimates of the volume of the flood 

wave and the erosion depth necessary to form lahar PC 

1 are discussed by Scott (1988 and in press).

NATURAL DAMS AT VOLCANOES

Considered together, the above evidence indicates 

that the flood surges that transformed to the lahars of 

Pine Creek age were formed by breaches in a natural 

dam or dams. A remarkably similar sequence of events 

probably would have followed emplacement of the 1980 

debris avalanche, except for human intervention. The 

abundance of dacite megaclasts in lahar PC 1 suggests 

that at least the initial natural dam may have been a 

debris avalanche, as in 1980. The geometry of the apron 

of volcanic and volcaniclastic deposits, where it abuts 

Harrys Ridge north of the peak, is conducive to the creation of natural dams. Spirit Lake had formed behind this 

apron of deposits by Smith Creek time (Crandell, 1987). 

On May 18,1980, the outlet of Spirit Lake was blocked 

by as much as 170 m of debris avalanche deposits, and 

the level of the lake was catastrophically raised by 63 m 

(Youd and others, 1981; Meyer and others, 1986).

Debris avalanches are especially likely to create large 

and deep lakes impounded by unstable dams because 

the avalanches are large (commonly more than 1 km3 ; 

Siebert, 1984) and mobile (median ratio of vertical 

descent to lateral distance = 0.11; Siebert, 1984), and
B22 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

their deposits are thick and hummocky. Debris flows 

may be formed at volcanoes by breakouts through other 

types of natural dams, including those formed by landslides, large pyroclastic flows (especially lithic types),

.-."' ,' /"-'' -.'.>'/,;.' ';:''' ' '> -'':-y-'-:^^::V^-;-H pc 4-;

lahars, rubbly lava flows, glaciers, moraines, and rock, 

ice, or snow avalanches. Some types of blocking 

deposits may be rapidly eroded and leave little evidence 

of their presence except for megaclasts in downstream 

debris flows.

A thick fill of pyroclastic flows and lahars of Pine 

Creek age in the Castle Creek valley extended into the 

valley of the North Fork Toutle River, where individual 

flows could have dammed the main drainage and then 

failed, releasing flow that formed lahars downstream 

(Crandell, 1987). The width of the valley near Castle 

Creek, the reestablishment of drainage between flows, 

and the fluid nature of both pyroclastic flow and lahar 

deposits all suggest that this fill could have been a 

source of multiple lahars, but none so large as the gigantic flows many kilometers downstream in the Pine Creek 

sequence. Flood surge release through pumiceous 

pyroclastic flow deposits would have yielded lahars with 

a characteristic pumice content not seen in large lahars 

later than Ape Canyon in age (although much pumice 

was incorporated in the small lahars formed by pumiceous pyroclastic flows during eruptions in 1980-81). 

Mobile lithic pyroclastic flows would have behaved 

much like lahars in their ability to block stream channels. As discussed in the next section, the surge released 

from a lahar blockage is likely to be of much lesser 

magnitude than the downstream continuation of the 

primary flow. Relatively fluid flows like lahars flow into 

lateral backwater areas, leaving broad, shallow deposits 

with gentle lateral slopes.

LAHAR-MARGIN LAKES MODERN AND ANCIENT

The inundation area of lahar PC 1 mapped by Scott 

and Janda (1982) was verified by the presence of small,

SCALE

0 1 m

FIGURE 8. Megaclasts in lahar PC 1 as exposed near the confluence 

of the North Fork Toutle River and Pullen Creek. A, Farthest 

downstream exposed dacite megaclast (da), resting on channel surface and extending above the surface of lahar PC 1. B, Dacite 

megaclast (da) that may have floated on the surface of lahar PC 

1, although the most recent exposures suggest that it may have 

grounded on a clast-supported diamicton of dacite megaclasts. C, 

Megaclast containing pre-lahar stratigraphic section of flood-plain 

stratigraphy, comprising the following units:

5. Lahar; cobble mode with high degree of roundness; 

identical in both roundness and texture to the incorporating lahar ........................ >1.8 m

4. Alluvium; sandy, stratified, fills channel cut mainly in unit 3 ............................. 1.1 m

3. Lahar; pebble mode, but contains cobbles and

boulders ................................ 1.1 m

2. Alluvium; cobble mode, size distribution and roundness shown in Scott (1988, fig. 25) .......... >1.2 m

1. Lahar; similar in texture and clast angularity to the

largest lahar of Smith Creek age ........... X).8 m
NATURAL DAMS AT VOLCANOES B23

slightly infilled lakes on or adjacent to the lateral surface of the peak flow deposits. These lakes, now mainly existing as marshy areas or ponds surrounded by 

marshes, were originally very shallow features 

analogous to the small lakes that existed temporarily 

along the margins of the 1980 North Fork and South 

Fork lahars. Most of the modern lakes did not exceed 

1 m in depth and were at least partly drained as runoff 

from the lateral tributaries filled the depressions and 

caused breaching. None yielded a discharge of any 

consequence.

The behavior of the 1980 lahars suggests that they 

were not capable of creating significant blockages and 

lake impoundments at the mouths of tributary streams. 

Their behavior varied greatly according to size of the 

tributary. The brief impoundment of the Cowlitz River 

by the 1980 North Fork lahar is described in Scott 

(1988). Similar brief blockages occurred at the confluence of the Green and North Fork Toutle Rivers and 

at the confluence of the forks of the Toutle River.

Silver Lake, formed as the result of blockage by the 

Pine Creek lahars, is the exception to the generally small 

size of lahar-margin lakes. The lake has a surface and 

lateral marsh area of more than 15 km2. However, with 

a maximum measured depth of little more than 3 m, the 

feature is an analog of the small lahar-margin lakes. It 

seems unlikely that downcutting of the broad natural 

dam has ever been rapid enough to yield a significant 

surge. In fact, before construction of a weir and drainage channel in 1971, a serious problem for lakeshore 

residents was the inability of the lake to drain rapidly 

through its weed-choked natural outlet after storm inflow. Wind-generated waves in combination with the 

sustained high water level periodically caused damage. 

A modern lake-breakout lahar from Spirit Lake possibly 

could raise the level of the water in Silver Lake, thereby 

adding lakeshore flooding to the list of hazards associated with such a lahar.

COMPARISON OF LAHAR PC 1 

WITH MODELED FLOWS

The peak discharges determined for lahar PC 1 (fig. 

7), which resulted from an ancient breakout of Spirit 

Lake, can be used to assess the accuracy of model 

discharges resulting from hypothetical modern breakouts of the lake. An ideal model should simulate the actual PC 1 lahar, yet the comparison shows that the 

behavior of that lahar was significantly different from 

that predicted by any of the three models to date.

Between 30 and 50 km from the volcano, the depths 

and inundation levels of lahar PC 1 (Scott and Janda, 

1982) and of a hypothetical modern lahar from a breakout of Spirit Lake (Swift and Kresh, 1983) are nearly

the same. Both levels are plotted by the Cowlitz County Department of Community Development (1983). 

Flow dynamics and the subsequent downstream attenuation are very different, however. Mean velocities 

of the modeled lahar are much less than those of lahar 

PC 1, and the rate of attenuation is lower. Velocities 

of the modeled flow are locally similar to or even less 

than those of the 1980 North Fork lahar (6 to 12 m/s; 

Fairchild and Wigmosta, 1983). The latter lahar was exceptional in its origin and its relatively high clay content (Scott, 1988). The differences in behavior between 

lahar PC 1 and the modeled analog arise partly because 

the clay content of the 1980 flow was used for comparison (see Scott, in press), and the model, therefore, 

assumed too much internal flow resistance. The 

behavior of the ancient lahars in the river system indicates that a modern lahar derived from a Spirit Lake 

breakout would be granular in character and would contain less than 3 percent clay (Scott, in press). That clay 

content is the approximate transformation-limiting clay 

content of lahars in the river system. Consequently, an 

actual flood wave would have undergone the same series 

of transformations shown in figure 7. It would not 

behave like the uniform, relatively unchanging mudflow 

represented by the model.

A second model of a modern Spirit Lake breakout 

flood surge projects that the surge would not generate 

sediment concentrations above 15 percent by volume 

(Bissell and Hutcheon, 1983). This concentration is 

below the hyperconcentrated range. The largest of 

several modeled flood waves is relatively small (fig. 7). 

This model has no similarity with the real-life event 

represented by lahar PC 1.

The third model assumed the formation of hyperconcentrated flow, but no lahar, from a modern Spirit 

Lake breakout (Sikonia, 1985). This assumption was 

based on the mistaken idea that the only sediment 

available to the flood surge would be from nearly 

saturated deposits of the debris avalanche. The high 

LBF of lahar PC 1 shows the assumption to be 

unrealistic, even in a case where transformation of the 

modern flood surge to a debris flow was not completed 

on the debris avalanche. The LBF's of the Pine Creek 

lahars clearly indicate that bulking to a lahar would continue below the modern avalanche terminus.

Large lahars like PC 1 also behave differently from 

many of the small-scale mudflows described in the 

literature. The large lahars at Mount St. Helens are less 

viscous and have less cohesive sediment than the 

smaller flows. Also, their progressive mixing with perennial streamflow may be pronounced, leading to distal 

flow transformations (Scott, 1988). Such transformations probably occur in debris flows of alpine and 

semiarid environments but have not been reported. The
B24 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

transferability of data on these flows to the behavior 

and deposit characteristics of lahars is generally slight.

CONCLUSIONS

A lahar or lahar-runout flow originating at Mount St. 

Helens is the chief hydrologic hazard to be considered 

in land-use planning for the Toutle and lower Cowlitz 

Rivers, their flood plains, and their higher, laharemplaced terraces.

It is highly probable that a major lahar will inundate 

flood plains and, locally, terraces throughout the Toutle River system within the next 100 years, a typical 

time frame on which engineering and land-use decisions 

are based. The most extreme lahar is larger than the 

"maximum possible mudflow" calculated by the Corps 

of Engineers, in part because the risk of gigantic lakebreakout lahars like those occurring in the Pine Creek 

eruptive period, although now reduced by engineering 

works, will increase over the long term.

All previous attempts to model a hypothetical lahar 

or volcanically induced flood surge have addressed 

single origins either (1) meltwater from a pyroclastic 

flow or (2) breakouts of avalanche-impounded lakes. 

However, the other formative mechanisms for lahars 

documented by Scott (1988) cannot be ignored in the 

modeling and planning processes. In estimating probable future lahar types, hazards, and probabilities, the 

importance of the past behavior and record of flows at 

a volcano cannot be overemphasized.

The largest lahars in the river system have originated 

in the North Fork Toutle River. The 1980 crater 

debouches into that drainage and will probably be the 

source of numerous lahars of snowmelt origin in the 

modern eruptive period. The South Fork, however, has 

also been the source of many lahars throughout the 

history of Mount St. Helens. When explosive activity 

analogous to that of 1980 recurs, the steep volcano flank 

in the headwaters of that drainage may induce the large 

(though uncommon) lahars formed by catastrophic ejection or slope failure.

The most probable lahar will be of the granular, noncohesive variety and will undergo the transformations 

shown in figure 7. It will attenuate more rapidly than 

debris flow waves such as the 1980 North Fork lahar 

and the modeled lahar resulting from a modern Spirit 

Lake breakout. As a result, flood waves consisting of 

hyperconcentrated lahar-runout flow will probably constitute the chief hazard in the lower Cowlitz River. 

Models that simulate this behavior, in the distal part 

of a lahar flood wave, can yield the magnitudes most 

appropriate for design purposes along the lower Cowlitz 

River. The most typical lahar flood wave in the system

was a flood surge in both its proximal and distal 

portions.

REFERENCES CITED

Bissell, V. C., and Hutcheon, R. J., 1983, Forecasting and data collection preparedness for streams affected by the Mount St. Helens 

volcano: Portland, Oreg., U.S. National Weather Service Northwest River Forecast Center, 72 p.

Call, W. A., 1974, Soil survey of Cowlitz area, Washington: 

Washington, D.C., U.S. Soil Conservation Service, 112 p.

Cowlitz County Department of Community Development, 1983, 

Toutle-Cowlitz Watershed Management Plan: Kelso, Wash., 416 p.

Crandell, D. R., 1980, Recent eruptive history of Mount Hood, Oregon, 

and potential hazards from future eruptions: U.S. Geological 

Survey Bulletin 1492, 81 p.

____1987, Deposits of pre-1980 pyroclastic flows and lahars from 

Mount St. Helens Volcano, Washington: U.S. Geological Survey 

Professional Paper 1444, 94 p.

Crandell, D. R., and Hoblitt, R. P., 1986, Lateral blasts at Mount St. 

Helens and hazard zonation: Bulletin of Volcanology, v. 48, 

p. 27-37.

Crandell, D. R., and Mullineaux, D. R., 1978, Potential hazards from 

future eruptions of Mount St. Helens volcano, Washington: U.S. 

Geological Survey Bulletin 1383-C, 26 p.

Crandell, D. R., Mullineaux, D. R., Rubin, Meyer, Spiker, Elliot, and 

Kelley, M. L., 1981, Radiocarbon dates from volcanic deposits at 

Mount St. Helens, Washington: U.S. Geological Survey Open-File 

Report 81-844, 16 p.

Dunne, Thomas, and Leopold, L. B., 1981, Flood and sedimentation 

hazards in the Toutle and Cowlitz River system as a result of the 

Mount St. Helens eruption: Washington, D.C., U.S. Federal 

Emergency Management Agency, 92 p.

Fairchild, L. H., and Wigmosta, Mark, 1983, Dynamic and volumetric 

characteristics of the 18 May 1980 lahars on the Toutle River, 

Washington: Proceedings of the Symposium on Erosion Control 

in Volcanic Areas; Seattle and Vancouver, Washington, July 6-9, 

1982, Technical Memorandum of Public Works Research Institute 

(Japan), no. 1908, p. 131-153.

Franklin, J. F., Cromack, K., Jr., Denison, W., McKee, A., Maser, C., 

Sedell, J., Swanson, F., and Juday, G., 1981, Ecological 

characteristics of old-growth Douglas fir forests: U.S. Forest Service, Pacific Northwest Forest and Range Experiment Station, 

General Technical Report PNW-118, 48 p.

Glicken, Harry, 1986, Potential future volcanic activity at Mount St. 

Helens Impact on Spirit Lake: Abstracts, International Volcanological Congress, Auckland-Hamilton-Rotorua, New Zealand, 

February, 1986, p. 243.

Hyde, J. H., 1975, Upper Pleistocene pyroclastic-flow deposits and 

lahars south of Mount St. Helens volcano, Washington: U.S. 

Geological Survey Bulletin 1383-B, 20 p.

Kochel, R. C., and Baker, V. R., 1982, Paleoflood hydrology: Science, 

v. 215, p. 353-361.

Leopold, L. B., and Maddock, Thomas, Jr., 1953, The hydraulic 

geometry of stream channels and some physiographic implications: U.S. Geological Survey Professional Paper 252, 57 p.

Major, J. J., and Scott, K. M., 1988, Volcaniclastic sedimentation in 

the Lewis River valley, Mount St. Helens, WashingtonProcesses, extent, and hazards: U.S. Geological Survey Bulletin 

1383-D, 38 p.

Meyer, William, Sabol, M. A., Glicken, H. X., and Voight, Barry, 1985, 

The effects of ground water, slope stability, and seismic hazard 

on the stability of the South Fork Castle Creek blockage in the 

Mount St. Helens area, Washington: U.S. Geological Survey Professional Paper 1345, 42 p.
REFERENCES CITED B25

Meyer, William, Sabol, M. A., and Schuster, Robert, 1986, Landslide 

dammed lakes at Mount St. Helens, Washington, in Schuster, 

R. L., ed., Landslide dams Processes, risk, and mitigation: 

American Society of Civil Engineers Geotechnical Special Publication 3, p. 21-41.

Mullineaux, D. R., 1986, Summary of pre-1980 tephra-fall deposits 

erupted from Mount St. Helens, Washington State, USA: Bulletin 

of Volcanology, v. 48, p. 17-26.

Mullineaux, D. R., and Crandell, D. R., 1962, Recent lahars from 

Mount St. Helens, Washington: Geological Society of America 

Bulletin, v. 73, p. 855-870.

____1981, The eruptive history of Mount St. Helens, in Lipman, 

P. W., and Mullineaux, D. R., eds., The 1980 eruptions of Mount 

St. Helens, Washington: U.S. Geological Survey Professional 

Paper 1250, p. 3-15.

Newhall, C. G., 1982, A prehistoric debris avalanche from Mount St. 

Helens: Eos, v. 63, no. 45, p. 1141.

____1983, Anticipating volcanic hazards at Mount St. Helens, 

Washington: Proceedings of the Symposium on Erosion Control 

in Volcanic Areas; Seattle and Vancouver, Washington, July 6-9, 

1982, Technical Memorandum of Public Works Research Institute 

(Japan), no. 1908, p. 295-328.

Oltman, R. E., Sternberg, H. O'R., Ames, F. C., and Davis, L. C., Jr., 

1964, Amazon River investigations, reconnaissance measurements 

of July 1963: U.S. Geological Survey Circular 486, 15 p.

Pierson, T. C., 1985, Initiation and flow behavior of the 1980 Pine Creek 

and Muddy River lahars, Mount St. Helens, Washington: Geological Society of America Bulletin, v. 96, p. 1056-1069.

Pierson, T. C., and Scott, K. M., 1985, Downstream dilution of a 

lahar Transition from debris flow to hyperconcentrated streamflow: Water Resources Research, v. 21, p. 1511-1524.

Scott, K. M., 1985, Lahars and flow transformations at Mount St. 

Helens, Washington, U.S.A.: Proceedings of the International 

Symposium on Erosion, Debris Flow, and Disaster Prevention, 

1985, Tsukuba, Japan, p. 209-214.

____1988, Origins, behavior, and sedimentology of lahars and laharrunout flows in the Toutle-Cowlitz River system: U.S. Geological 

Survey Professional Paper 1447-A, 78 p.

____in press, Origin, behavior, and sedimentology of catastrophic

lahars at Mount St. Helens, Washington, in Clifton, H. E., ed., 

Sedimentologic consequences of convulsive geologic events: 

Geological Society of America Special Paper 229.

Scott, K. M., and Janda, R. J., 1982, Preliminary map of lahar inundation during the Pine Creek eruptive period in the Toutle-Cowlitz 

River system, Mount St. Helens, Washington: U.S. Geological 

Survey Water Resources Investigation Report 82-4067.

Siebert, Lee, 1984, Large volcanic debris avalanches Characteristics 

of source areas, deposits, and associated eruptions: Journal of 

Volcanology and Geothermal Research, v. 22, p. 163-197.

Sikonia, W. G., 1985, Impact on the Columbia River of an outburst 

of Spirit Lake: U.S. Geological Survey Water Resources Investigations Report 85-4054, 29 p.

Stuiver, Minze, 1978, Radiocarbon timescale tested against magnetic 

and other dating methods: Nature, v. 273, p. 271-274.

Suess, H. E., 1970, Bristlecone-pine calibration of the radiocarbon timescale 5200 B.C. to the present, in Olsson, I. U., ed., Radiocarbon 

variations and absolute chronology, Nobel Symposium, 12th, Uppsala, 1969, Proceedings: New York, John Wiley, p. 303-309.

Swift, C. H., Ill, and Kresh, D. L., 1983, Mudflow hazards along the 

Toutle and Cowlitz Rivers from a hypothetical failure of Spirit 

Lake blockage: U.S. Geological Survey Water Resources Investigations Report 82-4125, 10 p.

U.S. Army Corps of Engineers, 1982, A comprehensive plan for responding to the long-term threat created by the eruption of Mount 

St. Helens, Washington: Portland, Oreg., U.S. Army Corps of 

Engineers, 2 vols., individually paginated sections.

Yamaguchi, D. K., 1983, New tree-ring dates for recent eruptions of 

Mount St. Helens: Quaternary Research, v. 20, p. 246-250.

Yamaguchi, D. K., and Hoblitt, R. P., 1986, Tree-ring dating of Mount 

St. Helens flowage deposits emplaced in the valley of the South 

Fork Toutle River, Washington, during the last 350 years, in 

Keller, S. A. C., ed., Mount St. Helens, five years later: Cheney, 

Wash., Eastern Washington University Press, p. 441.

Youd, T. L., Wilson, R. C., and Schuster, R. L., 1981, Stability of 

blockage in North Fork Toutle River, in Lipman, P. L., and 

Mullineaux, D. R., eds., The 1980 eruptions of Mount St. Helens, 

Washington: U.S. Geological Survey Professional Paper 1250, 

p. 821-828.

APPENDIX
B28 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

SELECTED MEASURED 

STRATIGRAPHIC SECTIONS

Green Mountain Mill Section (Channel Fades)

[Location: cutbank on south side of North Fork Toutle River, 1.2 km downstream from mill; 

SW/4 sec. 16, T. 10 N., R. 1 E.]

Deposits of Pine Creek age: Meters

7. Lahar PC 4: medium pebble mode, gray silty 

sand matrix; mainly matrix supported; 

basal 25 percent of unit is inversely graded, 

upper 60 percent is normally graded; fills 

shallow channels on surface of lahar PC 3. 

Exposures at this locality were removed by 

erosion in late 1981 ................. 0.0-1.5

6. Lahar PC 3: cobble mode, mainly similar to 

underlying units, with 15- to 25-cm-thick 

gray sandy sole layer at base; limb and 

trunk molds common; the basal 10 percent 

of the unit is inversely graded, the upper 

40 percent is normally graded. The vertical 

tree mold present at the margin of the 

channel facies penetrated all four lahars. 

Load and flame structures are evidence of 

postdepositional mobility ............ 1.4-4.0

5. Lahar PC 2: pebble or cobble mode, mainly 

similar to underlying unit but thins laterally to a locally pumice-rich phase that lenses 

out laterally between PC 1 and PC 3, where 

it locally is replaced with a 1-cm ash-rich 

layer; the basal 10 to 20 percent of the unit 

is inversely graded, the upper 30 percent 

is normally graded .................. 0.0-2.0

4b. Lahar PC 1: cobble or locally boulder mode 

with silty sand matrix that is light gray to 

tan; extensive areas of framework support 

in the coarser part of the unit; basal 20 percent of unit is inversely graded, upper 40 

percent of unit is normally graded; contains 

megaclasts of hydrothermally altered 

breccia. Subhorizontal limb and trunk 

molds are common in upper half of unit. By 

early 1983 a vertical tree mold was exposed 

as erosion moved the cutbank exposure 

toward the flood-plain facies of the unit 4.5-8.2 

4a. "Ball-bearing bed" of lahar PC 1: concentration of marble-size pebbles in frameworksupported sole layer; overall slight inverse 

grading; contact with overlying lahar was 

observed to be transitional only at this 

locality ........................... 0.0-1.1

3. Lahar-runout deposit: light-gray, mediumgrained sand without the iron stain typical 

of the underlying units, otherwise similar 

to unit 1; unit surrounds stumps of trees 

knocked down by lahar PC 1 ......... 0.0-0.4

Deposits of Smith Creek or Pine Creek age:

2. Fluvial sand: brownish-tan to reddish-brown 

sand with local coarsening to granule or 

pebble gravel; stratified and has welldeveloped crossbedding; probably represents reworked runout sand .......... 1.0-2.3

1. Lahar-runout deposit: brownish-tan coarse 

sand; massive, ungraded except for slight 

normal grading in upper 0.1 m ....... 0.0-0.7

Green Mountain Mill Section (Flood Plain Facies)

[Location: southeastward extension of cutbank containing the channel facies section described

above]

Deposits of probable Kalama age: Meters

6. Lahar-runout deposit: dark-gray medium

sand with pumice stringers .......... 0.0-1.8

5. Lahar, close to transition to lahar-runout 

flow: granule mode, dark-gray (reflecting 

andesite and basalt content) silty sand 

matrix; massive; graded ............. 0.0-1.9

Deposits of Pine Creek age:

4. Lahar PC 3: generally similar to unit 2; fine 

pebble mode, slightly darker gray matrix 

than unit 2; entirely matrix-supported; 

poor inverse grading at base; upper 50 percent is normally graded to sand without 

dispersed coarser clasts; widely dispersed 

cobbles or boulders in basal half. Tree 

molds observed at this locality in 1981 

penetrated units 2, 3, and 4, with evidence 

of bark attached where surrounded by unit 

4 ................................ 2.0-2.8

3. Ash-rich layer: pinkish-tan clay-rich unit, 

grading laterally to pumice-rich lahar (PC 

2) ................................ 0.01-0.6

2. Lahar PC 1: pebble mode, light gray silty 

sand matrix; unit is entirely matrix supported; locally sandy at base but "ballbearing bed" or other variety of sole layer 

is absent; lower 30 percent has poor inverse 

grading, upper 50 percent has locally welldeveloped normal grading; contains pumice 

cobbles in upper 0.5 m .............. 2.0-2.5

Deposits of Pine Creek or Smith Creek age:

1. Fluvial sand: identical to unit 2 of the channel facies section. The level from which the 

trees forming the tree molds grew could not 

be determined ..................... >1.4

Harry Gardner Park Section

[Location: cutbanks on northeast side of South Fork Toutle River 150-200 m downstream 

from main bridge above confluence of the forks of the Toutle River; in NEVi sec. 29, 

T. 10 N., R. 1 E.]

Deposits of Pine Creek age: Meters 

4. Lahar PC 3: pebble and cobble mode, gray 

silty sand matrix; matrix supported; lower 

40 percent of the unit is inversely graded, 

upper 50 percent is normally graded . . 1.6-2.0 

3. Fluvial sand and gravel: gray stratified and 

crossbedded coarse sand and pebble gravel; 

locally has a few pumice clasts mineralogically similar to tephra set P (table 1) 0.1-0.6 

2. Lahar PC 1: cobble mode with local boulders 

and light gray or tan silty sand matrix; extensive areas of framework support in basal 

60 percent of unit; slight degree of inverse 

grading in basal 15 percent, normal grading 

in upper 50 percent; contains megaclasts 

of hydrothermally altered dacite breccias 

mainly less than 3 m in maximum exposed 

dimension, although one such megaclast, 

at the north end of the exposure, extended 

from the base to above the top of the unit,
APPENDIX B29

Deposits of Pine Creek age Continued Meters 

where it formed a mound. Mean size at this 

locality is -3.5 <t> (11.2 mm), sorting is 3.5 <j>, 

and skewness is 4-0.57 .............. 6.5

1. "Ball-bearing bed" of lahar PC 1: concentrated marble-size pebbles in frameworksupported sole layer; not graded at this 

locality (continuously exposed in 1980-81); 

sharp upper contact; limb mold 10 cm in 

diameter in upper half of unit; roundness 

and size distribution are very similar to 

those of the same unit (4a) in the channel 

f acies of the Green Mountain Mill. Underlain by blue clay, possibly of Ape Canyon 

age, at this locality. Exposures of this unit 

were covered by bank slumping in 1981 1.1

Coal Bank Bridge Section

[Location: valley-side slopes above left bank Toutle River 0-200 m upstream from Coal Bank 

Bridge; in SE1A sec. 19, T. 10 N., R. 1 E.; lowest units in section were described from 

excavations for bridge reconstruction in 1982]

Deposits of Pine Creek age: Meters 

21. Lahar PC 3: cobble mode with light-gray 

sandy matrix (similar in color to unit 16 but 

lighter than that unit and with more of a 

bluish cast); locally clast-supported in lower 

part; basal 0.9 m inversely graded, normal 

grading above; slight relief (5 to 10 cm) on 

mainly planar lower contact; mean roundness of coarse mode estimated (on cliff face) 

at 0.50. Pumice from the basal part of the 

unit contains hypersthene and hornblende 

and is mineralogically similar to set P. Soil 

developed on surface is consistent with a 

Pine Creek age ..................... 2.0

20. Fluvial sand: gray medium-grained sand, indistinctly stratified, graded; formed during 

the inter-lahar episode of flood-plain sedimentation represented by unit 18 and interrupted by deposition of the ash-rich unit

19 ............................... 0.0-0.2

19. Ash-rich clayey silt: probably mainly fluvial 

but is so consistent at this stratigraphic 

level in sections of the lahar flood-plain 

facies (unit 3 of Green Mountain Mill section) that it may locally be an air-fall 

deposit that is not reworked; distinctive 

pink color, massive to indistinctly laminated; at this locality contains fine-pebblesize pumice clasts that contain cummingtonite and hornblende and are derived from 

tephra set Y, but include hypersthenehornblende pumice like that of set P (identifications by D. R. Mullineaux) ...... 0.0-0.12

18. Fluvial sand: gray medium-grained sand, 

massive to poorly stratified; similar to unit

20 and probably part of the same sedimentation episode; contains locally abundant 

pebble-size pumice clasts that are mainly 

derived from set Y (identification by D. R. 

Mullineaux) ........................ 0.05-0.15

17. Lahar (probably locally thin equivalent of PC 

1): cobble mode with light-gray sandy 

matrix, distinctly lighter and less bluish

than unit 16 (but color could be affected by 

moisture content); mainly matrix supported but has local framework support; inverse grading only in lowest 0.1 m, upper 

1.7 m is normally graded; mainly planar 

lower contact; mean roundness of coarse 

mode estimated (on cliff face) at 0.55 . . 2.8 

Deposits of Smith Creek age:

16. Lahar: cobble mode with muddy sand matrix 

(unit has a higher clay content than any 

other lahar in section); medium gray with 

a bluish cast when moist; coarser parts 

have areas of framework support; poor inverse grading in basal 0.3 m, normal 

grading in upper l.lm; mean roundness of 

modal class is 0.36±0.049. Unit appears 

more weathered and iron stained than the 

younger lahars in this section ........ 3.2

15. Deposit either of a lahar-runout flow or a 

dilute lahar (clearly laharic in origin): 

brownish-gray coarse sand and granule 

gravel with no distinct coarse mode; inverse grading in basal 0.2 m, normal 

grading in upper 0.3 m; basal contact 

slightly irregular (relief less than 10 cm) but 

mainly planar; roundness of granule class 

estimated at 0.10-0.15. Upper 0.1 m is 

locally rich in pumice clasts mineralogically 

similar to the Y set ................. 1.0

Deposits of Swift Creek age:

14. Lahar with paleosol: contains a few subangular pebbles and cobbles in earthy 

brown silty sand matrix; basal contact 

locally shows slight channeling with relief 

less than 15 cm; mean roundness of 

4-8 mm class is 0.15±0.022, that of the 

8-16 mm class is 0.20±0.031; local accumulations of pebbles that appear rounder 

may represent interbedded fluvial sediment. Finely comminuted, carbonized wood 

fragments are concentrated in uppermost 

part of unit and yield a radiocarbon date 

of 3,760± 180 years B.P. Many live roots 

penetrate this unit because of the degree 

of weathering ...................... 0.5

13c. Fluvial sand: brownish-gray (moist) to tan 

(dry) fine to medium sand with a few 

pebbles, stratified in beds 0.25 to 8 cm 

thick ............................. 0.6

13b. Lahar-runout deposit: granule and fine pebble 

gravel with medium and coarse sand, 

grayer in color than overlying and underlying sand; crudely stratified but generally 

massive ........................... 0.6

13 a. Fluvial sand: brownish gray (moist) to tan 

(dry) medium sand with significant granule 

component; well stratified in beds 0.2 to 8 

cm thick. Unit thins to northwest ..... 0.1-0.7

12. Lahar: lacks well-developed coarse mode and 

may be transitional to a runout deposit; 

brownish-gray (moist) to light-gray (dry) 

coarse sand and granule gravel; generally 

massive, inverse grading in basal 0.3 m, 

normal grading in upper 0.7 m; lower contact sharp and nearly planar. Top 3 cm
B30 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

Deposits of Swift Creek age Continued Meters 

locally includes reworked lahar sediment; 

top 5 cm has local concentrations of iron 

stain, clay, and carbonate, which occur in 

areas of high permeability and apparently 

result from ground-water movement rather 

than surface weathering. Upper 0.5 m of 

unit contains widely scattered clasts of 

pumice that are either Swift Creek or basal 

Smith Creek in age (identification by D. R. 

Mullineaux) ........................ 1.4

11. Silty clay: pink to tan, contains ash but is at 

least partly of alluvial origin. Unit contains 

molds of small wood fragments ....... 0.0025-0.015

10. Sand: medium to coarse gray sand mainly of 

colluvial origin, believed to be reworked 

lahar sediment derived from unit 9; locally impregnated with ash and with clay and 

carbonate of phreatic origin. Other lahars 

in this section have analogous, less well 

developed units at upper contacts ..... 0.0-0.06

9. Lahar: medium pebble mode with brownishgray (moist) to light-grayish-brown (dry) 

sand matrix; mainly matrix supported; 

basal 15 to 20 cm locally is relatively 

uniform sand and is analogous to a sole 

layer but lacks cohesiveness; slight inverse 

grading in upper 0.3 m; lower contact sharp 

and planar; mean roundness of coarse mode 

is 0.35±0.063 ...................... 1.2

8. Lahar: rubbly appearing with coarse pebble 

or fine cobble mode and silty sand matrix 

that is, like units 2 and 4, distinctively 

brownish gray with a slight pink cast; 

matrix supported; locally contains megaclasts of hydrothermally altered dacite 

breccia, which are as much as 1.9 m in maximum exposed dimension and show internal deformation of lithologic contacts. 

Here, as in unit 4, the coarse mode contains 

a high proportion of pre-Mount St. Helens 

lithologies. Central 60 percent of unit is not 

graded; grading is inverse below this central core and normal above it (unit is symmetrically graded). Basal contact sharp and 

planar in flood-plain facies. Clasts of all 

sizes are distinctly more angular than those 

in the lahars of Pine Creek age; mean 

roundness of clasts within the older dacitebreccia megaclasts is 0.17±0.037; roundness of the megaclasts themselves is 

estimated as 0.60-0.80. Basal 0.25 to 1.00 

cm is silty sand and silty clay containing 

small carbonized wood fragments. Unit 

thickens in exposures upstream and fills a 

paleochannel more than 6.0 m deep in exposed section, then thins abruptly on the 

south side of the paleochannel to its characteristic flood plain thickness of 1.0 m 

before thinning to a wedge edge against the 

paleo-valley-side slope cut in deposits mainly of Ape Canyon age ............... 1.0->6.0

Deposits of probable Swift Creek age:

7. Lahar: fine pebble mode with medium-gray 

(moist) fine sand matrix, upper 8 cm

distinctly browner; mainly matrix supported, local clast support at center of unit; 

well-developed symmetrical grading; basal 

contact sharp and planar; mean roundness 

of coarse mode is 0.21±0.029. This unit is 

a twin of the underlying lahar ........ 0.3-0.5

6. Lahar: medium pebble mode with mediumgray (moist) fine to medium sand matrix; 

mainly matrix supported, local clast support in center of unit; well-developed symmetrical grading; basal contact sharp and 

slightly undulating, has relief of 5 cm on 

minor scour channels in surface of underlying lahar; mean roundness of coarse mode 

is 0.39±0.052. This unit is a twin of the 

overlying lahar ..................... 0.3

5. Sand: medium-grained gray sand developed 

as colluvium on and derived from the 

underlying lahar ................... 0.0-0.05

4. Lahar: medium-coarse pebble mode with fine 

to medium sand matrix, distinctively 

brownish gray (moist) or tan (dry) with 

slight pinkish cast; similar in color to units 

2 and 8 and, like unit 8, includes a relatively 

high proportion of pre-Mount St. Helens 

rocks in the coarse mode; clasts widely 

dispersed, unit entirely matrix supported; 

good symmetrical grading, 3 to 4 mm silty 

sand at base; slightly undulating lower contact; mean roundness of coarse mode is 

0.41 ±0.062. The weathered surface of this 

unit is distinctive the dispersed coarse 

clasts fall out, resulting in a pockmarked 

appearance ........................ 0.6

3. Lahar: granule and fine pebble mode with 

gray silty sand matrix; mainly matrix supported but has some clast support in 

coarsest part; inverse grading in basal 15 

percent of unit, normal grading in upper 60 

percent; sharp, planar basal contact. Uppermost part of unit contains local concentrations of pumice mineralogically similar 

to the J set of Swift Creek age (identification by D. R. Mullineaux) ............ 0.0-1.2

2. Lahar: medium pebble mode with silty sand 

matrix that is brownish gray but has a 

slight pinkish cast like units 4 and 8; mainly matrix supported but locally clast supported in coarsest part; inverse grading in 

basal 0.3 m, irregular normal grading 

above; upper 15 cm is distinctly sandier 

and has more dispersed clasts; sharp, nearly planar basal contact; mean roundness of 

coarse mode is 0.36± 0.047 ........... 0.9-2.5

1. Bar facies of lahar: boulder gravel with 

brownish-gray silty sand matrix; lower 

coarser part of unit has clast support transitional to matrix support as unit fines upward; marked overall normal grading, 

upper 12 cm is mainly sand, like matrix of 

rest of unit; basal contact not seen; mean 

roundness of boulder-size clasts estimated 

as 0.55-0.65 ....................... 3.5

Deposits of Ape Canyon age:

Approximately 10 m of incompletely exposed
APPENDIX B31

Deposits of Ape Canyon age Continued

deposits are present upstream from and stratigraphically beneath the above section. The basal 

unit is a probable lahar-runout deposit more than 

1.0 m thick. That unit is overlain by 1.5 to 2.0 m 

of well-stratified pumiceous alluvium with characteristic crossbedding in some strata. The alluvium 

is overlain by a probable pumiceous lahar more 

than 1.5 m thick. The pumice in both the alluvium 

and the probable lahar is mineralogically similar 

to tephra set C.

Outlet Creek Section

[Location: valley-side slopes above left bank of Outlet Creek near the quarry that formerly 

contained exposures of the Silver Lake lahar assemblage described by Mullineaux and 

Crandell (1962); in NE'/i sec. 30, T. 10 N., R. 1 E.]

Deposits of Pine Creek age: Meters 

7. Lahar PC 3: medium to coarse pebble mode 

with a medium-gray sandy matrix; matrix 

supported; sandy sole layer at base, 15-20 

cm thick, is overlain by 0.3-m inversely 

graded zone; upper part of the unit is 

ungraded to slightly graded; lower contact 

is sharp and slightly undulating in existing 

exposures unit appears to unconformably 

overlie unit 4b in the nearby quarry; mean 

roundness of coarse mode is 0.52+0.10; 

mean roundness of 8-16 mm class is 

0.19+0.094; mean roundness of 4-8 mm 

class is 0.13+0.022. Degree of pedogenesis 

in upper 0.5 to 0.8 m of unit is consistent 

with a Pine Creek age ............... 2.6->4.4

6. Fluvial sand: tan-gray sand, locally including 

fine and medium pebbles; stratified with 

local crossbedding; a few pumice clasts in 

the unit resemble those in the locally 

pumice-rich lahar (PC 2) at the Green 

Mountain Mill section and are mineralogically similar to tephra set P ....... 0.0-0.3

5b. Lahar PC 1: medium to coarse cobble mode 

with sandy matrix that is lighter gray and 

has more of a light-brown cast than unit 

7 but less of a bluish cast than unit 1; contains boulders of hydrothermally altered 

dacite breccia as much as 0.9 m in maximum exposed dimension; much of coarsest 

part of unit has a clast-supported framework; reverse grading in basal 25 percent 

of unit, good normal grading above, with 

widely dispersed boulders 1.5 to 2.0 m 

below top; basal contact with unit 5a is 

transitional; mean roundness of coarse 

mode is 0.56±0.12; mean roundness of 

8-16 mm class is 0.17+0.029; mean roundness of 4-8 mm class is 0.12+0.016 . . . 4.2-6.7 

5a. "Ball-bearing bed" at base of lahar PC 1: unit 

interpreted as sole layer of overlying lahar; 

fine to medium pebble mode (a striking concentration of clasts in this size range) with 

brownish-gray sandy matrix like that in 

overlying unit; framework is mainly clast 

supported; not graded; upper contact transitional with main body of flow (unit 5b); 

lower contact is sharp and undulating;

mean roundness of 16-32 mm class is 

0.27+0.051; mean roundness of 8-16 mm 

class is 0.21+0.042; mean roundness of 

4-8 mm class is 0.15+0.058. The presence 

of this distinctive unit establishes definite 

correlation of units 5a and 5b with lahar 

PC 1 at the Harry Gardner Park and Green 

Mountain Mill sections. Unit 5a is thinner 

and less well developed here than at those 

sites, because of its position on a valley-side 

slope well above the thalweg of the Toutle 

River. Observation of this unit at this section requires excavation ............. 0.05-0.2

Deposits of Smith Creek or Pine Creek age [radiocarbon 

date of 2,810±200 years was obtained from wood 

fragments from sand probably equivalent to unit 

3 or 4 (Crandell and others, 1981); pumice probably 

from unit 3 a is most likely from set Y, indicating 

a Smith Creek age (Crandell and others, 1981), but 

could be reworked material]: 

4b. Silt (interpreted as the uppermost finegrained part of a lahar-runout deposit): 

modal class is 5-6 </> (0.016-0.031 mm); 

sample from middle of graded unit contains 

16 percent sand and 8 percent clay; mottled 

light gray and tan in color; abundant carbonized wood fragments in upper part and 

at upper contact; evidence of soil-forming 

processes; slight normal grading ...... 0.03-0.15

4a. Silty sand (interpreted as lower part of a 

lahar-runout flow deposit): light gray to 

tan; generally massive but shows some 

weak stratification; local inverse grading 

in basal 0.1 m; locally rich in pumice like 

that of unit 3a ..................... 0.5

3c. Silt (interpreted as the upper graded part of 

a lahar-runout flow deposit) with possible 

slight degree of soil formation: very similar 

to unit 4b; locally rich in carbonized wood 

fragments; lower contact transitional . . 0.1-0.2 

3b. Sand (main body of lahar-runout flow 

deposit): modal class is 1-2 </> (0.25-0.50 

mm), contains 15 percent silt and 3 percent 

clay; light gray-brown to brown; uniform 

and generally massive with slight inverse 

grading at base, locally crudely stratified 

and includes lenses of pumice similar to 

that of unit 3a ..................... 0.3

3 a. Pumice gravel (basal deposit of lahar-runout 

flow): pebble and fine-cobble gravel, locally openwork; commonly has matrix support and interstitial sand like that of unit 

3b. Fe-Mg phenocrysts in the pumice are 

mainly cummingtonite and hornblende. 

The pumice is probably from tephra set Y 

(Crandell and others, 1981) ........... 0.0-0.2

Deposits of Smith Creek age:

2. Fluvial sand: coarse, stratified, iron-stained 

sand with lenses of pebble gravel and 

pebbly sand; clearly of normal fluvial 

origin, unlike the sequence of unit 3; 

moderately well sorted; roundness of clasts 

in pebble size range is relatively low 

(0.20-0.40) because clasts were derived by 

erosion of the underlying lahar ....... 0.5-0.7
B32 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

Deposits of Smith Creek age Continued Meters 

1. Lahar: medium pebble mode with a muddy 

sand matrix that is medium gray and has 

a characteristic bluish cast; the muddy appearance is also characteristic (a sample 

0.3 m from the top of the unit contains 15 

percent silt and 3 percent clay); entirely 

matrix supported; slight overall normal 

grading in exposed part of unit; upper contact slightly channeled, lower contact not 

exposed; mean roundness of coarse mode 

(16-32 mm) is 0.24±0.041; mean roundness 

of 8-16 mm class is 0.15±0.024; mean 

roundness of 4-8 mm class is 0.12±0.015. 

Size distribution from the clast-poor upper 

part of the unit has the following characteristics: mean, 1.22 <t> (0.44 mm); sorting, 

3.10 <£; and skewness, +0.11 ......... 1.2

Bear Creek Section

[Last extensive sequence of lahar exposures along South Fork Toutle River before bedrock 

gorge. Location: high cutbank on north side of South Fork 0.5 km upstream from confluence with Bear Creek, near center of sec. 29, T. 9 N., R. 3 E.; approximately 20 km 

from 1980 crater]

Deposits of modern eruptive period: Meters 

13. Lahar: pebble mode, wood-rich. Contains 

andesite and basalt. Unit represents the 

peak flow deposits of the first lahar in the 

South Fork Toutle River on May 18, 1980 0.0-0.3 

Deposits of Pine Creek age:

12. Lahar: pebble mode, similar to underlying 

unit. Contains no andesite or basalt. 

Mantles slopes like underlying unit . . . 0.0-0.5 

11. Lahar: coarse pebble mode with a brownishgray sandy matrix; mainly matrix supported; good normal grading throughout 

most of unit above local, basal 12-15 cm 

sole layer. Contains little or no andesite or 

basalt. Slight soil development in upper 0.3 

m consistent with a Pine Creek age. Unit 

mantles slope, and is inset against the sequence of older deposits exposed in the 

main cliff face. It cuts down into section 

as far as unit 3. In spite of the young date 

(see unit 10) from soil developed on the 

underlying erosional surface, the erosion 

producing the valley-side slopes mantled 

by unit 11 probably occurred during the 

main part of the dormant interval between 

Swift Creek and Smith Creek time .... 2.0-4.5 

mainly of post-Swift Creek, pre-Pine Creek

Soil: well developed and clay rich, yellowish 

orange brown, containing abundant 

angular rubble of pre-Mount St. Helens 

lithologies derived from valley-side slopes. 

Large charcoal fragments near upper surface of soil yield a date of 2630±185 

radiocarbon years. The fragments were obtained from soil beneath the sloping contact with unit 11 but from a location well 

below the top of the terrace .......... 1.3-4.0

Deposits of Swift Creek age:

9. Lahar: pebble mode with a few boulders and

Deposits 

time: 

10.

a brownish-gray sandy matrix; entirely 

matrix supported; pronounced inverse 

grading contrasts with the well-developed 

normal grading in the otherwise similar 

unit 7 ............................ 1.5-4.0

8. Soil: sandy, light orange brown; not as extensively developed as the soils of units 5 and 

10 ............................... 0.1-0.4

7. Lahar: coarse pebble mode with brownishgray silty sand matrix; clast support only 

in coarsest part; basal 0.7 m is inversely 

graded, excellent normal grading above 

that level ......................... 4.0-6.5

6. Ash-rich layer: locally present; contains no

pumice; probably of fluvial origin ..... 0.0-0.02 

Deposits of Cougar age:

5. Lacustrine silt and sand, with some soil development locally extending into unit 4: 

clayey, stratified, blue-gray where not oxidized. Abundant plant stems are partially 

replaced by iron oxide. Wood fragments 

yield an age of 19,700±550 radiocarbon 

years ............................. 0.0-0.5

4. Fluvial sand, silt, and fine pebble gravel: well 

stratified, tan to gray; unit probably 

related to underlying sequence ....... 0.2-1.5

3. Alluvium: clast-supported boulders common 

in brownish-gray sandy matrix; rubbly appearance. Coarsest material is in middle of 

unit, but deposit lacks the distinctive 

grading seen in most lahars. Fine pebbles 

are markedly rounded, showing that the 

unit is not the bar facies of a lahar, 

although in other respects the unit is 

similar to the 1980 lahar deposits in the adjacent channel. Contains significantly more 

lithologies from Mount St. Helens than the 

underlying glacial sequence and therefore 

is distinctly younger. Unit dissimilar to 

fluvial deposits of Ape Canyon age, which 

are common upstream and downstream of 

this section ........................ 5.5

Deposits of pre-Cougar age (and probably of pre-Ape 

Canyon age):

2. Sand and gravel, probably of glaciofluvial 

origin: Interbedded sand and pebble gravel; 

chocolate brown and weathered to a degree 

similar to that of the underlying unit; well 

stratified; clasts are mainly pre-Mount 

St. Helens metavolcanics. Distinctly different from deposits of known Ape Canyon 

age ............................... 1.2

1. Probable glacial till: cobble mode with some 

boulders; matrix weathered brown to reddish brown; entirely matrix supported; 

nongraded and very poorly sorted; clasts 

are pre-Mount St. Helens metavolcanics; 

unit rests directly on pre-Mount St. Helens 

metavolcanics. Unit may correlate with 

basal unit of sections on opposite side of 

river; some till-like units in those sections 

contain a small proportion of Mount St. 

Helens rock types .................. >2.0
APPENDIX B33

Disappointment Creek Section

[Location: exposures in high cutbanks on south side of South Fork Toutle River opposite 

an interval from 0.2 to 0.9 km downstream from Disappointment Creek, in NWW sec. 4, 

T. 8 N., R. 4 E., and SEV4 sec. 32, T. 9 N., R. 4 E.]

Deposits of modern eruptive period: Meters 

11. Lahar: thin coating of peak-flow deposits of

first lahar of May 18, 1980 .......... 0.0-0.4

Deposits of Kalama age:

10. Lahar: thin coating of a lahar containing

andesite and basalt ................. 0.0-0.3

Deposits mainly of post-Swift Creek, pre-Smith Creek 

age:

9. Soil: the most prominent soil in the watershed 

of the upper South Fork Toutle River; 

marked at most locations by a significant 

clay content and angular clasts of locally 

derived pre-Mount St. Helens rock types. 

Same as unit 10 of the Bear Creek section 0.3-1.6

Deposits of Swift Creek age:

8. Lahar: texturally like unit 6, but extensively weathered; underlies the soil of unit 9 

(the soil is developed on other units elsewhere). Pumice from this unit is a 

hypersthene-hornblende type and, given 

the additional evidence of the depth of soil 

development, probably can be identified as 

tephra set J ....................... 0.0-1.2

7. Lahar: texturally like unit 6 but not

weathered ......................... 0.0-1.0

6. Lahar: cobble mode, boulders locally concentrated near center of unit; gray silty sand 

matrix; good inverse grading in basal 

1.0-2.5 m, fair normal grading above. Local 

sole layer 0.0-0.3 m thick has widely 

dispersed pebbles that weather out, leaving a pockmarked surface like that of unit 

4 of the Coal Bank Bridge section. Essentially one unit, but local crude stratification suggests deposition in surges. Upper 

weathered zone about 1 m thick has locally been removed by erosion of top surface. 

Clasts are dominated by rock types from

ancestral Mount St. Helens, and at least 

30-40 percent exhibit shapes indicating 

direct origin from disintegration of 

prismatically jointed clasts. Pumice, widely 

dispersed in the main body of the unit, is 

mineralogically similar to set S, like that 

locally concentrated in the uppermost part 

of the unit ........................ 8.0-25.0

5. Fluvial sand: brownish gray, well stratified,

lenticular .......................... 0.0-0.2

4. Fluvial silty sand with ash-rich layers: 

weathered (prior to succeeding units), 

orange-brown, massive, contains finely 

comminuted pumice fragments; local 0- to 

3-cm-thick ash-rich layers at top and bottom of unit. The pumice is mineralogically similar to tephra set S, an identification 

made more probable by the degree of soil 

formation represented by unit 9 ...... 0.0-0.4

3. Possible lahar-runout flow deposit: massive

to poorly stratified sand ............. 0.0-1.5

Deposits of probable Cougar age (or pre-Cougar but 

post-Ape Canyon age):

2. Lahar: rubbly, cobble mode with a few 

boulders in a gray sandy matrix; clast supported only locally at the coarsest level; 

bottom 0.3 m is inversely graded; normal 

grading above that level. Upper 0-20 cm 

shows soil development ............. 0.0-1.8

1. Alluvium: cobble mode with boulders in a 

gray sandy matrix; crudely stratified; not 

obviously bimodal. Sandy subunits show 

channeling and crossbedding. The texture 

and structure of the unit are identical in all 

obvious respects to those of the modern 

stream alluvium. Rock types are mainly 

those of pre-Castle Creek Mount St. 

Helens. Clasts and matrix are not similar 

to those of known Ape Canyon deposits, 

but are similar to those of the Cougar-age 

alluvium downstream ............... >4.5

Lahars and Lahar-Runout Flows in the 

Toutle-Cowlitz River System, 

Mount St. Helens, Washington

By KEVIN M. SCOTT

A. Origins, Behavior, and Sedimentology of Lahars and 

Lahar-Runout Flows in the Toutle-Cowlitz River System

B. Magnitude and Frequency of Lahars and Lahar-Runout 

Flows in the Toutle-Cowlitz River System

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1447

Chapters A and B were 

originally published separately

,V U.S. GOVERNMENT PRINTING OFFICE: 1989 673-047/86,053 REGION NO. 8

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