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Hydrologic Consequences of 

Hot-Rock/Snowpack Interactions*^ 

at Mount St. Helens Volcano, Washington 

1982-84 '*

Professional Paper 158<

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Hydrologic Consequences of 

Hot-Rock/Snowpack Interactions 

at Mount St. Helens Volcano, 

Washington 1982-84

THOMAS C. PIERSON, Editor

Professional Paper 1586
UNITED STATES DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director

For sale by U.S. Geological Survey 

Information Services 

Box 25286, Federal Center 

Denver, CO 80225-0286

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement 

by the U.S. Government

Graphics and layout by Christine G. Janda 

Reston, Virginia 1999

LIBRARY OF CONTRESS CATALOGING IN PUBLICATION DATA

Hydrologic consequences of hot-rock/snowpack interactions at Mount St. Helens 

Volcano, Washington, 1982—84 /Thomas C. Pierson, editor.

p. cm. — (U.S. Geological Survey professional paper ; 1586) 

Includes bibliographical references and index 

Also available in digital form through the World Wide Web 

URL: http://vulcan.wr.usgs.gov/Volcanoes/ — T.p. verso. 

Supt. of Docs, no.: I 19.16: 1586 

1. Lahars—Washington (State)—Saint Helens, Mount. 

2. Mount. 3. Landslides—Washington (State)—Saint Helens, Mount. 

4. Snow—Washington (State)—Saint Helens, Mount. I. Pierson, Thomas C. 

II. Series.

QE559.U5H93 1998 

551.3'07_dc21

97-46864

CIP

ISBN.0-607-89056

Cover: Southeast crater rim at Mount St. Helens formed during the eruption of May 18, 1980. Snow- and ice-covered 

outer slope is covered by tephra and shows erosion (closely spaced furrows) by pyroclastic flows and surges. Photo by 

Austin Post, June 19, 1980.
CONTENTS

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

Thomas C. Pierson and Richard B. Waitt

1. Nature of Depositional Contacts between Pyroclastic Deposits 

and Snow or Ice .................................................................. 9

Joseph S. Walder

2. Transformation of Water Flood to Debris Flow following the 

Eruption-Triggered Transient-Lake Breakout from the 

Crater on March 19, 1982 .................................................. 19

Thomas C. Pierson

3. Sediment Transport in the Hyperconcentrated Phase of the

March 19, 1982, Lahar ....................................................... 37

Randal L. Dinehart

4. Dome-Collapse Rockslide and Multiple Sediment-Water Flows 

Generated by a Small Explosive Eruption on 

February 2-3, 1983............................................................. 53

Thomas C. Pierson and Richard B. Waitt

5. Rock Avalanches, Rockfalls, and Associated Processes Induced 

by Spreading of the Lava Dome, March 1984 ..................... 69

Jon J. Major and Patrick T. Pringle

6. Eruption-Triggered Lahar on May 14, 1984........................ 81

Patrick T. Pringle and Kenneth A. Cameron

Appendixes

LA ................................................................................ 109

1.B ................................................................................ 115

III
CONTENTS 

ILLUSTRATIONS

INTRODUCTION

FIGURE 1. Photograph showing bombardment of slopes of Sakurajima volcano with incandescent rock

fragments during nighttime ............................................................................................. 2

2. Photograph showing pyroclastic surges descending snow- and ash-covered slopes of the 

south sector of Mount St. Helens at a velocity of about 50 meters per second during 

first moments of the May 18, 1980, explosive eruption ................................................... 3

3. Photograph showing small rockslide-avalanche in progress from Mount St. Helens lava

dome, June 10, 1985 ....................................................................................................... 4

4. Photograph showing lobe of rock avalanche deposit showing thorough mixing of rock

debris and snow............................................................................................................... 5

5. Location map of Mount St. Helens area showing major streams draining the volcano .......... 6

6. Photograph showing snow-covered lava dome and crater of Mount St. Helens on

March?, 1985................................................................................................................. 6

7. Map of Mount St. Helens crater and Pumice Plain, which acts as head of the

North ForkToutle River .................................................................................................. 7

CHAPTER 1

FIGURE 1.1 Photograph showing pyroclastic deposits lying on snow; Mount St. Helens, Washington ..... 10

1.2 Photograph showing pyroclastic deposits lying on snow; Redoubt Volcano, Alaska............... 11

1.3 Schematic cross section through a pyroclastic deposit of mean thickness L overlying

snow and ice.................................................................................................................... 12

1.4 Photograph showing results of experiment to illustrate differential melting on an

irregular interface............................................................................................................. 13

1.5 Definition of coordinate system for Fourier-series description of step in contact

between snow and overlying pyroclasts ............................................................................ 15

1.6 Photograph showing ice-cemented mixed avalanche deposits on Drift Glacier, Redoubt

Volcano, Alaska ............................................................................................................... 16

CHAPTER 2

FIGURE 2.1 Flow path of the March 19, 1982, eruption-triggered flood and subsequent debris flow....... 21

2.2 Position of highest transient-lake level, as interpreted from pumice-block strandlines and

other shoreline features mapped from aerial photography of March 23, 1982 .................. 22

2.3 Photograph showing view to the northeast out of the crater breach, where the water flood

left the crater floor and entered the deeply incised ravines on the north flank .................. 23

2.4 Photograph showing view into crater from head of Loowit Ravine ....................................... 24

2.5 Photograph showing oblique aerial view (looking south) of the crater floor and breach, the 

eroded topography of the volcano's lower north flank (the Steps), and the head of 

the Pumice Plain (Step Fan)............................................................................................. 25
ILLUSTRATIONS——Continued

FIGURE 2.6 Longitudinal profile of flood flow path

A. On crater floor (from east spillway to head of Loowit Ravine).................................... 26

B. Continuation of A on north flank of the volcano (along axis of Loowit Ravine) ......... 26

2.7 Photograph of unit 1 debris-flow deposit at head of Pumice Plain, about 6 kilometers

downstream from flood source......................................................................................... 27

2.8 Photograph showing lobe of Unit 2 debris-flow deposit at head of Pumice Plain .................. 27

2.9 Cumulative frequency curves of samples of debris-flow deposits (clasts larger than

64 millimeters not sampled) from upper North ForkToutle River ................................... 28

2.10 Photograph showing thin (few centimeters thick) Unit 2 debris-flow deposit on lower 

Pumice Plain; boulders stranded when finer-grained slurry continued flowing 

downstream..................................................................................................................... 29

2.11 Photograph showing gullying left by erosion of 1980 pumiceous pyroclastic-flow

deposits by debris flow on the lower Pumice Plain ........................................................... 31

2.12 Cross sections of North ForkToutle River channel where it is cut into the 1980 debrisavalanche deposit at distances of about 13 and 20 kilometers downstream from the 

transient lake ................................................................................................................... 33

2.13 Stratigraphic relations of Unit 1 to Unit 2 at distal end of channel reach carrying debris flow 34

CHAPTER 3

FIGURE 3.1 Course of the March 19, 1982 lahar past gaging stations on the Toutle River

and Cowlitz River............................................................................................................ 39

3.2 Photograph of standing waves in the Toutle River upstream from the Highway 99 bridge .... 42

3.3 Photograph showing newly deposited sand at right side of Toutle River channel, as

viewed upstream from the Highway 99 bridge................................................................. 43

3.4 Photograph of water-sediment mixture of fine material escapes from bank deposits

and returns to main flow.................................................................................................. 44

3.5 Sediment concentration and stream discharge derived from measurements of the

hyperconcentrated phase of the lahar at the Toutle River gaging stations.......................... 45

3.6 Sediment concentration in two sand-size classes for the hyperconcentrated phase of the

lahar, fit with an exponential recession curve ................................................................... 48

3.7 Sediment concentration of the lahar-generated flood wave measured at the gaging

station at the Cowlitz River at Castle Rock ...................................................................... 49

CHAPTER 4

FIGURE 4.1 Flow paths and deposits of sediment-water flows triggered by an explosive burst at

Mount St. Helens on February 3, 1983 during a dome-building eruption ....................... 54

4.2 Photograph of southward oblique aerial view of crater, Loowit Ravine, and Pumice Plain 

on February 3, 1983, showing darkened, debris-covered east crater wall and path of 

mixed avalanche and succeeding flows ............................................................................. 56

4.3 Seismograph records for parts of the 2339 (February 2, 1983) and the

0256 (February 3, 1983) eruptive bursts from seismic stations YEL and EDM................ 58

4.4 Photograph of northwestward view of dome on February 3, 1983, showing

rockslide deposit and mixed-avalanche deposit on crater floor ......................................... 59

4.5 Plan view of flow paths and deposits in crater and breach areas............................................. 60

V
ILLUSTRATIONS——Continued

FIGURE 4.6 Photograph showing rough-surfaced mixed-avalanche deposit, 80 centimeters thick,

overlying snow in lower crater breach at head of Loowit Ravine..................................... 61

4.7 Photograph showing outer margin of west depositional lobe on Pumice Plain ...................... 61

4.8 Components of mixed-avalanche deposit:

A. Plot of sample bulk density versus distance from avalanche source

(rock buttress on east crater wall)........................................................................... 62

B. Rock fragments in mixed-avalanche deposit as a fraction of total solids

(rock + ice) versus distance from avalanche source ................................................. 62

4.9 Grain-size frequency histograms for mixed-avalanche, mixed-slushflow,

and debris-flow deposits .................................................................................................. 63

4.10 Photograph showing nested nature of mixed-avalanche, mixed-slushflow, and

water-flow deposits, visible from subtle tonal and textural changes, and watereroded channel, in crater breach at head of Loowit Ravine............................................... 65

CHAPTER 5

FIGURE 5.1 Drainage network at Mount St. Helens ................................................................................ 71

5.2 Photograph of oblique aerial view of Mount St. Helens crater showing lava dome

and March 29, 1984, rock-avalanche fan 72

5.3 Seismograph record of March 29, 1984, from seismic station HOA ..................................... 73

5.4 Photograph showing time-lapse photograph of incandescent rockfalls on March 31, 1984,

resulting from spalling of newly extruded lava ................................................................. 74

5.5 Photograph of oblique southward aerial view of March 29, 1984, rock-avalanche fan........... 75

5.6 Geologic map of rock avalanche-rockfall complex................................................................. 76

5.7 Photograph showing undulating surface of snow at the base of unit A .................................. 77

5.8 Photograph showing exposure of unit Bl illustrating the poorly sorted, loose,

dry texture of the deposit................................................................................................. 77

5.9 Profiles of the north part of the Mount St. Helens dome on January 31, 1984,

and April 14, 1984 .......................................................................................................... 78

CHAPTER 6

FIGURE 6.1 Path of May 14, 1984, lahar and lahar runout and locations of selected

measured cross sections.................................................................................................... 82

6.2 Panorama from Johnston Ridge, about 7 kilometers north of the crater showing

approximate extent of May 14 mixed snow and rock avalanche and slushflow and

paths of various lobes of the lahar .................................................................................... 83

6.3 Photograph of upstream view showing lahar recessional flow in the North ForkToutle

channel at about 1013 on May 14, 1984 ......................................................................... 84

6.4 Photograph showing side view of a recessional-flow knickpoint 14 kilometers from

vent at about 1014........................................................................................................... 85

6.5 Photograph of downstream view of North ForkToutle River at 1016 (9 minutes after 

passage of lahar flow front) showing a series of oblique and normal wave fronts 

(hydraulic jumps?) ........................................................................................................... 86

6.6 Photograph showing close up of bow-shaped wave front (hydraulic jump?) .......................... 87

6.7 Photograph showing upstream view of the North Fork Toutle River and sinuous, dunelike features (bed forms?) 16 km downstream from the vent immediately after passage of 

recessional surge shown in figures 6.5 and 6.6 ................................................................. 87

VI
ILLUSTRATIONS—Continued

FIGURE 6.8 Sampling of the sediment-discharge peak at 1046, 24 kilometers downstream of the crater

(at measured cross section NF335).................................................................................. 90

6.9 Longitudinal profile of the North Fork Toutle River from the crater to Elk Rock showing 

A, transition interval from slushflow to debris flow, and B, the transition interval 

from debris flow to hyperconcentrated flow .................................................................... 91

6.10 Sequential schematic stratigraphic sections showing upstream-downstream textural

changes and depositional facies associated with the May 14, 1984 lahar.......................... 91

6.11 Photograph of aerial oblique view eastward at 1530 showing confluence of Steam Creek

with North Fork Toutle River, about 9 kilometers downstream of the crater .................. 92

6.12 Photograph showing diamict deposited as backfill in hanging tributary shown in fig. 6.11

(8 kilometers from the crater) ......................................................................................... 93

6.13 Photograph of downstream aerial oblique view of Steam Creek (about 6 kilometers 

from the lava dome) at 1336 showing ponding 3 hours after passage of the lahar 

flow front........................................................................................................................ 94

6.14 Photograph of terrace-capping deposits on the right bank of the North Fork Toutle River 

about 16 kilometers from the lava dome show evidence of the beginning of the 

transition from debris flow to hyperconcentrated flow .................................................... 94

6.15 Cross section of dacite-breccia fragile clast rafted by the May 14, 1984 lahar,

16 kilometers from the lava dome................................................................................... 95

6.16 Sediment concentration (by weight) changes in a series of dip samples obtained

on May 14, 1984, near the periphery of the lahar runout flow 24 kilometers from

the Lava Dome (near cross section NF335)..................................................................... 97

6.17 Photograph showing downstream oblique view of knickpoint cut into debris-flow

surface, 16 kilometers from the lava dome ...................................................................... 98

6.18 Diagrammatic formation of bed forms in transition reach: A, Surging waves and

scour during recessional flow on fresh lahar deposit; and B, knickpoint bed form

and pumice-rich veneer remaining after tail of lahar drains away..................................... 99

6.19 Photograph showing depth of peak flow at (A) mudline 4 kilometers from the lava dome 

and (B) streamlined ridges and smoothed surface scoured by the May 14, 1984, 

debris flow...................................................................................................................... 99

6.20 Selected cross-section plots of the North Fork Toutle River basin showing net erosion

and deposition caused by the May 14, 1984, lahar.......................................................... 100

6.21 Section of a seismogram from the Elk Rock Three seismograph (ERT), located about

24 kilometers downstream of the vent near the North Fork Toutle River ........................ 101

APPENDIX 1.A

FIGURE l.A Definition of coordinate system for heat-flow analysis .......................................................... 109

l.B Inversion of lapace transform................................................................................................ 117

VII
CONTENTS 

TABLES

CHAPTER 2

TABLE 2.1 Erosion index values for monumented cross sections on the North Fork Toutle River............... 30

2.2 Characteristics of flood channels at Mount St. Helens and Mount Rainier where

water floods have transformed to debris flows .................................................................... 32

CHAPTER 3

TABLE 3.1 Peak stream discharges at gaging stations along the course of the lahar of May 19, 1982........... 46

3.2 Average velocity of flood-wave front along the course of the lahar of March 19, 1982,

in the Toutle River, Washington ........................................................................................ 47

3-3 Sediment discharges of lahar of March 19-20, 1982, in the Toutle River.................................. 50

3.4 Sediment discharge in sand-size classes at two-gaging stations along the course of lahar of

March 19-20, 1982, in the Toutle River ............................................................................ 50

3.5 Comparison of stream discharge and sediment transport on February 20 and

March 20, 1982, at Toutle River at Highway 99 ................................................................ 51

CHAPTER 4

TABLE 4.1 Properties and composition of mixed-avalanche deposit............................................................ 62

4.2 Grain-size parameters of mixed-avalanche, mixed-slushflow, and debris-flow deposits............... 64

CHAPTER 6

TABLE 6.1 Flow observations and erosional and depositional features for the May 14, 1984, lahar ............ 88

6.2 Velocity, discharge, and associated channel data for the May 14, 1984, lahar ............................ 96

VIII
CONTENTS

CONVERSION FACTORS

Multiply By To obtain

millimeter (mm)

centimeter

meter (m)

kilometer

Length

0.03937

0.3937

3.281

0.6214

inch

inch

foot

mile

square meter (m ) 

square kilometer (km )

liter (L)

cubic centimeter

cubic meter (m3)

cubic kilometer (km3)

Area 

10.76 

1.196 

0.0002471 

0.3861

Volume 

0.2642 

0.06102 

35.31 

1.308 

0.0008107 

0.2399

square foot 

square yard 

square acre 

square mile

gallon 

cubic inch 

cubic foot 

cubic yard 

acre-foot 

cubic mile

Volume per unit time (includes flow)

meter per second (m/s) 3.281 foot per second 

cubic meter per second (m3 /s) 35.31 cubic foot per second

15,850. gallon per minute

kilogram (kg)

megagram (Mg) or metric ton

joule (J)

joule per kilogram (J/kg) 

joule per degree Kelvin 

per kilogram ([J/°K]/kg)

degree Celsius (°C)

Mass 

2.205 

1.102

Thermal energy 

9.478 xlO'4 

4.298 x 10'4 

2.388 x 10'4

Temperature 

D F = 1.8 (°C) + 32

pound avoirdupois 

short ton (2,000 Ib)

British thermal unit

British thermal unit per pound avoirdupois 

British thermal unit per degree Rankine 

per pound avoirdupois

degree Fahrenheit

IX
HYDROLOGIC CONSEQUENCES OF

HOT-ROCK/SNOWPACK INTERACTIONS

AT MOUNT ST. HELENS VOLCANO,

WASHINGTON, 1982-84

By THOMAS C. PIERSON and RICHARD B. WAITT

INTRODUCTION

CONTENTS

Background ................................................................... 1

Types of hot-rock/snowpack interactions ....................... 2

Processes of explosive eruptions................................ 2

Processes of lava-dome mass failures ......................... 3

Types and characteristics of sediment-water flows .......... 3

Volcanic activity and research at Mount St. Helens ........ 5

Acknowledgments.......................................................... 7

References cited ............................................................. 7

BACKGROUND

Emplacement or flowage of hot pyroclastic rock debris 

on or into thick snowpacks on volcanoes can trigger 

hazardous rapid flows of sediment (including ice grains) 

and water. Such rapid flows can extend far beyond the 

flanks of a volcano, as has been observed at volcanoes in 

many parts of the world (Major and Newhall, 1989). 

Commonly these sediment-water flows achieve discharges 

and velocities that produce catastrophic consequences 

more than 100 km downstream from source areas. The 

hazardous nature of such flows was most recently 

demonstrated in 1985 at Nevado del Ruiz volcano in 

Colombia, where snowmelt-generated sediment-water 

flows killed about 23,000 people (Lowe and others, 1986; 

Naranjo and others, 1986; Pierson and others, 1990).

Snow-covered stratovolcanoes like those in the 

Cascade Range and in Alaska commonly erupt explosively 

and produce potentially unstable lava domes. On such 

volcanoes volcanic explosions and mass failures from lavadomes are two primary mechanisms by which a hot rock 

having a large surface area is brought into contact with 

snow. Through this contact heat is effectively transferred 

to the snow, and large volumes of rapidly melted snow 

can trigger a variety of flows involving sediment (initially 

the hot volcanic debris) and water. Once triggered, the 

sediment-water flows are likely to erode and incorporate 

additional material from older deposits and thus increase 

in volume. In addition to providing the heat to melt 

snow, stratovolcanoes commonly provide other 

conditions favorable to the initiation of sediment-water 

flows: (1) high topographic relief and steep slopes; (2) the 

presence of poorly consolidated or unconsolidated, 

erodible pyroclastic rocks on slope surfaces; and 

(3) weakened structural integrity caused by hydrothermal 

alteration of rocks forming the edifice.

The details of interactions between hot pyroclastic 

rock fragments and snowpacks are poorly understood. 

Hydrologic consequences of eruptive activity are probably 

fairly common at snow-clad volcanoes but have largely 

gone unnoticed in the past owing to the remoteness of 

most such volcanoes, to the small-scale, short-lived nature 

of most of the phenomena, and to the sometimes 

enigmatic nature of deposits that once consisted partly 

of ice particles.
CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

TYPES OF HOT-ROCK/SNOWPACK 

INTERACTIONS

Dynamic mixing of hot pyroclastic material with 

snow seems to be a critical factor in effective heat transfer 

and the production of large sediment-water flows. 

Highly energetic processes that can occur from explosive 

eruptions and from mechanical failure of lava domes 

cause mixing through impact and penetration of snow 

by hot rock fragments and through turbulent shear at 

flow boundaries. Less energetic interactions between hot 

rock and snow, such as the laminar flow of lava over 

snow surfaces or the fall of hot tephra on snow with 

little or no penetration, produce relatively little meltwater 

(Major and Newhall, 1989). During the period 1982-84 

hot-rock/snowpack interactions at Mount St. Helens 

were predominantly of the highly energetic type.

PROCESSES OF EXPLOSIVE ERUPTIONS

Explosive eruptions can add large quantities of heat 

and can apply destabilizing mechanical stresses to slopes 

adjacent to volcanic vents. Primary physical processes 

that can melt or destabilize snow and ice during these 

eruptions include: (1) explosive ejection of hot ballistic 

fragments, which can penetrate the snowpack to various 

depths, cause some physical mixing, and apply mass 

loading to slopes (fig. 1); (2) pyroclastic surges and 

pyroclastic flows, which thermally and mechanically 

erode and apply shear stresses to overridden snow surfaces 

(fig. 2); and (3) acoustic or seismic shock from volcanic 

explosions, which can momentarily cause a decrease in 

frictional resistance to failure in snow slopes. Even small 

explosive eruptions can be highly effective in fostering 

hot-rock/snowpack interactions because low-energy

FIGURE 1.—Bombardment of slopes of Sakurajima volcano with incandescent rock fragments during nighttime eruption on 

February 11, 1983. This volcano is not snow-covered, but photograph shows density of ballistic surface impacts and the 

subsequent dry flow of hot particles (incandescent streams in valleys) during an explosive eruption. Photograph courtesy of 

Kosuke Kamo, Sakurajima Volcano Observatory, Japan.
INTRODUCTION

explosions may not disperse pyroclastics very widely, 

resulting in a focusing of hot debris on snow-covered 

slopes near the vent. Recent examples of explosive 

eruptions producing large sediment-water flows on snowcovered volcanoes include Mount St. Helens in 1980 

(Pierson, 1985; Major, and Voight, 1986; Fairchild, 

1987; Scott, 1988; Waitt, 1989) and Nevado del Rufz in 

1985 (Lowe and others, 1986; Naranjo and others, 1986; 

Pierson and others, 1990).

PROCESSES OF LAVA-DOME 

MASS FAILURES

Stratovolcanoes that erupt silicic magmas commonly 

produce lava domes. These bulbous masses of highly 

viscous lava grow as lava is erupted onto the surface or as 

it is injected into the interior. As a dome grows, the 

cooling exterior forms a brittle carapace that encloses a 

ductile, gas-pressurized lava core (Iverson, 1990). During 

periods of growth, or when a dome deforms under its 

own weight, gravitational stresses can cause mechanical

failure of an unstable part of the dome (fig. 3). Such 

mass failures may be accompanied by minor explosions 

from the pressurized interior of the dome. The failed 

mass commonly moves out away from the dome as a 

hot, disintegrating rock avalanche, which usually 

transforms into a type of pyroclastic flow known as a 

block-and-ash flow. If the surrounding ground is snow 

covered, hot rock debris can effectively mix with the snow 

(fig. 4). Dome-collapse rock avalanches and explosions 

generated block-and-ash flows at Redoubt Volcano, 

Alaska, that triggered very large sediment-water flows in 

late 1989 and early 1990 (Major and others, 1990; 

Dorava and Meyer, 1994; Trabant and others, 1994; 

Waitt and others, 1994).

TYPES AND CHARACTERISTICS OF 

SEDIMENT-WATER FLOWS

Rapid release of meltwater on the flanks of a volcano 

will generally result in streams of water (or slurries of

FIGURE 2.—Pyroclastic surges (dark turbulent clouds with steep flow fronts) descending snow- and ash-covered slopes of the 

south sector of Mount St. Helens at a velocity of about 50 meters per second during first moments of the May 18, 1980, 

explosive eruption (Pierson, 1985). Highly mobile sediment-water flows were generated. Photograph courtesy of Harold 

Fosterman.
CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

water and pyroclastic rock fragments) flowing downhill 

over easily credible material—snow or loose, 

unconsolidated rock debris. Initial meltwater flows can 

readily scour and incorporate these types of material. 

Flow volume then increases due to this erosional bulking 

and can increase by up to an order of magnitude (Gallino 

and Pierson, 1985; Pierson and others, 1990). In the 

opposite sense, flows overtaking and mixing with 

streamflow in a river channel can become diluted (Pierson 

and Scott, 1985). Sediment-water mixtures containing 

ice also may be diluted as ice grains melt due to heat 

from incorporated volcanic debris or, to a lesser extent, 

from heat generated by friction during flow (Pierson and 

others, 1990).

A variety of types of sediment-water flow are possible, 

depending on the relative proportions of water and solid 

particles. Flow rheology depends on the grain-size 

distribution of the solid fraction as well as on the 

sediment: water ratio (Varnes, 1978; Pierson and Costa, 

1987). Unsaturated (nonliquefied) granular mass flows

of rock debris are classified as debris avalanches and 

are usually generated by gravitational collapse ot some 

part of the volcanic edifice (Hsii, 1975; Voight, 1978; 

Ui, 1983; Siebert, 1984). Similar, predominantly 

granular flows of snow (with or without fragments of 

glacier ice) and admixed rock debris, classified as mixed 

avalanches, can occur as a consequence of explosive 

eruptions (Pierson and Janda, 1994; Waitt and others, 

1994). Normal snow avalanches also can be triggered. 

Flows involving saturated (liquefied) slurries of rock 

debris and water are termed debris flows (Costa, 1984; 

Johnson, 1984). Such flows have significant shear 

strength and are capable of transporting large boulders. 

If large flows involve mostly water, they are generally 

simply described as floods, although flows that have 

sufficient entrained sediment to provide a small amount 

of shear strength and to dampen turbulence are termed 

hyperconcentrated flows (Beverage and Culbertson, 

1964; Pierson and Scott, 1985; Scott, 1988). 

Hyperconcentrated flows commonly occur as the more

FIGURE 3.—Small rockslide-avalanche (block and ash flow) in progress from Mount St. Helens lava dome, June 10, 1985. 

Avalanche comprises a hot, granular, basal flow and a hot, turbulent ash cloud. Lava dome is approximately 300 meters 

high.
INTRODUCTION

dilute downstream runout flows of some large debris 

flows. Flows involving undifferentiated debris flows, 

hyperconcentrated flows, or muddy floods, or all three 

at volcanoes are termed lahars ( Smith and Lowe, 1991).

VOLCANIC ACTIVITY AND 

RESEARCH AT MOUNT ST. HELENS

Mount St. Helens (fig. 5) has been the most active 

Cascades volcano and a prolific producer of snowmeltgenerated sediment-water flows throughout post-glacial 

time (Scott, 1988). During the first minutes of the 

cataclysmic eruption on May 18,1980, devastating lahars

FIGURE 4.—Lobe of rock avalanche deposit showing thorough 

mixing of rock debris and snow. This cold rock avalanche 

originated from the west crater wall at Mount St. Helens 

on January 13,1989, and flowed partway across the snowcovered crater floor. Ice-axe handle is approximately 

80 centimeters long. Photograph courtesy of Kenneth 

Cameron.

were generated suddenly on the east, south, and west 

flanks of the volcano. Field evidence indicated that the 

main cause was interaction of turbulent, high-velocity 

pyroclastic surges (and possibly underriding pyroclastic 

flows) with the snowpack. Yet some of the evidence is 

contradictory (Pierson, 1985; Major and Voight, 1986; 

Scott, 1988; Waitt, 1989), and the exact cause of the 

rapid snowmelt is not fully understood. Research on 

the 1980 flows was carried out only intermittently over 

months and years. Critical evidence, such as erosive 

features of snow surfaces or deposits containing snow or 

glacier ice, disappeared within weeks; therefore present 

understanding of the 1980 flows is unavoidably 

incomplete.

Between 1981 and 1986, the crater at Mount 

St. Helens (fig. 6) was an excellent place to examine hotrock/snowpack interactions and resultant sediment-water 

flows because (1) the shaded northern exposure of the 

crater allowed perennial snow to accumulate on the steep 

crater walls, talus slopes, and much of the crater floor, 

(2) those snow-covered surfaces were close to the dome, 

and (3) relatively frequent small explosive eruptions and 

minor dome collapses occurred. Eighteen small explosive 

eruptions, some accompanied by partial mass failures 

from the growing lava dome, were recorded during this 

period. A number of different interactions between hot 

rocks and snowpack were recorded by deposits and snowsurface morphologic features, and diverse types of 

sediment-water flows were triggered. Helicopter access 

allowed the field evidence of source areas and flow 

deposits to be quickly examined by U.S. Geological 

Survey teams, usually within a day of each event. Rapid 

response afforded unprecedented opportunities to 

examine the transient geologic records and to interpret 

the nature of these hot-rock/snowpack interactions.

The chapters of this volume examine the 

consequences of relatively small explosive eruptions and 

hot dome-collapse avalanches occurring in the presence 

of a snowpack at an active stratovolcano. Chapter 1 

presents a theoretical and experimental analysis of the 

nature of depositional contacts between hot pyroclastic 

deposits and snow or ice. Chapters 2 through 6 present 

aspects of four volcanic events that occurred at Mount 

St. Helens and caused varying amounts of snowmelt in 

1982,1983, and 1984. One small eruption melted snow 

so rapidly and in such volume that a transient meltwater 

lake formed, then breached, sending sediment-water 

flows as far as 90 km downstream. Seismic monitoring, 

eyewitness accounts, and detection of eruptive plumes 

by civil-aviation radar supplemented geologic and
CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

123°30' 122°00'

46°15'

46<W

FIGURE 5.—Location map of the Mount St. Helens area showing major streams draining the 

volcano Inset rectangle shows area of map in figure 7.

FIGURE 6.—Snow-covered lava dome and crater of Mount St. Helens on March 7, 1985. 

Lava dome is approximately 300 meters high.
INTRODUCTION

hydrologic field evidence. Although some of the events 

produced effects and generated sediment-water flows that 

seemed similar, no two events were the same.

Locations of key geographic features near the Mount 

St. Helens crater are depicted in figure 7. Chapters in 

this volume use the place names shown, which include 

both formal and informal names of features. The drainage 

features are continuing to evolve on this young landscape;

FIGURE 7.—Map of Mount St. Helens crater and Pumice Plain, 

which acts as head of the North Fork Toutle River. 

Geographic features and locations are indicated using both 

formal and informal geographic names.

the features are drawn (fig. 7) approximately as they were 

in of 1984.

ACKNOWLEDGMENTS

Winter field investigations at an active volcano in 

mountainous terrain would not be possible without 

pilots, radio dispatchers, seismic observers, electronic 

technicians, and other colleagues in crucial support roles. 

Most areas affected by eruptions at Mount St. Helens in 

1981-86 were accessible only by helicopter, and pilots 

made skillful landings under sometimes challenging 

conditions. Night-flight airplane pilots and observers, 

volcano-monitoring crews, and seismic interpreters 

provided information on the eruptions and provided a 

measure of safety for field crews in or near the crater. 

Some interpretations have been influenced by discussions 

over the years with colleagues both within and outside 

the U.S. Geological Survey. The editor and authors wish 

to thank all these unnamed individuals for their 

assistance.

REFERENCES CITED

Beverage, J.P., and Culbertson, J.K., 1964, 

Hyperconcentrations of suspended sediment: American 

Society of Civil Engineers Proceedings, Journal of the 

Hydraulics Division, v. 90, no. HY-6, pt. 1, p. 117-128.

Costa, J.E., 1984, Physical geomorphology of debris flows, 

in Costa, J.E., and Fleisher, P.J., eds., Developments and 

applications of geomorphology: Berlin, Springer-Verlag, 

p. 268-317.

Gallino, G.L., and Pierson, T.C., 1985, Polallie Creek debris 

flow and subsequent dam-break flood of 1980, East Fork 

Hood River basin, Oregon: U.S. Geological Survey WaterSupply Paper 2273, 22 p.

Dorava, J.M., and Meyer, D.F., 1994, Hydrologic hazards in 

the lower Drift River basin associated with the 1989-1990 

eruptions of Redoubt Volcano, Alaska: Journal of 

Volcanology and Geothermal Research, v. 62, p. 387-407.

Fairchild, L.H., 1987, The importance of lahar initiation 

process, in Costa, J.E., and Wieczorek, G.F., eds., Debris 

flows/avalanches: Process, recognition, and mitigation: 

Geological Society of America, Reviews in Engineering 

Geology, v. 7, p. 51-62.

Hsii, K.J., 1975, Catastrophic debris streams (Sturzstroms) 

generated by rockfalls: Geological Society of America 

Bulletin, v. 86, no. 1, p. 129-140.

Iverson, R.M., 1990, Lava domes modeled as brittle shells that 

enclose pressurized magma, with application to Mount
CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

St. Helens, in Fink, Jonathan, ed., Lava flows and domes; 

emplacement mechanisms and hazard implication: Berlin, 

Springer-Verlag, p. 47-69.

Johnson, A.M., 1984, Debris flow, in Brunsden, Denys, and 

Prior, D.B., eds., Slope instability: Chichester, England, 

Wiley, p. 257-361.

Lowe, D.R., Williams, S.N., Leigh, Henry, Connor, C.B., 

Gemmell, J.B., and Stoiber, R.E., 1986, Lahars initiated 

by the 13 November 1985 eruption of Nevado del Rufz, 

Colombia: Nature, v. 324, no. 6092, p. 51-53. 

Major, J.J., and Newhall, C.G., 1989, Snow and ice 

perturbation during historical volcanic eruptions and the 

formation of lahars and floods: Bulletin of Volcanology, 

v. 52, no. 1, p. 1-27.

Major, J.J., and Voight, Barry, 1986, Sedimentology and clast 

orientations of the 18 May 1980 southwest-flank lahars, 

Mount St. Helens, Washington: Journal of Sedimentary 

Petrology, v. 56, no. 5, p. 691-705.

Major, J.J., Janda, R.J., Pierson, T.C., Waitt, R.B., LaHusen, 

R., Scott, WE., Trabant, D., Brabets, X, and Miller, XP, 

1990, Debris flows and floods generated by eruption of 

Redoubt Volcano, Alaska—Consequences of interaction 

between snow and ice and volcanic debris [abs.]: 

Geological Society of America Abstracts with Programs, 

v. 22, no. 7, p. 55.

Naranjo, J.L., Sigurdsson, H., Carey, S.N., and Fritz, W.J., 

1986, Eruption of Nevado del Rufz Volcano, Colombia, 

on 13 November 1985—Xephra fall and lahars: Science, 

v. 233, p. 961-963.

Pierson, X.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, no. 8, p. 1056-1069.

Pierson, X.C., and Costa, J.E., 1987, A rheologic classification 

of subaerial sediment-water flows, in Costa, J.E., and 

Wieczorek, G.F., eds., Debris flows/avalanches; process, 

recognition, and mitigation: Geological Society of 

America, Reviews in Engineering Geology, v. 7, p. 1-12. 

Pierson, X.C., and Janda, R.J., 1994, Volcanic mixed 

avaknches: A distinct eruption-triggered mass-flow process 

at snow-clad volcanoes: Geological Society of America 

Bulletin, v. 106, p. 1351-1358.

Pierson, X.C., Janda, R.J., Xhouret, J.C., and Borrero, C.A., 

1990, Perturbation and melting of snow and ice by the 

13 November 1985 eruption of Nevado del Rufz, 

Colombia, and consequent mobilization, flow, and 

deposition of lahars: Journal of Volcanology and 

Geothermal Research, v. 41, p. 17-66.

Pierson, X.C., and Scott, K.M., 1985, Downstream dilution 

of a lahar—transition from debris flow to 

hyperconcentrated streamflow: Water Resources 

Research, v. 21, no. 10, p. 1511-1524.

Scott, K.M., 1988, Origins, behavior, and sedimentology of 

lahars and lahar-runout flows in theXoutle-Cowlitz River 

system [Mount St. Helens, Washington]: U.S. Geological 

Survey Professional Paper 1447-A, 74 p.

Siebert, Lee, 1984, Large volcanic debris avalanches— 

characteristics of source areas, deposits, and associated 

eruptions: Journal of Volcanology and Geothermal 

Research, v. 22, no. 3/4, p. 163-197.

Smith, G.A., and Lowe, D.R., 1991, Lahars—volcanohydrologic events and deposition in the debris-flow/ 

hyperconcentrated-flow continuum, in Fisher, R.V., and 

Smith, G.A., eds., Sedimentation in volcanic settings: 

SEPM (Society for Sedimentary Geology) Special 

Publication 45, p. 59-70.

Xrabant, D.C., Waitt, R.B., and Major, J.J., 1994, Disruption 

of Drift glacier and origin of floods during the 1989-1990 

eruptions of Redoubt Volcano, Alaska: Journal of 

Volcanology and Geothermal Research, v. 62, p. 369-385.

Ui, Xadahide, 1983, Volcanic dry avalanche deposits— 

identification and comparison with nonvolcanic debris 

stream deposits: Journal of Volcanology and Geothermal 

Research, v. 18, no. 1/4, p. 135-150.

Varnes, D.J., 1978, Slope movement types and processes, in 

Schuster, R.L., and Krizek, R.J., eds., Landslides; analysis 

and control: Washington, D.C., National Research 

Council, Xransportation Research Board Special 

Report 176, p. 11-33.

Voight, Barry (ed.), 1978, Rockslides and avalanches, 1.— 

natural phenomena: Developments in geotechnical 

engineering I4a: Amsterdam, Elsevier, 833 p.

Waitt, R.B., 1989, Swift snowmelt and floods (lahars) caused 

by great pyroclastic surge at Mount St. Helens volcano, 

Washington, 18 May 1980: Bulletin of Volcanology, 

v. 52, no. 2, p. 138-157.

Waitt, R.B., Gardner, C.A., Pierson, X.C., Major, J.J., and 

Neal, C.A., 1994, Unusual ice diamicts emplaced during 

December 15,1989 eruption of Redoubt Volcano, Alaska: 

Journal of Volcanology and Geothermal Research, v. 62, 

p. 409-428.
1.

NATURE OF DEPOSITIONAL CONTACTS

BETWEEN PYROCLASTIC DEPOSITS

AND SNOW OR ICE

By JOSEPH S. WALDER

CONTENTS

Abstract ......................................................................... 9

Introduction................................................................. 9

Heat-transfer considerations and illustrative

experiments............................................................. 11

Implications of theoretical and experimental result

for interpretation of field observations..................... 15

Conclusions.................................................................. 17

Acknowledgments......................................................... 17

References cited ............................................................ 17

Appendix LA................................................................ 109

Appendix l.B................................................................ 115

ABSTRACT

Pyroclastic flows and surges moving over snow or ice 

commonly scour their substrates, as shown by deposits 

containing snow and glacier ice and by ravines cut into glacier 

surfaces. Yet observed contacts between pyroclastic deposits 

and snow or ice are in at least some places and at some scales 

sharply defined and smooth. Traditional stratigraphic 

principles provide equivocal guidance in interpreting such 

contact geometries: Do they reflect passive deposition without 

erosion, or uniform planing of the substrate by the flowing 

mass? I show that inferring process from form in this situation 

is problematic because of the unique circumstance of a substrate 

that can be melted by the overlying deposit. Analysis of heat 

transfer between a hot deposit and a frozen substrate indicates 

that for plausible values of initial deposit temperature, the 

depth of melting can easily be as much as the thickness of the 

deposit. Furthermore, the amplitude of irregularities on the

contact between pyroclastic debris and substrate is inevitably 

modified by spatial variability in melt rate: closely spaced 

irregularities (those with wavelength less than 2 71 times the 

deposit thickness) tend to be destroyed, whereas more widely 

spaced ones tend to be amplified. Simple experiments illustrate 

the validity of the theoretical predictions, which are also 

consistent with observations made at Mount St. Helens and 

Redoubt Volcano. Accordingly, for the particular case at hand, 

an observed contact cannot be that which existed at the time 

of deposition, so one cannot make unequivocal inferences 

about amount or mechanics of interfacial scour on the basis 

of contact geometries.

INTRODUCTION

Pyroclastic flows moving over snow or ice commonly 

scour their substrates, as evidenced by deposits containing 

blocks of snow and ice, and by ravines cut into the 

surfaces of glaciers that mantle volcanoes (Mellors and 

others, 1988; Pierson and others, 1990;Thouret, 1990; 

Waitt and others, 1994; Major and Newhall, 1989). Yet 

contacts between pyroclastic deposits and snow or ice 

vary greatly: some contacts show substantial small-scale 

relief (fig. 1.1), whereas others are free of irregularities 

and are almost planar (fig. 1.2). One would hope to use 

such contact relations to learn about the mechanics of 

interaction between the pyroclastic flow and the frozen 

substrate, but application of traditional stratigraphic 

principles leads to somewhat of a quandary in this regard. 

One possible line of argument involves postulating 

process and predicting form, as follows: As a mass of 

rock clasts flows down a slope, some particles will collide 

with the slope; by analogy with the processes that form
10 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

FIGURE 1.1.—Pyroclastic deposits lying on snow. Mount St. Helens, Washington. Cross section of linear 

ridge within pyroclastic deposit deposited on snow, May 9, 1986. The ruler is 15 cm long. Photograph 

by R.B. Waitt, U.S. Geological Survey.
NATURE OF DEPOSITIONAL CONTACTS BETWEEN PYROCLASTIC DEPOSITS AND SNOW OR ICE 11

striations or sole marks in muddy sediment (Alien, 1985), 

one might reasonably expect the frozen substrate to be 

scoured as a result. A smooth contact between the 

pyroclastic deposit and the substrate then must reflect 

uniform erosion of the substrate. An alternative, but 

equally plausible, line of argument takes a different tack, 

beginning with form and inferring process: The presence 

of grooves in the snow/ice substrate reflects vigorous 

erosion of that substrate, whereas the absence of grooves 

means that the substrate was not scoured, and that the 

hot rock clasts flowed more or less passively over the snow 

or ice. Both lines of argument have been presented to 

me by colleagues and reviewers of this manuscript, with 

strong opinions as to the inadequacy of the "opposing" 

view; I therefore conclude that the issue is not a simple 

one!

I believe that the element missing from both of the 

above arguments is a clear recognition of the 

consequences of having a substrate that can be thermally 

eroded—or more simply, melted—by an overlying

deposit even when that deposit is at rest. Postdepositional modification of the contact geometry is 

inevitable in this circumstance; the pertinent physics are 

summarized in the next section and elaborated in the 

analysis in Appendix LA. A perplexing, somewhat 

negative conclusion is that post-depositional 

modification of the contact is typically so great that 

neither of the aforementioned interpretations is likely to 

have much bearing on the effort to understand the 

mechanics of the interaction between hot flowing clasts 

and a frozen substrate. Carefully designed experiments 

are needed to reach that goal and are in progress.

HEAT-TRANSFER CONSIDERATIONS 

AND ILLUSTRATIVE EXPERIMENTS

Consider the situation depicted in figure 1.3, which 

shows a hypothetical cross section through a pyroclastic 

deposit and its substrate (either snow or ice) at the instant

FIGURE 1.2.—Pyroclastic deposits lying on snow. Redoubt Volcano, Alaska. The exposure shown was excavated at a site 

visited within several hours of an eruption on March 23, 1990. Photograph by R.G. McGimsey, U.S. Geological Survey.
12 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

at which the pyroclastic flow has come to rest. In the 

absence of direct knowledge of the mechanics of erosion, 

the least restrictive assumption one can make is that if 

the substrate is eroded by the pyroclastic flow, the eroded 

interface will be irregular, with some areas of the substrate 

scoured more deeply than others. My goal here is to 

assess whether, once flow ceases, topography at the 

interface (that is, the contact) will be modified by melting 

of the substrate, and if so, whether irregularities on the 

contact will tend to be enhanced or muted. I emphasize 

that the focus here is solely on post-depositional 

modification of the contact.

A hot pyroclastic deposit can melt a substantial 

quantity of snow or ice. An upper bound on the amount 

of melting caused by a pyroclastic layer of uniform 

thickness Lis easily found if one assumes that all thermal 

energy of the deposit goes into melting. Equating the 

specific heat change of the deposit to the latent heat 

required to melt, one finds

d.i) -0 PI//

where /m is the thickness of melted substrate, pp is the 

density of the deposit, cp is the specific heat of the 

deposit, 0is the substrate porosity (zero for pure ice), PJ 

is the density of ice, H is the heat of fusion, and 

AT = Tfi — rm, with TJ the initial temperature of the 

deposit and Tm the melting temperature of snow or ice; 

measuring temperatures in degrees Celsius (°C), one has 

Tm = 0 and thus AT = TJ. To estimate lm/L, take 

Pp/Pi =1.5 (corresponding to a moderately inflated 

deposit), 0= 0.5 (corresponding to dense, old snow),

Air

Pyroclastic deposit 

\

B

Snow

FIGURE 1.3.—Schematic cross section through a pyroclastic 

deposit of mean thickness L overlying snow or ice. Heatflow vectors are shown schematically. A and B represent 

trough and peak, respectively, of a long-wavelength 

irregularity on the pyroclast/snow contact; C and D are 

peak and trough for a short-wavelength irregularity.

H= 3.35 x 10 5 J/kg, and cp « 103 0/°K)/kg, whence

(1.2)

where T, is expressed in degrees Celsius; thus /m = L for 

7j ~ 110°C. Pyroclastic deposits are commonly 

emplaced at temperatures of 600°C or greater (Banks 

and Hoblitt, 1981; Cas and Wright, 1987), and thus 

lmIL ~ 6 is a plausible upper bound. Even taking into 

account likely variations in Pp/Pi, along with the fact 

that sufficiently hot pyroclastic deposits might vaporize, 

rather than melt, underlying snow (Walder, 1992), it is 

clear that a substantial thickness of the substrate could 

be removed, with lm l'L - 1 a plausible orderof-magnitude estimate.

Simple experiments of the sort described below, in 

which a layer of hot sand was poured onto artificial snow 

(actually shaved ice), give amounts of melting consistent 

with the estimate above. This estimate is also consistent 

with observations related to the March 19, 1982, 

eruption of Mount St. Helens, Washington, USA (Waitt 

and others, 1983; Waitt and MacLeod, 1987; Pierson, 

chap. 2, this volume). During this eruption, pumice was 

deposited on the snow-covered crater floor and crater 

walls. Rapid melting resulted in the formation of a lake 

within the crater; the lake subsequently breached its dam 

and drained. Based on subsequent estimates of the lake 

volume and study of the pumice deposits, the thickness 

of snow melted was roughly equal to the thickness of 

the pumice deposit. Finally, note that melting may 

continue for a substantial period of time; observations 

at Nevado del Ruiz, Colombia, indicated runoff from 

snow and ice melted by pyroclastic deposits lasted for at 

least two days after deposition (Pierson and others, 1990).

I therefore conclude that, barring exceptional 

circumstances, any observed contact between pyroclastic 

deposits and either snow or ice cannot be the contact 

that existed instantaneously at the time of deposition. 

Yet with the exception of a brief comment by Mellors 

and others (1988), this retrospectively "obvious" result 

does not seem to have been explicitly recognized by field 

workers at snow- and ice-clad volcanoes. It is the first 

indication that traditional stratigraphic- and 

sedimentologic methods are at best suspect, and at worst 

inapplicable, in studying (formerly) hot deposits on snow

or ice.

The argument presented above shows that the absolute 

position of the contact must change due to postdepositional melting, but says nothing about whether 

the shape of the contact will be changed. A simple
NATURE OF DEPOSITIONAL CONTACTS BETWEEN PYROCLASTIC DEPOSITS AND SNOW OR ICE 13

D

FIGURE 1.4.—Results of experiment to illustrate differential melting on an irregular interface. Hot sand (150°C) was poured 

onto a bed of slightly packed, finely shaved ice. The plexiglas box containing the shaved ice was fitted with a false bottom to 

allow meltwater drainage. Horizontal lines on the box are at 10-millimeter intervals. A, Initial state for a wavy irregularity. 

B, Modified wavy irregularity after 10 minutes contact with hot sand. Amplitude of the relief has been decreased by about 

one-third as peaks and troughs have migrated vertically downward. C, Initial state for a step-like irregularity. D, Modified 

step-like irregularity after 10 minutes contact with hot sand. Amplitude of the step has increased slightly, and the slope of the 

step has been reduced.
14 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

experiment, described next, demonstrates that such 

change should in fact occur.

A plexiglas box was filled nearly to the top with "shaved 

ice" made with an Echols Microflake II ice shaver, a 

machine used commercially for making a confection 

commonly known in the United States as "Hawaiian 

ice." The ice shavings were quite fine, not gritty, and at 

above-freezing temperatures looked and tended to clump 

much like wet snow; to break up the largest clumps, the 

shaved ice was put through a 5.6-mm sieve while filling 

the box, which was fitted with a false bottom to allow 

meltwater drainage. The shaved ice so prepared had a 

density of approximately 500 kg/m3 . The surface of the 

shaved ice was then shaped and slightly compacted, and 

the box was wrapped on three sides with foam insulation, 

the fourth side being left unobstructed to allow viewing. 

Hot, dark-colored sand was then poured onto the shaved 

ice up to the top of the box and photographs were taken 

to show temporal change of the contact between sand 

and shaved ice. In some experiments, a layer of foam 

insulation was placed on top of the sand, but this made 

no difference in the results, reinforcing the conclusion 

that heat loss to the melting substrate far exceeded loss 

to the atmosphere.

Figure 1.4 shows typical results of the experiments, 

which were of two sorts. For the case illustrated by 

Figs. 1 AA and 1 AB, a piece of corrugated metal was 

pressed against the surface of the shaved ice to create a 

roughly sinusoidal form that had an amplitude of about 

10 mm and a wavelength of about 75 mm. The sand 

had an initial temperature of 150°C and a mean thickness 

of about 40 mm. Melting caused the peaks and troughs 

of the contact to translate vertically downward. The 

amplitude of the relief on the contact was reduced by a 

factor of about one-third after about 10 minutes, by 

which time the sand had become noticeably wet.

For the experiment illustrated by Figs. 1.4 C and 

1 AD, a nearly vertical step was cut into the shaved ice, 

the height of the step being 20 to 40 mm in the several 

runs with this geometry. The temperature of the sand 

was in the range 150°C to 200°C, with a thickness of 20 

to 40 mm over the upper step surface. In all runs with a 

vertical step, there was a slight increase with time in the 

step amplitude. The sand eventually became wet because 

of meltwater drawn up by capillary suction; little change 

in amplitude of the step occurred once capillary rise 

became noticeable. More noticeable than the change in 

the step amplitude was a change in its shape, which was 

invariably altered to a ramp.

The alterations in contact shape in these experiments

can be explained by considering heat transfer between 

the hot sand and the shaved ice. (The same line of 

reasoning will apply for pyroclastic deposits resting on 

snow.) One may reasonably expect that whether or not 

contact irregularities are enhanced by melting depends 

on their spacing X. In line with standard results in heatconduction theory (Carslaw and Jaeger, 1959), the time 

scale for the deposit to lose all of its heat is about L2/K, 

whereas the time scale for local heat redistribution 

(fig. 1.3) is about X2/K, where K is the deposits thermal 

diffusivity; accordingly, one may expect peaks on the 

contact to capture heat from troughs if 'k/L is less than 

about one, whereas for more widely spaced irregularities, 

the depth of melting should depend simply on the local 

deposit thickness (eq. 1.1). This qualitative expectation 

is supported by results of the analysis in Appendix 1 .A, 

where I show that for small-amplitude irregularities — 

that is, for the case in which initial irregularities at the 

contact are much less than the mean thickness of the 

pyroclastic deposit—irregularities with wavelength 

A, < 2nL will tend to be destroyed, whereas those with 

A, > 2nL will tend to be enhanced. For the experiment 

illustrated in figures 1 AA and 1 AB, \~ L, and therefore 

one would expect the irregularities to be damped, as 

indeed was observed. For the experiment shown in 

figures 1.4Cand I AD, even though the amplitude of 

the step is obviously not small compared to the mean 

thickness of the sand, the small-amplitude theory is still 

qualitatively applicable. To see this, note that the shape 

of an ideal vertical step may be represented by a Fourier

series:

Af ^ 2h ^

A(x) = IT z n

m = 0

sm (2m

W

n ^

(L3)

where h is the height of the step, Wis the width of the 

box, and A(x) is the (mathematical) amplitude of the 

contact at a horizontal position x (fig. 1.5). Equation 

1.3 has the property that A(x) = -h!2 for - W72 < x < 0, 

A(x) = h!2 for 0 < x < W/2, and A (x) = 0 for x = 0. 

Thus the shape represented by equation 1.3 contains 

components of wavelength \m = 2WI(2m +1), 

m = 0,1,2,3 .... For large enough m, h^JL < 271, so those 

short-wavelength components will be damped. 

Conversely, the longest wavelength components 

Q^mIL < 27l)will tend to be enhanced. Because the shortwavelength components are what give the step its corners, 

the step should tend to become ramp-like, as observed. 

The foregoing discussion leads to the conclusion that
NATURE OF DEPOSITIONAL CONTACTS BETWEEN PYROCLASTIC DEPOSITS AND SNOW OR ICE 15

in general the shape, as well as the absolute position, of 

any contact between a pyroclastic deposit and an 

underlying substrate of either snow or ice will be 

substantially altered by melting; in particular, melting 

may exaggerate some irregularities on the contact while 

at the same time damping or destroying others. Almost 

certainly, then, the observed contact relation no longer 

contains information pertaining to scour of the substrate, 

and any attempt to make inferences about extent or 

mechanics of scour based on the contact relation is likely 

to be illusory. This frustrating conclusion seems 

unavoidable.

IMPLICATIONS OF THEORETICAL

AND EXPERIMENTAL RESULTS FOR

INTERPRETATION OF FIELD

OBSERVATIONS

Field observations illustrate some of the phenomena 

discussed in the preceding section, as well as some 

potential pitfalls of traditional stratigraphic 

interpretation. A useful initial example is provided by 

the observations of Mellors and others (1988) at Mount 

St. Helens of effects of a hot-rock avalanche from the 

dome that formed within the crater left by the 1980 

eruption. The avalanche occurred on May 9,1986, while 

substantial spring snowpack remained. Mellors and 

others (1988, p. 18-19) observed linear ridges within 

pyroclastic-flow deposits and noticed that some of these

-W/2

t

w

W/2 W

•>- x

FIGURE 1.5.—Definition of coordinate system for Fourier-series 

description of step in contact between snow and overlying 

pyroclasts.

ridges "coincided with furrows in the snow," as shown 

in figure 1.1. They went on to suggest that "differential 

erosion and melting of underlying snow may have 

contributed to this topography."

The observed contact relation in figure 1.1 could in 

principle have resulted simply from differential erosion. 

On the basis of the heat-transfer considerations discussed 

in the preceding section, however, one would also expect 

differential melting after deposition; an initial furrow 

beneath a ridge of hot material should tend to become 

both deeper and broader—that is, a V-shaped furrow 

should tend to become U-shaped—because of the same 

sort of spatial variability of heat conduction that altered 

a step to a ramp in the experiments illustrated by figures 

1.4 Cand 1 AD. Thus in the example drawn from Mellors 

and others (1988), differential erosion—or indeed any 

amount of erosion—cannot be unequivocally inferred 

simply on the basis of the contact geometry.

Observations from Redoubt Volcano in Alaska also 

illustrate the sort of interpretational ambiguities 

associated with hot deposits that melt their substrate. 

The eruption of December 14-15, 1989, produced 

unusual diamicts containing clasts of rock and glacier 

ice (Pierson and Janda, 1994; Waitt and others, 1994). 

When first observed (within a few days of the eruption), 

these deposits already were solidly frozen; the matrix 

largely comprised ice, apparently from eroded snow and 

refrozen meltwater. This deposit blanketed most of the 

piedmont lobe of Drift Glacier [informal name of Sturm 

and others (1988)], which drains northward from the 

summit crater, as well as parts of glaciers on other flanks 

of the volcano. Pierson and Janda (1994) and Waitt and 

others (1994) estimated that only about 10 to 20 percent 

of the deposit consisted of rock; accordingly, its presence 

on moderately steep slopes (more than 10°) strongly 

suggests that its liquid-water content at the time of 

deposition must have been small—otherwise the material 

would have easily flowed (cf. Nobles, 1966). It is hard 

to avoid the conclusion that the diamict was on average 

relatively cold, probably only slightly warmer than the 

freezing point, when it ceased to flow. [Local hot spots 

due to large rock clasts may have existed, however, and 

caused melting that reworked the surface of the diamict 

(Waitt and others, 1994).] Therefore, little melting of 

the substrate would be expected, consistent with my 

observations at Drift Glacier in July 1990, when 

subsequent volcanic activity had cut numerous deep 

ravines into the ice. Exposures along such ravines 

(fig. 1.6) showed the still-frozen diamict partly filling 

some crevasses, but bridging others; a plausible
16 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

interpretation is that the deposits had filled the snowfree upper parts of crevasses without melting the 

subjacent snow. Local relief at the contact between 

diamict and glacier ice was as much as a few meters, 

consistent with the scale of local irregularities that 

probably existed on the undisturbed glacier. Thus, the 

contact between the diamict and the underlying glacier 

ice probably underwent little, if any, post-depositional 

thermal modification. The same conclusion was reached 

by Waitt and others (1994).

The icy diamict deposits at Redoubt Volcano were 

subsequently blanketed by snow, which was in turn 

overridden and scoured by pyroclastic flows and surges 

that left deposits on top of the diamict. Contacts 

between pyroclastic deposits and diamict were relatively 

smooth compared to those between the diamict and 

glacier ice, an observation that lead Waitt and others 

(1994) to conclude that the "mechanically resistant" 

diamict" had undergone little if any erosion by the 

moving pyroclasts. By way of comparison, glacier ice, 

which is probably about as "mechanically resistant" as 

the icy diamict, is known to develop furrow-like

erosional scours during passage of hot flows that leave 

no deposits (Pierson and others, 1994; Waitt and others, 

1994).

The interpretation of Waitt and others (1994) is 

plausible, but there are alternatives. The diamict might 

have been continuously protected from flowing 

pyroclasts by a layer of snow that subsequently melted, 

thereby lowering the pyroclastic deposit onto the diamict. 

Yet another possibility is that the moving pyroclasts 

scoured completely through the blanket of snow and 

then moved directly over the diamict; in this case, melting 

of the diamict during passage of the pyroclasts, as well 

as after depostion thereof, might have occurred. Such 

melting would smooth out much of any erosional 

roughness that might have been created. It is difficult 

to see how the nature of the pyroclast-diamict contact 

permits an unambiguous reconstruction of physical 

processes at the pyroclast-diamict boundary.

Finally, I mention my observations near the western 

edge of the Drift Glacier's piedmont lobe, where 

pyroclastic deposits rested on snow. Such contacts 

showed very little small-scale relief and were much like

FIGURE 1.6.-Ice-cemented mixed-avalanche deposits on Drift Glacier, Redoubt Volcano, Alaska. The exposure is along a 

deep ravine carved into the glacier by repeated eruptions. Glaciological constraints (Paterson, 1994) suggest the exposed 

crevasses are probably about 20 meters deep.
NATURE OF DEPOSITIONAL CONTACTS BETWEEN PYROCLASTIC DEPOSITS AND SNOW OR ICE 17

that shown in Fig. 1.2. (I did not find exposures that 

would permit assessment of the relief over horizontal 

distances of many meters.) Snow immediately beneath 

the deposits, over a thickness of about 0.5 m, was typically 

rather hard, appeared to contain a lot of refrozen 

meltwater, and sometimes had a dirty cast. This hard 

snow was in places underlain by discontinuous ice lenses 

several centimeters thick, and these were in turn underlain 

by soft, clean snow. Because the thick blanket of 

pyroclastic deposits effectively insulates the snow from 

solar radiation and heat transfer from the atmosphere 

(Driedger, 1981), another heat source must be invoked 

to explain the apparent melting. I suggest that the 

pyroclastic deposits melted underlying snow to a depth 

of about 0.5 m subsequent to deposition (assuming a 

snow density of about 500 kg/m3), and that the melt 

water percolated downward before refreezing. Such a 

large amount of melting would have destroyed any 

contact features that could have provided information 

about mechanical interaction of the pyroclastic flow and 

the snow substrate. Alternatively, the refrozen meltwater 

within the snowpack might have been derived from 

melting of snow that fell on top of the pyroclastic 

deposits.

CONCLUSIONS

Eruptions of snow- and ice-clad volcanoes often cause 

substantial disruption of that snow and ice. To try to 

understand the mechanical interaction between a hot 

clastic flow and a snow- or ice substrate, inspecting the 

subsequent deposits and the contact geometry would 

seem to be standard, useful field methods. Unfortunately, 

traditional modes of stratigraphic and sedimentologic 

interpretation are questionable for the particular, and 

perhaps peculiar, case of pyroclastic deposits laid down 

on snow or ice. Post-depositional melting will tend to 

destroy precisely the sort of features, such as grooves or 

other indicators of scour, that might yield useful 

information. There seems to be no way around this 

phenomenon in the field.

Laboratory studies, albeit at much smaller scale than 

actual volcanic events, do hold promise for understanding 

the physics of interaction between hot clastic flows and 

snow or ice. Deposits can be literally frozen in place 

immediately after emplacement (for example, with a blast 

of liquid nitrogen) and subsequently dissected. Such 

laboratory sedimentologic studies, along with appropriate 

measurements made while hot flows are in motion, are 

in progress (Walder, 1992, 1995).

ACKNOWLEDGMENTS

I wish especially to thank Cynthia Gardner, Paul 

Delaney, and Roger Denlinger for their incisive reviews 

of earlier versions of this paper. Discussions with Jon 

Major and Chris Paola while at Redoubt Volcano, and 

with Richard Iverson, Richard Waitt and Edward Wolfe 

at the Cascades Volcano Observatory, helped clarify my 

thinking. Waitt and Wolfe each provided a photograph 

used in this chapter.

REFERENCES CITED

Alien, J.R.L., 1985, Principles of physical sedimentology: 

London, George Alien and Unwin, 272 p.

Banks, N.G., and Hoblitt, R.P., 1981, Summary of 

temperature studies of 1980 deposits, 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. 295-313.

Baer, M.R, and Nunziato, J.W., 1986, A two-phase mixture 

theory for the deflagration-to-detonation transition (DDT) 

in reactive granular materials: International Journal of 

Multiphase Flow, v. 12, no. 6, p. 861-889.

Carrier, G.R, and Pearson, C.E., 1976, Partial differential 

equations—theory and technique: New York, Academic 

Press, 320 p.

Carslaw, H.S., and Jaeger, J.C., 1959, Conduction of heat in 

solids (2d ed.): Oxford, Clarendon Press, 510 p.

Gas, R.A.F., and Wright, J.V., 1987, Volcanic successions, 

modern and ancient—A geological approach to processes, 

products and successions: London, Alien and Unwin, 

528 p.

Driedger, C.L., 1981, Effect of ash thickness on snow ablation, 

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. 757-760.

Levinson, Norman, and Redheffer, R.M., 1970, Complex 

variables: San Francisco, Holden-Day, 429 p.

Major, J.J., and Newhall, C.G., 1989, Snow and ice 

perturbation during historical volcanic eruptions and the 

formation of lahars and floods: a global review: Bulletin 

of Vblcanology, v. 52, p. 1-27.

Mellors, R.A., Waitt, R.B., and Swanson, D.A., 1988, 

Generation of pyroclastic flows and surges by hot-rock 

avalanches from the dome of Mount St. Helens volcano, 

USA: Bulletin of Vblcanology, v. 50, no. 1, p. 14-25.

Nobles, L.H., 1966, Slush avalanches in northern Greenland 

and the classification of rapid mass movements, in 

International symposium on scientific aspects of snow and 

ice avalanches, Davos, Switzerland, 1965: International 

Association of Scientific Hydrology Publication 69, 

p. 267-272.

Paterson, W.S.B., 1994, The physics of glaciers (3rd ed.): 

Oxford, Pergamon Press, 480 p.
18 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

Pierson,T.C., andJandaRJ., 1994, Volcanic mixed avalanches: 

A distinct eruption-triggered mass-flow process at snowclad volcanoes: Geological Society of America Bulletin, 

v. 106, no. 10, p. 1351-1358.

Pierson, T.C., Janda, R.J., Thouret, J.C., and Borrero, C.A., 

1990, Perturbation and melting of snow and ice by the 13 

November 1985 eruption of Nevado del Ruiz, Colombia, 

and consequent mobilization, flow, and deposition of 

lahars: Journal of Volcanology and Geothermal Research, 

v. 41, p. 17-66.

Ryan, M.P., Banks, N.G., Hoblitt, R.P., and Blevins, J.Y.K., 

1990, The in-situ thermal transport properties and the 

thermal structure of Mount St. Helens eruptive units, in 

Ryan, M.P. ed., Magma transport and storage: New York, 

John Wiley and Sons, p. 137-155.

Sturm, Matthew, Benson, C.S., and MacKeith, Peter, 1988, 

Recent glacier-volcano interactions on Mt. Redoubt, 

Alaska: Alaska Department of Natural Resources, Division 

of Geological and Geophysical Surveys, Report of 

Investigations, v. 88-9, 18 p.

Thouret, J.-C, 1990, Effects of the November 13, 1985 

eruption on the snow pack and ice cap of Nevado del Rufz 

volcano, Colombia, in Williams, S.N., ed., Nevado del

Rufz Volcano, I: Journal of Volcanology and Geothermal 

Research, v. 41, no. 1-4, p. 177-201.

Waitt, R.B., Jr., Pierson, T.C., MacLeod, N.S., Janda, R.J., 

Voight, Barry, and Holcomb, R.T., 1983, Eruptiontriggered avalanche, flood and lahar at Mount St. Helens— 

effects of winter snowpack: Science, v. 221, no. 4618, 

p. 1394-1397.

Waitt, R.B., Gardner, C.A., Pierson, T.C., Major, J.J., and 

Neal, C.A., 1994, Unusual ice diamicts emplaced during 

the December 15, 1989 eruption of Redoubt Volcano, 

Alaska: Journal of Volcanology and Geothermal Research, 

v. 62, nos. 1-4, p. 409-428.

Walder, J.S., 1992, Movement of pyroclastic flows over "snow": 

An experimental and theoretical study [abs.]: Eos 

(Transactions, American Geophysical Union), v. 73, no. 43 

(supplement), p. 612.

Walder, J.S., 1995, Role of fluidization in pyroclast/snow 

interactions and lahar generation [abs.]: Eos (Transactions, 

American Geophysical Union), v. 76, no. 46 (supplement), 

p. F199.

Whitaker, Stephen, 1977, Fundamental principles of heat 

transfer: Malabar, Florida, R.E. Krieger Publishing Co., 

556 p.
2:

TRANSFORMATION OF WATER FLOOD TO 

DEBRIS FLOW FOLLOWING THE ERUPTIONTRIGGERED TRANSIENT-LAKE BREAKOUT

FROM THE CRATER ON MARCH 19, 1982

By THOMAS C. PIERSON

CONTENTS

Abstract ........................................................................ 19

Introduction ................................................................. 19

Lake-breakout flood...................................................... 20

Transient-lake formation ......................................... 20

Transient-lake breakout........................................... 23

Discharge estimation............................................... 23

Flow transformation of the flood .................................. 24

Channel environment for flow transformation ........ 24

Sedimentologic evidence for transformation to

debris flow ............................................................ 28

Continued erosion and bulking by debris flow.............. 29

Factors for transformation to debris flow....................... 30

Factors for the March 19, 1982 flood...................... 30

Factors for other flow events.................................... 32

Mount St. Helens crater—Loowit Ravine,

smaller discharge ............................................. 33

North Fork Toutle River valley—Breakout of

small landslide-dammed lake ........................... 33

Mt. Rainier—Glacier-outburst floods................ 33

Summary and conclusions ............................................ 33

Acknowledgments......................................................... 35

References cited ............................................................ 35

ABSTRACT

A critical but poorly understood aspect of flash floods in 

mountainous terrain is their potential to incorporate large

volumes of eroded sediment and to transform to debris flows, 

thereby increasing in flood volume, peak discharge, flow 

velocity, erosional capacity, and hazard to life and property. 

A well-documented example of such a flow transformation 

occurred at Mount St. Helens on March 19, 1982, when an 

explosive eruption rapidly melted snow and ice, producing a 

transient lake that grew to a volume of 3 to 4 x 106 cubic 

meters before rapidly draining. The consequent flood, which 

was released in two major flood waves, transformed from water 

flow to debris flow as it progressed down the steep, deeply 

incised north flank of the volcano. Peak flood discharge 

approximately quadrupled and flow volume tripled in this 

transformation, which was accomplished over a distance of 

less than three kilometers.

Critical factors for the transformation of a water flood to 

debris flow in this type of setting seem to be high discharge 

(relative to channel capacity) and channel-bank height, slope, 

and erodibility. In this and several other examples, 

transformations have occurred after the initial water floods 

passed bank-to-bank through narrow channel reaches bounded 

by high (>20 meters), oversteepened (>45°) banks consisting 

of noncohesive sands and gravels—reaches where large-scale 

bank failures could introduce substantial volumes of 

unconsolidated sediment directly into the flow.

INTRODUCTION

Flash floods or sudden releases of large volumes of 

water, such as from dam failures, are in themselves

19
20 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

significantly hazardous hydrologic events. Under certain 

circumstances, however, water floods have eroded and 

incorporated sufficient sediment to transform to debris 

flows (O'Shea, 1954; Richardson, 1968; Jackson, 1979; 

Yesenov and Degovets, 1979; Blown and Church, 1985; 

Clague and others, 1985; Laenen and others, 1987; 

Driedger and Fountain, 1989). A flow transformation1 

from water to a dense, viscous slurry of water and rock 

debris can increase flood volume, peak discharge, flow 

velocity, and erosion capacity (bed shear stress) all by at 

least a factor of 5 (Pierson, 1980; Costa, 1984; Gallino 

andPierson, 1985; Pierson and others, 1990). Whether 

such transformations occur is of particular interest to 

those modeling dam-break floods in mountainous terrain 

(for example, Swift and Kresch, 1983; Laenen and Orzol, 

1987; Laenen and others, 1987). At present (1994), the 

factors responsible for transformation to debris flow are 

not understood and debris-flow transformations cannot 

be predicted.

A laterally directed blast from the south side of the 

Mount St. Helens lava dome heralded the onset of the 

March 19, 1982, eruption, during which hot 

volcaniclastic debris dynamically interacted on a massive 

scale with crater snow (Waitt and others, 1983; Waitt 

andMacLeod, 1987). A meltwater transient lake formed 

within tens of minutes and then drained rapidly, 

producing a lahar—a rapidly flowing mixture of water 

and sediment from a volcano (Smith and Lowe, 1991). 

The lahar initially had the characteristics of a water flood; 

it subsequently eroded and incorporated enough 

sediment to transform to a debris flow, which continued 

flowing downstream for 25 kilometers before 

transforming to hyperconcentrated flow (Waitt and 

others, 1983; Pierson and Scott, 1985). The debris-flow 

phase of the lahar continued to erode and incorporate 

sediment along most of this flowpath.

The fresh deposits left by the events of March 19 

provided an opportunity to document the conditions 

under which a transformation from water flow to debris 

flow occurred. This chapter documents in more detail 

than in previous accounts the formation of the transient 

lake that produced the flood, and it examines the factors

1 "Flow transformation" is used here to connote fundamental changes 

in rheology and flow behavior caused by an increase in sediment 

concentration. In contrast to this use, Fisher (1983) has limited 

the term to describe changes in the flow state (laminar or 

turbulent flow) and flow continuity (homogeneous or segregated 

flow).

constraining flood discharge. Channel characteristics 

then are described for the reaches in which 

transformation occurred and in which the debris flow 

continued to evolve. The purpose of the study was to 

determine which factors seemed most directly related to 

the transformation of a water flood to a debris flow.

LAKE-BREAKOUT FLOOD 

TRANSIENT-LAKE FORMATION

The initial explosion of the March 19,1982, eruption 

was directed against the steep south crater wall, where it 

dislodged most of the snow over a 120° sector on that 

500-m-high slope (Waitt and MacLeod, 1987). Snow 

and hot blast debris avalanched down off the crater wall, 

coalescing to form a 106 to 107 m3 mixed avalanche of 

snow and hot debris (see Pierson and Janda, 1994) that 

traveled around the dome and north out of the crater, 

reaching as far as 9 km from its source (Waitt and others, 

1983) (fig. 2.1). More explosions followed within the 

next 2 to 3 minutes, during which time dacite pumice 

began erupting. Hot pumice landed directly on the south 

crater floor and on the crater walls, from where it slid 

onto the crater floor. Heat from the pumice rapidly 

melted snowpack and snow-avalanche deposits on the 

crater floor, and large quantities of meltwater also flowed 

directly off the crater walls (Waitt and MacLeod, 1987).

Meltwater ponded over a thick snowpack in the moat 

formed at the south end of the crater between the lava 

dome and the crater walls, and it was filled with rapidly 

accumulating meltwater and juvenile deposits from the 

eruption. The water eventually formed a lake about 

0.3 km2 in area that varied from 8 to 15 m in depth and 

contained 3 to 4 x 106 m3 of water and floating pumice 

(fig. 2.2). The transient lake was partly impounded by 

the lava dome and by talus cones impinging from the 

east and west crater walls, but no obvious evidence of 

damming by other deposits was preserved at the outlets 

following lake drainage (Waitt and others, 1983).

Altitudes of the highest pumice-block strandline 

around the transient-lake shore generally ranged between 

about 1,938 m and 1,942 m; maximum lake level was 

approximately 1,942 m. Locally, pumice deposited 

directly by dome explosions or by landslides down the 

crater walls was present above the highest strandline. The 

area of the transient lake was covered by 4 to 8 m of 

juvenile pumice blocks, lapilli, ash, and sparse lithic 

fragments (Waitt and others, 1983). Collapse structures
CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84 21

Spirit Lake

123°00'

Hyperconcentrated streamflow Transition

, Transient lake 

1 shoreline approximate)

B

Mixed-avalanche deposit

Flow path of flood surges; arrow 

indicates direction of flow

Knickpoints or heads of very 

steep slopes

Snow avalanche tracks 

Measured cross section 

Talus cone

2 KILOMETERS

SCALE APPROXIMATE

FIGURE 2.1.—Flow path of the March 19, 1982, eruption-triggered flood and subsequent debris flow. Flow transformation to 

debris flow occurred on the north flank of the volcano (the Steps). Subsequently, the debris flow transformed into a 

hyperconcentrated water flood farther downstream. Map traced from uncorrected aerial photographs.
22 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

Pumice-block 

strandline

EXPLANATION

Lava dome 

Dome talus

Mixed-avalanche deposit 

--^f-f: Spillway location

Transient lake (maximum extent)

Crater wall darkened by 

blast deposits

Pumice-block accumulations 

Cataract headcuts in channel

FIGURE 2.2.—Position of highest transient-lake level, as interpreted from pumice-block strandlines and 

other shoreline features mapped from aerial photography of March 23, 1982.
CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84 23

up to 100 m in diameter formed in the snowpack beneath 

these deposits.

TRANSIENT-LAKE BREAKOUT

The transient lake drained approximately 

simultaneously through two vertically sided outlet 

channels up to 30-m deep on either side of the lava dome. 

The lack of physical evidence for the formation of natural 

dams suggests that the lake formed by hydraulic 

ponding—meltwater was supplied faster than it could 

drain out (Waitt and others, 1983). However, outflow 

was probably hindered significantly by floating pumice 

blocks, which may have jammed in the outlets. There 

were remnants of meters-thick subaerial pumice deposits 

preserved immediately above both outlets (Fig. 2.2), the 

lower parts of which were eroded away by lake outflow. 

Above 3—5 m of relatively dense volcanic deposits on 

the lake-floor were 1-3 m of rounded, openwork, very 

low-density pumice blocks, which had floated on the 

lake surface (Waitt and others, 1983). Rounding was 

apparently accomplished by abrasion during jostling 

while floating. Outflow through the spillways must have 

been at least partly impeded by rafts of interlocking 

pumice blocks several meters thick. Slush from 

descending snow avalanches also may have decreased the 

fluidity of the outflow (Waitt and others, 1983). In 

addition, some water percolated into and through the 

crater-floor snowpack, exiting at the east and west outlets 

through tunnels in the snow. Strandlines of pumice 

blocks also formed at levels lower than 1,938-1,942 m, 

indicating that drainage of the transient lake was not 

steady.

DISCHARGE ESTIMATION

Transient-lake breach geometry was determined from 

a large-scale topographic map (scale 1:2,000, 2-m 

contour interval) drawn from June 23, 1982 aerial 

photography and from low-level aerial photographs taken 

four days after the eruption. Cross sections of the outlets 

were intermediate in shape between triangles and 

trapezoids; the east breach had a top width of about 135 

m, a bottom width of about 15m, and a height of about 

10m, and the west breach had a top width of about 158 

m, a bottom width of, at most, 50 m, and a height of 

about 7 m. Discharge equations for triangular and for 

trapezoidal breaches were used to bracket maximum 

possible discharge through the two outlets. Use of either

equation infers that maximum possible instantaneous 

discharge can be calculated by assuming critical flow 

through the outlets. The trapezoidal equation used here 

is a general form of the broad-crested weir equation, 

modified for dambreaks through trapezoidal sections 

(Price and others, 1977; Costa, 1988):

= 0.3 g°-5 H1 - 5 (0.4 £ + 0.6 T) (2.1)

The general equation for critical flow through triangular 

channels (King, 1954, p. 8-10) is:

(2.2)

For both equations

Q is discharge (Q is maximum possible discharge), 

g is the acceleration of gravity, 

l/z is average slope of the breach side wall, 

H is height of the water surface above breach base

upstream from spillway prior to failure, 

b is the width of the breach base, 

and

T is top width of the breach at the reservoir water 

level.

FIGURE 2.3.—View to the northeast out of the crater breach, 

where the water flood left the crater floor and entered the 

deeply incised ravines on the north flank. Dark bands at 

flow margins in foreground are mixed-avalanche deposits, 

which are about 500 meters apart, measuring from outer 

edges. The flood swath and Spirit Lake are visible in the 

distance.
24 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

The two equations yielded discharges of 2,585 and 

2,689, rounded to 2,600 m3/s for flow through the east 

spillway, and 1,828 and 1,688, rounded to 1,800 m3/s 

for flow through the west spillway. If the spillways were 

partly clogged by pumice blocks, initial cross-sectional 

dimensions may have been smaller than the final 

dimensions, and the computed discharges would be 

maximum values. Had the outflows been synchronous, 

maximum lake discharge would have been about 

4,400 m3 /s, but the peak outflows from these two 

breaches are assumed not to have coincided, because two 

distinct flood surges were interpreted farther downstream. 

Therefore, the maximum outflow rate from the transient 

lake was between 2,600 and 4,400 m3/s.

FLOW TRANSFORMATION 

OF THE FLOOD

The flood followed the path of the mixed avalanche 

around the dome, coalesced into a single channel, and 

flowed north across the crater floor and out the crater

breach (figs. 2.1, 2.3, 2.4). On both sides of the dome 

the flood cut channels 10 m to 20 m deep and about 

120 m wide. Trains of cataracts as much as 40 m deep 

formed in both channels. North of the dome the unified 

flood channel was about 200 m wide. While confined 

in the crater (average gradient 0.164 m/m), the flood 

incorporated much of the avalanche deposit, the preemption snowpack, and the crater-floor deposits that it 

scoured, but it remained a water flood, as evidenced by 

deposit sedimentology.

CHANNEL ENVIRONMENT FOR 

FLOW TRANSFORMATION

After exiting the crater, the flood split again and 

entered two deeply incised and slightly steeper channels 

(average gradient 0.184 m/m), that guided it down the 

north flank of the volcano, still largely following the 

avalanche path (figs. 2.1, 2.5, 2.6). The small canyon 

on the east side of the flow path, Loowit Ravine, was 

completely overwhelmed by the flood, and the channel

FIGURE 2.4.—View south into crater from head of Loowit Ravine. Lava dome, west spillway of transient lake (between 

dome and crater wall), and part of the darkened (eroded) rear crater wall are visible. Crater-floor flood channel visible 

in foreground. Flood and avalanche swath is about 500 meters wide.
CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84 25

FIGURE 2.5.—Oblique aerial view (looking south) of the crater floor and breach, the eroded topography of the 

volcano's lower north flank (the Steps), and the head of the Pumice Plain (Step Fan). Flood and avalanche 

swath on crater floor is about 500 meters wide.
26 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

and a wide swath of surrounding ground surface were 

scoured. Step Ravine, on the west side, carried a large 

part of the mixed avalanche but only a small fraction 

(probably less than 10 percent) of the flood.

The first kilometer of Loowit Ravine has a gradient of 

0.141 m/m and is deeply and narrowly incised in 

unconsolidated volcaniclastic deposits. Ravine sides are 

50 m to 70 m high and almost continuously steeper than 

45°, which means they are steeper than the angle of repose 

of the material. Post-flood reconnaissance indicated that 

massive bank failures occurred in this reach during the 

flood. On exiting the deeply incised reach, much of the 

flow then plunged over a 70-m-high waterfall and 

continued down a less deeply incised channel (gradient 

0.149 m/m). In this lower reach of Loowit Ravine, flow 

continued to scour and spilled over into the lower end of

Step Ravine. For the entire length of Loowit Ravine, 

channel banks ranged from about 10 m to 70 m high, 

and for at least 74 percent of its length, banks were steeper 

than 45°. The largest share of incorporated sediment 

probably came from mass failures of these high, steep 

banks.

At the head of the pyroclastic debris fan (Pumice Plain) 

where both ravines debouched (fig. 2.1), characteristic 

debris-flow deposits were present instead of fluvial 

gravels, indicating that flow transformation had occurred 

over a distance of only 2.7 km. Lithologically and 

texturally distinct depositional units, Unit 1 and Unit 2 

(figs. 2.7, 2.8), were in sharp contact wherever they 

occurred together. The units are inferred to have been 

deposited by two separate flow pulses (Flow 1 and 

Flow 2), which are assumed to have originated from

METERS

2000^

1900 — 

1800 — 

1700- 

1600- 

1500

Head of

Loowit

Ravine

1000 1000

METERS 

1800 —

1700 — 

1600 — 

1500 — 

1400 — 

1300 — 

1200 — 

1100 —

LOOWIT RAVINE

B

Pumice 

Plain

2000 3000 4000

DISTANCE, IN METERS 

Vertical exaggeration x1.6

FIGURE 2.6.—Longitudinal profile of flood flow path: A, on crater floor (from east spillway to head of Loowit 

Ravine) and B, continuation of A on north flank of the volcano (along axis of Loowit Ravine).
CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84 27

FIGURE 2.7.—Unit 1 debris-flow deposit at head of Pumice Plain, about 6 kilometers downstream from flood source. 

Shovel (75 centimeters long) is resting against a flow lobe, a primary debris-flow depositional feature.

FIGURE 2.8.—Lobe of Unit 2 debris-flow deposit at head of Pumice Plain. Shovel is 75 centimeters long.
28 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

unsynchronized breaching of the transient lake through 

the two outlets. Flow 1 descended both ravines and 

scoured much of the lower interfluve between the ravines; 

Flow 2 had a smaller discharge and was confined to 

Loowit Ravine. When first examined in the field, less 

than 24 hours after deposition, the two distinct (and 

now frozen) debris-flow deposits overlay or cut across 

the mixed-avalanche deposit. The debris-flow units were, 

in turn, locally overlain or cut by thin fluvial deposits, 

which were apparently emplaced by small dilute 

discharges during the waning stages of the flood. This 

depositional sequence is typical of debris-flow tails 

(Pierson, 1986).

Flow exiting Step Ravine (Flow 1) left mudlines that 

defined a channel cross section about 230 m wide and 

about 4 m deep (cross section^, fig. 2.1), but not all the 

flow was conveyed past this cross section. Flow also exited 

Loowit Ravine, but a flow cross section was not 

established there. If a mean velocity of 10 m/s is assumed 

for the Step Ravine section (a conservative velocity for 

flow of this magnitude in a steep channel, see Janda and 

others, 1981; Gallino and Pierson, 1985; Pierson, 1985; 

Pierson and others, 1990), an instantaneous discharge 

of 9,200 m3/s is obtained for that part of the flow. Farther 

out on the Pumice Plain where all of Flow 1 combined 

to form an irregular unconfined sheet of flowing debris 

roughly 1,800 m wide, mudlines left on remnants of the 

mixed-avalanche deposit indicated a flow depth of as 

much as 3 m. If mean depth were 1.0m and mean 

velocity 5 m/s (also a conservative value for a large flow), 

peak discharge for Flow 1 across the upper Pumice Plain 

would be 9,000 m3/s. These rough estimates indicate 

that debris-flow peak discharge was on the order of 104 

m3/s for Flow 1, at least double the maximum possible 

outflow discharge from the transient lake. Total volume 

of the debris flow as it traversed the upper Pumice Plain, 

assuming a sediment concentration by volume of at least 

0.70 from the incorporation of eroded debris and adding 

this amount to the water flood (Pierson and Scott, 1985), 

would have been about 10.0 to 13.3 x 106 m3 , 

approximately three times the original volume of the 

transient lake.

SEDIMENTOLOGIC EVIDENCE FOR 

TRANSFORMATION TO DEBRIS FLOW

Debris-flow sediments differ from normal fluvial 

sediments in their characteristic lack of stratification, 

poorly sorted texture, random or poorly developed clast

fabric, fluid shear strength (evidenced by transportation 

of large clasts over low gradients and steep terminal lobes), 

and boulder levees (Costa, 1984). These criteria were 

largely met in the deposits present on the Pumice Plain 

(Pierson and Scott, 1985). The differences between the 

two deposits also provide evidence for two distinct flow 

pulses (fig. 2.9).

The stratigraphically lower Pumice Plain deposit 

(Unit 1) was a dark- to medium-gray, muddy, sandy 

gravel consisting predominantly of lithic (nonvesicular) 

volcanic rock debris. Sampled deposits (excluding clasts 

>64 mm) were unstratified, ungraded, and very poorly 

sorted (Oj >3.00). Unit 1 was deposited as a 10- to 

50-cm-thick sheet over much of the Pumice Plain 

(average gradient 0.04 m/m), but boulders as large as 

2 m in diameter were stranded on broad areas of this fan 

(Pierson and Scott, 1985), which was evidence that the 

flow depth was greater than the deposit thickness. Lobate 

flow fronts and boulder levees were preserved on the west 

edge of the Pumice Plain where this unit was not 

overridden by Flow 2. Similarities in color and lithology 

between Unit 1 and the mixed-avalanche deposit indicate 

that the first flood surge from the crater obtained much 

of its sediment load by remobilizing the mixed-avalanche 

deposit that lay in its path.

100

99.9

P 

I 99

95

80

E 50

20

5

1

0.1

_ Unit 1(15) 

Unit 1 (5) -v \ ^^s?

l{ \- Unit 2 (15) 

•Unit 2 (5)

-8 

256

-6 

64

-A 

16

-2 

4

2468

0.25 0.0630.0160.004

oo phi 

0 mm

GRAIN DIAMETER

FIGURE 2.9.—Cumulative frequency curves of samples of 

debris-flow deposits (clasts larger than 64 millimeters not 

sampled) from upper North ForkToutle River. Numbers 

in parentheses are distances in kilometers from transient 

lake in the crater. Y-axis is normal probability scale. Grain 

diameter in phi units is the negative logarithm to base 2 

of grain diameter in millimeters.
CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84 29

The second Pumice-Plain deposit (Unit 2) was a 

grayish brown, pumice-rich, muddy, gravelly sand, having 

more fine material in its matrix than Unit 1 (fig. 2.9). 

Like Unit 1, sampled deposits (excluding clasts >64 mm) 

were unstratified, ungraded, and very poorly sorted. 

Unit 2 was generally thinner (5 to 20 cm) than Unit 1, 

but the flow that deposited it also had sufficient depth 

and competence to transport numerous boulders 

(fig. 2.10). Pumice fragments in this deposit were mostly 

yellowish brown, weathered clasts that had been scoured 

from the crater floor and north-flank channels.

CONTINUED EROSION AND BULKING 

BY DEBRIS FLOW

Average erosion measured over two North Fork 

Toutle River channel cross sections on the Pumice Plain 

(LO030 and LO040) was 0.5 m and 1.2 m net scour

(table 2.1). Where debris flow was unconfined, 

significant erosion did not occur (fig. 2.10), but where 

flow was confined, it incised the pumiceous fan deposits 

and formed steep-sided gullies (fig. 2.11). Parts of both 

debris flows entered Spirit Lake (fig. 2.1); the rest 

followed anastomosing channels down the North Fork 

Toutle River valley. The multiple channels eventually 

coalesced about 13 km downstream.

Peak flow depths (Flow 1) in the North Fork Toutle 

River channel from 13 km to 26 km downstream from 

the crater ranged from about 4 m to 8 m, and net 

downcutting of the channel (with no widening) averaged 

about 1 m to 3 m (table 2.1), locally exceeding 6 m 

(fig. 2.12). Along this reach, which has an average 

gradient of 0.020, the North Fork Toutle River channel 

was incised into the highly erodible 1980 debrisavalanche deposit (Voight and others, 1983; Pierson and 

Scott, 1985). Peak flow velocities, indirectly computed 

using superelevations at two cross sections at the upper

FIGURE 2.10.—Thin (few centimeters thick) Unit 2 debris-flow deposit on lower Pumice Plain; boulders stranded 

when finer-grained slurry continued flowing downstream. Shovel is 75 centimeters long.
30 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

TABLE 2.1 .—Erosion index values (positive = net erosion) for monumented cross sections on the North Fork Toutle River. Cross-sectional 

area changes computed from preemption surveys in early March and again following the eruption on March 20, 1982

[Data supplied by D.F. Meyer, U.S. Geological Survey]

Cross section

LO030

LO040

NF130

NF310

NF320

NF350

Channel distance 

downstream from 

crater (kilometers)

4.2

6.4

15.7

21.1

22.5

25.1

Channel width 

(meters)

538

238

111

424

280

486

Change in 

cross section area 

(square meters)

618

122

313

688

455

104

Erosion index 

(meters)

1.2

0.52

2.8

1.6

1.6

0.2

2 This channel carried only a minor part of the flow, so value not directly comparable to other cross sections, which carried all the flow.

end of the reach, were 8.5 m/s and 10.0 m/s. Although 

peak flow decreased through this reach from 6,800 m3/s 

to 3,700 m3/s (Pierson and Scott, 1985), total volume 

(both flows) increased to about 1.1 to 1.9 x 107 m3 , 

nearly five times the original lake outflow volume 

(R.J. Janda, U.S. Geological Survey, written commun., 

1983; Pearson, 1986, p. 90) as the flows continued to 

bulk up with eroded wet sediment. Along this entire 

reach, Units 1 and 2 were separated by a sharp 

stratigraphic contact and maintained distinctive textural 

differences (fig. 2.13).

Downstream from the toe of the 1980 debrisavalanche deposit, flow entered the catch basin of the 

N-l sediment dam (fig. 2.1), which induced deposition 

of about 7.7 x 106 m3 of debris slurry (Pearson, 1986, 

p. 90). From that point on, the remaining debris flow 

(mostly Flow 2) began to transform through dilution to 

hyperconcentrated streamflow (Waitt and others, 1983; 

Pierson and Scott, 1985).

FACTORS FOR TRANSFORMATION 

TO DEBRIS FLOW

For flowing water to increase its volumetric sediment 

concentration from less than 0.20 to 0.65 or more and, 

thus, become a debris flow, a number of conditions must 

be satisfied: (1) enough sediment, in proportion to 

available water, must be available for incorporation; 

(2) sediments must be sufficiently poorly sorted to trap

pore water and facilitate partial liquefaction of the 

sediment-water mass; and (3) there must be an adequate 

mechanism for effectively mixing the sediment and water.

FACTORS FOR THE 

MARCH 19, 1982 FLOOD

On March 19, 1982, necessary and sufficient 

conditions for transformation of water flood to debris 

flow were present in the 2.7-km-long Loowit Ravine, 

on the north flank of the volcano (fig. 2.1). 

Approximately 7 to 9 x 106 m3 of sediment were added 

to the floodwater at a minimum average rate of 

2.5 x 106 m3/km as the flood exited the crater. The 

added sediments were loose and very poorly sorted, and 

the steep channel gradient (including one large waterfall) 

ensured turbulent mixing. It is noteworthy that 

transformation did not occur along the 1.7-km-long 

channel still within the crater, which was actually steeper 

than the two individual subreaches of Loowit Ravine 

(0.164 compared to 0.141 and 0.149) and which had 

bed and bank material that was equally as credible as 

the material in the banks in Loowit Ravine. Although 

the crater reach was slightly shorter than Loowit Ravine, 

other floods have transformed to debris flows along an 

even shorter channel reach at Mt. Rainier (table 2.2).

The key differences, between reaches that induce 

transformation of water flow to debris flow and those 

that do not, seem to be related to channel morphology.
CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84 31

FIGURE 2.11.—Gullying left by erosion of 1980 pumiceous pyroclastic-flow deposits by debris flow 

on the lower Pumice Plain. Shovel is 75 centimeters long.
32 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

TABLE 2.2.-Characteristics of flood channels at Mount St. Helens and Mount Rainier, where water floods

have transformed to debris flows

Approximate 

peakQ 

(cubic meters 

Event per second)

Mount St. Helens, 1500 - 

3-19-82, 4500 

Loowit Ravine

Mount St. Helens, <500

2-4-83, (est.) 

Loowit Ravine

Elk Rock dam- 450 

break flood, 

8-27-80

Mt. Rainier, 550

Tahoma Creek, 

7-26-88

Reach 

Reach gradient Average 

length (meters per Bed/bank bank height 

(meters) merer) material (meters)

2,670 0.184 Unconsolidated 10-70 

gravelly sand

2,670 0.184 Unconsolidated 10-70

gravelly sand

6,500 0.027 Unconsolidated 40-60 

gravelly sand

-1,000 0.14 Unconsolidated 20-30

sandy gravel, 

locally ice-cemented

Bank 

conditions

74 percent 

of reach has 

banks steeper 

than 45°

Same as

3-19-82

80 percent of 

reach for first 

2 km has

banks steeper 

than 45°

Banks near

vertical

Loowit Ravine was deeply incised, having bank heights 

that ranged from 10 to 70 m overall and that were higher 

than 50 m along most of the first 1,000 m. Channel 

narrowness, relative to the discharge of flow passing 

through, allowed the flood to undercut and destabilize 

both banks. More than 74 percent of the total reach 

length (and almost all of the first 1,000 m) had banks of 

Unconsolidated volcaniclastic deposits that had slopes 

steeper than 45°. This steepness means that the slopes 

were very susceptible to mass failure when undercut. 

Post-flood reconnaissance revealed that numerous bank 

failures had occurred. In contrast, the channel in the 

crater floor was 10 to 40 m deep but offered fewer 

unstable banks and little bank erosion occurred there. 

Large-scale mass failure of the high, oversteepened 

channel banks in Loowit Ravine seems to have been 

critical in transforming the water flood to debris flow. 

Temporary ponding of flow by bank collapses may have 

occurred, as has been inferred for debris-flow

transformations at Mt. Shasta (Osterkamp and others, 

1986), but because of the steep channels and high 

discharge, any such ponding could only have been 

momentary.

FACTORS FOR OTHER FLOW EVENTS

Three other examples of transformation from water 

flood to debris flow at Mount St. Helens and Mt. Rainier 

have occurred within the last decade (table 2.2), and high, 

unstable channel banks seem to have been important at 

these sites, as well. Peak discharges and channel gradients 

were variable and not particularly high, so especially steep 

channels or anomalously high flood discharges apparently 

were not required to effect the transformation. All the 

reaches did have poorly sorted, Unconsolidated bed and 

bank materials that were highly erodible, and the 

channels were cut in recent, unvegetated materials. 

Additionally, all of the reaches had narrow cross sections
CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84 33

that had high (>20 m), oversteepened (>45°) banks. High 

flows in these channels impinged upon and destabilized 

the banks, causing large volumes of sediment to collapse 

into the channels.

MOUNT ST. HELENS CRATER—LOOWIT 

RAVINE, SMALLER DISCHARGE

A much smaller water flood generated by an explosive 

eruption in February 1983 (Pierson and Waitt, chap. 4, 

this volume) followed the same flow path as did the flood 

of March 19, 1982 (table 2.2). This flood, that had a 

peak discharge of probably a few hundred cubic meters 

per second, remained a water flood in the crater reach 

and transformed to a debris flow in Loowit Ravine. 

However, the debris-flow volume was so small that the

LU

710

705

1 I I I I \

13 kilometers downstream

from transient lake

LUCO 70°

LLI

O 695

m

LIJ 690

Q

D

b 685

_l

--• 680 

CO 0

S 57°

LLI

!> 565

Uj 560 

U

I

CO

Q

50 100 150 200 250 300 350 400

555

550

:± 545

CC. 

Ill 

> 540

I I I I I

20 kilometers downstream 

from transient lake

February 24, 1982 

March 20, 1982 

Net erosion 

Net deposition

I I I

50 100 150 200 250 300 350 

HORIZONTAL DISTANCE, IN METERS

400

FIGURE 2.12.—Cross sections of North Fork Toutle River 

channel where it is cut into the 1980 debris-avalanche 

deposit at distances of about 13 kilometers and 

20 kilometers downstream from the transient lake. 

Average gradient between the cross sections is 0.020.

flow did not travel beyond the Pumice Plain.

NORTH FORK TOUTLE RIVER VALLEYBREAKOUT OF SMALL 

LANDSLIDE-DAMMED LAKE

Near the downstream end of the 1980 debrisavalanche deposit, Elk Rock Lake breached on 

August 27, 1980 and produced a flood wave that was 

hyperconcentrated (volumetric sediment concentration 

42 percent) 30 km farther downstream (Randy Dinehart, 

U.S. Geological Survey, written commun., 1980). The 

flood wave almost certainly reached debris-flow 

concentration initially. Peak discharge at the spillway 

was estimated to have been 450 m3/s (table 2.2) (Jennings 

and others, 1981). The flood started by flowing down a 

narrow, 2-km-long, 0.027-gradient channel reach, where 

more than 80 percent of the channel banks were 40 to 

60 m high and steeper than 45°.

MT. RAINIER—GLACIER-OUTBURST FLOODS

Glaciers at Mt. Rainier have long been producing 

glacier-outburst floods that transform to debris flows 

(Richardson, 1968; Driedger and Fountain, 1989). One 

that originated from South Tahoma Glacier on 

July 26, 1988 (Walder and Driedger, 1994) had a peak 

discharge of about 550 m3/s at a point several kilometers 

downstream from the glacier (table 2.2). Transformation 

occurred in a 1-km-long, O.l4-gradient reach bounded 

by 20- to 30-m-high, near-vertical banks consisting of 

partly ice-cemented glacial till.

SUMMARY AND CONCLUSIONS

The March 19, 1982, explosive eruption at Mount 

St. Helens produced a prodigious volume of mehwater 

in a matter of tens of minutes. The topography of the 

crater floor, combined with the eruption of a large 

amount of low-density pumice, created a situation where 

meltwater was accumulated and then released in two 

major flood waves. After flowing out of the crater, the 

meltwater flood waves transformed to debris flows in a 

steep, narrow, 2.7-km-long channel reach on the north 

flank of the volcano. During the transformation, peak 

discharge and flow volume more than doubled. Once 

transformed, the debris-flow flood (still in two separate 

flood waves) continued to erode and bulk up in volume
34 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

30

20

DC

^

O 10

LJJ

UNIT 2

Sample 32082-2

-8 -6 -4 -2 

256 64 16 4

0 2 4 6 8 °°phi 

1 0.25 0.063 0.016 0.004 0 mm

30

20

15

10

I I I I I I I I

UNIT1

Sample 32082-3

-8 -6 -4 -2 

256 64 16 4

0 2 4 6 8 °°phi 

1 0.25 0.063 0.016 0.004 0 mm

Fluvial 

gravels

Terrace surface

Unit 2 KS*o^.SteS

O O O O

CO 

DC 

LJJ ID

oQoo

MARATTA CREEK CONFLUENCE

15 kilometers downstream

from crater

0 2 4 6 8 °°phi 

1 0.25 0.063 0.016 0.004 0 mm

Mudline

i i i i

UNIT 2

Sample 32582-2B

O

Lahar deposit 

May 18, 1980

o

0.25 0.063 0.016 0.004 0 mm

Channel bed 

March 25, 1982

Mudline o o

Unit 2

O

o o

Fluvial 

gravels

NEAR TOE OF DEBRIS-AVALANCHE DEPOSIT 

28 kilometers downstream from crater

COtr

LLJ 

LLJ

FIGURE 2.13.—Stratigraphic relations of Unit 1 to Unit 2 at distal end of channel reach carrying debris flow. Histograms of 

sample matrix and small-clast grain-size distributions are shown; clasts larger than 64 millimeters not sampled.
CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84 35

for the next 25 km, eventually reaching a flow volume 

roughly 5 times that of the initial flood.

The March 19, 1982 water flood transformation to 

debris flow is one of several to have occurred at northwest 

volcanoes in the last decade. Not all of these other cases 

occurred in especially steep channels and none had large 

(>500 m3/s) discharges. But, like the March 19, 1982 

flood, all of them did occur after the initial water floods 

passed through narrow channel reaches bounded by high 

(>20 m), oversteepened (>45°) banks consisting of poorly 

sorted, unconsolidated, and unvegetated deposits. 

Therefore, channels that have such morphology and bank 

materials may be prime sites for transformation of water 

floods to debris flows.

ACKNOWLEDGMENTS

This study has benefitted from numerous discussions 

with U.S. Geological Survey colleagues at the Cascades 

Volcano Observatory. Waite Osterkamp and Jim 

O'Connor provided very helpful reviews of the 

manuscript.

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1990, Perturbation and melting of snow and ice by the 

13 November 1985 eruption of Nevado del Ruiz, 

Colombia, and consequent mobilization, flow, and 

deposition of lahars: Journal of Volcanology and 

Geothermal Research, v. 41, p. 17-66.

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, no. 10, 

p. 1511-1524.

Price, J.T., Lowe, G.W, and Garrison, J.M., 1977, Unsteady 

flow modeling of dam-break waves, in Proceedings of dambreak flood routing model workshop, Bethesda, MD, 1977: 

Washington, D.C., U.S. Water Resources Council, 

Hydrology Committee, p. 89-130. [available from National 

Technical Information Service, Springfield, VA 22161 as 

NTIS Report PB-275-437.]

Richardson, Donald, 1968, Glacier outburst floods in the 

Pacific Northwest, in Geological Survey research 1968: 

U.S. Geological Survey Professional Paper 600-D, 

p. D79-D86.

Smith, G.A., and Lowe, D.R., 1991, Lahars—Volcanohydrologic events and deposition in the debris flowhyperconcentrated flow continuum, in Fisher, R.V., and 

Smith, G.A., eds., Sedimentation in volcanic settings: 

SEPM (Society for Sedimentary Geology) Special 

Publication 45, p. 59-70.

Swift, C.H., III, and Kresch, 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.

Voight, Barry, Janda, R.J., Glicken, Harry, and Douglass, P.M., 

1983, Nature and mechanics of the Mount St. Helens 

rockslide-avalanche, 18 May 1980: Geotechnique, v. 33, 

no. 3, p. 243-273.

Waitt, R.B., and MacLeod, N.S., 1987, Minor explosive 

eruptions at Mount St. Helens dramatically interacting 

with winter snowpack in March-April 1982, in Schuster, 

J.E., ed., Selected papers on the geology of Washington: 

Washington Department of Natural Resources, Division 

of Geology and Earth Resources Bulletin 77, p. 355-379.

Waitt, R.B., Jr., Pierson, T.C., MacLeod, N.S., Janda, R.J., 

Voight, Barry, and Holcomb, R.T., 1983, Eruptiontriggered avalanche, flood, and lahar at Mount St. Helens— 

Effects of winter snowpack: Science, v. 221, no. 4618, 

p. 1394-1397.

Walder, J.S., and Driedger, C.L., 1994, Geomorphic change 

caused by outburst floods and debris flows at Mount 

Rainier, Washington, with emphasis on Tahoma Creek 

valley: U.S. Geological Survey Open-File Report 93-4093, 

93 p.

Yesenov, U.Ye., and Degovets, A.S., 1979, Catastrophic 

mudflow on the Bol'shaya Almatinka River in 1977: Soviet 

Hydrology-Selected Papers, v. 18, no. 2, p. 158-160.
3:

SEDIMENT TRANSPORT IN THE HYPER

CONCENTRATED PHASE OF THE

MARCH 19, 1982, LAHAR

By RANDAL L. DiNEHART1

CONTENTS

Abstract ........................................................................ 37

Introduction ................................................................. 38

Purpose and scope ................................................... 38

Flow characteristics observed at gaging stations ............. 38

Kid Valley ............................................................... 40

Tower Road............................................................. 40

Highway 99 ............................................................ 41

Castle Rock............................................................. 42

Vblcanism-related observations................................ 42

Analyses of sediment-transport measurements............... 43

Computation of stream discharge............................ 43

Analyses of sediment concentration......................... 46

Computation of sediment discharge........................ 47

Comparison of lahar with storm flow............................ 50

Prediction and measurement of lahar behavior.............. 50

Summary ...................................................................... 51

References cited ............................................................ 52

ABSTRACT

Hyperconcentrated lahars occur rarely, and are even more 

rarely observed. Synoptic measurements of an eruptiontriggered lahar from Mount St. Helens were made on 

March 19—20, 1982, at three gaging stations encompassing 

37 kilometers on the Toutle River. Measurements were made 

of surface velocity, stream depths and widths, stream discharge, 

stream temperature, and river stage; flow samples (depth 

integrated or dipped) were collected periodically. Decreasing

Current address: U.S. Geological Survey, Water Resources Division, 

Placer Hall, 6000 "J" Street, Suite 2012, Sacramento, CA 95819

amounts of sediment, from 4.9 to 3.1 million megagrams, 

were transported past the three gaging stations, and 1.8 million 

megagrams of sediment were deposited in the Toutle River 

between the gaging stations at Kid Valley and Highway 99.

Much of the sediment transport occurred at 

hyperconcentrated levels (40 to 80 percent sediment by weight) 

during passage of the lahar, and the flow characteristics were 

different from commonly observed sediment transport in 

several respects. An extraordinary amount of sand was 

transported at stream discharges typical of storm flow. The 

high-density flow supported large quantities of woody debris 

and hot, pumiceous dacite clasts. Rapid deposition of sand, 

dramatic transitions in flow regimes, and dampened flow 

turbulence were described in eyewitness accounts at each 

gaging station.

Rapid fluctuations in channel cross-section geometry and 

in flow composition were measured as the lahar passed the 

gaging stations. Peak stream discharge decreased by more than 

50 percent between Kid Valley and Highway 99. The peak 

sediment concentrations also decreased from an estimated 

1,600,000 milligrams per liter at Kid Valley to 980,000 

milligrams per liter at Highway 99. The fine-material 

concentration persisted in the range 250,000 to 350,000 

milligrams per liter for three hours, which enabled the watersediment mixture to transport unusually large sediment loads. 

Computations of sediment discharge for sand and fine-material 

size classes showed that sand discharges decreased in a 

downstream direction; however, fine-material discharges were 

similar at three Toutle River gaging stations. These trends 

confirmed the observed magnitude of sand deposition along 

the channel and accounted for the paucity of fine material in 

the lahar deposits.

37
38 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

INTRODUCTION

On the evening of March 19, 1982, the explosive 

phase of a dome-building eruption at Mount St. Helens 

generated a snowmelt water flood that transformed to a 

lahar and entered the North ForkToutle River as a debris 

flow (Waitt and others, 1983; Pierson and Scott, 1985; 

Pierson, chap. 2, this volume). The debris flow phase of 

the lahar evolved into a hyperconcentrated flow (40—80 

percent sediment by weight; 530,000-1,590,000 mg/L, 

Beverage and Culbertson, 1964). The front of the lahar 

transformed from debris flow to hyperconcentrated flow 

along a channel reach 27 to 43 km from the crater and 

source area (Pierson and Scott, 1985). The debris flow 

had transformed before reaching the U.S. Geological 

Survey (USGS) gaging station on the North ForkToutle 

River at Kid Valley and traveled about 50 km downstream 

from the gage before dilution below hyperconcentration. 

Hyperconcentrated phases of lahars were termed "laharrunout flow" by Pierson and Scott (1985) and Scott 

(1988). The term lahar has been assigned broader 

meaning than it had previously, which now includes the 

debris-flow and the hyperconcentrated phases (Smith and 

Lowe, 1991).

Stream-gaging stations were established along the 

Toutle River following the mudflow-producing eruption 

of Mount St. Helens on May 18, 1980, in order to 

monitor sediment discharge from the rapidly eroding 

debris avalanche near the crater. Sediment-laden storm 

flows were measured often for stream and sediment 

discharge by USGS crews at the stream gaging stations 

during the two years prior to March 19, 1982. 

Equipment and techniques were well developed, and 

channel geometry and flood behavior had been recorded 

often before the measurements of the hyperconcentrated 

flow.

This chapter documents the lahar observations and 

the analyses of hydraulic and sediment transport 

measurements of the lahar as it passed three gaging 

stations in the Toutle River. The farthest-upstream stage 

hydrograph of the lahar was recorded on the North Fork 

Toutle River at Kid Valley. Stage hydrographs of the 

lahar were also recorded at two gaging stations on the 

main stem of the Toutle River. The flow remained in 

the hyperconcentrated range for several hours throughout 

the Toutle River system. The flow was finally diluted 

below the hyperconcentrated range by rapid 

sedimentation and by mixing with streamflow of the 

Cowlitz River. The diluted flow was measured and 

sampled at a fourth gaging station located on the Cowlitz 

River at Castle Rock.

PURPOSE AND SCOPE

Before March 19, 1982, the flow behavior of the 

hyperconcentrated phase of a lahar had never been 

observed and measured in detail in a natural stream. In 

fact, hyperconcentrated flows of any kind were poorly 

recognized in the geologic record or in field observations. 

Therefore, the purpose of this description of sediment 

transport in the hyperconcentrated phase of the 

March 19, 1982, lahar is to present quantitative evidence 

for flow behavior based mainly on interpretation of field 

observations. Sediment samples of the flow, collected 

over time, show variations in the concentration and size 

distribution of sediment that may prove to be diagnostic 

of hyperconcentrated flows in general. Measurements 

of the flow passage along the Toutle River show both the 

high velocity of the flow and the rapid decrease in stream 

discharge; such flow behavior had previously been 

speculative or unknown. Fluid-dynamical analyses or 

inferences from sedimentary deposits can be found in 

other references (Laenen and Hansen, 1988; Scott, 

1988).

FLOW CHARACTERISTICS 

OBSERVED AT GAGING STATIONS

Direct observations, measurements, and continuous 

river-stage records of the March 19, 1982 lahar were 

obtained at gaging stations located 49,77, 86, and 92 km 

downstream from the crater (fig. 3.1). In downstream 

order, the gaging station names are referred to in the text 

as follows:

North Fork Toutle River at

Kid Valley: Kid Valley

Toutle River at Tower Road near

Silver Lake: Tower Road

Toutle River at Highway 99 near

Castle Rock: Highway 99

Cowlitz River at Castle Rock: Castle Rock

The lahar of March 19,1982, entered the lower North 

Fork Toutle River when base flow was about 25 m3/s. 

At a distance 42 km downstream from the mountain, 

the tributary Green River added about 17 m3/s to the 

passing flow. At a downstream distance of 61 km (17 km 

upstream from Tower Road), the South ForkToutle River 

added another 15 m3/s. At a downstream distance of 

88 km (1.6 km downstream from Highway 99) the lahar
SEDIMENT TRANSPORT IN THE HYPERCONCENTRATED PHASE OF THE MARCH 19,1982, LAHAR 39

entered the Cowlitz River where base flow was 357 m3/s.

Channel conditions of theToutle River immediately 

prior to the arrival of the lahar of March 19, 1982 were 

determined from gaging-station records. Alluvial sand 

(D5Q = 0.5 mm) had accumulated to depths of about 

1 m near the mouth of the Toutle River following a storm 

flow on February 20,1982. Stream discharge had receded 

steadily after the storm, and shifting channel geometry 

had been measured weekly at the gaging stations. The 

records of channel geometry were used to estimate peak 

discharge of the lahar.

During March 1982, geologic and hydrologic 

personnel of the USGS had been monitoring the seismic 

activity at Mount St. Helens continuously. Immediately 

following the explosive eruption at 1927 Pacific Standard 

Time (PST) on March 19, 1982, a temperature sensor 

at the crater floor indicated a rise of several degrees. 

Concern for the possibility of snowmelt prompted the 

mobilization of several flood crews. Field personnel 

arrived at the upstream-most gaging station (Kid Valley)

after the lahar peak discharge began to recede. 

Observations at the other gaging stations were made 

during most or all of the passage of the lahar. The lahar 

briefly filled the channel to bankfull level with the muddy, 

debris-laden sediment load. The chief visual 

characteristics of the lahar common among the three 

gaging stations on the Toutle River include:

1. An oily, brown-gray appearance with a smooth, 

reflective surface where the lahar was not modified 

by turbulence associated with antidune activity.

2. Floating woody debris ranging from small litter 

to cut logs and tree stumps with large root balls.

3. Extensive, rapid deposition of sand and gravel in 

lateral berms immediately after the peak discharge.

4. A progressive transition from a plane surface to 

standing waves to upstream-migrating antidune 

waves during the lahar recession.

122°00'

46°15' —

Kid Valley GS 

^Highway 99 GS 14241100

/ 14242580

Castle

RockGS

14243000 Toutle Rive

above

Bear Creek GS

14240400

' Castle }} Silver

MOUNT 

ST. HELENS

Gaging station (GS)

WASHINGTON Course of March 19, 1982 lahar

Study area I I I I 

02468 KILOMETERS

46°00' —

FIGURE 3.1.—Course of the March 19, 1982, lahar past gaging stations on theToutle River and Cowlitz River.
40 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

5. Relatively little deposition of woody debris along 

the Toutle River stream banks compared with the 

amount of debris transported past the gaging 

stations.

Hydraulic and sediment-transport measurements of 

the hyperconcentrated and diluted lahar were made 

synoptically at the four gaging stations during the flow. 

At all sites, measurements were made of surface velocity, 

stream depths and widths, stream discharge, stream 

temperature, and river stage; flow samples (depth 

integrated or dipped) were collected periodically. These 

measurements have been interpreted by researchers to 

describe the transition in modes of sediment transport 

(Dinehart, 1983; Pierson and Scott, 1985; Scott and 

Dinehart, 1985; Scott, 1988). Sediment-concentration 

data for the lahar are reported in Dinehart (1986).

At all sites, the rise in river stage was rapid and peak 

stage was attained in less than 30 minutes. During peak 

discharge, much of the stream surface was obscured by 

floating logs, tree stumps, and pumiceous dacite clasts 

of all sizes. The rapid rise and the hazards associated 

with the floating debris allowed only intermittent 

measurements of depth, indirect estimates of velocity, 

and sporadic sediment sampling during the interval of 

peak discharge.

KID VALLEY

Initial observations of the lahar were made at the Kid 

Valley gaging station 49 km downstream from the crater 

(fig. 3.1), beginning at 2315 PST on March 19, 1982, 

1.3 hours after the arrival of the lahar front. The lahar 

was in recession from a peak stream discharge estimated 

at 963 m3/s, the highest peak discharge at this gaging 

station since the great May 18, 1980, lahar. (For lahars, 

the stream discharge is the combined flow-rate of water 

and sediment past a point on the river.) Lateral berms of 

massive, poorly sorted medium to coarse sand [described 

by Scott (1988)] were deposited at streambanks before 

the arrival of field personnel. Dip samples (Guy and 

Norman, 1970) of the receding flow were collected 

immediately by field personnel. Technicians who placed 

their hands in the flowing lahar felt walnut-sized particles 

suspended throughout the vertical column and felt the 

streambed in motion (W.W. Larson and K.K. Lee, USGS, 

written commun., 1982). Larger clasts (as much as 0.5-m 

diameter) were transported and deposited through the

reach. These clasts were primarily dacite pumice, angular 

to subrounded, that had varying degrees of porosity. 

Some clasts were completely suspended in the flowing 

lahar near midchannel, while clasts near the bank would 

settle in the shallow depth and tumble intermittently 

until stopping. Sand continued to accumulate around 

the deposited pumice clasts as the lahar discharge receded. 

As described above, sediment samples were collected first 

by dip sampling, followed by standard sampling with a 

USD-49 sampler and standard discharge measurements.

TOWER ROAD

Field personnel at the Tower Road gaging station, 

77 km downstream from the crater, were informed by 

radio that the approaching lahar was a slurry of mud, 

and they prepared to observe and sample the lahar. Three 

observers stood at the right streambank, while the fourth 

collected sediment samples and measured depth and 

velocity from a cablecar suspended over the stream. The 

first pieces of woody debris passed the Tower Road gaging 

station at 2349 PST on March 19, 1982. The flow 

reached peak stage in 19 minutes, at 0008 PST on March 

20, and then stream discharge began gradually to recede. 

Sediment samples were first collected by taping pint 

bottles to the hanger bar above the USD-74 suspendedsediment sampler and lowering the bottle through the 

flow. As concentrations decreased, depth-integrated 

samples were collected with bottles inside the sampler.

Although small fluctuations of stage were recorded 

at the time of peak flow, no evidence of a secondary pulse 

in the lahar was apparent. After the peak stage, an 

increasing percentage of sediment in the lahar rapidly 

changed the appearance of the stream surface as 

deposition rates accelerated. The arrival and passage of 

the lahar was described in detail for the Tower Road 

gaging station by the author and another field person 

(unpublished data; S.A. Gustafson, USGS, written 

commun., 1982) and those observations were 

summarized by Pierson and Scott (1985) and Scott 

(1988). A part of the summary by Pierson and Scott 

(1985, p. 1520-1521) follows:

"...river stage began to rise steadily with little 

visible change in water turbidity. Within 3 

min, the surface of the river was covered with 

logs and woody debris, but the water was 

still not muddy. The only significant noise
SEDIMENT TRANSPORT IN THE HYPERCONCENTRATED PHASE OF THE MARCH 19,1982, LAHAR 41

from the flow was from debris striking the 

banks. Seven minutes later discharge was 

275 m3/s, and the flow was noticeably 

muddier; suspended sediment concentration 

was measured at 60,000 mg/L, and water 

temperature had risen to 7.2°C.

Peak discharge (about 650 m3/s) occurred 

only 19 min after the arrival of the flood

wave...

Suspended sediment concentration at 0014 was 

158,000 mg/L, and at about this time downstream 

pointing V-shaped standing waves began to develop. 

Wave crests were originally about 0.3 m high, and 

distance between crests was about 6 m. Between 0020 

and 0030 hours the V-shaped waves extended all the way 

across the channel and grew to as high as 2.5 m. At this 

time the surface of the flow was thickly covered with 

pumice lapilli and blocks, and deposition of sand-size 

material was occurring on the river banks; suspendedsediment concentration was about 500,000 mg/L.

About 10 minutes after deposition on the banks 

began, the large oblique standing waves started breaking 

vigorously in the upstream direction. Observers noted 

that they looked like breaking antidunes, only not 

oriented transverse to the flow. This turbulence roughly 

corresponded with the time of peak sediment 

concentration (1,160,000 mg/L; 67 percent solids by 

weight, 43 percent by volume) and it was accompanied 

by a constant very loud roar, similar to a jet engine. Also, 

at this time the sound of boulders moving as bed load 

below the surface could be distinguished. While the 

antidune activity was vigorous, the flow surface had a 

smooth, oily appearance. Throughout the recession the 

flow surface carried pumice and fine woody debris while 

antidunes continued to form and disappear. Deposition 

of sand continued. Unlike the upstream debris-flow 

deposits that remained fluid for days after deposition, 

these hyperconcentrated-flow deposits drained 

immediately after deposition and within seconds were 

firm enough to walk on.

The standing waves of the flow appeared V-shaped 

in plan view, possibly because of the pronounced lateral 

velocity gradient in the cross section. Refraction of the 

waves near the stream banks appeared to accentuate the 

V-shape. The lateral velocity gradient caused debris in 

the center of the channel to travel faster than debris 

floating near the banks. After berm deposition had begun 

and the lahar discharge had waned noticeably, two of

the Tower Road observers drove to the gaging station at 

Highway 99 to begin measurements at that site.

HIGHWAY 99

The arrival and peak flow of the lahar was not directly 

observed at the Highway 99 gaging station, which is 

86 km downstream from the crater. An abrupt increase 

in river stage was recorded on the continuous chart trace 

at 0030 PST on March 20, 1982. Observers arrived at 

about 0130 PST and immediately collected sediment 

samples using a US D-49 depth-integrating sediment 

sampler that had a 0.64-cm inlet nozzle. Analyses of 

sediment data indicated that sampling began about the 

time of peak sediment concentration. Time between 

samples ranged from 5 to 15 minutes, because sampling 

attempts were often delayed by the presence of samplerthreatening woody debris.

Extensive standing waves occurred during the lahar 

recession at Highway 99, which was similar to the 

transition of flow regimes observed at Tower Road. 

Between 10 and 20 standing waves were counted along 

the approach to the Highway 99 bridge (fig. 3.2). The 

train of waves would cyclically break upstream, then the 

stream surface would become nearly plane, and then it 

would begin to undulate and form standing waves again. 

Absence of light-colored froth, small eddies, and water 

bubbles associated with storm flow was taken as evidence 

of reduced flow turbulence.

A visible contrast of the hyperconcentrated flow with 

typical storm flows was evidenced by the greater ease of 

sampling on March 20, 1982. The 28.2-kg sediment 

sampler, which was suspended by cable from a stationary 

point on the bridge, remained steady in the 

hyperconcentrated flow. During a typical storm flow of 

similar depth, immersion of the sampler in the highvelocity flow would cause the sampler to swing erratically. 

Steadiness of the sampler in the hyperconcentrated flow 

because of limited flow separation along the sampler body 

would be consistent with the observations of reduced 

flow turbulence.

Bank deposition occurred at the outside of the channel 

bend approaching the bridge. The deposition formed a 

shallow beach at the right side of the bridge opening, 

and it eventually extended about 30 m from the original 

bank (fig. 3.3). Surface waves broke obliquely across the 

newly deposited banks, propagating short-crested waves 

in an upstream direction, which resembled undular bores 

common to shore flow on sandy ocean beaches. Wave 

action decreased as the receding flow began to incise the
42 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

newly deposited banks. Patches of mottled, tan color 

and some surface relief marked locations where the fluid 

mixture of water and fine sediments was escaping from 

the sandy bank deposits (fig. 3.4). The new banks 

repeatedly collapsed, which eroded the beach at the right, 

outside of the bend. The sediment deposited on the bed 

at the Highway 99 cross section was removed only after 

several months of common streamflow, during which the 

channel thalweg returned to the right from the left side 

of the bridge opening.

CASTLE ROCK

At the gaging station at Castle Rock, 92 km 

downstream from the crater, the hyperconcentrated lahar 

from the Toutle River was substantially diluted by the 

clear stream discharge of the Cowlitz River. Nonetheless, 

the peak sediment concentration (157,000 mg/L) was 

three times higher than the highest sediment

concentration measured previously during storm flow at 

that gaging station (47,200 mg/L). The logs and woody 

debris arrived at the Castle Rock bridge at 0115 PST on 

March 20, 1982, hampering measuring procedures for 

about 30 minutes (W.P. Johnson, USGS, written 

commun., 1982). The water surface displayed sediment 

boils throughout the passage of the flood wave, and the 

streambed elevation fluctuated about 1 m because of 

migrating sand dunes.

VOLCANISM-RELATED OBSERVATIONS

Dacite clasts in the lahar were hot to the touch at Kid 

Valley, at the Toutle River below the confluence with the 

South Fork Toutle River (M.A. Uhrich, USGS, written 

commun., 1982) and at Tower Road. Several dacite clasts 

removed from the flowing lahar were too hot to hold 

bare-handed, and one retrieved clast melted the plastic 

coating on a glove. The air temperature at Kid Valley

FIGURE 3.2.—Standing waves in the Toutle River upstream from the Highway 99 bridge. Picture taken during waning of the 

hyperconcentrated phase of the lahar. Flow is from left to right in foreground.
SEDIMENT TRANSPORT IN THE HYPERCONCENTRATED PHASE OF THE MARCH 19,1982, LAHAR 43

was 5.5°C at 0500 PST. The cool air temperatures 

allowed steam to form around the hot clasts and to rise 

briefly from the lahar surface.

A water sample was collected at Tower Road about 

1.5 hours before lahar arrival, and water temperature was 

6.5°C. Flow temperature increased to 10.5°C about 

2 hours after lahar arrival. At Kid Valley, the first 

measured temperature of the lahar was 16.5°C at 

midnight, and flow temperature declined to 8.0°C by 

0700 PST. At Highway 99, flow temperature was 

probably near 6°C before lahar arrival (as at Tower Road), 

and rose to between 11°C and 15°C after flow arrival. A 

strong odor of wet earth was associated with the lahar, 

but no sulfurous odors were noted. At Kid Valley, airborne ash particles drifted past observation lights like 

fine snow, and similar fine ash was seen at the town of 

Toutle by the Tower Road observers.

ANALYSES OF SEDIMENTTRANSPORT MEASUREMENTS

Graphs of stream discharge and sediment 

concentration derived from measurements at gaging 

stations on the Toutle River are presented in figure 3.5. 

Downstream attenuation was observed in peak stream 

discharge, peak sediment concentration, and total 

sediment discharge. Because calculation of stream and 

sediment discharge of a lahar is unique in hydrologic 

analyses, the collection and analysis of the data are 

discussed in the following three sections.

COMPUTATION OF STREAM DISCHARGE

Stream discharge was computed for the lahar from a 

combination of stage readings and intermittent discharge

FIGURE 3.3.—Newly deposited sand at right side of Toutle River channel, as viewed upstream from the Highway 99 bridge. The 

shallow flow formed small bores over the wide (30 meters), planar deposits. Main flow is from upper right to lower left.
44 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

estimates and measurements. River stage was measured 

by periodic wire-weight gage readings and by nitrogen 

manometers linked to analog and digital continuous 

recorders. The wire-weight readings are direct 

measurements of the water-surface elevation relative to 

a known datum; the river stage recorded by the nitrogen 

manometer is based on the height of a column of clear 

water above the sensing orifice in the stream. The 

manometer readings were exaggerated by the extreme 

fluid density of the lahar, so the manometer stage records 

do not correspond directly to changes in the water-surface 

elevation. Peak sediment concentrations lagged the peak 

river stage at Tower Road by 34 minutes. The peak stages 

that were recorded by the manometers at Tower Road 

and at Highway 99 before the peak sediment 

concentration are, therefore, close to the true waterFIGURE 3.4.—Water-sediment mixture of fine material escapes 

from bank deposits (bottom of photograph) and returns 

to main flow (top of photograph). View is across 

upstream side of Highway 99 bridge over Toutle River, 

taken during waning of lahar. Main flow is from left to 

right.

surface elevations. Subsequent stage records that were 

obviously affected by increased fluid density were 

corrected to wire-weight readings.

Standard discharge measurements were attempted 

during the hyperconcentrated phase of the lahar. Depth 

measurements were accomplished with sounding 

weights, but velocity measurements with Price AA current 

meters were unsuccessful in the sandy, dense lahar. The 

bucket wheels clogged with sediment and woody debris 

and rotated very slowly or not at all. As the lahar 

discharge receded and sediment concentrations 

decreased, velocity measurements were made with current 

meters.

Changes in water-surface elevation during the passage 

of the lahar were due partly to sediment deposition on 

the streambed. The measured mean bed elevations at 

the three Toutle River cross sections were raised between 

0.5 and 0.7 m above bed elevations measured before the 

lahar occurred. Therefore, the stage-discharge relation 

existing at the arrival of the lahar changed continuously 

during recession of the lahar when deposition and 

subsequent incision occurred. The rate at which the 

stage-discharge relation changed as the lahar discharge 

receded could not be determined, and interpolative 

methods were used to compute stream discharge.

The approximate cross-sectional area at the time of 

peak discharge was computed by extending the area from 

previous discharge measurements to high-water levels at 

the cross sections. The streambed geometry was assumed 

to be stable at the time of peak discharge. This 

assumption was based on the low sediment concentration 

sampled at Tower Road at the time of peak discharge. A 

range of velocities (derived from estimated surface 

velocities or empirical velocity-area relations) was 

multiplied by the peak cross-sectional area to obtain peak 

stream discharge. Where numerous discharge 

measurements are available for gaging stations with 

unstable cross sections, the area of the cross section may 

be related to the mean velocity for the discharge 

measurement to extrapolate a mean velocity for a greater 

cross-sectional area. Peak discharge was estimated at Kid 

Valley by rating curve extension, and at Tower Road and 

Highway 99 by rating curve extension and velocity-area 

methods. The peak stream discharges are summarized 

in table 3.1. (The peak discharge at Castle Rock was 

greater than the peak discharge at Highway 99 because 

of the greater base flow in the Cowlitz River.)

To prepare stream-discharge hydrographs, the time 

and magnitude of peak discharges were plotted, and the 

discharge recession curve was interpolated between the
SEDIMENT TRANSPORT IN THE HYPERCONCENTRATED PHASE OF THE MARCH 19, 1982, LAHAR 45

DC 

LJJ 

b 200,000

DC 

LJJ 

D_

CO 1,200,000

g 1,000,000

O

Z

O 

o

LJJ

I \ I 

North Fork Toutle River at Kid Valley 

March 19-20, 1982

EXPLANATION

Total sediment concentration 

Coarse material (>0.062 millimeters) 

— Fine material (<0.062 millimeters) 

Stream discharge

Toutle River at Tower Road 

March 19-20, 1982

Toutle River at Highway 99 

March 19-20, 1982

< 400,000

0

1,200,000

Q 1,000,000

LJJ 

CO

800,000

600,000

400,000

200,000

0:00 3:00 6:00 9:00 

TIME, IN HOURS

12:00 15:00

FIGURE 3.5.—Sediment concentration and stream discharge derived from measurements of the 

hyperconcentrated phase of the lahar at the Toutle River gaging stations.
46 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

TABLE 3.1. -Peak stream discharges at gaging stations along the course of the lahar of March 19, 1982

Location

Stream discharge 

(cubic meters per second)

North Fork Toutle River at Kid Valley 

Toutle River at Tower Road 

Toutle River at Highway 99 

Cowlitz River at Castle Rock

963

651

453

513

peak discharge and subsequent discharge measurements. 

The time of peak stage (equal to the time of peak 

discharge) was observed directly only at Tower Road, at 

19 minutes after the first arrival of debris and beginning 

of stage rise. A similar time of 20 minutes for the interval 

between lahar arrival and peak discharge was assigned to 

the Kid Valley discharge hydrograph, on the basis of the 

Tower Road observation. A time interval of 25 minutes 

was assigned to the Highway 99 discharge hydrograph, 

which coincided with the first recorded peak on the stage 

hydrograph for that station.

To interpolate the discharge recession curve for the 

gaging station records, an initial rapid decrease in 

discharge was assumed for the first 30 minutes after the 

peak discharge, based on observations of the lahar at 

Tower Road. An exponentially decreasing line was drawn 

from the estimated peak discharge through the discharge 

measurements made 3 to 6 hours after the peak discharge. 

The actual rate of recession was uncertain because the 

streambed rose measurably during recession as the stream 

discharge of the lahar decreased through sediment 

deposition.

Sediment deposition from the lahar was the primary 

cause of downstream attenuation in peak stream 

discharge. Stream discharge decreased by about onehalf of the peak discharge between Kid Valley and 

Highway 99. As sediment from the lahar was deposited, 

the stream discharge decreased. As described under 

Computation of Sediment Discharge, a large proportion 

of the stream discharge was sand that decreased rapidly 

in concentration during channel deposition.

The velocity of the flood wave between the four 

gaging stations was determined by identifying the time 

of debris arrival or initial stage rise at each gaging station. 

Because the extreme lahar density distorted the stage 

recordings by manometer, the arrival times were the most 

reliable times available. The arrival times and velocities 

are summarized in table 3.2. From Kid Valley to Tower 

Road, the front of the flow traveled 4.4 m/s, and slowed

slightly to 4.1 m/s between Tower Road and Highway 99. 

The Toutle River enters the larger, slower moving Cowlitz 

River at 1.6 river km downstream from Highway 99. 

The flood wave traveled 2.4 m/s between Highway 99 

and Castle Rock.

ANALYSES OF SEDIMENT 

CONCENTRATION

The highest measured sediment concentrations were 

about 1 million mg/L at the three Toutle River gaging 

stations; the highest measured sediment concentration 

at Castle Rock was 157,000 mg/L. Hyperconcentrations 

are, by definition, greater than 40 percent sediment by 

weight (Beverage and Culbertson, 1964), which is 

equivalent to 530,000 mg/L. Therefore, the lahar that 

passed the Toutle River sites was hyperconcentrated. 

Sediment samples were collected at the Toutle River 

gaging stations with standard samplers described by Guy 

and Norman (1970), except for periods when the sampler 

nozzles would clog with large sediment particles. At those 

times, dip samples were collected with open-mouth pint 

bottles (Guy and Norman, 1970). Some dip sampling 

was necessary at Kid Valley and Tower Road, but all 

sediment samples from Highway 99 and Castle Rock 

were collected with standard samplers. Sediment 

concentration and particle size were determined 

according to methods described by Guy (1969).

The peak sediment concentration at the four gaging 

stations decreased in downstream order. A peak 

concentration of 1,600,000 mg/L at Kid Valley was 

extrapolated from samples collected 70 minutes after the 

approximate time of the peak concentration. The peak 

concentrations at Tower Road (1,170,000 mg/L) and at 

Highway 99 (980,000 mg/L) were interpolated from 

samples collected about the time of peak concentration.

The peaks in sediment concentration occurred 

between 25 and 80 minutes after the peak discharges at 

the Toutle River gaging stations. Sediment concentration
SEDIMENT TRANSPORT IN THE HYPERCONCENTRATED PHASE OF THE MARCH 19, 1982, LAHAR 47

TABLE 3.2.—Average velocity of flood-wave front along the course of the lahar of March 19, 1982

in the Toutle River, Washington

Location

Kid Valley

Tower Road

Highway 99

Castle Rock

Time

2155

2351

0030

0115

Difference 

(minutes)

116

39

45

River 

kilometer

38.8

10.5

1.6

-4.3

Difference 

(kilometers)

28.3

8.9

5.9

Velocity 

(m/s)

4.4

4.1

2.4

also lagged peak stream discharge during storm flow in 

the Toutle River during 1981 and 1982. The primary 

sediment sources were several tens of kilometers from 

the gaging stations where the concentrations were 

measured, and the distance between sediment source and 

measuring point is a major factor in the lagging of peak 

sediment concentration behind peak discharge (Williams, 

1989).

Sediment-concentration graphs for the Toutle River 

sites (fig. 3.5) show an order of magnitude decrease in 

concentration within 10 hours of peak concentration. 

During that period, more than 100 samples were 

collected at the Toutle River sites. The total sediment 

concentration and the coarse-material (sizes from 

0.062 mm to 2.0 mm) concentration followed similar 

rise and decay curves. The coarse material reached peak 

concentrations that were twice the fine material (sizes 

finer than 0.062 mm) and receded in concentration more 

rapidly. Fine-material concentration persisted in the 

range 250,000 to 350,000 mg/L for three hours during 

the main passage of the lahar.

Particle-size distributions in the coarse-material size 

range were obtained for all sediment samples collected 

during the flow. Concentration graphs of individual size 

classes (fig. 3.6) were fitted well by exponential-decay 

curves (Dinehart, 1983). The concentration of the 

coarser sands receded more rapidly, indicating that the 

hyperconcentrated phase was characterized by differential 

settling. By contrast, sediment samples of debris flows 

show that most particle sizes deposit at a uniform rate.

The persistence of fine-material concentration in a 

range narrower than sand concentration during flow 

passage was noted by Dinehart (1983). The finite source

of sediment in the fine-material size range may have 

limited the attainment of fine-material concentrations 

higher than 350,000 mg/L. Because the settling velocity 

of silts and clays is less than that of sands, it is reasonable 

to deduce that fine material was transported through the 

Toutle River with less deposition and thus with less 

reduction in concentration. The fine material also 

continued to drain from coarse-material deposits and 

return to the stream for transport (Scott, 1988).

The sediment-concentration graph for Castle Rock 

(fig. 3.7) shows that most of the suspended-sediment 

load transported past this gaging station was fine material. 

Although much of the sediment load at the upstream 

gaging stations was sand, the deposition of sand along 

the Toutle River attenuated suspended-sand discharge 

at Castle Rock. Sediment discharge of fine material past 

Highway 99 was about the same as the total suspendedsediment discharge past Castle Rock for March 20,1982. 

Substantial sand deposition occurred between the two 

stations, especially at the mouth of the Toutle, and some 

sand presumably became unsampled bedload in the 

Cowlitz River. In the following section, sediment 

discharges in various size classes are compared to show 

the amount of sediment deposition between stations.

COMPUTATION OF SEDIMENT 

DISCHARGE

Sediment discharge at each gaging station (tables 3.3, 

3.4) was computed by subdividing the stream discharge 

and the concentration graphs into appropriate time 

intervals. The sediment discharge for each interval was
48 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

DC 

LJJ

LJJ 

CL

CO

1,000,000

100,000

10,000

1,000

0

1,000,000

CT 100,000 0

- 10,000

<

DC

LJJ

o

o 

o

LJJ ^.

Q 

LJJ 

CO

1,000

0

1,000,000

100,000

10,000

1,000

0:00

North Fork Toutle River at Kid Valley 

March 20, 1982

EXPLANATION

O Very fine sand (0.062-0.125 millimeters

—— Log C = A*T + B (regression)

• Medium sand (0.250-0.500 millimeters)

- - - - - Log C = A*T + B (regression)

\ V~ Toutle River at Tower Road 

March 20, 1982

Toutle River at Highway 99 

March 20, 1982

3:00 6:00 9:00

TIME, IN HOURS

FIGURE 3.6.—Sediment concentration in two sand-size classes for the hyperconcentrated phase 

of the lahar, fit with an exponential recession curve.
SEDIMENT TRANSPORT IN THE HYPERCONCENTRATED PHASE OF THE MARCH 19,1982, LAHAR 49

computed by the equation

where Qs is sediment discharge, C is sediment 

concentration in mg/L, Q(w+s) is stream discharge of the 

water-sediment mixture, and a is a coefficient for units 

of measurement and time interval. The values for each 

interval were summed to obtain the suspended-sediment 

discharge for the day. This procedure was applied to 

concentration curves of total suspended sediment, sand 

fraction, fine fraction, and (for Tower Road and 

Highway 99) sediment-size classes from 0.062 mm to 

1.00 mm. In table 3.3, the total suspended-sediment 

discharge at Kid Valley for March 19 plus March 20 is 

compared with the totals at Tower Road and Highway 99 

for March 20 only, using data from Dinehart (1986). 

The sediment discharge at Kid Valley on March 1 9 before 

the lahar was 0.2 percent of the 2-day total. During the 

last hour of March 20, 1982, the lahar was in complete 

recession everywhere, and sediment discharges for that 

time interval were again less than 1 percent of the total 

sediment discharge for the flow.

Although only 50 to 70 percent of the sediment 

sampled in the lahar at Kid Valley was sand size and larger 

(Dinehart, 1986), the lahar deposits near the site were

95 percent sand size and larger (Scott, 1988). The 

difference in sediment proportions is due to the passage 

of much of the fine material through the Toutle River to 

the Cowlitz River without deposition. Sediment 

discharge values (table 3.3) show a 46-percent loss in 

sand discharge between Kid Valley and Highway 99. A 

10-percent loss in fine-material discharge was measured 

for the same reach.

Sediment discharges for four sand-size classes also 

were computed from concentration curves based on 

particle-size analyses of sediment samples from Tower 

Road and Highway 99 (Dinehart, 1986). Downstream 

differences in sediment discharge ranged from 2 to 

14 percent (table 3.4). The sand loss computed from 

totals of individual sand-size classes from 0.062 mm to 

1.00 mm was 201,000 Mg, whereas the loss computed 

from the entire sand fraction was 280,000 Mg. Erratic 

variation in the sand-concentration curves and the 

additional, uncomputed sediment discharge for size 

classes greater than 1.00 mm can account for the 

disparity. The samples collected during the lahar may 

not have defined the true trends in sand concentrations, 

but the downstream differences in sediment discharge 

of individual sand-size classes all confirm the magnitude 

of sediment deposition between the two gaging stations.

200,000

^ 150,000

2 LU 

LU Q_

o DC 

O

LU

Q -, 

LJJ 2

CO

100,000

^ 50,000

0:00

I I

Cowlitz River at Castle Rock 

March 19-20, 1982

EXPLANATION

-•— Total sediment concentration 

A— Coarse material (>0.062 millimeters) 

o— Fine material (<0.062 millimeters) 

Stream discharge

600

500

400

3:00 6:00 9:00 12:00 

TIME, IN HOURS

15:00

300 

15:00

O 

O

LJJ LJJ 

O CO 5°= < LJJ o^ 

w w

2s

EE2

CO GO 

O

FIGURE 3.7.—Sediment concentration of the lahar-generated flood wave measured at the gaging station on the Cowlitz River 

at Castle Rock. Most of the suspended-sediment load transported past this gaging station was fine material.
50 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

TABLE 3.3.-Sediment discharges oflahar of March 19—20, 1982, in the Toutle River (in megagrams [Mg]). Discharges of fine and 

sand sediment may not equal total sediment discharge, because all discharges were computed independently from separate concentration

curves

Location

Kid Valley

Tower Road

Highway 99

Difference from Kid Valley 

to Highway 99

Total sediment 

discharge

4,930,000

3,550,000

3,130,000

1,800,000

Fine sediment 

discharge (less 

than 0.062 mm)

1,420,000

1,390,000

1,250,000

170,000

Sand sediment 

discharge (greater 

than 0.062 mm)

3,500,000

2,160,000

1,880,000

1,620,000

TABLE 3A.—Sediment discharge in sand-size classes at two gaging stations along the course oflahar of March 19—20, 1982

in the Toutle River (in megagrams [Mg])

Size class 

(mm)

0.062-0.125

0.125-0.250

0.250 - 0.500

0.500-1.00

Tower Road

431,000

589,000

624,000

318,000

Highway 99

370,000

575,000

543,000

273,000

Difference

61,000

14,000

81,000

45,000

Percent 

difference

14

2

13

14

COMPARISON OF LAHAR 

WITH STORM FLOW

The hyperconcentrated phase of the March 19,1982, 

lahar was highly effective in transporting sediment from 

Mount St. Helens. A storm flow on February 20, 1982, 

in the Toutle River basin transported about 2 million 

more megagrams of sediment past the Highway 99 

gaging station than did the March 19, 1982, lahar (table 

3.5). Although the sediment discharge was 1.7 times 

greater than the lahar, the daily mean stream discharge 

was 7.5 times greater than on March 20. The high 

concentration of fine material that persisted during the 

hyperconcentrated phase probably accounted for the 

greater total sediment discharge. Fine-material 

concentration in the storm flow probably did not exceed 

150,000 mg/L, on the basis of samples collected during 

the brief period of peak discharge (Dinehart, 1986). In 

the lahar, fine material at concentrations near 250,000

to 350,000 mg/L were transported by a smaller volume 

of streamflow because unreworked deposits were eroded 

near the crater. Additional fine material was acquired by 

the lahar as it eroded extensive fine-grained reservoir 

deposits impounded behind the 1980 sediment-retention 

structure (fig. 3.1). The resulting dense, yet highly 

mobile, mixture of water and fine sediments reduced the 

fall velocities of coarser sediments so that sand 

concentrations remained far greater than observed at the 

higher stream discharges of typical sediment-laden storm 

flows.

PREDICTION AND MEASUREMENT 

OF LAHAR BEHAVIOR

The lahar from Mount St. Helens reached the mouth 

of the Toutle River with unexpected rapidity. Traveling 

86 km in 5 hours, the lahar front moved at an average
SEDIMENT TRANSPORT IN THE HYPERCONCENTRATED PHASE OF THE MARCH 19, 1982, LAHAR 51

TABLE 3.5.- Comparison of stream discharge and sediment transport on February 20 and March 20,1982, at Toutle River at Highway 99

Date

February 20

March 20

Daily 

mean 

discharge 

(m3/s)

603

80

Daily 

suspendedsediment 

discharge (Mg)

5,430,000

3,130,000

Daily 

mean sediment 

concentration

(mg/L)

81,100

234,000

velocity of 4.8 m/s. This is nearly twice the velocity of 

2.5 m/s that had been estimated for a debris flow 

originating from a pyroclastic flow on snow at Mount 

St. Helens (Dunne and Leopold, 1981). Although the 

kinematic viscosity of a sediment-laden flow increases 

with increasing silt and clay concentration (Simons and 

others, 1963), the increased viscosity did not retard flow 

velocity significantly. The lahar front traveled from 

Kid Valley to Highway 99 in about the same time as 

flood waves of several previous storm flows.

Travel times of debris flows in the Toutle River were 

predicted from digital models (Swift and Kresch, 1983; 

Laenen and Hansen, 1988). Long travel times for Toutle 

River debris flows that were computed by Swift and 

Kresch (1983) appeared to result from overly large 

assigned values of the consistency index "jU." Laenen 

and Hansen (1988) modeled the March 19, 1982 lahar 

and reported that the travel times, among other 

hydrologic parameters, were sensitive to the values of 

"//' and Manning's "n." Their model indicated that flood 

waves for storm flows of the same magnitude would take 

approximately twice as long as the modeled lahars to 

travel from Kid Valley to Highway 99. However, the 

hyperconcentrated flood wave traveled at a celerity very 

near that of five storm-flow flood waves that were 

measured during 1980 and 1981 (Dinehart, 1982).

Behavior of a flow like the March 19, 1982, lahar is 

difficult to predict because hyperconcentrated lahars 

occur rarely and are even more rarely observed. In 

environments where lahars may be expected and an 

opportunity for observation is presented, several types 

of nonstandard measurements are desirable. Constant 

sampling of the flowing lahar will provide a useful 

sediment-concentration time series. Measurements of 

flow depth throughout lahar passage can be used to 

estimate rates of deposition. Recorded observations of 

water temperature, flow regime, and bank deposits are

also valuable. High-angle video recordings of the flow 

will provide evidence for estimates of surface velocity 

and flow regime. For volcaniclastic sediments, laboratory 

analyses may include clay mineralogy and measurements 

of the specific gravities of various size classes. These 

measurements will yield a more accurate description of 

the dynamic changes in sediment transport that occur 

during lahar passage.

SUMMARY

Rapid snowmelt in the crater of Mount St. Helens, 

caused by a minor volcanic eruption on March 19, 1982, 

eroded sediment that was transported rapidly down the 

Toutle River. Immediate deployment of field personnel 

to three gaging stations along the Toutle River enabled 

the hyperconcentrated phase of the flowing lahar to be 

measured in detail. The sediment transport and stream 

discharge data indicated that sand deposition was 

extensive along the course of the lahar. Particle-size 

distributions of sediment samples were used to determine 

the changes in concentration of various sediment-size 

classes. Knowledge of the settling behavior was used to 

interpret the lahar deposits that were examined by Scott 

(1988). The main results of the study are:

1. Lahars, in their hyperconcentrated phase, are 

overwhelmingly depositional, yet are capable of 

transporting large quantities of sediment several 

tens of kilometers from their source.

2. Standard techniques of discharge measurement are 

not adequate to define discharge hydrographs for 

a lahar passing a gaging station. Direct, 

continuous measurements of cross-section 

geometry, river stage, and flow velocity can aid in 

discharge estimation.

3. Fine-material concentration remained constant in 

a range from 250,000 to 350,000 mg/L, which
52 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

was coincident with the duration of the 

hyperconcentrated range of the lahar.

4. Coarse-material concentration reached higher 

levels than fine-material concentration.

5. Differential settling of coarse sediments was 

evident, and may be characteristic of 

hyperconcentrated flows in general.

6. The peak stage coincided with peak discharge of 

the flow, although streambed deposition after peak 

stage was greater than 0.5 m at the gaging stations 

along the Toutle River.

REFERENCES CITED

Beverage, J.P., and Culbertson, J.K., 1964, 

Hyperconcentrations of suspended sediment: American 

Society of Civil Engineers Proceedings, Journal of the 

Hydraulics Division, v. 90, HY6, p. 117-128.

Dinehart, R.L., 1982, Sediment-discharge characteristics of 

the Toutle River following the Mount St. Helens eruption 

[abs.]: Conference on Mount St. Helens—Effects on water 

resources, Jantzen Beach, Or., 1981, Proceedings, p. 149.

Dinehart, R.L., 1983, Patterns of sediment concentration in 

hyperconcentrated flows at Mount St. Helens [abs.]: Eos, 

American Geophysical Union, Transactions, v. 64, no. 45, 

p. 707.

Dinehart, R.L., 1986, Sediment data for streams near Mount 

St. Helens, Washington, v. 2—water years 1981-83: U.S. 

Geological Survey Open-File Report 85-632, 438 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, 

1980—Review and assessment for the Federal Emergency 

Management Agency (FEMA): Federal Emergency 

Management Agency, 92 p.

Guy, H.P., 1969, Laboratory theory and methods for sediment 

analysis: U.S. Geological Survey Techniques of WaterResources Investigations, book 5, chap. Cl, 58 p.

Guy, H.P., and Norman, V.W, 1970, Field methods for 

measurement of fluvial sediment: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 3, 

chap. C2, 59 p.

Laenen, Antonius, and Hansen, RP, 1988, Simulation of three 

lahars in the Mount St. Helens area, Washington, using a 

one-dimensional, unsteady-state streamflow model: U.S. 

Geological Survey Water-Resources Investigations Report 

88-4004, 20 p.

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, no. 10, p. 

1511-1524.

Scott, K.M., 1988, Origins, behavior, and sedimentology of 

lahars and lahar-runout flows in the Toutle-Cowlitz River 

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

Survey Professional Paper 1447-A, 74 p.

Scott, K.M., and Dinehart, R.L., 1985, Sediment transport 

and deposit characteristics of hyperconcentrated 

streamflow evolved from lahars at Mount St. Helens, 

Washington: International Workshop on Flow at 

Hyperconcentrations of Sediment, Beijing, China, 1985, 

Beijing, China.

Simons, D.R,Richardson, E.V., and Haushild, W.L., 1963, 

Some effects of fine sediment on flow phenomena, U.S. 

Geological Survey Professional Paper 1498-G, 47 p.

Smith, G.A., and Lowe, D.R, 1991, Lahars—Volcanohydrologic events and deposition in the debris flowhyperconcentrated flow continuum, in Fisher, R.V., and 

Smith, G.A., eds., Sedimentation in volcanic settings: 

SEPM (Society for Sedimentary Geology) Special 

Publication 45, p. 59-70.

Swift, C.H., III, and Kresch, 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.

Waitt, R.B., Pierson, T.C., MacLeod, N.S., Janda, R.J., Voight, 

Barry, and Holcomb, R.T., 1983, Eruption-triggered 

avalanche, flood, and lahar at Mount St. Helens—effects 

of winter snowpack: Science, v. 221, p. 1394-1397.

Williams, G.P., 1989, Sediment concentration versus water 

discharge during single hydrologic events in rivers: Journal 

of Hydrology, v. Ill, p. 89-106.
4:

DOME-COLLAPSE ROCKSLIDE AND MULTIPLE

SEDIMENT-WATER FLOWS GENERATED BY A

SMALL EXPLOSIVE ERUPTION

ON FEBRUARY 2-3, 1983

By THOMAS C. PIERSON and RICHARD B. WAITT

CONTENTS

Abstract ........................................................................ 53

The eruption of February 2-3, 1983............................. 55

Eyewitness accounts ................................................ 55

Eruption seismicity ................................................. 55

Study methods ........................................................ 55

Volcanic processes affecting snowpack........................... 55

Ballistic shower........................................................ 57

Rockslide from lava dome ....................................... 57

Ash cloud................................................................ 57

Tephra fall............................................................... 57

Flows mobilized by pyroclast—snow interactions ......... 57

Mixed avalanche...................................................... 57

Mixed slushflow...................................................... 62

Water flow and debris flow...................................... 62

Sequence and timing of flows........................................ 66

Source and effects of meltwater..................................... 66

Similarity to events of March 19, 1982 ......................... 67

Mixed avalanche initiation mechanisms ........................ 67

Summary ...................................................................... 67

References cited ............................................................ 68

ABSTRACT

There is mounting evidence that significantly hazardous 

hydrologic consequences can result from small volcanic 

eruptions when ice and snow are present near the vent. Such 

was the case at Mount St. Helens during a small, dome-building 

eruption on February 2-3,1983. Two explosive eruptive bursts

began the eruption in the snow-filled crater, each sending 

plumes to about 6,100 meters in altitude. The plume from 

the initial burst, beginning at 2339 Pacific Standard Time on 

February 2, was dark-colored and was probably accompanied 

by minor ejection of ballistic fragments. The second burst 

began at 0256 on February 3 and had a plume that was lighter 

in color. Within about 5 minutes the second burst successively 

caused: (1) a ballistic shower that rapidly loaded the east crater 

wall with hot volcaniclastic debris; (2) a 105-cubic-meter 

avalanche of wet snow and hot rock fragments from the ballistic 

impact area; and (3) a gravitational collapse of part of the lava 

dome, resulting in a hot rockslide, 104 to 105 cubic meters in 

volume, onto the snow-covered crater floor. For the next onehalf hour, sustained meltwater production at relatively low 

discharge saturated and remobilized part of the avalanche 

deposit and then eroded a channel through the avalanche 

deposit and into the crater floor. Some meltwater flow 

transformed to a debris flow in a steep, deeply incised northflank channel that drained the crater.

Ballistic impacts of hot pyroclastic debris apparently 

triggered the initial snow avalanches, although the exact 

triggering mechanism is not clear. Heat from the rockslide 

deposit, conducted to the underlying snowpack and possibly 

supplemented by radiant heating from the incandescent 

rockslide headwall, produced the sustained meltwater flow that 

followed. It was the smaller, less energetic of two explosive 

eruptive bursts that mobilized the sediment-water flows, 

indicating that the risk of snowmelt-generated flows may be 

more directly related to the relative effectiveness of hot-rock 

emplacement on snow slopes than to eruption magnitude. 

From a hydrologic perspective therefore, a larger eruption is 

not necessarily more hazardous than a smaller one.

53
54 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

EXPLANATION

Lava dome

Talus slopes

Crater wall (steep rock face)

Area affected by ballistic 

fragments and avalanching

Rockslide deposit

Mixed-avalanche and 

mixed-slushflow deposits

Debris-flow and water-flood 

deposits

F Sample location 

YEL® Seismic station

T

1 MILES

2 KILOMETERS 

J

FIGURE 4.1.—Flow paths and deposits of sediment-water flows triggered by an explosive burst at Mount St. Helens 

on February 3, 1983, during a dome-building eruption. Seismic station YEL is shown; station EDM is 

located just off the map, 2.5 kilometers east of the lava dome.
THE ERUPTION OF 

FEBRUARY 2-3, 1983

At the onset of a sustained dome-building eruption 

in early February 1983, a hot rockslide from the lava 

dome and a sequence of flows consisting of snow grains, 

volcaniclastic sediment, and water (figs. 4.1, 4.2) were 

triggered by the second of two relatively similar explosive 

bursts from the side of the dome that were separated by 

about 3.5 hours. Prior to these events, the steep crater 

walls were coated by snow and ice, and the crater floor 

was covered by as much as several meters of snow. A 

storm on January 27 had added 15 cm of new snow at 

Spirit Lake and at least as much in the crater. For five days 

before the beginning of the eruption, temperatures in 

the crater stayed slightly below freezing. At a few minutes 

after midnight on February 3, the air temperature at Spirit 

Lake (at about 1,040 m altitude) was -0.6°C (U.S. Army 

Corps of Engineers, radio log, February 3, 1983). When 

the standard lapse rate of 0.6°C/100 m is applied, air 

temperature at the level of the crater walls would have 

been about -7°C to -9°C at the time of the explosive 

events.

EYEWITNESS ACCOUNTS

Personnel at the Spirit Lake pumping barge 8 km 

from the dome observed some of the events by moonlight 

(U.S. Army Corps of Engineers, radio log, 

February 2-3, 1983; Harder Engineering, daily logs, 

February 2-4, 1983; N.R. Larson, U.S. Army Corps of 

Engineers [retired], written commun., 1987). At about 

2339 Pacific Standard Time on February 2, observers 

saw a large, dark eruption plume rise and drift to the 

northwest, obscuring the crater. Visibility improved after 

about 30 minutes, but nothing else unusual was observed 

until a second eruption plume, lighter in color than the 

first, rose at 0256 on February 3. The National Weather 

Service at Portland Airport detected both plumes at about 

6,100 m altitude. Within about five minutes, the crater 

cleared again, and a pulsing incandescent glow near the 

top of the dome indicated to the observers that part of 

the dome had failed. Shortly thereafter, the view into 

the crater was obscured by what looked to be steam, and 

then a dark mass appeared to move out of the crater and 

down the north flank of the volcano. The next day, field 

crews confirmed that part of the east sector of the lava 

dome had collapsed as a rockslide that extended out 

toward the east crater wall, and a large, dark, mixedavalanche deposit covered part of the crater floor and 

extended down onto the Pumice Plain, the pyroclastic 

debris fan at the foot of the volcano.

ERUPTION SEISMICITY

Seismometers located about 1,000 m north (station 

YEL) and 2,500 m east (station EDM) of the lava dome 

recorded both eruptive bursts (fig. 4.3). The YEL site 

was narrowly missed (within meters) by the avalanche 

(fig. 4.1). For the first eruptive burst, the seismic signal 

began abruptly, increasing from zero to full scale in 

7 seconds. The YEL recorder remained at full scale for 

6 minutes; the more distant EDM remained at full scale 

for only 90 seconds. The YEL and EDM seismic records 

continued with about 5 and 8 minutes of fluctuating 

moderate-amplitude signal and then about 50 and 

21 minutes of decreasing low-amplitude signal. Gain 

on the YEL seismograph was set at about one-half that 

for the EDM seismograph.

The second eruptive burst was shorter in seismic 

duration and slightly less intense than the initial burst. 

It began more gradually, taking about 20 seconds to attain 

full scale, but dropped off much more abruptly. The 

YEL seismograph was at full scale for about 6 minutes; 

the EDM instrument was at full scale for about 

50 seconds. Decreasing moderate-amplitude signals were 

recorded for 6 minutes at YEL and for only 100 seconds 

at EDM.

STUDY METHODS

Helicopter access allowed field crews to quickly reach 

deposits over a broad area, beginning at about 1000 on 

February 3. For the next few days, all deposits remained 

solidly frozen while field surveys and site examinations 

were carried out. Initial depositional features were well 

preserved throughout the field examinations. Frozen 

bulk samples were insulated and flown back to the 

laboratory for bulk-density and particle-size analyses.

VOLCANIC PROCESSES 

AFFECTING SNOWPACK

Several different types of volcanic processes produced 

pyroclastic material that interacted with the snowpack 

during this eruption. Interactions were most intense near
56 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

FIGURE 4.2.—Southward oblique aerial view of crater, Loowit Ravine, and Pumice Plain on February 3, 1983, 

showing darkened, debris-covered east crater wall and path of mixed avalanche and succeeding flows. 

The crater breach is about 1 kilometer wide.
the lava dome and were relatively minor outside the 

crater.

BALLISTIC SHOWER

Ballistic fragments of hot vesicular dacite at least as 

large as 0.5 rn in diameter were hurled explosively from 

a vent on the upper east side of the lava dome (at the 

head of what subsequently became the rockslide failure 

scar)(figs. 4.1, 4.4). The fragments were thrown 

primarily to the northeast, apparently during both of 

the eruptive bursts. Most of the larger blocks landed 

within about 350 m of the vent. During the 0256 burst, 

a dense pattern of ballistics struck the lower two-thirds 

of the east crater wall, centered on a prominent rock 

buttress just northeast of the dome (fig. 4.1). A large 

proportion of these ballistics were incorporated into the 

subsequent sediment-water flows. Melt holes from the 

hot clasts were as much as 1 m deep and were more 

abundant outside than inside the margins of the 

avalanche deposit.

ROCKSLIDE FROM LAVA DOME

The second eruptive burst on February 3 caused a 

rockslide of incandescent dacite from the east side of the 

lava dome to travel about 200 m out onto the snowcovered crater floor (fig. 4.4). A volume of 104 to 105 m3 

spread out over an area of about 50,000 m2 . The 

rockslide originated from a deep failure scar heading at 

the eruptive vent on the upper east side of the dome, 

into which lava later flowed to form a new lobe on the 

lava dome.

The pulsing red glow observed near the top of the 

dome probably emanated from the newly exposed 

headwall of the landslide scar, and the white cloud that 

filled the crater soon afterward presumably resulted from 

increased fumarolic venting from the dome and from 

steam produced by incandescent rocks contacting snow. 

The bright incandescence of the new headwall and 

temperatures measured in cracks in the dome just prior 

to the eruption indicate that much of the rockslide debris 

may have been as hot as 900°C.

ASH CLOUD

A small ash cloud was generated by the rockslide. 

The ash-cloud deposit, symmetrical with respect to and 

lying on top of the rockslide deposit, consisted of thin

(1 to 3 cm), gray lapilli and ash, and it coated about 

0.5 km2 of the lower crater wall and crater floor east of 

the dome in a nearly 90° sector (fig. 4.4). The deposit 

overlay and postdated the main snow-avalanche scars. 

Lapilli proximal to the rockslide scar on the dome graded 

to coarse ash at the distal margin of the ash-cloud deposit. 

Most of the lapilli were dense lithic or poorly vesiculated 

fragments (dome carapace), but the deposit also 

contained a trace of moderately vesiculated white pumice 

(presumed juvenile material). The ash-cloud deposit had 

a relatively sharp outer boundary, indicating flowage 

rather than pyroclastic fall, and it generally lacked an 

underlying ice layer, indicating that the presumably hot 

ash cloud caused little melting of the snow surface and 

therefore had little effect on the generation of sedimentwater flows.

TEPHRA FALL

A 1- to 2-km-wide band of thin (<1 cm) tephra-fall 

deposit extended northwestward from the lava dome. 

Fine lithic lapilli near the dome graded outward on the 

outer flank of the volcano to become coarse ash. Like 

the thin pyroclastic deposit east of the dome, the proximal 

tephra-fall deposit just northwest of the dome contained 

a trace of angular white pumice. The tephra deposit 

overlay the mixed-avalanche and slushflow deposits, but 

not the central water-scoured channel. It caused litde to 

no melting of the snow surface and did not contribute 

to the generation of sediment-water flows.

FLOWS MOBILIZED BY 

PYROCLAST—SNOW INTERACTIONS

Four different types of sediment-water flows (some 

of which contained a significant component of granular 

ice) resulted from the explosive venting of hot pyroclastic 

rock debris during the second eruptive burst: mixed 

avalanche, mixed slushflow, water flow, and debris flow. 

Flows reached as far as 8 km from the dome (fig. 4.1).

MIXED AVALANCHE

The first and largest flow was a mixed avalanche (see 

Pierson and Janda, 1994), a wet (but apparently 

unsaturated) avalanche of rock debris and snow, similar 

to the large avalanche triggered by the March 19, 1982, 

eruption (Waitt and others, 1983). The main avalanche 

mass originated as a wet-slab snow avalanche from the
00

YEL RECORD

J^Begin 2339 event =

1 minute \———————\ s

8Cfl §I IBS

I Begin 0256 event=

EDM RECORD

1 minute

= Begin 2339 event;

I Begin 0256 event

FIGURE 4.3.-Seismograph records for parts of the 2339 (February 2, 1983) and the 0256 (February 3, 1983) eruptive bursts from seismic stations YEL and EDM.
prominent crater-wall buttress northeast of the dome 

(fig. 4.1) after the snowpack had been bombarded by 

explosively ejected, hot, pyroclastic debris. The mass of 

wet snow and incorporated rock debris swept down and 

spread out on the crater floor, turned clockwise to follow 

the low topography, and flowed out through the crater 

breach (figs. 4.1, 4.2, 4.5). Most of the homogenized 

mixture (roughly 90,000 m3) was deposited on the floor 

of the crater and of the breach, but about 10 percent of 

the total volume entered Loowit Ravine on the north 

flank and flowed down that channel. Velocity through 

the initial turn in the crater was roughly 20 m/s 

(72 km/h), as estimated from 9.5 m of superelevation. 

Other small snow avalanches were subsequently released 

off the east crater wall north of the buttress. Only one 

secondary avalanche reached the main flow path, where 

it overlay the grooves etched by the main avalanche 

(fig. 4.5, labelled "late wet-slab snow avalanche").

The mixed-avalanche deposit on the crater floor and 

in the breach, 12 hours after deposition, was a porous, 

rough-surfaced sheet (0.5 to 2 m thick) of ungraded, 

unstratified diamict, consisting of boulder-sized chunks 

of refrozen, dirty snow studded with dacite boulders 

(fig. 4.6). It was similar to mixed-avalanche deposits 

described elsewhere (Pierson and Janda, 1994; Waitt and 

others, 1994). The mixed-avalanche deposit was firmly 

ice-cemented, indicating the prior presence of liquid 

water as a pore fluid. Many hot boulders had melted 

through the avalanche deposit into underlying snowpack; 

the larger ones remained warm 12 hours and longer after 

emplacement. Along most of its route, the avalanche 

was not significantly erosive, although thin slabs of snow 

crust were broken and plowed up in places along the 

margin. In the crater, the deposit appeared to have been 

emplaced in an unsaturated state; small stones and snowdebris aggregates had broken ofif from the deposit margin

FIGURE 4.4.—Northwestward view of dome on February 3, 1983, showing rockslide deposit (foreground) and mixedavalanche deposit (background) on crater floor. Lava dome is 1,050 meters in diameter.
60 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

in numerous places, indicating a somewhat dry, friable 

consistency, and no pore water had seeped from the 

deposit to the adjacent snow.

About 104 m3 of the avalanche material flowed down 

Loowit Ravine and fanned out on the Pumice Plain as 

far as 6 km from the dome (fig. 4.1, 4.2). There the 

deposit contained lithic clasts as large as 30 cm. As in 

the crater, the mixed-avalanche deposit had an irregular 

hummocky surface, although somewhat smoother, and 

had sporadic flat, very smooth-surfaced areas resulting 

from localized puddling of saturated slurry, indicating a 

higher pore-water content on the Pumice Plain than in 

the crater. Further evidence of a higher degree of 

saturation of the Pumice Plain deposits were vertical 

dewatering pipes— zones of coarser material along which 

pore water had escaped to the surface of the deposit. In 

addition, seepage of muddy pore water from the deposit 

along the margin had stained adjacent undisturbed snow 

(fig. 4.7). Despite the higher pore-water content and 

lobate flow margins, both of which are indicative of 

flowage, the thicker parts of the deposit were disrupted 

by longitudinal shears and en echelon tensional faults that 

indicate translational (sliding) movement.

The mixed-avalanche deposit was initially composed 

of poorly sorted, hot lithic volcanic fragments ranging 

from clay to boulder size, snow grains as much as several 

millimeters in diameter, and liquid water. The day after 

deposition, 2- to 4-kg samples obtained from various 

locations (fig. 4.1) consisted of about one-quarter rock 

debris, about one-half granular snow and ice cement, 

and about one-quarter void space, by volume. Mean 

bulk density of the samples (excluding clasts larger than 

64 mm) was 1.10 g/cm3 (table 4.1), but deposits in the 

crater and in the breach had lower bulk densities and 

lower proportions of rock debris than those on the 

Pumice Plain (table 4.1, fig. 4.8). The bulk-density 

increase in the Pumice Plain deposits, together with 

higher rock-fragment concentrations and evidence of 

extensive pore-water drainage at deposit margins prior 

to refreezing, seem to indicate that a progressive 

downslope melting of snow occurred in the mixed 

avalanche during flow.

Matrix samples of the mixed-avalanche deposit 

(excluding clasts >64 mm) had an average mean grain 

size (Mz) of -1.1 (j) (2.2 mm) and an average sorting 

coefficient (CFj) of 3.30. They were the coarsest and most

Mixed-avalanche deposit 

Striated erosion surface (on snow) 

Mixed-slushflow deposit 

Water-eroded channel

Deeply incised channel

100 200 300 METERS

APPROXIMATE SCALE

FIGURE 4.5.—Plan view of flow paths and deposits in crater and breach areas. Map made directly from 

aerial photographs taken by Oregon Air National Guard.
FIGURE 4.6.-Rough-surfaced mixed-avalanche deposit, 80 centimeters thick, overlying snow in 

lower crater breach at head of Loowit Ravine.

FIGURE 4.7.—Outer margin of west depositional lobe on Pumice Plain; view up valley 

showing wet mixed-avalanche deposit and snow stained by escaping muddy pore water.
62 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

TABLE 4.1 .—Properties and composition of mixed-avalanche deposit. Sample locations (designated by letter) shown in figure 4.1

[Volume computations made for particle density of 2.4 g/cm3 and ice density of 0.9 g/cm3; g, grams; cm3, 

cubic centimeters; km, kilometers; vol. pet., volume percent]

Sample

Jl

11

Dl

Cl

C2

El

Distance

from 

source1 

(km)

0.9

1.5

4.4

4.4

4.4

4.7

Bulk 

density 

(g/cm3)

0.84

1.01

1.10

1.32

1.18

1.14

Rock

clasts, 

R 

(vol. pet.)

18

15

28

36

36

32

Ice, I 

(vol. pet.)

46

71

48

50

36

42

Void 

space 

(vol. pet.)

36

14

24

14

28

26

R

R + I

0.28

0.17

0.37

0.42

0.50

0.43

Source arbitrarily defined as base of east crater wall at the rock buttress.

poorly sorted of the sediment-water flows produced 

during this eruption (table 4.2, fig. 4.9). The character 

of the mixed-avalanche deposit changed drastically within 

a few weeks, due to melting of the ice component. After 

complete melting and drying, the deposit was only a few 

tens of centimeters thick and was extremely loose and 

friable.

MIXED SLUSHFLOW

Another deposit of water, snow, and rock debris was 

found refrozen and nested within levees of the mixedavalanche deposit; its form indicated that it had been a 

mixed slushflow—wetter and more fluid than the mixed

2.0

cc m CD 2E 1-5

1.0

0.6

0.4

0.2

B

I I I I

0123456 0123456 

DISTANCE FROM AVALANCHE SOURCE, IN KILOMETERS

FIGURE 4.8.—Components of mixed-avalanche deposit: A, 

Plot of sample bulk density versus distance from avalanche 

source (rock buttress on east crater wall); B, rock fragments 

in mixed-avalanche deposit as a fraction of total solids 

(rock + ice) versus distance from avalanche source.

avalanche. Contacts between the two units were 

gradational in some places and sharp in others (fig. 4.5). 

The mixed slushflow, probably a wetter, tail-end phase 

of the mixed avalanche, did not seem to have been totally 

liquefied while flowing in the crater; its deposit, 

compared to the mixed-avalanche deposit, was only 

moderately smoother, thinner, and more digitate at its 

lateral margins. On the Pumice Plain, however, the 

mixed slushflow behaved as a fully liquefied slurry. The 

contacts were sharp where mixed-slushflow deposits were 

confined by levees of the mixed-avalanche deposit. 

Mixed-slushflow deposits were thinner than the mixedavalanche deposits and extended farther down the Pumice 

Plain, where they spread out as thin, lobate sheets beyond 

the confining levees. Copious amounts of muddy water 

seeped into the snow all along the margins of the mixedslushflow, which indicates that the mixed-slushflow 

deposit dewatered far more than did the mixed-avalanche 

deposit. Matrix samples of the mixed-slushflow deposit 

(particles >64 mm not analyzed) had variable grain-size 

distributions but were finer grained and generally better 

sorted than the mixed-avalanche samples; they had an 

average mean grain size of 0.90 (0.5 mm) and average 

sorting coefficient of 2.70 (table 4.2).

WATER FLOW AND DEBRIS FLOW

A flow consisting almost entirely of water followed 

the mixed slushflow in the crater, incising a channel 2 to 

4 m through the mixed-slushflow deposits and the
MIXED-AVALANCHE DEPOSITS

40

LU

O 30 

DC 

LU 

0.

Z 20

CO 10 

LU

I I I I I I I I I 

Jl

0.9km

||

i I I I r

II 

1.5km

Dl 

4.4km

-6-4-2024 68 10 -6-4 -2 024 68 10 -6 -4 -20 24 68 10

40

LU

O 30 

DC 

LU 

Q.

^ 20

CO 10 

LU

I I I I I

Cl

4.4 km -

li.

C2 

4.4km -

illllllli..

El 

4.7km

ll

-6-4-2024 6 8 10 -6 -4 -2 0 2 4 6 8 10 -6 -4 -2 0 24 6

GRAIN SIZE, IN <|> UNITS

10

MIXED-SLUSHFLOW DEPOSITS

40

O 30 

DC

20

CO 10 

LU

Bl 

4.7km

\ i i i i i i i i i

B2 

4.7km

I 40

Al

4.8km

-6-4-2024 6 8 10 -6 -4 -2 0 2 4 6 8 10 -6 -4 -2 0 24

GRAIN SIZE, IN <|> UNITS

10

DEBRIS-FLOW DEPOSITS

40

LU

O 30 

DC

20

CO 10 

LU

D2 

4.6km

Illlllll

HI 

6.1km

i i i i r

Gl 

6.6 km

-6-4-2024 6 8 10 -6 -4 -2 0 2 4 6 8 10 -6 -4 -2 0 24 6 8 10

GRAIN SIZE, IN <|> UNITS

FIGURE 4.9.—Grain-size frequency histograms for mixed-avalanche, mixed-slushflow, and debris-flow deposits. Clasts coarser 

than 32 mm (-50) not sampled. Distances downslope from rock buttress on east crater wall are indicated.
64 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

Sample

TABLE 4.,2.-Grain-size parameters* of mixed-avalanche, mixed-slushflow, and debris-flow deposits

[km, kilometers; 0, negative logrithm of grain diameter in millimeters to base 2.]

Distance 

from 

source2

(km)

Mean 

grain 

size,

Sorting,

Skewness, Kurtosis, 

KG

Jl

11

Dl

Cl

C2

El

Average

0.9

1.5

4.4

4.4

4.4

4.7

Mixed Avalanche

-1.47 3.38

-1.11 3.35

-1.52 3.22

-0.14 3.00

-1.97 3.46

-0.26 3.13

-1.08 3.26

-0.03

-0.07 

0.09 

0.04 

0.27

-0.03 

0.05

0.65

0.82

0.75

0.86

0.67

0.88

0.77

Slushflow

B2

Bl

Al

Average

4.7

4.7

4.8

-0.08 

0.54 

2.36 

0.94

2.88

2.90

2.38

2.72

0.03 

0.05 

-0.02 

0.02

0.92

0.91

0.99

0.94

Debris Flow

D2

HI

Gl

Average

4.6

6.1

6.6

-1.56

2.23

2.34

1.00

3.18

2.45

2.26

2.63

-0.07

0.11

0.09

0.04

0.76

1.02

1.09

0.96

1 Units of Folk and Ward (1957); see also Folk (1966, 1974).

2 Source arbitrarily defined as base of east crater wall at the rock buttress.

underlying snowpack into the pumiceous crater-floor 

deposits (fig. 4.10). Channel sides were glazed with dirty, 

dense ice as thick as 5 cm. Localized deposition of 

noncohesive sandy gravel as thick as 0.5 m on the channel 

floor indicates that the agent of this downcutting was 

normal streamflow; flow magnitude was sufficient to 

transport boulders as large as 1.5 m in diameter. The 

incised channel could be traced back almost to the 

margin of the rockslide deposit, which indicates that 

much of the flow was meltwater produced by heat from 

this debris.

On the Pumice Plain, evidence for a substantial water 

flow was lacking. Instead, the mixed-slushflow deposits

seemed to have been eroded by dilute debris flow, as 

evidenced by thin mud coatings on boulders in the 

channels. The debris flow presumably evolved when 

water being discharged from the crater bulked up with 

debris that the water eroded as it descended Loowit 

Ravine. Like the mixed-slushflow, the debris flow also 

contained some snow granules, although these formed a 

smaller proportion of the solid component. Unlike the 

other deposits, however, the debris-flow deposit 

contained oxide-stained pumice derived from 

downcutting at the crater breach (fig. 4.2). Samples of 

the debris-flow deposit finer than 64 mm were similar 

to the mixed-slushflow deposits in texture, had an average
FIGURE 4.10.—Nested nature of mixed-avalanche, mixed-slushflow, and water-flow deposits, visible from subtle tonal and 

textural changes, and water-eroded channel, in crater breach at head of Loowit Ravine. View is upslope and southward; see 

figure 4.5 for scale. Some meltwater had piped through the snowpack to emerge as a muddy spring (upper center of 

photograph).
66 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

mean grain size of 1.00 (0.5 mm), and had an average 

sorting coefficient of 2.60 (table 4.2). The sorting values 

of the distal debris-flow samples indicate that the debris 

flow was very dilute and almost transitional to 

hyperconcentrated flow. The debris-flow deposits also 

were well drained and fairly porous; bulk density of one 

frozen sample (site H) was 1.3 g/cm3 .

The debris flow issued from the steep channel of 

Loowit Ravine at a fairly low discharge and did not have 

the competence to transport boulders much beyond the 

confining levees of the mixed-avalanche and mixed 

slushflow deposits, although fine-grained slurries flowed 

as far as Spirit Lake (fig. 4.1). Strand lines at the head of 

the Pumice Plain (site Dl in fig. 4.1) showed a flow 

width of about 25 m and a peak-flow depth of a little 

more than 1 m. A slope-conveyance calculation for this 

cross section, assuming Manning's n = 0.03, gives an 

approximate discharge of 250 m3/s and a velocity of 

8.8 m/s. Runup of debris on a large boulder and channel 

superelevation at sites H and G (fig. 4.1) indicated 

minimum velocities of 3.4 and 3.3 m/s. A relatively low 

discharge for the debris flow is consistent with the 

observation that, while descending Loowit Ravine, the 

debris flow did not overflow into the channel to the west 

as the mixed avalanche and mixed slushflow did.

SEQUENCE AND TIMING OF FLOWS

The inferred sequence and timing of flows is 

constrained by the seismic records, the eye-witness 

accounts, and the field relations of eroded areas and 

deposits. The mixed-avalanche deposit contained hot 

blocks that had been emplaced prior to deposition of 

the mixed avalanche, implying that the ballistic shower 

preceded avalanching from the crater wall. Snow 

striations left by the mixed avalanche terminated at the 

foot of the rockslide deposit, and the surface of that 

deposit did not seem to be eroded (fig. 4.5), indicating 

that the dome-collapse rockslide must have followed the 

mixed avalanche. Thus, the sequence must have been 

(1) ballistic shower, (2) mixed avalanche, and (3) dome 

rockslide.

All three events must have occurred within about 

5 minutes of 0256 because that was the duration of the 

high-amplitude seismicity at the YEL seismic station in 

the crater; each of the three events is presumed sufficient 

to have caused a seismic signal but their signals would 

have been largely masked by the high-amplitude signals 

caused by the explosive venting during the second burst. 

The timeframe within which the three events occurred

is also constrained by the eyewitness report of the 

appearance of the pulsing red glow (glowing rockslide 

headwall) "about 5 minutes" after sighting the emission 

plume. The 50 seconds of high-amplitude seismicity 

recorded by the EDM seismometer is probably the 

vigorous venting that caused the ballistic shower, and a 

30-second, cigar-shaped seismic signal about 4 minutes 

later (not shown in fig. 4.3) may be the rockslide from 

the dome. The observed white cloud within the crater, 

noted after the pulsing red glow was seen on the dome, 

is interpreted to be steam produced by very hot rockslide 

debris contacting snow. This cloud then was followed 

by the sighting of a dark flow (time not recorded) exiting 

the crater.

Some type of flow, presumably the mixed slushflow 

or debris flow or both, continued for about one-half hour 

after the intense seismicity at the beginning of the 0256 

eruptive burst, because both seismometers recorded a 

gradually diminishing, low-amplitude, high-frequency 

signal until about 0328. This signal was similar to 

seismograms known to be caused by debris flows 

(Pierson, 1986). Thus, all major flow events seem to have 

occurred by about 0330; the mixed-avalanche deposit 

was confirmed to be in place during a pre-dawn 

observation flight at 0530.

SOURCE AND EFFECTS OF 

MELTWATER

The main meltwater channel headed at the hot 

rockslide deposit. Meltwater could have been produced 

by a combination of (1) melting crater snowpack from 

beneath the hot rockslide deposit, (2) late snow 

avalanches that landed on the deposit, and (3) melting 

from the crater walls that were being subjected to radiant 

heating from the -900°C rockslide headscarp only 700 

to 800 m away. Rilling from running water was evident 

at the base of the east crater wall.

Meltwater flowing from the east side of the dome 

seemed to have encountered the mixed-avalanche deposit 

blocking its path. The upstream end of the avalanche 

deposit and parts adjacent to the eroded channel appeared 

to have been much wetter than the rest of the mixedavalanche deposit (fig. 4.5). Some meltwater was diverted 

under the snowpack and emerged as a spring farther 

downslope (fig. 4.5). The mixed-avalanche deposit 

seemed to have served simultaneously as a sponge and as 

a dam that temporarily impeded the flow of meltwater. 

Once sufficiently saturated, the core of the mixedavalanche deposit along the axis of meltwater flow became
partly liquefied and remobilized as the mixed slushflow. 

Saturation of the original, relatively dry deposit by later 

meltwater would account for the gradational contact 

between the dry- and wet-looking deposits in the crater 

and for the sharp contacts between these deposits farther 

downslope on the Pumice Plain. Once the flow path 

through the mixed-avalanche deposit had been cleared, 

flowing meltwater began eroding into the crater floor.

SIMILARITY TO EVENTS OF 

MARCH 19, 1982

The February 2-3, 1983, eruption was similar to the 

eruption that occurred on March 19, 1982 (Waitt and 

others, 1983; Pierson and Scott, 1985; chap. 2, this 

volume), but the earlier eruption was larger and occurred 

without a rockslide from the dome. The other main 

difference between these eruptions was that meltwater 

produced in 1982 accumulated in a large transient lake 

in the crater before being released; consequently, the 

resulting flood and debris flow were substantially larger. 

Such a lake might have formed in 1983, as well, if the 

eruptive bursts had been aimed more southward. Egress 

of sediment-water flows from the southern end of the 

crater is partly blocked by the dome itself, and greater 

accumulations of snow at that end provide a potentially 

abundant source of meltwater.

MIXED-AVALANCHE INITIATION 

MECHANISMS

Five mechanisms could have triggered the snow-slope 

failures that produced the mixed avalanche (Pierson and 

Janda, 1994): (1) seismic or acoustic destabilization, 

(2) generation of destabilizing pore-water pressures in 

the snowpack from melting by imbedded ballistic 

fragments, (3) destabilization by stress imparted to the 

snow by ballistic impacts (inertial loading), 

(4) destabilization by the added weight of the ballistic 

fragments to the slope (gravitational loading), and (5) loss 

of snow-slab tensile strength due to decreases in cohesion 

(from localized melting and meltwater infiltration). The 

last three of these mechanisms are probably responsible 

for snow avalanches that commonly occur immediately 

after rain begins falling on snow slopes, as suggested by 

Conway and Raymond (1993). A single mechanism 

could have triggered the mixed avalanches, or several 

mechanisms possibly could have acted together.

The first mechanism seems improbable because the

earlier 2339 eruptive burst, which was seismically and 

presumably acoustically more vigorous than the 

0256 burst, did not produce significant avalanching. 

Remnant deposits of early snow avalanches (occurring 

prior to the mixed avalanche) were not observed, which 

indicates that stress-wave propagation in the snowpack 

from earthquakes or air-pressure waves would have been 

insufficient to cause snow-slope failures during the 

smaller 0256 eruptive burst. Furthermore, seismic or 

acoustic shock should have released avalanches in 

locations unrelated to ballistic-impact patterns on the 

crater wall, but in this case, snow avalanche scars were 

concentrated in the areas most heavily bombarded by 

ballistics during the 0256 eruptive burst.

The spatial coincidence of ballistic impacts with 

snow-avalanche failure scars supports a causal connection 

between ballistic impact and snowpack destabilization. 

The rapidity of seismic events after the beginning of the 

0256 eruptive burst indicates that less than five minutes 

elapsed from the time ballistic fragments burrowed into 

the snow slopes until failures occurred. A similar, very 

rapid response of snow avalanching to rainfall has been 

documented by Conway and Raymond (1993), where 

snow-slab failures 30 to 50 cm deep occurred sometimes 

within minutes of the onset of rain, when liquid water 

and heat had penetrated only 5 to 10 cm into the snow. 

Weakening of snow-grain bonds by meltwater or buildup 

of pore-water pressures in the snowpack might have a 

destabilizing effect within 5 minutes only if the snow 

was already thoroughly wetted (Conway and Raymond, 

1993). The slightly subfreezing temperatures on 

February 2-3 probably would not have allowed the upper 

snow layers on the crater walls to be that wet.

The rapid onset of avalanching following the 

beginning of the 0256 eruptive burst indicates that the 

avalanches were triggered by inertial loading by ballistics 

striking the snow surface, gravitational loading by the 

weight of the accumulating ballistic particles, or 

reduction of tensile strength in the snowpack by melting 

and weakening of the snow surface (or some 

combination of the three).

SUMMARY

Like the March 19, 1982, eruption, the 

February 2-3, 1983 eruption in the crater of Mount 

St. Helens triggered a rapid sequence of sediment-water 

flows involving snow, hot rock debris, and water. Also 

like the 1982 eruption, the 1983 eruption first produced 

a massive mixed avalanche that reached as far as 6 km
68 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

and temporarily held back subsequently produced 

meltwater in the crater. In 1983, the smaller of two, 

otherwise similar, explosive bursts produced the flows, 

indicating that snowmelt flow hazards may not be directly 

related to eruption magnitude but rather to the relative 

effectiveness of hot-rock emplacement on snow slopes. 

Therefore, a larger eruption is not necessarily more 

hazardous than a smaller one. Although a single, primary 

avalanche-release mechanism could not be identified, 

avalanche triggering occurs rapidly and seems to be 

directly related to the ballistic fragments that hit the snow 

slopes.

REFERENCES CITED

Conway, Howard, and Raymond, C.F., 1993, Snow stability 

during rain: Journal of Glaciology, v. 39, no. 133, 

p. 635-642.

Fisher, R.V., and Schmincke, H.-U., 1984, Pyroclastic rocks: 

New York, Springer-Verlag, 472 p.

Folk, R.L. 1966, A review of grain-size parameters:

Sedimentology, v. 6, no. 2, p. 73-93. 

Folk, R.L., 1974, Petrology of sedimentary rocks (2nd ed.):

Austin, HemphiU's, 182 p. 

Folk, R.L., and Ward, W.C., 1957, Brazos River bar

[Texas]—A study in the significance of grain size

parameters: Journal of Sedimentary Petrology, v. 27,

no. 1, p. 3-26.

Pierson, T.C., 1986, Flow behavior of channelized debris 

flows, Mount St. Helens, Washington, in Abrahams, 

A.D., ed., Hillslope processes: Boston, Alien and 

Unwin, p. 269-296. [Binghamton Symposia in 

Geomorphology—International Series, v. 16.]

Pierson, T.C., and Janda, R.J., 1994, Volcanic mixed 

avalanches: A distinct eruption-triggered mass-flow 

process at snow-clad volcanoes: Geological Society of 

America Bulletin, v. 106, no. 10, p. 1351-1358.

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, no. 10, p. 1511-1524.

Waitt, R.B. Jr., Pierson, T.C., MacLeod, N.S., Janda, R.J., 

Voight, Barry, and Holcomb, R.T., 1983, Eruptiontriggered avalanche, flood, and lahar at Mount St. 

Helens—effects of winter snowpack: Science, v. 221, 

no. 4618, p. 1394-1397.

Waitt, R.B., Gardner, C.A., Pierson, T.C., Major, J.J., and 

Neal, C.A., 1994, Unusual ice diamicts emplaced 

during the December 15, 1989 eruption of Redoubt 

Volcano, Alaska: Journal of Volcanology and 

Geothermal Research, v. 62, no. 1-4, p. 409-428.

Wentworth, C.K., 1922, A scale of grade and class terms 

for clastic sediments: Journal of Geology, v. 30, no. 5, 

p. 377-392.
5:

ROCK AVALANCHES, ROCKFALLS, AND 

ASSOCIATED PROCESSES INDUCED BY 

SPREADING OF THE LAVA DOME, MARCH 1984

By JON J. MAJOR and PATRICK T. PRiNGLE1

CONTENTS

Abstract ......................................................................... 69

Introduction .................................................................. 69

Events in March 1984 .............................................. 70

Terminology............................................................. 70

Timing of rock avalanches and rockfalls ......................... 72

Deposits of the rock avalanche-rockfall complex ............ 73

Unit A...................................................................... 73

Units Bl and B2 ...................................................... 74

Unit C ..................................................................... 75

Unit D..................................................................... 76

Boundary deposits.................................................... 76

Causes of rock avalanches and rockfalls .......................... 76

Implications for future mass failures............................... 79

Conclusions................................................................... 79

Acknowledgments.......................................................... 80

References cited ............................................................. 80

ABSTRACT

On March 29,1984, intrusion of new magma caused rapid 

spreading and extensive cracking of the north flank of the 

Mount St. Helens lava dome, which culminated in a series of 

small rock avalanches and rockfalls (total volume approximately 

105 cubic meters). The resulting avalanche and fall deposits 

consist of five main units: a thin, coherent, basal diamict 

(unit A); several overlapping, dry, noncohesive sand-rich 

deposits that contain dispersed pebbles and boulders (broadly 

grouped as units Bl and B2); a thin, generally massive deposit 

of silty, fine sand that mantles much of the avalanche fan

(unit C); and talus composed of boulders several meters in 

size (unit D). The suite of deposits is bounded by a fringe of 

fine sand and dirty snow.

The initial avalanche, recorded by unit A, is attributed to 

failure of the cold, snow-clad, brittle shell of the dome; units 

Bl and B2 record several subsequent failures of newly exposed 

hot rock that lay beneath the shell; and unit D records spoiling 

of older dome rock and newly extruded lava for several days 

after the initial mass failure. The mantle of massive, silty sand 

(unit C) is attributed to a small pyroclastic flow and to fallout 

of fine tephra generated by disaggregation of the hot-rock 

avalanches. The boundary deposit is attributed to a 

combination of pyroclastic flow, tephra fall, and runout of a 

dilute slurry and dirty water.

Snow provides an abundant water source that exacerbates 

the potential lahar hazard to sites down valley if hot-rock 

avalanches and associated disaggregation processes cause the 

snow to melt rapidly. However, more energetic, much hotter, 

or much larger rock avalanches than those of March 1984 are 

needed to produce associated meltwater flows that would affect 

areas much beyond the crater. Without the presence of a 

snowpack, rock avalanches must be substantially larger or more 

energetic than those of March 1984 to pose a threat beyond 

the crater.

INTRODUCTION

Rock avalanches and rockfalls originating from the 

composite lava dome of Mount St. Helens since 1980 

typically occur in one of three ways: (1) during periods 

of renewed eruptive activity as a result of extensive 

fracturing and weakening of the dome caused by

1 Current address: Washington Division of Geology and Earth Resources, 

P. O. Box 47007, Olympia, WA 98504-7007 69
70 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

intrusion of new magma; (2) by rock spalling from the 

leading edge of newly extruded lava that has encroached 

on the steep slopes of the dome; or (3) by earthquakeinduced collapse unrelated to eruptive activity. Typically, 

the volume of failed material is small (<104-105 m3) 

compared to the volume of the dome (over 108 m3 in 

1986), and most of the debris that is shed is deposited as 

talus at the base of the dome. However, rock avalanches 

and rockfalls from the lava dome occasionally are 

sufficiently large and mobile that they reach onto and 

spread across a part of the crater floor. Rock avalanches 

and rockfalls from the dome that occur while the crater 

floor is snow free affect only the crater and generally pose 

little hazard beyond the immediate area, even if they are 

hot. However, when hot-rock avalanches and rockfalls 

occur while snowpack is present they can induce 

meltwater flows that may reach beyond the crater. Since 

1980, several small eruptions of Mount St. Helens have 

generated flood surges, slushflows, or mixed avalanches 

of snow and debris that commonly have eroded large 

amounts of sediment and transformed into lahars 

(volcanic debris flows) that flowed beyond the crater and 

into the North ForkToutle River (fig. 5.1) (Waitt and 

others, 1983; Pierson and Scott, 1985; Cameron and 

Pringle, 1990; Pierson andjanda, 1994; Pierson, chap. 2, 

this volume; Pierson and Waitt, chap. 4, this volume; 

Pringle and Cameron, chap. 6, this volume).

EVENTS IN MARCH 1984

Rock avalanches and rockfalls accompanying an 

eruption on March 29,1984, illustrate failures that occur 

by weakening of the lava dome when new magma is 

actively injected into the dome, and by spalling of rock 

from the leading edge of a lobe of newly extruded lava 

that has encroached on the steep slopes of the dome. 

The eruption was preceded by about eight days of 

precursory seismicity and geodetic deformation. Shallowdepth seismicity and deformation of the dome at Mount 

St. Helens increased beyond background level on 

March 22. The rate of deformation increased more than 

twenty-fold, from about 12 mm per day to more than 

30 cm per day, between March 22 and March 27, 

(P.M. Otway, New Zealand Geological Survey, written 

commun., 1984). By March 27, earthquakes that could 

be felt and heard occurred frequently within the crater. 

Geodetic measurements indicated the northern part of 

the dome was spreading most rapidly in response to the 

intrusion of new magma. The rapid rate of deformation

and extensive cracking of the north flank of the dome 

caused concern that a small explosion and (or) rock 

avalanche might occur, which in turn could generate 

flood surges or lahars that could affect downstream areas.

On March 29 a segment of the north flank of the 

dome failed and disaggregated into a rock avalanche that 

covered about 2 x 105 m2 of the crater floor. Subsequent 

rock avalanches from exposed hot rock and rockfalls from 

the spalling carapace of the dome and from newly 

extruded lava (first observed on March 30) added to the 

deposit of debris during the next several days (fig. 5.2). 

No explosion, significant snowmelt, or flood occurred, 

however.

This chapter describes the complex deposit that 

resulted from the rock avalanches and rockfalls of 

March 29, 1984, deduces the geologic processes that 

produced various components of that aggregate deposit, 

and suggests why a flood was not generated. The events 

described in this chapter contrast a winter-condition 

eruption having only a local effect to other wintercondition eruptions that had far-reaching effects 

downstream (for example, Waitt and others, 1983; 

Pierson and Scott; 1985; Pierson, chap. 2, this volume; 

Pierson and Waitt, chap. 4, this volume; Pringle and 

Cameron, chap. 6, this volume). Since the emplacement 

of the rock avalanches and rockfalls of March 1984, other 

similar events have occurred and their deposits have 

buried the deposits described in this chapter.

TERMINOLOGY

The term rock avalanche is applied somewhat 

broadly in this chapter. Some masses of failed material 

probably were initiated by sliding, others by falling. 

However, most of the failed rock masses quickly 

disaggregated into predominantly sand-sized particles, 

and the resulting deposits have characteristics of flow. 

The term avalanche connotes flow behavior. Initiation 

of an avalanche by slide or fall does not significantly affect 

the characteristics of the resulting deposit (McSaveney, 

1978). Rockfall refers to rock masses that became 

dislodged, remained essentially intact or disaggregated 

into predominantly boulder-sized fragments, and 

contributed primarily to a deposit of bouldery talus that 

does not have characteristics of flow. Pyroclastic flow 

refers to a hot, rapidly moving, high-concentration 

dispersion of gas and solid particles that commonly flows 

in a nonturbulent manner. Pyroclastic-flow deposits 

commonly are characterized by massive diamicts that can
123°00' 45' 30' 15' 122°00'

46° 

15'

46° 

00'

I I 

0246 8 KILOMETERS

/-Study Area

Mount St. Helens

FIGURE 5.1.—Drainage network at Mount St. Helens. The letters HOA indicate the location of the seismometer referred to in figure 5.3.
72 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

exhibit vertical variation in the distribution of coarse 

particles (Carey, 1991). A continuous spectrum of 

transport and depositional mechanisms exists between 

hot-rock avalanches and pyroclastic flows. Pyroclastic 

surge refers to a hot, low-concentration dispersion of 

gas and solid particles that flows in a turbulent manner, 

and whose deposit is commonly thinly to thickly 

laminated and has planar to cross stratification (Fisher 

and Schminke, 1984; Carey, 1991).

TIMING OF ROCK AVALANCHES 

AND ROCKFALLS

At approximately 0319 Pacific Standard Time (PST) 

on March 29, 1984, the U.S. Weather Service radar at 

Portland, Oregon, recorded a plume to an altitude of

4.6 km over the crater of Mount St. Helens. 

Simultaneously, seismograph signals received at the U.S. 

Geological Survey in Vancouver, Washington, suggested 

that a series of rock avalanches or rockfalls had occurred. 

As inferred later from the seismic signals, the first rock 

avalanche occurred shortly after 0318, and during the 

next several minutes was followed by several similar events 

that produced avalanche-like seismic signals (fig. 5.3).

Apre-dawn reconnaissance flight on March 29,1984, 

verified the presence of a rock-avalanche deposit north 

of the dome. During the flight, several incandescent 

rockfalls were observed moving down the north face of 

the dome. Photographs taken during a later flight at 

1030 indicated the rock-avalanche deposit resulted from 

emplacement of multiple avalanches, an observation 

consistent with the inferences drawn from the seismic 

signals. During the next several days lava extruded and

FIGURE 5.2.—Oblique aerial view of Mount St. Helens crater showing lava dome and March 29, 1984, rock-avalanche 

fan. View is south-southwest. Lava dome is about 1000 meters in diameter.
ROCK AVALANCHES, ROCKFALLS, AND ASSOCIATED PROCESSES INDUCED BY SPREADING OF THE LAVA DOME 73

spread across the top of the dome. When the lava reached 

the steep north flank of the dome, it spalled and produced 

rockfalls that moved down the north face of the dome 

and deposited bouldery talus on the rock-avalanche 

deposits. Rockfalls from this lava lobe persisted for several 

days. Many incandescent rockfalls visible beyond the 

crater were recorded on March 31 in a dramatic timelapse photograph taken from a U.S. Army Corps of 

Engineers pump station located near the outlet of Spirit 

Lake (fig. 5.4).

DEPOSITS OF THE ROCK 

AVALANCHE-ROCKFALL COMPLEX

The deposits that resulted from the initial rock 

avalanches and later rockfalls of March 29,1984, formed 

a crudely oval fan about 700 m long and 500 m wide 

(figs. 5.5 and 5.6). Preliminary observations of the 

deposits were made by aerial reconnaissance on the 

morning of March 29; the weather was clear, the deposits 

were fresh, and physical features of the fan were easily 

recognized. The fan appeared to be bordered by two 

distinctly concentric zones: a dark-colored inner zone, 

and a light-colored outer zone that graded into a deposit 

plastered on the east crater wall (figs. 5.2 and 5.5). 

Inclement weather prevented ground investigation of the 

deposits until April 2. By then, several inches of new 

snow had fallen and covered all but the central area of 

the fan, which apparently remained warm.

UNIT A

A brief examination of deposit stratigraphy and 

morphology indicated the fan consisted primarily of five

depositional Units. The basal deposit, Unit A, was a 

compact, dark-brown diamict that covered an area of 

about 2 x 105 m2. It was composed of a thin (less than 2 

cm thick) basal layer of discontinuous, commonly frozen, 

silty, fine sand that contained scattered coarse sand to 

granules. The basal layer was overlain gradationally by 2 

to 7 cm of nonstratified silty sand that contained 5 to 

10 percent lithic granules. This nonstratified sandy layer 

was in turn overlain in gradational-to-sharp contact by 

20 to 30 cm of poorly sorted, pebbly, silty sand that was 

inversely graded locally and supported clasts as large as 

20cm.

The basal facies lay in sharp contact with the 

underlying snow surface. Near the deposit margin, the 

contact was locally erosional (fig. 5.7). Where exposed 

the deposit was 30 to 40 cm thick, and subjacent snow 

appeared to be eroded at least 20 cm deep (see Walder, 

chap. 1, this volume). However, the contact with 

underlying snow clipped mostly toward the center of the 

fan; therefore, the maximum thickness of the deposit 

occurred near the central part of the fan. If the deposit 

thickness along the margin is used as an average value 

for the whole deposit, then an estimated minimum 

volume for unit A is 8 x 104 m3 .

A layer of granular ice 1 to 2 cm thick mantled the 

top of the snow surface beneath unit A at the distal margin 

of the deposit. A similar, but thinner, layer of granular 

ice also mantled the snow beyond the area affected by 

the rock-avalanche deposit. The difference in thickness 

between the ice layer beneath and beyond the avalanche 

deposit may be evidence for minor melting of snow 

during emplacement of unit A, caused perhaps by friction 

between the snow and the overriding debris, or perhaps 

by warm rock that was incorporated in the rock 

avalanche. Warm rock could have come from fumaroles

« -... .;**&* '.f^t., *~

FIGURE 5.3.—Seismograph record of March 29, 1984, from seismic station HOA. The first significant rock-avalanche signal 

occurred about 0318 Pacific Standard Time (arrow) and was followed closely by similar signals during the next several 

minutes. Note the difference between short earthquake signals (EQ) and long rock-avalanche signals.
74 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

on the north side of the dome or from segments of the 

recently extruded February 1984, lava lobe that were 

incorporated in the initial failure.

The poorly sorted nature of unit A, local inverse 

grading, and basal erosion indicate that this unit formed 

by a flow process. We therefore conclude that unit A 

resulted from a rock avalanche and not simply from a 

rockfall. The wet, compact nature of the deposit, the 

absence of significant melting of snow, and the absence 

of evidence for hot emplacement suggest that unit A 

formed from a source of relatively cool material and was 

emplaced at low temperature.

UNITS Bl AND B2

Overlying unit A, in sharp contact, was a rockavalanche deposit consisting of dry, loose, lithic-rich sand 

in which pebble- to boulder-sized clasts were dispersed

(unit Bl). The upper several centimeters of this loose 

sand commonly was damp (fig. 5.8). Several of the large 

boulders in the middle and distal reaches of the fan, 

some many meters in diameter, probably were 

transported by this rock avalanche.

The surface morphology of unit Bl contrasted 

markedly with that of unit A. Several small pits as wide 

as 1 to 2 m were evident locally on the surface (figs. 5.5; 

5.6). These pits were the result of either small phreatic 

(steam) explosions caused by hot particles interacting 

with snow or collapse around hot rocks that melted 

downward into underlying snow. The pits did not result 

from ballistic impacts, for if they had, pits also would 

have been present in the snow beyond the deposit, which 

they were not. Even though new snow had fallen 

subsequent to the emplacement of the rock-avalanche 

deposits, impact pits in the original snow surface would 

have been obvious. In addition to pits, the surface of

Figure 5.4.—Time-lapse photograph of incandescent rockfalls on March 31, 1984, resulting from spalling of newly extruded 

lava. Lava dome is about 350 meters high. Photograph taken from U.S. Army Corps of Engineers pump station near Spirit 

Lake, 8 kilometers north-northeast of crater. Copyrighted photograph by Jack Hash; published with permission.
ROCK AVALANCHES, ROCKFALLS, AND ASSOCIATED PROCESSES INDUCED BY SPREADING OF THE LAVA DOME 75

unit Bl was studded with clasts averaging about 0.5 m 

in diameter. The unit also consisted locally of lobate 

deposits from smaller rock avalanches, indicating that 

this unit resulted from emplacement of multiple pulses 

rock avalanche. Individual lobes as thick as 2 to 3 m 

contained meter-sized clasts.

In several localities, pockets of debris had 

temperatures greater than 50°C (the thermometer limit) 

on April 2. Owing to the loose, dry texture of the deposit, 

the pockets of hot debris, and the pits that probably 

originated by small phreatic explosions or collapse, we 

conclude that unit B1 was hot when it was deposited.

Unit B1 covered an area much smaller than did unit A 

(about 1 x 105 m2); most of unit Bl was deposited near 

the base of the dome (fig. 5.6). Volume estimates of 

unitBl are not well constrained because estimates of 

the thickness of the deposit are poor. A volume estimate 

for unit B1 of 3 x 105 m3 of debris is based on an assumed 

average thickness of about 3 m.

Emplacement of a third phase of rock avalanches 

(unit B2) was interpreted primarily from aerial 

photographs of the debris fan. The loose, dry, sand-rich

deposits resulting from the rock avalanches that produced 

unit B2 were similar to those of unit B1. The rock 

avalanches that formed unit B2 eroded into deposits 

along the margin of the debris fan and melted into the 

snow substantially below the pre-fan surface. As a result, 

unit B2 deposits overlie, and were inset against, older 

deposits. Along the east margin of the fan debris from 

unit B2 was deposited in a cove about 2.5 m below the 

surrounding surface. Exposure in the walls of the cove 

showed that unit A rested on a grooved snow surface 

and was partly eroded by unit B2. During emplacement, 

avalanches that deposited unit B2 apparently scoured 

enough debris from unit A that they were able to transfer 

sufficient heat to melt down through about 2 m of snow. 

Deposits of unit B2 spread across an area of about 4 x 

104 m2 . An estimated volume of 1 x 105 m3 for unit B2 

is based on an assumed average deposit thickness of 3 m.

UNITC

The dry, loose, sandy deposits of units B1 and B2 

were overlain sharply by a few centimeters of massive,

FIGURE 5.5.—Oblique southward aerial view of March 29, 1984, rock-avalanche fan.
76 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

silty, fine sand that contained scattered granules to a few 

mm in diameter (unit C; fig. 5.8). Similar massive, silty 

sand, probably contiguous with the sand overlying units 

Bl and B2, constitutes part of the boundary deposits 

discussed below. The massive, silty sand generally did 

not mantle the large boulders near the head of the debris 

fan (see unit D).

The lack of seismic signals that indicate any 

geomorphic processes other than rock avalanches and 

rockfalls during the events described in this chapter (such 

as explosions from the dome), the mantle of silty sand 

on units Bl and B2, and the general lack of a sand deposit 

on the many boulders that compose unit D indicate a 

close association between the hot-rock avalanches that 

deposited units Bl and B2 and the geologic process that 

deposited unit C. The generally massive texture of the 

silty sand indicates that it was deposited by a relatively 

high-concentration flow and not by a low-concentration 

surge. We attribute unit C to emplacement by an areally 

restricted pyroclastic flow associated with the complete 

disaggregation of rock involved in the hot-rock 

avalanches that deposited units Bl and B2.

0 100 METERS

EXPLANATION

A Rock-avalanche deposit (cold)

Aw Watery slurry

B1 Rock-avalanche deposits (hot)

B2 Rock-avalanche deposits (cold)

C Rockfall deposits

Fs Silty sand fallout

Sd Dirty snow

Contact

... inferred contact 

3 Failure scarp 

• Boulders > 10m

;;; Pits

Figure 5.6.—Geologic map of rock avalanche-rockfall 

complex compiled from figure 5.5.

UNIT D

Rockfall deposits that consist of boulders several 

meters in diameter, unit D, formed a talus apron near 

the head of the debris fan (fig. 5.6). This apron of talus 

resulted from persistent rockfall activity during the several 

days that followed the rock avalanches on March 29. We 

did not estimate the volume of this bouldery talus

BOUNDARY DEPOSITS

A deposit consisting of a dark-colored inner zone and 

a light-colored outer zone bounded the rock-avalanche 

fan. The dark inner zone consisted of about 1 mm of 

frozen silt overlain by 1 to 2 cm of massive, fine to 

medium sand that contained scattered granules. The 

sand, in turn, was overlain locally by a few centimeters 

of graded, silty fine sand. Except for the surficial silty 

sand, the dark deposit has two possible origins: (1) a dilute 

slurry fades related to unit A, or (2) a facies of pyroclastic 

flow related to disaggregation of the hot-rock avalanches 

(units Bl and B2). The light-colored outer zone consisted 

of dirty snow, caused by muddy water seeping through 

the snowpack, and of a mantle of massive to moderately 

graded silty sand overlying the snow. The mantle of silty 

sand, similar to the sand that caps much of the debris 

fan, is attributed to a small pyroclastic flow, and perhaps 

an associated surge-like cloud, and to fallout dust 

generated by fragmentation of the rock avalanches, 

particularly the hot-rock avalanches that produced units 

B1 and B2. Fallout dust coated the east crater wall. The 

plume detected by the National Weather Service radar 

during the early phase of the events was composed of 

ash convecting from the rock avalanches and the 

associated pyroclastic flow, and not from an explosion. 

Fallout of very fine ash was reported as far as 70 km 

south-southwest of Mount St. Helens (Bobbie Myers, 

U.S. Geological Survey, oral commun., 1984).

CAUSES OF ROCK AVALANCHES 

AND ROCKFALLS

The initial rock avalanche on March 29 was the 

culmination of intense fracturing and weakening of the 

north side of the dome, which resulted from lateral 

spreading induced by the intrusion of new magma. On 

March 22, the rate at which the dome deformed increased 

beyond background level. By March 27, the north side 

of the dome deformed at an average rate greater than
ROCK AVALANCHES, ROCKFALLS, AND ASSOCIATED PROCESSES INDUCED BY SPREADING OF THE LAVA DOME 77

FIGURE 5.7.—Undulating surface of snow at the base of unit A. View looking northward in direction of flow. 

Scale divisions in centimeters (right) and inches (left).

FIGURE 5.8.—Exposure of unit Bl illustrating the poorly sorted, loose, dry texture of the deposit. The upper 

several centimeters of unit Bl is damp, and the unit is overlain by about 2 centimeters of fine silty sand 

(unit C) deposited by a pyroclastic flow associated with the avalanche. Scale divisions in centimeters (left) 

and inches (right).
78 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

30 cm per day, whereas the west and south sides of the 

dome deformed at rates of 8 and 2 cm per day 

(P.M. Otway, New Zealand Geological Survey, written 

commun., 1984). Reflectors mounted on the dome were 

displaced northward by 42 m between midday on 

March 27, and midday on March 29; total displacement 

of the north sector of the dome during this eruption was 

estimated at 57 m. Intrusion of new magma into the 

dome thus severely disrupted, and mechanically 

weakened, the northern sector of the lava dome in the 

few days prior to the rock avalanches. By 1700 on 

March 27, a new arcuate crack, 1 to 2 meters wide and 

about 10 meters deep, appeared on the north-northwest 

sector of the February 1984 lava lobe, which had been 

extruded high on the northeast part of the dome (Daniel 

Dzurisin, U.S. Geological Survey, written commun., 

1984). At about 0318 on March 29 segments of the 

north side of the dome and of the February lobe failed. 

The geometry of the north flank of the dome before and 

after the March rock avalanches, based on profiles 

measured from contour maps from January 31, 1984, 

and April 14,1984, is illustrated in figure 5.9. Disruption 

of the dome and emplacement of extruded lava in 

February 1984 precluded direct measurement of the 

initial volume of failed rock and of the total thickness of 

the March rock-avalanche deposits, but the failure plane, 

transport slope, and the region of deposition are clearly 

indicated by the profiles.

On the basis of the stratigraphy and morphology of 

the deposits and on the recorded seismic signals, we offer 

the following interpretation of the events of March 29. 

Segments of the outer shell of the dome dislodged, and 

rapidly disaggregated, after the north flank was severely 

weakened by intruding magma. The disaggregated mass 

flowed down the north face of the dome, incorporated 

snow and possibly saturated debris from the talus apron, 

and then spread out to become unit A of the rockavalanche fan. That large segments of rock from lava 

domes can fail, disaggregate almost instantaneously, and 

transform into flowing masses has been observed at 

Mount St. Helens (N.S. MacLeod, U.S. Geological 

Survey, oral commun., 1985) and at Unzen volcano, 

Japan (Ishikawa and others, 1992). Removal of a part of 

the outer shell destabilized other mechanically weakened 

segments of the dome high on the north side and induced 

retrogressive failure. The ensuing piecemeal failure 

(indicated by development of diamict stratigraphy in 

unit A) consequently exposed hot rock in the dome. 

Continued retrogressive failure of hot rock produced 

pulses of hot-rock avalanches closely associated in time

with the initial cold rock avalanche, and resulted in 

units Bl and B2. As these hot-rock avalanches 

disaggregated, they generated a small pyroclastic flow (or 

flows) that swept over the avalanche debris and deposited 

a mantle of massive, silty sand (unit C). When magma 

reached the surface of the dome, small rockfalls, involving 

older pieces of the dome as well as new debris, continued 

intermittently for several days and produced unit D.

Processes similar to those described in this chapter 

occurred during a hot-rock avalanche in May 1986 

(Mellors and others, 1988). That avalanche produced a 

fan-shaped deposit that consisted of coarse, bouldery, 

juvenile debris, more extensive pyroclastic-flow and 

pyroclastic-surge deposits, and fine-grained tephra that 

fell several kilometers downwind. As with the March 

1984 episode of rock avalanches, no explosion occurred 

and all of the observed facies resulted from the 

disaggregation of rock during the emplacement of the 

avalanche.

There are two major differences between the 

March 1984 and May 1986 events, however. During 

the rock avalanches of May 1986, hot debris mixed with 

snow and produced debris flows in the crater and a small 

flood and debris flow that left the crater (Cameron and 

Pringle, 1990). Also, no deposit corresponding to an 

avalanche of cold rock was recognized from the May 1986 

event. The May 1986 event was caused primarily by the 

failure of a segment of rock from a new lava lobe and not 

by the failure of rock from the older, colder carapace of 

the dome.

At volcanoes where lava domes have been extruded 

onto slopes much steeper than the floor of the Mount 

St. Helens crater, gravitational collapse of segments of 

the domes has occurred without precursory activity or 

warning and has generated hot-rock avalanches and

SW March 1984 lobe NE

Failure 

plane Rock-avalanche 

fan

L

50m —— January 1984

—- April 1984

FIGURE 5.9. —Profiles of the north part of the Mount St. 

Helens dome on January 31, 1984, and April 14, 1984.
ROCK AVALANCHES, ROCKFALLS, AND ASSOCIATED PROCESSES INDUCED BY SPREADING OF THE LAVA DOME 79

pyroclastic flows that have traveled far beyond the lava 

dome, sometimes with tragic consequences. Hot-rock 

avalanches, pyroclastic flows, and pyroclastic surges 

occurred during the 1990 eruptions of Redoubt volcano, 

Alaska, when disrupted, thermally stressed rock from the 

lava dome suddenly failed and rapidly disaggregated 

(Miller, 1994; Gardner and others, 1994). Pyroclastic 

flows that resulted in 43 deaths occurred when the lava 

dome at Unzen volcano, Japan, suddenly collapsed and 

hot rock disaggregated explosively (Fink, 1991; Ishikawa 

and others, 1992). Similar dome collapses and associated 

pyroclastic flows also have occurred during past eruptions 

of Mount Hood volcano, Oregon (Brantley and Scott, 

1993).

IMPLICATIONS FOR FUTURE 

MASS FAILURES

Rock avalanches and rockfalls have occurred 

infrequently from the lava dome at Mount St. Helens 

during periods of eruption and of quiescence. If episodic 

growth of the lava dome persists during the next several 

decades, new lava lobes probably will be extruded, and 

growth by intrusion of magma will certainly recur. 

Regardless of how growth proceeds, rock avalanches and 

rockfalls should occasionally occur owing to deformation 

and lateral spreading of parts of the dome. Rock 

avalanches such as those described in this chapter may 

be a visible external expression of future volcanic activity. 

However, rock avalanches of the magnitude described 

in this chapter are likely to affect only the crater.

Comparison of the March 1984 rock avalanches with 

other similar events at Mount St. Helens, particularly 

the May 1986 event, suggests that the deposit from the 

initial, cold rock avalanche prevented the subsequent hotrock avalanches and rockfalls from mixing with snow 

and generating debris flows and floods. Although the 

hot-rock avalanches of March 1984 probably were similar 

in temperature to those of May 1986, the March 1984 

rock avalanches did not generate pyroclastic flows that 

extended far beyond the limits of the cold avalanche. 

Thus there was litde, if any, opportunity for quantities 

of hot debris to mix with snow.

Although small rock avalanches and rockfalls from 

the dome usually are not vigorous enough or hot enough 

to melt snow rapidly and cause flooding beyond the 

crater, exceptions do "occur (Mellors and others, 1988; 

Pringle and Cameron, chap. 6, this volume). Most of 

the volcanic events that affected areas beyond the crater

were accompanied by small explosions of hot gas and 

rocks (Waitt and others, 1983; Brantley and others, 1985; 

Pierson, chap. 2, this volume; Pierson and Waitt, chap. 4, 

this volume). During those events, snow from the crater 

walls and floor melted suddenly and produced varied 

combinations of flood surge, slushflow, and mixed 

avalanches of snow and debris that flowed out of the 

crater. Such flows commonly incorporated sediment as 

they moved across the steep slopes beyond the crater, 

and some of the flows subsequently transformed into 

lahars that traveled tens of kilometers downstream (Waitt 

and others, 1983; Pierson and Scott, 1985). Without a 

snowpack, the likelihood of rock avalanches similar to 

those described in this chapter spawning flows that would 

affect areas beyond the crater is vastly decreased. Potential 

hot-rock avalanches and related pyroclastic flows and 

surges precipitated by a substantially larger failure of the 

dome, however, could pose a future hazard beyond the 

crater regardless of snowpack conditions.

CONCLUSIONS

Rapid spreading and extensive cracking of the lava 

dome at Mount St. Helens related to intrusion of new 

magma in March 1984, generated a series of small rock 

avalanches. The total volume of deposited debris was 

about 5 x 105 m3 . The initial rock avalanche probably 

was cold, and its deposit consisted of a thin, dense 

diamict. Subsequent rock avalanches had their source 

in hot material. When the hot rock failed it rapidly 

disaggregated into dominantly sand-sized particles and 

generated not only hot-rock avalanches but also a small 

pyroclastic flow(s). The hot debris was unable to mix 

with snow, however, owing to the underlying avalanche 

deposit.

Snow provides an abundant water source that can 

exacerbate the lahar hazard down valley if hot-rock 

avalanches and associated processes cause rapid melting. 

But more energetic, much hotter, or much larger rock 

avalanches than those of March 1984 are needed to 

produce associated meltwater flows that would affect 

areas beyond the crater. Without a snowpack, rock 

avalanches must be substantially larger than those of 

March 1984 to pose a threat beyond the crater.
80 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

ACKNOWLEDGMENTS

We thank our USGS colleagues for discussions and 

reviews of earlier drafts of this paper, and Muriel Hash 

for allowing us to publish figure 5.4.

REFERENCES CITED

Brantley, S.R., Power, J.A., andTopinka, L.J., 1985, Reports 

from the U.S. Geological Survey's Cascades Volcano 

Observatory at Vancouver, Washington: U.S. Geological 

Survey Earthquake Information Bulletin, v. 17, no. 1, 

p. 20-32.

Brandey, S.R., and Scott, WE., 1993, The danger of collapsing 

lava domes: Lessons for Mount Hood, Oregon: 

U.S. Geological Survey Earthquakes and Volcanoes 

Information Bulletin, v. 24, no. 6, p. 244-269.

Cameron, K.A., andPringle, P.T., 1990, Avalanche-generated 

debris flow of 9 May 1986 at Mount St. Helens, 

Washington: Northwest Science, v. 64, no. 3, p. 159-164.

Carey, S.N., 1991, Transport and deposition of tephra by 

pyroclastic flows and surges, in Fisher, R.V, and Smith, 

G.A., eds., Sedimentation in volcanic settings: SEPM 

(Society for Sedimentary Geology), Special Publication no. 

45, p. 39-57.

Fink, J.H., 1991, Volcano warning needed: Nature, v. 351, 

no. 6328, p. 611.

Fisher, R.V., and Schminke, H.-U., 1984, Pyroclastic rocks: 

Springer-Verlag, Berlin, 472 p.

Gardner, C.A., Neal, C.A., Waitt, R.B., andjanda, R.J., 1994, 

Proximal pyroclastic deposits from 1989-90 eruption of 

Redoubt Volcano, Alaska—stratigraphy, distribution, and 

physical characteristics: Journal of Volcanology and 

Geothermal Research, v. 62, p. 213-250.

Ishikawa, Yoshiharu, Yamada, Takashi, and Yajima, Shigemi, 

1992, A study on the pyroclastic flows and debris flows 

associated with the Mount Unzen eruption in 1991: Sabo 

Department, Public Works Research Institute, Ministry 

of Construction (Japan), PWRI Technical Memorandum 

no. 3141, 56 p.

McSaveney, M.J., 1978, Sherman glacier rock avalanche, 

Alaska, U.S.A., in Voight, Barry, ed., Rockslides and 

avalanches-1. Natural phenomena in the collection 

developments in geotechnical engineering: Amsterdam, 

Elsevier,v. 14A, p. 197-258.

Mellors, R.A., Waitt, R.B., and Swanson, D.A., 1988, 

Generation of pyroclastic flows and surges by hot-rock 

avalanches from the dome of Mount St. Helens volcano, 

USA: Bulletin of Volcanology, v. 50, no. 1, p. 14-25.

Miller, T.P, 1994, Dome growth and destruction during the 

1989—90 eruption of Redoubt Volcano: Journal of 

Volcanology and Geothermal Research, v. 62, p. 197-212.

Pierson, T.C., and Janda, R.J., 1994, Volcanic mixed 

avalanches: A distinct eruption-triggered mass-flow process 

at snow-clad volcanoes: Geological Society of America 

Bulletin,v. 106, p. 1351-1358.

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, no. 10, 

p. 1511-1524.

Waitt, R.B., Jr., Pierson, T.C., MacLeod, N.S., Janda, R.J., 

Voight, Barry, and Holcomb, R.T., 1983, Eruptiontriggered avalanche, flood, and lahar at Mount 

St. Helens—Effects of winter snowpack: Science, v. 221, 

no. 4618, p. 1394-1397.
6

ERUPTION-TRIGGERED LAHAR ON MAY 14, 1984

By PATRICK T. PwNGLE1 and KENNETH A. CAMERON2

CONTENTS

Abstract........................................................................ 81

Introduction ................................................................. 81

Study methods ........................................................ 82

Terminology............................................................ 84

Eyewitness observations on May 14 .............................. 84

Lahar deposits............................................................... 86

Proximal-reach deposits........................................... 86

Transition-reach deposits......................................... 91

Estimated deposit volume, water content, and heat

energy implications ................................................. 93

Velocity and discharge................................................... 93

Discussion .................................................................... 95

Transformation of mixed avalanche and 

slushflowto debris flow........................................ 95

Transformation of debris flow to hyperconcentrated flow................................................. 97

Dramatic wave forms (hydraulic jumps?)

observed in the lahar recessional flow ................ 97

Scour markings and depositional features of 

the lahar............................................................... 98

Post-lahar geomorphic changes in the upper

North ForkToutle River basin........................... 100

Summary ...................................................................... 101

Acknowledgments......................................................... 102

References cited ............................................................ 102

ABSTRACT

On May 14, 1984, a shallow-seated explosion from the 

dome at Mount St. Helens sprayed the west crater wall with 

hot rock debris and gases and triggered a mixed snow-androck avalanche and slushflow that exited the crater through 

the Breach into upper Loowit Ravine. This mixture 

transformed into a debris flow on the Pumice Plain as it

1 Current address: Washington Division of Geology and Earth Resources, 

P. O. Box 47007, Olympia, WA 98504-7007

2 Current address: 14708 SE Rupert Dr. Apt. A., Portland, OR 97267

accumulated sediment and heat from the still-hot 1980 

pyroclastic flow deposits and added water through melting of 

admixed snow and ice granules and through incorporation of 

water from stream channels and saturated sediments. The 

resulting debris flow was erosional throughout the Pumice 

Plain but caused aggradation of the active channel of the North 

ForkToutle River from 8 to 16 kilometers from the lava dome 

before transforming to a hyperconcentrated flow. Temporal 

and downstream changes in the flow behavior and correlative 

textural changes in deposits were documented by eyewitness 

observations, velocity measurements, sampling of the lahar, 

and examination of deposits after they became exposed. At 

least 7.1 x 10 10 joules was required to generate the estimated 

4 x 105 cubic meters of meltwater incorporated in the lahar 

from crater snow and ice.

INTRODUCTION

This report documents an unusual geologic 

opportunity, that of making eyewitness observations of 

a small lahar at close range and then subsequently 

examining the deposits and geomorphic effects of the 

lahar. This may be the first report that describes how 

visually observed flow transformations in a lahar have 

been correlated with downstream textural changes in its 

deposits.

A shallow-seated volcanic explosion from the upperwest flank of the Mount St. Helens lava dome triggered 

a lahar on May 14, 1984, just after 0937. Amixedsnowand-rock avalanche from the west crater wall and a 

recessional slushflow moved northeastward, exited the 

1980 crater through the Breach, and then entered Loowit 

Ravine, incising the surface of the crater floor and upper 

Steps as deep as 2 m along a 30-m-wide track (fig. 6.1). 

This mixture splayed into three distributary lobes on 

Step Fan at the mouth of Loowit Ravine, about 5 km 

from the lava dome (fig. 6.2).

81
82 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

The eastern lobe on the Step Fan reached Spirit Lake 

by Lake Creek as a debris flow. The middle and western 

lobes incorporated additional sediment, heat, and water 

as they flowed northwestward in a channel system across 

the still-hot 1980 pyroclastic-flow deposits on the Pumice 

Plain. Most of the material (the western lobe) entered 

the North Fork Toutle River by Steam Creek; a lesser 

amount (the middle lobe) flowed northwestward down 

the Pumice Plain and entered Pumice Creek. The two 

flows united in the North Fork Toutle channel about 

9 km from the vent and flowed 7 km down the North 

Fork Toutle River as a debris flow, before transforming 

to a hyperconcentrated flow. Hot rocks that were hurled 

onto the crater wall and crater floor (possibly including 

minor amounts of juvenile material) evidently melted 

enough snow and ice to generate a slushflow and a 

recessional discharge of muddy water from the crater that 

lasted for more than one hour.

STUDY METHODS

The earliest observation of the flow front was at 

0950 Pacific Daylight Time (PDT) by Jack Hash, who

was located at the U.S. Army Corps of Engineers 

temporary pumping station near Spirit Lake (fig. 6.1). 

He reported "...steaming... and a considerable mudflow 

flowing toward Spirit Lake"; and at 0952, "...quite a bit 

of water..." (6.6 km from the lava dome). At 1008, 

volunteer observers ("riverwatch") near Elk Rock 

reported a mudflow in the North Fork Toutle River near 

Coldwater Lake (16 km from the crater).

A helicopter carrying three U. S. Geological Survey 

personnel, arrived at the Mount St. Helens crater at 1012, 

but steam in the crater prevented us from making detailed 

observations of the vent area at that time. However, we 

flew downstream along the North Fork Toutle River 

valley and overtook the lahar flow front, sampling it as 

it passed three downstream locations. After several hours, 

we returned upstream to examine and sample deposits 

of the lahar.

More detailed study of the sedimentary characteristics 

of the lahar was conducted the following year as the 

deposits became dissected, which allowed interpretations 

of downstream rheological changes and estimates of 

deposit volume and distribution. The volume estimates 

allowed approximations of water content and of the

122°22'30"

46° 

15'

SP#3

Boundary of 1980

debris-avalanche

deposit

U.S. Army Corps

of Engineers

temporary pumping

station

Pumice 

pond

_ Harry's 

£: \_channd

NORTH FORK 

TOUTLE RIVER

SP

EXPLANATION

» Measured cross section 

Sampling point 

Debris flow

Path of snow and rock 

avalanche and slushflow

Transition interval, debris flow 

to hyperconcentrated flow

Hyperconcentrated flow 

Step Fan

May 14-

explosion

ejecta

Lake Creek __ 

Pumice Creek

Transition from 

snow-and-rock 

avalanche and 

slushflow to 

debris flow

-East Loowit Ravine 

L0010 

-Lava dome

3 KILOMETER 

i i MOUNT ST. HELENS

1 MILE

FIGURE 6.1.—Path of May 14, 1984, lahar and lahar runout and locations of selected measured cross sections.
ESE WSW

Ash and gas plume 

rose to 28,000 ft

Deposit of rock and 

snow avalanche and 

slushflow

Eastern lobe of 

lahar flowed down 

Lake Creek

Rock and snow avalanche 

and slushflow generated 

high frequency seismic si 

as it exited the Breach at 0941PDT

ST. HELENS

0937 PDT explosion sprayed 

bouldery ejecta as far as 

1.6 km from the lava dome

Middle lobe of lahar flowed down Pumice Creek

Western lobe of lahar 

reached cross section 

LO 040 by about 0952 PDT 1980 debris- avalanche deposit

Bank failures in 1980 pyroclastic-flow deposits v contributed to bulking \ by lahar and to ponding Cross section during recessional flow NF100 

Vs\ Hanging valley shown in fig. 6.11

Spirit Lake

Figure 6.2.—Panorama from Johnston Ridge, about seven kilometers north of the crater showing approximate extent of May 14 mixed snow and rock avalanche and 

slushflow and paths of various lobes of the lahar. Langes Crest is the drainage divide formed by the 1980 debris avalanche deposit that impounds Spirit Lake.
84 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

energy required to generate the incorporated water from 

crater snow and ice.

Channels affected by the lahar are crossed by measured 

cross sections that are regularly surveyed for channel 

evolution, geometry, and sediment budget. These cross 

sections were resurveyed shortly after May 14 and 

compared with pre-lahar data to determine the amount 

and nature of erosion and deposition caused by the lahar.

TERMINOLOGY

Sedimentology of Mount St. Helens lahars and 

associated terminology is discussed by Scott (1988). In 

slight contrast to his usage and that of Crandell (1971), 

the term lahar is used in a more general sense to mean 

a rapidly flowing mixture of rock debris and water from 

a volcano (Smith and Lowe, 1991). Lahars can have 

sediment concentrations of more than about 80 percent 

by weight (60 percent by volume) and, thus, may be 

classified as debris flows (Beverage and Culbertson, 

1964; Pierson and Costa, 1987). In distal reaches lahars 

commonly undergo dilution to hyperconcentrated 

flows, which have sediment concentrations in the range 

or about 40 to 80 percent solids by weight (20-60 percent 

by volume) (Beverage and Culbertson, 1964; Pierson and 

Costa, 1987) and which form as the result of debrisflow dilution (Pierson and Scott, 1985; Scott and 

Dinehart, 1985; Scott, 1988). Hyperconcentrated flows 

have a measurable yield strength but flow more like a 

liquid (Pierson and Costa, 1987). Muddy floods are 

highly or extremely concentrated streamflow but have 

less than about 40 percent sediment by weight (about 

20 percent by volume).

Backfill denotes slack-water deposition by debris flow 

in tributary channels or in the main channel (Patton and 

others, 1979; Kochel and Baker, 1982). Avalanche/ 

slushflow refers to the flow as it debouched from Loowit 

Ravine, where it had some qualities of a snow and rock 

avalanche and also those of a more fluid flow of mixed 

slush, rock, and snow. Diamict is a term used to describe 

unsorted deposits such as tills or debris-flow deposits.

EYEWITNESS OBSERVATIONS ON MAY 14

On arrival at Mount St. Helens by helicopter at 1012, 

we observed the muddy trace of the avalanche/slushflow 

extending from the crater floor west of the dome to the 

north-northeast and, farther to the north, a discharge of

muddy water flowing through Loowit Ravine and across 

Step Fan. On the Pumice Plain, superelevating, muddy 

surges continued to trigger bank failures throughout the 

length of Steam Creek. These bank failures exposed 1980 

pyroclastic-flow material that was still hot enough to 

generate small steam explosions upon contact with the 

flow (fig. 6.3).

In the North Fork Toutle River (from about 8 to 

16 km from the crater), the lahar deposits already had 

filled the active channel to the top of its banks by the 

time the lahar was observed (18 minutes behind the flow 

front). Recessional watery surges flowed over the fresh 

lahar-deposit surface, creating knickpoints (fig. 6.4). The 

capacity of the channel had been so decreased by initial 

deposition from the lahar that one of these surges 

overtopped a terrace that had not been inundated by the

FIGURE 6.3.—Upstream view showing lahar recessional flow 

in the North Fork Toutle channel at about 1013 on 

May 14, 1984. Steam Creek enters from the right. The 

channel of the North Fork Toutle River is about 50 meters 

wide and 15 meters deep at this confluence.
ERUPnON-TRIGGERED LAHAR ON MAY 14, 1984 85

lahar flow front. The surges apparently were debris flows 

because the flow surface was relatively smooth, except at 

hydraulic jumps at knickpoints, and boulders were 

suspended by the flow (fig. 6.4).

Steaming from the lahar surface as a result of 

incorporated hot debris decreased downstream of the 

1980 pyroclastic flow deposits, but isolated hot boulders 

continued to float or tumble along in the flow. Near the 

confluence of the North ForkToutle River and Coldwater 

Creek (16 km from the crater and only 9 minutes behind 

the flow front)(fig. 6.1), a series of transitory, 2- to 

7-m-high wave fronts surged oblique and normal to flow 

in the hyperconcentrated tail of the lahar (figs. 6.5, 6.6). 

These waves rapidly decreased in amplitude as the 

discharge decreased during passage of the tail of the lahar; 

20 to 30 seconds later a series of sinuous, dunelike bed 

forms appeared on the lahar deposit surface (fig. 6.7). 

Some of these regularly spaced features later were

measured; they had an amplitude of 0.5 m and a 

wavelength of 50 to 80 m (see "Transition-reach deposits" 

and "Discussion" sections of this chapter).

Within a few kilometers downstream of Coldwater 

Creek, the North ForkToutle channel widens to 250 to 

450 m, and during high-water periods, the stream 

normally has a braided form. A series of smaller standing 

waves were seen near cross section NF300 (19 km from 

the crater)(fig. 6.1), unlike the large waves in the 

Coldwater reach. A muddy flood characterized by choppy 

turbulence became evident at the head of the flow near 

NF300, and surges were observed both at the flow front 

and on the surface of the flow during peak stage. The 

surface-wave surges seemed to move downstream faster 

than the flow as a whole which indicates subcritical flow.

Overtaking the flow front, we landed 24 km 

downstream from the crater (near Elk Rock gage) at 1021 

to sample the flow and measure its velocity (sampling

FIGURE 6.4.-Side view of a recessional-flow knickpoint (upper left center of photograph) 14 kilometers from vent at about 1014. 

Flow is right to left. Boulder with splash ring (slightly upstream from knickpoint) is apparently supported within the flow. 

White spots (indicated by arrows) are hot boulders transported from the dome or exhumed from still-hot May 1980 deposits.
86 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

station l)(fig. 6.8). A rumbling noise was heard and felt 

for more than one minute before the flow came into sight 

600 m away. We began sampling at about 1032 near the 

center of the channel, then moved to the side as the stage 

increased. Occasional surges in the thalweg were less 

prevalent at this sampling locality than upstream. In 

fact, there was no well-developed flow front, only a rapid 

smooth increase in stage for about 6 minutes. We did 

wade out knee-deep to obtain samples during peak flow 

at about 1039 and during flow recession at about 1046 

(fig. 6.8), but these samples did not include the coarse 

material (tree trunks, rootballs, and boulders). 

Backsplashing standing waves were ephemeral and less 

than 0.5 m in height. Maximum flow temperature was 

estimated to be 25°C to 30°C (seasonal average is about

FIGURE 6.5.—Downstream view of North Fork Toutle River 

at 1016 (9 minutes after passage of lahar flow front) 

showing a series of oblique and normal wave fronts 

(hydraulic jumps?). Cross section NF130, 16 kilometers 

from crater, is near the middle of the photograph. Channel 

bottom is about 150 meters wide. Diffuse light spots are 

raindrops on the helicopter window.

One-half kilometer downstream of sampling 

station 1, the flow overtopped a low divide, and about 

40 percent of the flow had diluted considerably at this 

point and had the appearance of hyperconcentrated flow. 

Velocity measurements were made at three more sites 

(sampling points 2, 3, and 4) 30, 33, and 37 km 

respectively from the crater. A distinct flow front was 

discernible only at sampling station 2 in the dry and 

constricted Bear Creek channel. No surges were 

observed, and stage increased only gradually for about 2 

minutes. At sampling points 2 and 3, the first indication 

of the flow was an increase in turbidity followed by 

considerable floating debris. At all three of these 

downstream sites (sampling points 2, 3 and 4) the main 

body of the flow was dark brown in color and had a 

choppy, turbulent surface. Longitudinal bars were 

deposited rapidly during and after passage of the flow 

front.

LAHAR DEPOSITS

The changing character of the May 14 deposits 

downstream indicates that transformation from slushflow 

to debris flow occurred 2 to 7 km from the crater and 

that transformation from debris flow to 

hyperconcentrated flow occurred 15 to 22 km from the 

crater (table 6.1) (figs. 6.9,6.10). Void space from melting 

of constituent snow/ice granules, which formed about 5 

percent of the matrix in the Step Fan reach, disappeared 

within a few kilometers downstream. Fragile clasts of 

unconsolidated bank material as large as 2 m that were 

rafted by the lahar were present on the surface of the 

deposit between 3 and 16 km downstream of the lava 

dome, indicating high yield strength and probably plug 

flow (Johnson, 1970; Enos, 1977). Absence of the fragile 

clasts downstream of Coldwater Creek (fig. 6.1) indicates 

a decrease in the yield strength of the lahar that 

corresponds to a transformation from debris flow to 

hyperconcentrated flow. Detailed descriptions of the 

deposits in specific reaches are provided in the following 

paragraphs.

PROXIMAL-REACH DEPOSITS

From the mouth of Loowit Ravine, the 

microhummocky avalanche/slushflow deposit extended 

about 1 km onto Step Fan. The deposit averaged about 

0.5 m thick and was as much as 2 m thick where a 

preexisting channel was filled. Although the avalanche/ 

slushflow was predominantly depositional on the upper
ERUPTION-TRIGGERED LAHAR ON MAY 14, 1984 87

FIGURE 6.6.—Close up of bow-shaped wave front (hydraulic jump?) shown at right center of figure 6.5. 

Flow is lower right to upper left. Average height of wave is 2 to 3 meters.

FIGURE 6.7.—Upstream view of the North Fork Toutle River and sinuous, dune-like features (bed forms?) 

16 kilometers downstream from the vent immediately after passage of recessional surge shown in figures 6.5 

and 6.6. Measured cross section NF130 is located midway through the lower left quarter of the photograph.
TABLE 6.1. —Flow observations and erosional and depositional features for the May 14, 1984, lahar

Distance from

lava dome,

in kilometers

Observations Erosional and Depositional Features Interpretations

I

1

0.3

4.3

6.4

8 to 10

14

Not closely observed.

Not closely observed.

Not closely observed.

Recessional phase debris flow and watery surges 

20 minutes behind flow front; phreatic explosions when hot bank material slumps 

into channel (fig. 6.3).

North ForkToutle River clogged with sediment 

bank-to-bank; recessional surge overtops terrace not inundated by initial flow front of lahar.

Boulders suspended in flow create concentric 

splash rings; laminar flow; turbulence downstream from knickpoints (fig. 6.3); hot rocks tumbling along in flow.

Abraded pavement of pumice clasts at edge of 

incised channel on crater floor.

Abraded pavement of pumice clasts at edge of 

incised channel on crater floor.

Micro-hummocky, matrix-supported diamict 

0.5-2 m thick on Step Fan extends 1 km 

from mouth of Loowit Ravine; scour and small streamlined-molded features in 

channel walls on Step Fan.

Matrix-supported diamict (inverse coarse-tail 

grading); abundant bank slumps and ponding in channel for at least 3 hours after 

passage of the flow front.

Two-meter thick, matrix-supported diamict in 

reentrant, tributary gully; abundant fragile 

clasts as large as 1 m.

One-meter thick, matrix-supported diamict 

channel fill and terrace veneer; rubbly texture; fragile clasts.

Erosion and sediment bulking; avalanche/ 

slushflow sheared off pumice and incised as 

much as 2 m into crater floor.

Erosion and sediment bulking; avalanche/ 

slushflow sheared off pumice and incised as 

much as 2 m into crater floor.

Small, streamlined ridges situated downstream of 

rocks protruding from pre-May 14 fragmental 

deposits (fig. 6.21).

Ongoing watery flow from crater and ponding 

in Steam Creek generates surges seen in 

recessional flow.

Eight-meter-deep debris flow entered the North 

Fork Toutle River from Steam Creek; 

abundant fines incorporated by debris flow, 

mostly from May 1980 pyroclastic-flow deposits.

Debris flow stage attenuated in North ForkToutle 

River; increasingly rubbly texture downstream 

indicates lahar was incorporating bed material from North Fork Toutle River.

00
TABLE 6.1.-Flow observations and erosional and depositional features for the May 14, 1984, lahar — Continued

Distance from

lava dome,

in kilometers

Observations Erosional and Depositional Features Interpretations

16

19

2124

Oblique and normal hydraulic jumps (figs.6.5, 

6.6) in recessional flow (9 minutes after flow front); laminar conditions except at 

jumps; boulders rolling in flow.

Standing waves in thalweg of recessional flow; 

transition from laminar to turbulent flow during recessional phase.

Strong turbulence visible at flow front.

Rapid, smooth increase in stage (turb.); 

followed after about 5 minutes by a laminar, sediment rich pulse (fig.6.8); tail had 

slightly lower stage height and was warmer 

(25-30°C); antidunes.

Massive gravel bars abut valley wall; matrixsupported diamict with friable sole layer 

at base (fig. 6.15) is channel fill and terrace 

veneer—inset relative to bars; 10 cm 

pumice-rich zone at surface; fragile clasts 

(0.3 m) rest on diamict (fig. 6.16); poorly 

defined boulder levee at margin of diamict; 

knickpoints cut into surface of diamict.

Scattered boulders (some hot) on terrace but 

no matrix; inset channel fill is massive 

sandy diamict overlying openwork, clastsupported pebble gravel.

Sandy diamict having faint stratification at base 

and bubble vesicles locally near top; scour 

on terrace above diamict; poorly defined, 

high-angle cross beds underlying 

diamict(P).

No terrace-capping deposits noted.

Transformation from debris flow to 

hyperconcentrated flow began about here: gravel bars and sole layer deposited by turbulent surge at head of flow; diamict and 

fragile clasts deposited by trailing debris flow 

body; knickpoints in deposit surface created by hydraulic jumps in lahar tail

Flow transformation well developed here: 

boulders and pebble gravel deposited by turbulent surge at head of flow; sandy 

diamict deposited by debris flow body.

Transformation to hyperconcentrated flow 

almost complete: scour and cross beds 

associated with initial watery surge; bubble 

vesicles in diamict indicates last vestiges of 

debris flow.

Flow sampled near its edge was 33 percent 

sediment by wt.; probably >40 percent at thalweg.

I
90 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

reaches of Step Fan, recessional flow incised aim channel 

at cross section LO030 (fig. 6.1). Within 4 km of the 

crater, deposits consisted of a loose and openwork diamict 

because of the high void space and interstitial water 

content, both of which are attributed to melting of 

entrained snow and ice grains. The deposits 6 km from 

the vent had much less void space and were similar in 

appearance to the compact debris-flow deposits farther 

downstream.

Several distributary gullies from Step Fan join to form 

Steam Creek and slushflow material entering the gullies 

reached Steam Creek at slightly different times (fig. 6.2). 

As a result, deposits slightly downstream of cross section 

LO033 in Steam Creek include successive layers of 

texturally distinct diamicts, attributable to the different 

arrival times of the material flowing by different paths.

Nested terrace deposits were exposed at LO033. An upper 

1.5-m unit had a high percentage of void spaces (from 

constituent snow), but a lower (and later) 0.3-m unit 

had minimal void space.

Considerable recessional erosion in Steam Creek 

limited deposits to areas of backfill in gullies tributary to 

it and to overbank veneers on terraces. A 1.6-m-thick 

matrix-supported diamict was deposited as a backfill in 

a reentrant hanging tributary 7 m above the bed of Steam 

Creek (figs. 6.11, 6.12). This unit has inverse coarse-tail 

grading and contains boulders as large as 0.8 m in 

diameter. Texturally similar deposits are present several 

kilometers downstream where a diamict as thick as 2 m 

was deposited in mid-channel, near-overbank settings.

At the mouth of Steam Creek, numerous fragile clasts 

consisting of deformed blocks of channel-bank material

FIGURE 6.8.—Sampling of the sediment-discharge peak at 1046, 24 kilometers downstream of the crater (at measured cross 

section NF335). A velocity of 2.3 meters per second was calculated from flow runup on the sampler's leg. Metal grommets 

on the sampler's boot and on sieves that were used to process these samples became corroded. Apparently this corrosion was 

caused by dissolved salts from hydrothermally altered deposits incorporated by the lahar because pH of the dip samples 

ranged only from 6.4 to 7.4. Trees visible on the opposite shore were blown down during the May 18, 1980, eruption. 

Channel width is about 400 meters.
ERUPTION-TRIGGERED LAHAR ON MAY 14, 1984 91

averaged 0.5 m in diameter, although some clasts were 

as large as 2 m in diameter. These fragile clasts had floated 

on or near the surface of the flow.

Dissection of deposits near the mouth of Pumice 

Creek (fig. 6.1) exposed slump-block material from the 

channel walls that was overlain by 0.5 cm of silt and by a 

20-cm-thick layer of sandy, pebble- and cobble-rich 

diamict. This stratigraphic relation indicates that a 

secondary surge occurred after the slumping. Such 

surging behavior by debris flows is common in eyewitness

Lava 

dome

Phases

co 2,000

DC 

LU

tD

„- 1,000

Fragile clasts Bed forms

_L___ I

0 5 10 15 20 

DISTANCE, IN KILOMETERS

25

FIGURE 6.9.—Longitudinal profile of the North Fork Toutle 

River from the crater to Elk Rock showing A, transition 

interval from slushflow to debris flow, and B, the transition 

interval from debris flow to hyperconcentrated flow. Extent 

of bed forms preserved in the lahar deposit surface (as 

shown in figs. 6.17 and 6.18) is also shown.

reports where boulder snouts or channel-wall slumps dam 

up a flow (Blackwelder, 1928; Sharp and Nobles, 1953; 

Costa and Williams, 1984). Slump blocks at the 

confluence of the North Fork Toutle River with Steam 

Creek and Pumice Creek evidently dammed the flow 

temporarily in those gullies, for no large boulder levees 

were present. The silt between the units probably was 

deposited during temporary ponding. Failure of the 

debris dams probably contributed to the surging that we 

observed from the air (for example, Pierson, 1980, p. 

237) and probably provided considerable amounts of 

loose material that was easily incorporated by recessional 

stages of the lahar. Slumping and associated ponding 

continued for at least 3 hours after passage of the flow 

front (fig. 6.13). By 1530, most of this material had been 

flushed and a channel reestablished.

TRANSITION-REACH DEPOSITS

Two depositional facies occur 16 km downstream 

from the vent (cross section NF130)(fig. 6.1); coarse, 

openwork longitudinal gravel bars nested against the 

valley wall (fig. 6.10) and 0.2- to 1-m-thick sandy, matrixsupported diamict in mid-channel and channel-margin 

settings (fig. 6.14). The diamict overlies a discontinuous, 

poorly bedded sand and gravel which grades upward into 

10 to 20 cm of massive sandy gravel, also below the 

diamict. The openwork gravel bars and sub-diamict 

material apparently were deposited by the initial surge, 

and the diamict by the body of the lahar (see table 6.1.). 

Together, the two facies provide evidence that the lahar 

was transforming from debris flow to hyperconcentrated 

flow.

The bars (deposited first) were as thick as 0.5 m, 

slightly convex upward, and irregularly spaced. Erosional 

trimlines and splash marks on the channel wall were 

situated over the bars locally. Although the sorting value 

(2.79 0) for the bar sediments is low (Folk, 1974) and 

the deposit was structureless, the overall shape of the bars 

was streamlined. Apparently the bars were deposited 

rapidly at peak flow in a zone of mixing at the head of

Micro-hummocks

Pumiceous veneer 

deposited by 

recessional flow

Friable sand and 

gravel deposited by 

initial surge

^*.^ r-Openwork

NF100 NF130 NF300 NF320

8.1 15.7 18.6 225 

CHANNEL DISTANCE FROM LAVA DOME, IN KILOMETERS

EXPLANATION

[' . | Avalanche/slushflow ^ 

Debris-flow Q 

Hyperconcentrated flow

Gravel crossbeds 

Pre-May 14

FIGURE 6.10.—Sequential schematic stratigraphic sections 

showing upstream-downstream textural changes and 

depositional facies associated with the May 14,1984, lahar. 

Names of corresponding measured cross sections are shown 

below each section.
92 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

the lahar. The dunelike shape of the bars could have 

resulted from depositional processes and/or from 

erosional streamlining by trailing flow. These bars are 

smaller than, but probably analogous to the whaleback 

bars described by Scott (1988).

Fragile clasts of fragmental material from the 1980 

debris-avalanche were present on the surface of the 

diamict. As large as 25 cm, these fragile clasts were nested 

within the aforementioned bars (toward the center of 

the channel) at the extreme upstream reaches of the 

transition reach (fig. 6.15). The diamict at this location 

was deposited by a trailing flow that immediately followed 

the bar-depositing surge. This diamict is covered by 2 to 

10 cm of pumice and contains high-angle, knickpointlike features 20 cm high that are regularly spaced 50 to 

80 m apart on the surface of the diamict. A poorly defined 

boulder levee lies along one edge of the diamict on a

terrace. Matrix clay content for a sample of the diamict 

that was obtained while the deposit was still thixotropic 

was 1.5 percent by weight, the highest value for any sample 

of this lahar deposit.

Additional evidence of flow transformation is found 

three kilometers farther downstream, where the channel 

widens at cross section NF300 (fig. 6.1). There the 

channel-fill deposit has two textures: a massive, clastsupported, openwork basal layer, 5-13 cm thick, composed 

of pebble gravel; and an overlying, inversely graded, sandy 

diamict 30 cm thick. The gravel was deposited rapidly by 

the water-diluted surge as it spread out over the wider 

channel. The gravel was then immediately overrun by the 

less-diluted body of the flow. The gravel is probably 

correlative with the basal layer at NF130. Overbank 

deposits that were 13 to 19 km from the crater include 

longitudinal arrays of clustered boulders as large as 1.5 m.

FIGURE 6.11.—Aerial oblique view eastward at 1530 showing confluence of Steam Creek (right center) with North ForkToutle 

River, about 9 kilometers downstream of the crater. ^4. Hanging tributary gully is site of backfill deposit shown in fig. 6.12; 

B. Scoured right bank of Steam Creek; and C. Fragile clasts littering the surface of the lahar deposit. The gullies are cut into 

pyroclastic-flow deposits of May 18, 1980.
ERUPTION-TRIGGERED LAHAR ON MAY 14, 1984 93

These were left as overbank deposits when one or more 

boulders rolled to a stop and trailing clasts collided with 

them and stopped ("Keystone-cop" style).

The last evidence of flow transformation was found 

21 km from the vent (cross section NF310)(fig. 6.1) 

where the main depositional unit is a 15- to 25-cm-thick 

sandy diamict having faint stratification near the base. 

The lack of matrix-supported clasts larger than 3 cm at 

the NF310 cross section provides evidence that the yield 

strength of the flow had been significantly reduced, 

however, voids (inferred to be bubble vesicles) in the 

upper part of the deposit indicate the last vestiges of 

debris-flow emplacement. High-angle cross beds of gravel 

underlying the diamict may have been deposited by the 

watery surge that we observed ahead of the 

hyperconcentrated flow at this location.

FIGURE 6.12.—View of diamict deposited as backfill in hanging 

tributary shown in fig. 6.11 (8 kilometers from the crater). 

Left bank of Steam Creek in background. Average thickness 

of deposit is 1.7 meters. Note boulders that were suspended 

in the flow.

ESTIMATED DEPOSIT VOLUME,

WATER CONTENT, AND HEAT

ENERGY IMPLICATIONS

Total primary deposit volume (dry) for the lahar was 

5 x 105 m3 . This value was calculated by digitizing 

mapped channel deposits and by using changes in three 

manometer gage readings of the level of Spirit Lake to 

calculate the volume of the eastern lobe of the lahar. All 

Spirit Lake manometer gages showed sustained increases 

of 0.1 ft (0.03 m) immediately following the lahar (Dallas 

Childers, U.S. Geological Survey, written commun., 

1984). If water accounted for 50 percent of the initial 

(saturated) deposit volume (as interpreted for a debrisflow pulse followed by a smaller volume of 

hyperconcentrated recessional flow), then total water 

volume for all lobes of the May 14 lahar was about 

5 x 105 m3. Because of the much greater amount of 

sediment bulking by the middle and western lobes of 

the lahar, the significant water contained in those lobes 

could have been incorporated channel water and ground 

water. Analysis of discharge data (U.S. Army Corps of 

Engineers, 1984) indicates that approximately 4.2 x 104 

m3 of water occupied the North Fork Toutle River 

channel in the affected reach. An additional 6 x 104 m3 

of water was available as ground water within the 

interstices of eroded sediment, so at least 70 percent of 

the water (2.5 x 105 m3) in the middle and western lobes 

probably originated as melted snow granules from the 

crater. Because little channel water was available for the 

eastern lobe between the crater and Spirit Lake, water 

contained in that lobe amounted to 1.5 x 105 m3 . 

Therefore, the total volume of water derived from the 

crater was 4 x 105 m3, about 10 percent of the size of a 

transient lake that formed in the crater following the 

explosion of March 19, 1982 (Waitt and others, 1983; 

Pierson, chap. 2, this volume). The latent heat effusion 

for water is 1.8 x 105 J/m3 . Therefore, at least 7.1 x 1010 J 

were required to melt the crater ice/snow incorporated 

in the May 14 lahar.

VELOCITY AND DISCHARGE

Mean velocity values for the May 14 flow were 

calculated using seismic signals and times reported by 

observers. Inception of flow from the crater was inferred 

to be at 0941, at the onset of a second pulse of seismic 

signals having a slightly higher frequency than the first 

pulse. Calculated average velocities (table 6.2) ranged 

from 9.6 m/s (within 6.4 km of the crater) to 2.3 m/s
94 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

FIGURE 6.13.—Downstream aerial 

oblique view of Steam Creek 

(about 6 kilometers from the 

lava dome) at 1336 showing 

ponding 3 hours after passage 

of the lahar flow front. Note 

slumps of channel wall in the 

right and upper right part of 

the photograph and freshly 

deposited material. Height of 

the channel walls is about 

13 meters.

FIGURE 6.14.—Terrace-capping deposits on the right bank of the North ForkToutle River about 16 kilometers

from the lava dome show evidence of the beginning of the transition from debris flow to hyperconcentrated

flow. Depositional sequence includes: Subunit v4, a bedded sand and gravel grading upward into massive sand

and depleted in coarse clasts; subunit B, a friable massive sand and gravel; and subunit C, a thin (0-10 centimeter),

friable, pumice-rich sandy layer.. The sequence records a leading hyperconcentrated flow, (A), which transitioned

into a debris-flow phase (note boulders suspended in matrix) (B), and a final, more watery resessional surge (Q.

Rock hammer is 35 centimeters long. Flow direction is right to left.
ERUPTION-TRIGGERED LAHAR ON MAY 14, 1984 95

(38 km from the crater). The latter value correlates with 

surface velocities measured by timing flotsam. 

Measurements of flow superelevation from mudlines at 

a channel bend near cross section LO033 (6.2 km from 

the lava dome) yielded a calculated velocity of 9 m/s. 

Actual velocity values for specific reaches were 

undoubtedly higher or lower than the mean values 

derived from eyewitness observations.

Instantaneous peak discharges for the lahar were 

calculated from surveyed mudlines and cross-sectional 

areas using the computed mean velocities. The western 

distributary lobe had an instantaneous peak discharge of 

3,400 m3/s 8.1 km from the vent (table 6.2). The 

combined peak discharge for the other two lobes was 

about 500-1000 m3/s, most of which flowed into Spirit 

Lake (eastern lobe).

DISCUSSION

TRANSFORMATION OF MIXED

AVALANCHE AND SLUSHFLOW TO

DEBRIS FLOW

Initial sediment bulking by the slushflow occurred 

in Loowit Ravine, which is cut into 1980 fragmental 

deposits in the area of the Breach and into the older lava 

flows and volcaniclastics composing the Mount 

St. Helens edifice. However, the downstream textural 

changes, distribution of deposits, and sustained stage of 

the western distributary lobe of the lahar indicate that 

most sediment bulking on May 14 occurred within Steam 

Creek, a channel which is cut in unwelded pyroclasticflow material from the May 18,1980, eruption. Ackroyd

FIGURE 6.15.—Cross section of dacite-breccia fragile clast rafted by the May 14, 1984, lahar,

16 kilometers from the lava dome.
96 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

TABLE 6.2.—Velocity, discharge, and associated channel data for May 14, 1984, lahar. Method for calculating respective times and 

velocities is noted under values. All cross sections more than 5 km from the lava dome are in the North Fork Toutle River.

[km, kilometers; m/m, meters per meter; m/s, meters per second; m3/s, cubic meters per second; m2, square meters]

Cross 

section

LO010

LO020

LO030

LO033

LO040

NF100

NF110

NF113

NF120

NF130

NF300

NF310

NF320

NF325

NF330

NF335

NF345

NF365

NF375

NF390

Nl Dam

NF400A

NF420

Distance from 

lava dome (km)

0.0

0.3

4.3

6.2

6.4

8.1

10.4

14.1

15.7

18.6

21.1

22.5

23.5

23.7

24.2

24.9

28.3

29.5

33.0

35.5

37.7

Channel slope Flow front 

in 1984 (m/m) arrival, PDT

0.02

0.11

0.04

—

—

—

—

—

—

0.06

0.03

0.02

0.02

0.02

—

—

—

—

_

0941

seismic

—

—

—

0952

eyewitness

—

—

1008

eyewitness

—

—

—

1028

Elk Rock gage

—

1039

(flow front)

1045

(sediment slug)

—

1108

—

1138

1211

Velocity Discharge Net erosion (-) or 

(m/s) (m3/s) deposition (+), (m2)

— —

— —

— —

9 2,600

superelevation

9.6 2,000

interpolation

9 3,400

interpolation

— —

5-7 1,100-1,500

runup

— —

— —

— —

— —

— —

2-4 —

runup, — 

timed drift

— —

2.6 —

timed drift

— —

2.3 —

timed drift

— —

_ _

+436

-12

+233

-478

-43

-191

-644

-772

-20

-36

+64

-225

-16

+1

—

—

—

+322

—

+69

+113

+25

-88
ERUPTION-TRIGGERED LAHAR ON MAY 14, 1984 97

(1987) documents erosional features that resulted from 

a channelized snow avalanche, but other examples of 

debris flows containing substantial percentages of snow 

are rare. Although snowy debris flows could exert a 

significant shear stress on the bed, the substantial scour 

documented by the cross-section resurveys of channels 

swept by the May 14 slushflow was probably caused by 

entrained rock debris and watery recessional flow.

TRANSFORMATION OF DEBRIS FLOW TO 

HYPERCONCENTRATED FLOW

Flow transformations in sediment gravity flows have 

been discussed theoretically by Fisher (1983), and the 

flow transformations of lahars and the corresponding 

textural characteristics of their deposits have been 

documented by field studies (Pierson and Scott, 1985; 

Scott and Dinehart, 1985; Scott, 1988). The rapid 

downstream textural changes in May 14 deposits 

resemble those reported for the March 19, 1982 flows 

(Pierson and Scott, 1985) in which dilution is inferred 

to have occurred at the head of the lahar by deposition 

of solids and incorporation of overrun channel water in 

a stretch of channel that Scott (1985) called the transition 

reach (for a given flow).

The transition-reach textural changes in 

May 14, 1984, deposits were first observed in the bar 

deposit and basal layer 16 km from the crater and also 

2 km farther downstream, where the lower unit is better 

sorted and contains less fine material. Sixteen kilometers 

from the crater, the bar deposit is poorly sorted and has 

little evidence of matrix strength (for example, inverse, 

coarse-tail grading; suspended boulders; floating fragile 

clasts), unlike the mid-channel diamict deposited in the 

medial channel environment immediately following 

deposition of the bars. Because of their stratigraphic 

relations with the diamict, the bars that are 16 km from 

the vent and the lower units 16, 18, and 21 km from the 

vent may have been deposited in an environment of 

turbulent(?) mixing and dilution through incorporation 

of channel water at the head of the flow. The upper unit 

that is 21 km from the vent is texturally similar to 

hyperconcentrated-flow deposits of March 19, 1982 

(Pierson and Scott, 1985); this similarity indicates almost 

complete transformation from debris flow to 

hyperconcentrated flow.

Dip samples obtained as the flow passed cross section 

NF 335 (fig. 6.8) indicate that sediment concentrations 

were much less than those for debris flows (typically 

greater than 80 percent by weight for flows that have

less than 3 percent clay: Scott, 1988) and that peak 

sediment concentration lagged the peak discharge by 

about 5 minutes (fig. 6.16). The peak sediment discharge 

concentrations of 33 percent solids by weight of a sample 

obtained at the extreme periphery of the flow probably 

underestimated sediment concentrations nearer the 

thalweg where conditions were at least 

hyperconcentrated.

DRAMATIC WAVE FORMS

(HYDRAULIC JUMPS?) OBSERVED IN THE

LAHAR RECESSIONAL FLOW

Hydraulic jumps in rivers have been described as 

stationary shock waves or surges in which mechanical 

energy from upstream supercritical flow is abruptly 

converted to thermal energy through the action of 

turbulence, such that flow immediately downstream from 

a jump is deeper and travels at a slower velocity in the 

subcritical flow regime (Stoker, 1957; Chow, 1959; 

Komar, 1971). Large, almost evenly-spaced wave forms 

visible about 16 km from the crater on May 14 could be 

hydraulic jumps (figs. 6.5, 6.6). At this location, the 

hyperconcentrated(?) tail of the lahar apparently returned 

rapidly to a supercritical state after each successive wave 

form. This shifting flow state perhaps was caused by 

(1) an increased velocity due to an increase in slope or 

confinement by channel narrowing (Baker, 1978; Fisher, 

1983), (2) changes in fluid density caused by upward 

movement of pore fluid due to rapid deposition of large 

clasts, or (3) increased suspension of fine material because 

of the turbulence at a hydraulic jump (Middleton and 

Hampton, 1976; Lowe, 1979) or a combination of these

LU

QI

LUCD

40

30

20

10

1032 1036 1040 1044

SAMPLE TIME 

(Pacific Daylight Time)

1048 1052

FIGURE 6.16.—Sediment concentration (by weight) changes 

in a series of dip samples obtained on May 14, 1984, near 

the periphery of the lahar 24 kilometers from the lava dome 

(near cross section NF335).
98 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

factors. The regular amplitude of these features suggests 

that they could have been formed by antidunes; however, 

no stratigraphic evidence (such as backset beds) was noted 

(see below).

SCOUR MARKINGS AND DEPOSITIONAL 

FEATURES OF THE LAHAR

Sinuous mid-channel bed forms visible in figure 6.7 

(moments after the wave forms noted above) seemed to 

be coarse-grained dunes, but field investigations later 

revealed only a series of irregularly spaced steps on the 

surface of the May 14 debris-flow deposit (figs. 6.17, 

6.18). These steps, 50 to 80 m apart, apparently were 

knickpoints created during recessional flow. The convex 

upstream shape of several knickpoints in plan view 

mimicked a similar geometry in some of the wave forms 

noted in the above section (figs. 6.5, 6.6). These 

knickpoints may have been scoured by hydraulic jumps, 

which lasted less than about 30 seconds during passage 

of a recessional surge.

Numerous active knickpoints observed farther 

upstream (9-14 km from the vent), just after passage of 

the lahar, formed when the lahar deposit was being 

remobilized by recessional flow. Initial reestablishment 

of the thalweg occurred by removal of the lahar material 

by turbulent erosion and by corresponding rapidly 

retreating knickpoints.

Scour markings resulting from the May 14, 1984, 

lahar occur between the crater floor and 14 km from the 

vent. The avalanche/slushflow carved an abraded 

pavement consisting of grooved, faceted pumice boulders 

and cobbles in the crater floor. A medial part of upper 

Steam Creek contained longitudinal streamlined ridges 

that began downstream of rocks protruding from the 

underlying deposit (fig. 6.19). Evidence of scour, 

including flutes and smoothed surfaces of underlying 

fragmental deposits, were present elsewhere, particularly 

in areas of the Pumice pond (fig. 6.1) where recessional 

flow was minimal (see Schmincke, 1967, and Scott 

1988). However most of these features were destroyed 

or buried by talus within a few months of May 14.

FIGURE 6.17.—Downstream oblique view of knickpoint cut into debris-flow surface, 16 kilometers from the lava dome. 

Diffuse boulder levee from debris flow is in the upper-right background. Flow direction is lower left to upper right. 

Knickpoint was excavated near shovel for diagrammatic cross section shown in figure 6.18.
ERUPTION-TRIGGERED LAHAR ON MAY 14,1984 99

Hydraulic jump B

Deposit surface

just after passage

of lahar

1 meter

Lahar deposit 

Recessional surge 

Pumice-rich veneer

Scour

Figure 6.18.—Diagrammatic formation of bed forms in 

transition reach: A. Surging waves and scour during 

recessional flow on fresh lahar deposit; and B. knickpoint 

bed form (as shown in fig. 6.17) and pumice-rich veneer 

remaining after tail of lahar drains away.

Fragile clasts, on or within deposits, are evidence of 

plug flow within a debris flow (Johnson, 1970, p. 513; 

Enos, 1977). Fragile clasts may include unconsolidated 

or partly indurated debris and can originate as bankfailure blocks or terrace fragments incorporated by a debris 

flow. Such clasts (fig. 6.15) are transported more-or-less 

intact on or near the surface of the rigid plug, and they 

are commonly preserved in terrace-capping deposits. The 

downvalley limit of fragile clasts in the May 14 deposits 

(16 km from the vent) corresponds with the downstream 

limit of debris-plug deposits. The relative elevation and 

lateral extent of the fragile clasts within the valley at their 

downstream limit indicate that the rigid plug was lower 

in stage than the more-dilute, peak-flow surge, which 

had deposited the bars immediately before the passage of 

the debris plug.

FIGURE 6.19.—Depth of peak flow at mudline A Four kilometers from the lava dome and B. Streamlined ridges and smoothed 

surface scoured by the May 14, 1984, debris flow. Person in lower left corner shows scale.
100 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

POST-LAHAR GEOMORPHIC CHANGES

IN THE UPPER NORTH FORK

TOUTLE RIVER BASIN

An earlier study of geomorphic adjustment and 

development of the drainage system on the 1980 debrisavalanche deposit required the installation of 

56 measured cross sections across the North ForkToutle 

River and its major tributaries in 1980 and 1981. Most 

of these cross sections have been surveyed annually using 

theodolites and electronic distance meters, and a few have 

been surveyed monthly. Resurveying of several sections 

soon after May 14 allowed direct measurement of change 

at about 30 cross sections on the 1980 debris avalanche 

deposit crossed by the May 14 flow. Six sections were 

resurveyed on May 15, and 13 more within two months. 

The general areas of erosion and deposition are easily 

recognizable from the profile comparisons (fig. 6.20).

Although the avalanche/slushflow was erosive along 

the crater floor, no significant erosion was caused in the 

part of Loowit Ravine that is cut into lava flows. Net 

erosion was documented from 6 to 23 km from the crater. 

Incision of more than 5 m occurred locally, but in some 

places, notably at cross section LO 033, part of this 

erosion could be attributed to the action of winter storms 

before May 14. Recessional-phase erosion 6 to 10 km 

from the crater roughly matched erosion during peak 

flow. Most of the sediment incorporation by the lahar in 

this reach resulted from slumping of the steep channel 

walls. Most net erosion 10 to 15 km from the vent 

occurred as a result of channel reincisement and 

sidecutting during recessional flow. This reach had 

aggraded during the initial stages of the lahar.

In the reach 15 to 21 km from the vent, where the 

lahar was transforming from debris flow to 

hyperconcentrated flow, deposition dominated. As much 

as 1 m of material was deposited in near-overbank settings 

locally; the channel in this reach was not confined, but 

braided over a width 150 to 250 m. In addition to the 

dilution caused by overrunning channel water in the 

North Fork Toutle River and the added water at 

confluence of Coldwater Creek, spreading of the lahar 

by channel widening probably contributed greatly to the 

flow transformation and to the corresponding rapid 

deposition of material. A short reach of net erosion about 

21 km from the crater may have been the result of a 

locally steeper gradient.

No substantial channel changes were apparent beyond 

23 km from the crater. The dip samples obtained at cross 

section NF 335 indicated that the flow had diluted to a 

concentration close to the boundary between

— — October 18,1983 

June 12,1984

f— Mudline

— — February?, 1984 

May 15,1984

6.2km 

LO003

6.4km 

LO040

April. 18,1984 

May 15,1984

'— Mudline — — April 18,1984 

1984

• Slump block 

8.1km 

NF100

Steam Creek channel

14.1 km 

NF120

Slump flow —CZT — Mudline

15.7 km 

NF130

— — April 18,1984

——— May 15,1984

18.6 km 

NF300

10

METERS

10

Deposition 

Erosion

FIGURE 6.20.—Selected cross-section plots of the North Fork 

Toutle River basin showing net erosion and deposition 

caused by the May 14,1984, lahar. Channel distance from 

the lava dome is shown above each cross-section number.
ERUPTION-TRIGGERED LAHAR ON MAY 14, 1984 101

hyperconcentrated flow and muddy flood. Nevertheless, 

considerable coarse bedload transport occurred for at 

least one hour after the flow front had passed the Elk 

Rock area, indicated by the prolonged, high-frequency 

seismic signal from the Elk Rock seismograph (ERT) 

(fig. 6.21). Generally, less than 0.5 m of deposition 

occurred along the channel between Elk Rock and the 

Toutle Dam (N-l sediment dam, 23-33 km downstream 

from the vent) and in the upper reach of the Bear Creek 

channel. No significant changes attributable to the flow 

were seen downstream of the Toutle Dam.

SUMMARY

On May 14, 1984, a shallow-seated explosion from 

the dome at Mount St. Helens sprayed the west crater 

wall with hot rock debris and gases and triggered a mixed

snow-and-rock avalanche and slushflow that exited the 

crater through the Breach into upper Loowit Ravine. 

This mixture transformed into a debris flow on the 

Pumice Plain as it accumulated sediment and heat from 

the still-hot 1980 pyroclastic flow deposits and added 

water through melting of admixed snow and ice granules 

and through incorporation of water from stream channels 

and saturated sediments. The resulting debris flow was 

erosional throughout the Pumice Plain but caused 

aggradation of the active channel of the North Fork 

Toutle River from 8 to 16 km from the lava dome before 

transforming to a hyperconcentrated flow.

This report documents an unusual geologic 

opportunity, that of making eyewitness observations of 

a relatively small lahar at close range and then of 

subsequently examining the lahar deposits and 

geomorphic effects of its passage. This may be the first 

report in which observed flow transformations in a lahar

0937 PDT

May 14,1984 D135 Start of dome explosion

Figure 6.21.—Section of a seismogram from the Elk Rock Three seismograph (ERT), located about 24 kilometers downstream 

of the vent near the North Fork Toutle River. Elongated, high frequency seismic signal shows passage of the May 14, 1984 

lahar. Each horizontal line is separated by 15 minutes; 1-minute tic marks (small square humps) are visible in the top six 

lines.
102 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

have been correlated with downstream textural changes 

in its deposits.

Temporal and downstream changes in the flow 

behavior and correlative textural changes in deposits were 

documented by eyewitness observations, velocity 

measurements, sampling of the lahar, and examination 

of deposits after they became exposed. At least 7.1 x 1010 

joules were required to generate the estimated 4 x 105 

cubic meters of meltwater incorporated in the lahar from 

crater snow and ice.

ACKNOWLEDGMENTS

The authors thank K.M. Scott, D.W. Hubbell, and 

Martha Crawford for reviews of this manuscript. 

R.M. Iverson, K. M. Nolan, and J.E. O'Connor also 

had helpful comments on the text. We shared enthusiastic 

discussions about the May 14 explosion, lahar, and 

related processes and deposits with R.J. Janda (deceased), 

T.C. Pierson, D.F. Meyer, R.L Shreve, R.B. Waitt, Peter 

Otway, R.V. Fisher, P.M. Mango, W.W. Mast, 

M.P. Doukas, T.J. Casadevall, Lyn Topinka, Bobbie 

Myers and various staff members of the Cascades Volcano 

Observatory. Helicopter pilot Bob Edwards deftly flew 

us along side the actively flowing lahar and to 

downstream locations where we warned tourists and 

construction personnel about the approaching flood.

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implications for geomorphic stability, Torlesse Range, New 

Zealand: Arctic and Alpine Research, v. 19, p. 65-70.

Baker, V.R., 1978, Paleohydraulics and hydrodynamics of 

Scabland floods, in Baker, V.R., and Nummedal, P., eds, 

The Channeled Scabland: Washington, D.C., National 

Aeronautics and Space Administration, p. 59-79.

Beverage, J.P., and Culbertson, J.K., 1964, 

Hyperconcentrations of suspended sediment: American 

Society of Civil Engineers Proceedings, Journal of the 

Hydraulics Division, v. 90, no. HY6, pt. 1, p. 117-128.

Blackwelder, Eliot, 1928, Mudflow as a geologic agent in 

semiarid mountains [with a discussion by 

J.T. Singewald, Jr.]: Geological Society of America 

Bulletin, v. 39, no. 2, p. 465-484.

Chow, V.T., 1959, Open-channel hydraulics: New York, 

McGraw Hill, 680 p.

Costa, J.E., and Williams, G.P, 1984, Debris-flow dynamics 

(videotape): U.S. Geological Survey Open-File Report 

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Survey Training Center, Denver Federal Center, Mail Stop 

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volcano, Washington: U.S. Geological Survey Professional 

Paper 677, 75 p.

Enos, P., 1977, Flow regimes in debris flow: Sedimentology, 

v. 24, no. l,p. 133-142.

Fisher, R.V., 1983, Flow transformations in sediment gravity 

flows: Geology, v. 11, no. 5, p. 273-274.

Folk, R.L., 1974, Petrology of sedimentary rocks (2nd ed.): 

Austin, Hemphill s, 182 p.

Janda, R.J., Scott, K.M., Nolan, K.M., and Martinson, H A., 

1981, Lahar movement, effects, and deposits, 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. 461-478.

Johnson, AM., 1970, Physical processes in geology, a method 

for interpretation of natural phenomena, intrusions in 

igneous rocks, fractures, and folds; flow of debris and ice: 

San Francisco, Freeman Cooper, 577 p.

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

Science, v. 215, no. 4531, p. 353-361. 

Komar, P.D., 1971, Hydraulic jumps in turbidity currents: 

Geological Society of America Bulletin, v. 82, no. 6, 

p. 1477-1487.

Lowe, D.R., 1979, Sediment gravity flows—Their 

classification and some problems of application to natural 

flows and deposits, in Doyle, L.J., and Pilkey, O.H., eds., 

Geology of continental slopes: Society of Economic 

Paleontologists and Mineralogists Special Publication 27, 

p. 75-82.

Middleton, G.V., and Hampton, M.A., 1973, Sediment gravity 

flows: Mechanics of flow and deposition, mTurbidites and 

deep-water sedimentation: Los Angeles, Society of 

Economic Paleontologists and Mineralogists, Pacific 

section, p. 1-38.

Middleton, G.V., and Hampton, M.A., 1976, Subaqueous 

sediment transport and depostion by sediment gravity 

flows, in Stanley, D.J., and Swift, D.J.P, eds., Marine 

sediment transport and environmental management: John 

Wiley and Sons, Inc., p. 197-218.

Patton, PC., Baker, V.R., and Kochel, R.C., 1979, Slack-water 

deposits—A geomorphic technique for the interpretation 

of fluvial paleohydrology, in Rhodes, D.D., and 

Williams, G.P, eds., Adjustments of the fluvial system; 

tenth annual geomorphology symposium: Dubuque, Iowa, 

Kendall/Hunt, p. 225-253.

Pierson, T.C., 1980, Erosion and deposition by debris flows 

at Mt. Thomas, North Canterbury, New Zealand: Earth 

Surface Processes, v. 5, no. 3, p. 227-247.

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

of a lahar; transition from debris flow to hyper concentrated 

streamflow: Water Resources Research, v. 21, p. 1511- 

1524.
ERUPTION-TRIGGERED LAHAR ON MAY 14, 1984 103

Pierson, T.C, and Costa, J.E., 1987, A rheologic classification 

of subaerial sediment-water flows: Geological Society of 

America Reviews in Engineering Geology, v. VII, p. 1-12.

Schmincke, H.-U., 1967, Graded lahars in the type sections 

of the Ellensburg Formation, south-central Washington: 

Journal of Sedimentary Petrology, v. 37, no. 2, p. 438-448.

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

St. Helens, Washington, U.S.A., in International 

symposium on erosion, debris flows, and disaster 

prevention: Tsukuba, Japan, p. 209-214.

Scott, K.M., 1988, Origins, behavior, and sedimentology of 

lahars and lahar runout flows in the Toutle-Cowlitz River 

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

Survey Professional Paper 1447-A, 74 p.

Scott, K.M., and Dinehart, R.L., 1985, Sediment transport 

and deposit characteristics of hyperconcentrated streamflow 

evolved from lahars at Mount St. Helens, Washington, in 

Proceedings, International Workshop on Flow at 

Hyperconcentrations of Sediment: Sept. 10-14, Beijing, 

China, 1985, Proceedings, 33 p.

Sharp, R.P., and Nobles, L.H., 1953, Mudflow of 1941 at 

Wrightwood, southern California: Geological Society of 

America Bulletin, v. 64, no. 5, p. 547-560.

Smith, G.A., and Lowe, D.R., 1991, Lahars: Volcanohydrologic events and deposition in the debris flowhyperconcentrated flow continuum, in Fisher, R.V., and 

Smith, G.A., eds., Sedimentation in volcanic settings: 

SEPM (Society for Sedimentary Geology) Special 

Publication 45, p. 59-70.

Stoker, J.J., 1957, Water waves; the mathematical theory with 

applications: Wiley Interscience Publishers, 567 p.

U.S. Army Corps of Engineers, 1984, Alternative strategies 

for a permanent outlet for Spirit Lake near Mount 

St. Helens, Washington; final environmental impact 

statement: [Portland, Oregon], 1 v., various paging.

Waitt, R.B., Jr., Pierson, T.C., MacLeod, N.C., Janda, R.J., 

Voight, Barry, and Holcomb, R.T., 1983, Eruptiontriggered avalanche, flood, and lahar at Mount St. Helens— 

Effects of winter snowpack: Science, v. 221, no. 4618, 

p. 1394-1397.
APPENDIXES 1.A AND l.B
APPENDIX 1.A
APPENDIX 1A 109

APPENDIX 1.A — ANALYSIS

I examine here the physics of heat flow from a 

pyroclastic deposit to subjacent snow or ice, with the 

goal of determining whether irregularities at the interface 

will be amplified or muted by heterogeneities in heat 

flux. The mean thickness L of the deposit is assumed 

small compared to its lateral extent, so that for 

mathematical purposes the layer may be considered of 

infinite extent. The analysis is restricted to the case of 

small-amplitude irregularities. The large-amplitude case 

is not amenable to analysis, although it would be 

straightforward to use a finite-element numerical 

approach. Because the object here is to elucidate physical 

principles rather than tabulate numerical results of 

uncertain applicability, such a computational effort does 

not seem appropriate.

The coordinate system is shown in figure 1.A1. It is 

useful, although not necessary, to think of the contact as 

having been scoured by the pyroclastic flow before the 

flow came to rest; the cross section of figure 1.A1 is then 

normal to the flow direction. As the substrate melts, the 

entire deposit moves downwards relative to fixed 

coordinates. The rate of motion of the contact will be 

constrained by thermodynamics because the contact 

represents a phase change from ice to water or vapor. 

The aim here is to determine the spatial variability in 

that rate of motion and thus whether irregularities on 

the contact grow or decay

= rj(x,t) I

FIGURE 1 .A. 1.—Definition of coordinate system for 

heat-flow analysis.

I assume that heat transfer within the pyroclastic 

deposit is purely conductive; thus advective heat transfer 

by gases within the deposit's pores, and in particular, by 

water vapor generated at the deposits base, is ignored. 

This assumption certainly fails if the substrate consists 

of glacier ice or firn, in which case meltwater will not be 

able to escape readily. Ponded water at the contact might 

reasonably be expected to vaporize due to its proximity 

to hot pyroclastic material, and then to escape upward 

through the deposit. A similar phenomenon should occur 

even with a permeable snow substrate if the pyroclasts 

are hot enough. Although the detailed physics of heat 

transfer from a hot deposit to a frozen substrate are still 

not well understood, by drawing analogy to results from 

studies of boiling (Whitaker, 1977), I suggest that 

pyroclastic grains will generally be separated from the 

frozen substrate by a fluid film across which heat is 

transferred. Meltwater flows along this film towards the 

nearest pore, gaining heat from the pyroclasts along the 

way. If the pyroclast temperature 7j exceeds some critical 

value Tcrlt- , the meltwater will vaporize before it can reach 

the pore. The water vapor so generated will then tend to 

escape upward through the pyroclastic deposit. On the 

other hand, if 7j < Tcrlt- , the meltwater will remain liquid 

and tend to percolate downward into the snow, quickly 

losing its excess sensible heat by melting more snow.

If meltwater produced at the base of the pyroclastic 

deposit does vaporize and escape upward, cooling of the 

pyroclastic deposit would no longer be simply by 

conduction; advective heat transport by the vapor and 

thermal interaction between the vapor and the solid grains 

would have to be explicitly considered (Baer and 

Nunziato, 1986). Based on preliminary results of 

experiments (Walder, 1992, 1995), I suggest 

Tcrlt - ~ 200°C, so advective heat transport is in fact likely 

to be significant in many cases. However, as explained 

later, this will have no qualitative effect on conclusion 

drawn from the purely conductive model, so for 

simplicity I ignore advection.

The mathematical problem to be posed is in the 

general class of moving-boundary problems (Carslaw and 

Jaeger, 1959, p. 282-296). The temperature Tin the 

deposit is described by the equation

dt

(1.A1)

where K = /L/ppCp is the thermal diffusivity of the deposit, 

with kp its thermal conductivity, pp its density, and <:„ 

its specific heat; u is the particle velocity, x and y are
110 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

spatial coordinates, and t is time. The initial condition 

is simply

T(t=0) + Td (LA2)

The appropriate boundary conditions are less obvious. 

For plausible terrestrial conditions, the substrate (snow 

or ice) will exist near its melting point 7^ and must be 

melted by the much hotter pyroclasts. In line with earlier 

discussion in this appendix, temperature in the fluid film 

between pyroclasts and ice grains may vary tremendously 

on a short length scale, being as low as Tm or as great as, 

and perhaps greater than, the vapor point 7^. Lacking 

experimental data, one can state only that the average 

temperature 7], at the base of the pyroclastic deposit is 

bounded by Tj, > Tm, and that as the deposit cools, 

Tj, — » Tm. For purposes of the present analysis, I will 

therefore make the physically reasonable assumption that 

Tj, relaxes exponentially from an initial value of 7j toward 

the melting point:

T(y= £(*f)) =Tb =Tm+ ( Td - TJe-P',

where /3 is the reciprocal of the time scale over which the 

boundary temperature relaxes to that of the substrate; 

1//3 is sensibly much less than the characteristic time scale 

for the bulk of the deposit to cool substantially. (In other 

words, the base of the deposit approaches a temperature 

of Tm much more quickly than does the bulk of the 

deposit). Henceforth I take Tm = 0. The detailed form 

of the lower boundary condition, as long as it reflects the 

physical necessity that the temperature relaxes toward Tm 

over a finite period of time, has no effect on the 

conclusions as to growth or decay of irregularities.

The appropriate boundary condition to apply at the 

free surface y= T\(x,t) also requires careful consideration. 

Air is a poor conductor of heat, and heat loss from the 

free surface is probably dominated by convection due to 

wind and perhaps due to escaping vapor as well. A 

boundary condition of the form used by Ryan and others 

(1990), namely, T[y = ll(x,t)] = Ta, where Ta is the air 

temperature, will tend to overestimate heat loss to the 

atmosphere. Proper description of the convective 

boundary condition at y - t](x,t) would require 

specification of at least air temperature and wind speed 

[see any text on heat transfer, for example, Whitaker 

( 1 977) ] . In light of the uncertainties, for mathematical

simplicity I will assume the upper boundary to be 

insulated:

where dT/dn is the normal derivative, nearly equal to 

oT/oy if the free surface is relatively smooth 

( 1 9 7] fox « 1). This assumption leads to an overestimate 

of total heat loss from the base, but has no effect on the 

qualitative conclusions regarding growth or decay of 

contact irregularities.

Some justification for using equation (1.A4) comes 

from recent observations made at Mt. Redoubt, Alaska, 

by Edward Wolfe and Cynthia Gardner (oral 

communication, 1990), who examined a pyroclastic 

deposit that had been laid down on snow only a few 

hours previously. The deposit, roughly 0.5 m thick, was 

cold — in fact, partially frozen — at its base, yet hot and 

dry at its surface, indicating that heat was removed from 

the base of the deposit much more efficiently than from 

the free surface.

The existence of a moving boundary requires that 

one more boundary condition be specified, namely, that 

which relates the rate of motion of the contact £>(x,t) to 

the heat flux at the contact. To do so, I will assume that 

as the deposit melts its way downward, particles move 

strictly vertically. Thus, the only non-zero component 

of u> is the y- component, denoted by v. Further 

assuming that the density of the deposit remains constant, 

it follows that

t ^ v= v(x,t) = — .

dt 

The heat-flux condition at the contact is then

(1.A5)

4 = k — ] t p dy

where p^ is the density of ice, (j) is the snow's porosity, 

and His the latent heat of melting. Equation (1.A6) 

expresses the rate of movement of the contact due to 

melting. In general, the moving-boundary condition 

(equation 1.A6) renders equation 1.A1 nonlinear, 

because v depends on o T/oy. As will be shown below, 

however, under appropriate constraints the problem 

effectively linearizes.

In order to discuss the evolution of topography on 

the moving boundary ^(x,t), a mean boundary and local
APPENDIX 1A 111

deviations therefrom need to be defined. Formally one 

may write

lim

and similarly

£>->c

lim

-— C(x,t)dx , U.A7)

-D

D

ID J

-D

as long as ^(x,t) and T](x,t) are themselves finite for 

all x The mean speed of the contact is then simply

d\

~ . f ^-\ _ v^

dt

(1.A9)

It is now straightforward to transform the coordinate 

system to one that moves with the mean boundary |(?) 

by choosing

where L is the mean thickness of the deposit, that is,

D

2D J

-D

whence (1.A1) may be rewritten as

+ (^) = K ( + ), O.A12)

ot dy

with the initial condition

T(t=0)=Td for 7}-7}<£-

and the boundary conditions

(1.A15)

on L , (1.

For purposes of further analysis, I recast equation 

1.A12 into dimensionless form, using

x = KX

* = L\

t = (L2/K)i

T = rde

(1.A17)

where X is the characteristic length scale over which the 

deposit thickness varies in the x direction. The 

characteristic velocity scale v^^ is sensibly given by 

/m/(Z2/K), thus using equation (1.1) of the main text,

"char

p

kTd/L (1.A18)

whence equation 1.A12 becomes

f

where v and v are dimensionless velocity and mean 

velocity, respectively, both O(l); \i = LI'k, and

#a = c^U-x-??-) o-A2^)

is a dimensionless number indicating the relative 

importance of the advective term in the energy balance. 

Taking parameter values as in the main text, one can 

estimate 7Va ~ 4 - 10; accordingly, the advective term is 

negligible and the mathematical problem linearizes only 

if v- v « 1 . Conditions under which this approximation 

holds will now be elucidated.

Suppose that the contact roughness is described by 

an equation of the form

where k* = 271/ta, with A* the wavelength of the contact 

irregularities. Similarly, assume that the free-surface 

topography is given by

7] - rj = zL s'mk^x ( 1 .A22)

where k = 271/X , with X the wavelength of free-surface 

irregularities. Because my purpose here is to determine 

simply how variations in deposit thickness affect the local 

melt rate at the contact with the underlying snow, I can 

without loss of generality take the free surface as planar. 

This simplifies the mathematics without affecting the 

qualitative result. Thus, Intake 7] = fj, k = k, A,e = K. 

Evidently, v-v— > 0 as £ - ^ — > 0 because the melt rate 

must be uniform on a planar boundary. Therefore 

v - v - O(5), and in particular,

v - v~ §i> as 5 —> 0 . (1.A23)

Accordingly, for small-amplitude irregularities (I § «11), 

the advective term on the left-hand side of equation 

1 .Al 9 vanishes and the problem linearizes.

The mathematical problem to be solved is thus, in 

dimensionless form,
112 CONSEQUENCES OF HOT-ROCK/SNOWPACK INTERACTIONS AT MOUNT ST. HELENS VOLCANO, 1982-84

a20

(1.A24) 

w dr"

with the initial condition

0(1 = 0) = 1 for 0 <Y < E(X) , (1.A25) 

and the boundary conditions

Q = e^ on Y = E(X) ,

= on =

where y = PZ2/K (»1) and

E(X) = 1 + 8 sin 2nX .

(1.A26) 

(1.A27)

(1.A28)

The flux boundary condition, expressed in dimensionless 

form as

v = _|§_ on Y = E(X) , (1.A29)

now simply serves to provide an a posteriori means of 

calculating the speed at which the contact moves. Peaks 

on the contact correspond to sin27L^< 0, troughs to 

sin 2nX> 0.

The assumption of small-amplitude irregularities 

(I 8 « 11) means that the boundary condition applied 

on the wavy contact can be expanded using a Taylor 

series as

oY

, (1.A30)

thereby in effect transferring the boundary condition to 

the plane Y= 1. The form of equation 1.A30 suggests 

(Carrier and Pearson, 1976) a perturbation scheme of 

the form

Q(X,Y,i;$ = Q (0\Y,i) + 60(1) (XKT;|i) + O(82) , (1.A31)

where Q^°\Y,i) is simply the (dimensionless) temperature 

distribution for the limiting case of planar boundaries. 

The dimensionless temperature gradient at the contact 

may therefore be expressed as

(Y=l) ar

+ 0(82), (1.A32)

which is the quantity of eventual interest, as it is 

proportional to the melt rate and thus determines 

whether irregularities on the contact will grow or shrink. 

Using equation 1.A31 in equations 1.A24 through 

1 .A27 and separating terms of like order, one finds for 

the leading-order terms

90W= 920(°) 

9x 9F2

(1.A33)

with the conditions

and for the O(8) terms,

90(1) n 920(1)

with the conditions

9F2

(1.A34) 

(1.A35) 

(1.A36)

(1.A37)

0(D(0<F<1,T = 0) = 0, (1.A38)

(Y=l) + 0(62) , (1.A39)

= 0 , (1.A40)

9F

The lower boundary condition reflects the additional 

assumption that irregularities at the contact not only have 

small amplitude but also small slope , that is, ^(=JJ,8), in 

which case 90/9w = 90/9Fto within O(^2). This 

assumption simplifies the algebra without qualitatively 

altering the results.

Exact expressions for 0(0) and 0(1) may readily be found 

by the Laplace-transform method (Carslaw and Jaeger, 

1959). Solution by this method involves transform pairs 

that are not tabulated, so explicit inversion of the 

transforms requires evaluating contour integrals in the 

complex plane. Details are given in Appendix B. The 

quantity of principal interest here is Q, the heat flux at 

the contact Y = E(X) integrated over time, given in 

dimensionless terms as

Q- -f ||[r= SpO]^. (1-A41)
APPENDIX 1A 113

Using equation 1 .A3 3 and the results in Appendix 1 .B, I 

find

Q= 1 -8(- O(82 ) , (1.A42)

where the first term on the right-hand side of equation 

1.A42 is simply the mean heat flux. Equation 1.A42 

shows that peaks on the contact receive a greater flux 

than troughs — and thus irregularities tend to be 

damped — if 2n\i>\, that is, for irregularities with 

wavelength A, < 2nL. In contrast, long-wavelength 

irregularities (K > 2nL) tend to be enhanced by melting. 

For a pyroclastic deposit with a thickness of, say, a few 

tens of cm (as in figure 1.1 A), irregularities that have 

wavelengths less than about 1 m would tend to be 

damped. Small-scale "scour" features, should they in fact 

ever exist, are thus likely to be muted or destroyed by the

time an observer can arrive on the scene.

A final comment about heat advection by vapor, 

heretofore neglected in the analysis, is in order. 

Considering again figure 1.A1, for short-wavelength 

irregularities, the conductive heat flux to the snow at the 

trough A is less than at the peak B; accordingly, the melt 

rate, and thus (if the base of the deposit is hot enough) 

the vaporization rate, is less at A than at B. Because the 

vapor will tend to rise vertically and convectively cool 

the deposit as it rises (the vapor itself becoming 

superheated), the deposit above A will undergo less 

convective cooling than the deposit above B. The vapor 

flux therefore tends to keep the deposit above A hotter 

than the deposit above B, reinforcing the tendency for 

conduction to divert heat from A toward B. For longwavelength irregularities, A and B are, as before, 

effectively thermally isolated from each other.
APPENDIX 1.B
Selected Series of U.S. Geological Survey Publications

Books and Other Publications

Professional Papers report scientific data and interpretations 

of lasting scientific interest that cover all facets of USGS investigations and research.

Bulletins contain significant data and interpretations that are of 

lasting scientific interest but are generally more limited in 

scope or geographic coverage than Professional Papers.

Water-Supply Papers are comprehensive reports that present 

significant interpretive results of hydrologic investigations of 

wide interest to professional geologists, hydrologists, and engineers. The series covers investigations in all phases of hydrology, including hydrogeology, availability of water, quality of 

water, and use of water.

Circulars are reports of programmatic or scientific information 

of an ephemeral nature; many present important scientific 

information of wide popular interest. Circulars are distributed 

at no cost to the public.

Fact Sheets communicate a wide variety of timely information 

on USGS programs, projects, and research. They commonly 

address issues of public interest. Fact Sheets generally are two 

or four pages long and are distributed at no cost to the public.

Reports in the Digital Data Series (DDS) distribute large 

amounts of data through digital media, including compact discread-only memory (CD-ROM). They are high-quality, interpretive publications designed as self-contained packages for viewing and interpreting data and typically contain data sets, 

software to view the data, and explanatory text.

Water-Resources Investigations Reports are papers of an 

interpretive nature made available to the public outside the formal USGS publications series. Copies are produced on request 

(unlike formal USGS publications) and are also available for 

public inspection at depositories indicated in USGS catalogs.

Open-File Reports can consist of basic data, preliminary 

reports, and a wide range of scientific documents on USGS 

investigations. Open-File Reports are designed for fast release 

and are available for public consultation at depositories.

Maps

Geologic Quadrangle Maps (GQ's) are multicolor geologic 

maps on topographic bases in 7.5- or 15-minute quadrangle 

formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, 

surficial, or engineering geology. Maps generally include brief 

texts; some maps include structure and columnar sections only.

'*l|-Cleophysical Investigations Maps (GP's) are on topographic 

or planimetric bases at various scales. They show results of 

geophysical investigations using gravity, magnetic, seismic, or 

radioactivity surveys, which provide data on subsurface structures that are of economic or geologic significance.

Miscellaneous Investigations Series Maps or Geologic 

Investigations Series (I's) are on planimetric or topographic 

bases at various scales; they present a wide variety of format 

and subject matter. The series also incudes 7.5-minute quadrangle photogeologic maps on planimetric bases and planetary 

maps.

Information Periodicals

Metal Industry Indicators (Mil's) is a free monthly newsletter that analyzes and forecasts the economic health of five 

metal industries with composite leading and coincident 

indexes: primary metals, steel, copper, primary and secondary 

aluminum, and aluminum mill products.

Mineral Industry Surveys (MIS's) are free periodic statistical 

and economic reports designed to provide timely statistical data 

on production, distribution, stocks, and consumption of significant mineral commodities. The surveys are issued monthly, 

quarterly, annually, or at other regular intervals, depending on 

the need for current data. The MIS's are published by commodity as well as by State. A series of international MIS's is also 

available.

Published on an annual basis, Mineral Commodity Summaries is the earliest Government publication to furnish estimates 

covering nonfuel mineral industry data. Data sheets contain 

information on the domestic industry structure, Government 

programs, tariffs, and 5-year salient statistics for more than 90 

individual minerals and materials.

The Minerals Yearbook discusses the performance of the 

worldwide minerals and materials industry during a calendar 

year, and it provides background information to assist in interpreting that performance. The Minerals Yearbook consists of 

three volumes. Volume I, Metals and Minerals, contains chapters about virtually all metallic and industrial mineral commodities important to the U.S. economy. Volume II, Area Reports: 

Domestic, contains a chapter on the minerals industry of each 

of the 50 States and Puerto Rico and the Administered Islands. 

Volume III, Area Reports: International, is published as four 

separate reports. These reports collectively contain the latest 

available mineral data on more than 190 foreign countries and 

discuss the importance of minerals to the economies of these 

nations and the United States.

Permanent Catalogs

"Publications of the U.S. Geological Survey, 1879-1961" 

and "Publications of the U.S. Geological Survey, 1962- 

1970" are available in paperback book form and as a set of 

microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" is

available in paperback book form (two volumes, publications 

listing and index) and as a set of microfiche.

Annual supplements for 1982, 1983, 1984, 1985, 1986, and 

subsequent years are available in paperback book form.
ISBN

Printed on recycled paper