Origins, Behavior, and Sedimentology of 

Lahars and Lahar-Runout Flows in the 

Toutle-Cowlitz River System

LAHARS AND LAHAR-RUNOUTf fLQpS IN THE TC^TlJ^CLKX RI¥ER 

SYSTEM, MOUNT ST. HELENS, WASHINGTON

U.S, GEOLOGICAL SURVEY I>R;OFESS I ON[A;L P^JPER
Origins, Behavior, and Sedimentology of 

Lahars and Lahar-Runout Flows in the 

Toutle-Cowlitz River System

By KEVIN M. SCOTT

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

SYSTEM, MOUNT ST. HELENS, WASHINGTON

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1447-A

Analyses of modern flows 

lead to greater understanding 

of ancient flow deposits

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

DONALD PAUL HODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging-in-Publication Data

Scott, Kevin M., 1935- 

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

(U.S. Geological Survey professional paper; 1447-A- )

Includes bibliographies.

Contents: [1] Origins, behavior, and sedimentology of Lahars and Lahar-Runout flows in the ToutleCowlitz River System [2] Magnitude and frequency of Lahars and Lahar-Runout flows in the 

Toutle-Cowlitz River System.

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

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

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

4. Sediments (Geology) Washington (State) Cowlitz River Watershed. I. Title. II. Series: 

Geological Survey professional paper ; 1447-A, etc. 

QE598.2.S38 1988 551.3 86-600198

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

Federal Center, Box 25425, Denver, CO 80225
CONTENTS

	Page

Abstract ........................................... Al

Introduction ....................................... 2

Definitions and concepts .......................... 2

Lahar-runout flows ........................... 5

Lahar in the strict and general senses ........... 5

Flow transformations ......................... 5

Flow bulking ................................ 6

Levels of particle support ..................... 6

Boundary features ............................ 6

Acknowledgments ............................... 6

Lahars and lahar-runout flows of the modern eruptive period 7

Lahars and lahar-runout flow of May 18-19, 1980 .... 7

Lahar and lahar-runout flow of March 19-20, 1982 ... 8

Phases, flow types and divisions, and facies models ...... 8

Phases of the 1980 South Fork lahar ............... 8

Proximal phase .............................. 9

Medial phase ................................ 11

Distal phase ................................. 12

Lahar-runout phase ........................... 12

Flow types and divisions ......................... 13

Longitudinal flow transformations .............. 13

Flow divisions at a section basal flow and the whaleback bars ................................. 13

Lahar and lahar-runout facies models ............... 16

Lahar channel facies .......................... 17

Lahar flood-plain facies ....................... 19

Lahar-runout facies ........................... 19

Transition facies ............................. 19

Lahar-related steamflow deposits ............... 20

Facies of the largest lahar in the river system .... 21

Size distributions of the lahars ........................ 21

North Fork lahar peak-flow deposits of the flood-plain

facies ........................................ 23

North Fork lahar basal-flow deposits of the channel

facies ........................................ 25

North Fork lahar sole layer of the channel facies .... 26

South Fork lahar peak-flow deposits of the flood-plain

facies ........................................ 27

South Fork lahar avalanche deposits of the deflating 

pyroclastic surge and basal-flow deposits of the channel

facies ........................................ 29

South Fork lahar sole layer of the channel facies .... 30

Lahars of previous eruptive stages and periods ....... 32

Large lahar of the Pine Creek eruptive period .... 32

"Ball-bearing bed" ........................... 32

Alluvium source of sediment in lahars of the Pine

Creek eruptive period ....................... 34

Clast roundness and the lahar bulking factor ............ 35

Roundness in the 1980 lahars ..................... 36

Bulking factors of the 1980 lahars ................. 37

Roundness in the pre-1980 lahars .................. 38

Bulking factors of the pre-1980 lahars .............. 39

Roundness in the sole layers ...................... 40

Abrasion and cataclasis in lahars .................. 40

Clastlithology ...................................... 41

Pyroxene andesite and olivine basalt ................ 41

Clast lithology Continued

Pumice ......................................... A42

Low-density rock types from domes ................ 42

Juvenile dacite derived from facies of the debris avalanche 42 

Eroded soil ..................................... 42

Lahar boundary features ............................. 43

Lahar-abraded pavement .......................... 43

Sole layer ...................................... 44

Type I sandy sole layers lacking dispersed coarse 

clasts ..................................... 44

Type II sole layers with dispersed coarse clasts . . 45 

Type III sole layers of clast-supported gravel; the 

"ball-bearing bed" .......................... 46

Inverse grading ................................. 46

Origin and relations of the boundary features ........ 47

Flow transformations ................................ 51

Pyroclastic surge and avalanche to debris flow ....... 51

Temperature of the pyroclastic surge ............ 54

Origin of water in the lahar .................... 55

Comparison with other pyroclastic surge and flow 

deposits ................................... 55

Comparison with Kalama- and post-Kalama-age deposits 56 

Flood surge to debris flow ........................ 57

Debris flow to hyperconcentrated streamflow ......... 57

Evidence for transformation ................... 57

Characteristics of deposits of hyperconcentrated laharrunout flows ............................... 60

Lack of stratification in particle- and clastsupported deposits ...................... 61

Inverse grading ........................... 61

Coarse-tail grading of low-density clasts ...... 62

Concentration of low-density clasts near lateral 

flow boundaries ......................... 62

Sorting .................................. 62

Transitional cumulative curves .............. 62

Lack of coarse inflection point in cumulative curves 63 

Sediment transport in the lahar-runout flow of March 

19-20,1982, and correlation with deposit characteristics ....................................... 63

Effect of postdepositional drainage on deposit texture 66 

Origin and evolution of lahar-runout flows ........ 66

Conclusions on lahar texture as related to origin and behavior 69 

Lahars formed by flow transformations ............. 69

Flood surges transformed to lahars .............. 69

Catastrophically ejected surges and avalanches transformed to lahars ............................ 70

Lahars formed from failure of volcanic deposits not selectively sorted .................................. 70

Summary of the general relation of lahar origin to magnitude ........................................ 71

Interpretations of dynamics based on texture ........ 71

References cited ..................................... 72

Appendix Equations used in calculating lahar velocity and 

statistics of grain size distributions .................. 76

Velocity ........................................ 76

Grain size statistics .............................. 76

in
IV CONTENTS

ILLUSTRATIONS

Page

FIGURE 1. Map showing the major streams draining Mount St. Helens .............................................. A3

2. Graph of stage and recession sediment concentrations in the Cowlitz River at Castle Rock, May 18-19, 1980 .... 7

3. Photograph showing surface of the debris avalanche approximately 18 km downstream from the 1980 crater ..... 8

4. Photograph of pyroclastic flow formed by collapse of the Plinian eruption column ............................ 9

5. Photograph showing uppermost part of watershed of South Fork Toutle River, and tributary Sheep Canyon ..... 10

6. Profile of the South Fork Toutle River, showing longitudinal phases of the main lahar occurring on May 18, 1980 11

7. Diagram portraying origin of the 1980 South Fork lahar as the transformation of a pyroclastic surge ........... 14

8. Photograph showing longitudinal view of whaleback bar in South Fork Toutle River ......................... 16

9. Photograph showing transverse view of whaleback bar in South Fork Toutle River ........................... 17

10. Diagrams portraying facies types described in table 4 ................................................... 19

11. Cross section of Cowlitz River showing deposits of North Fork lahar of May 18, 1980, and subsequent streamflow 20

12. Diagrams of channel and flood-plain facies of 1980 North Fork lahar at Coal Bank Bridge and of lahars of Pine Creek

age near the Green Mountain Mill .................................................................. 22

13. Graphs showing downstream changes in particle-size distribution characteristics in peak-flow deposits of 1980 North

Fork lahar ....................................................................................... 23

14. Cumulative curves and histograms of peak-flow deposits of the North Fork lahar ............................ 24

15. Photograph of bar deposits of North Fork lahar at confluence of forks of Toutle River ........................ 26

16. Photograph showing channel facies of the North Fork lahar at Kid Valley .................................. 27

17. Cumulative curves of sole layer, bar, and peak-flow deposits of the North Fork lahar near Kid Valley ........... 28

18. Photograph showing two subunits in sole layer of North Fork lahar at the Highway 99 bridge ................. 29

19. Graphs showing downstream changes in mean size, sorting, skewness and percent clay in peak-flow deposits of the 1980

South Fork lahar ................................................................................. 30

20. Cumulative curves of the distal phase of the South Fork lahar and the lahar-runout flow derived from that lahar . 31

21. Graphs showing downstream changes in mean size and sorting of the avalanche and bar deposits of the South Fork

pyroclastic surge and lahar ........................................................................ 32

22. Cumulative curves and histograms for first lahar of Pine Creek age, including channel and flood-plain facies, the "ballbearing bed," and alluvium in a megaclast ........................................................... 34

23. Photograph of "ball-bearing bed" at base of first lahar of Pine Creek age ................................... 35

24. Photograph of megaclast in the first lahar of Pine Creek age .............................................. 36

25. Graph showing mean roundness of particles in first lahar of Pine Creek age, alluvium in a megaclast in that lahar, and

bar and flood-plain deposits of 1980 North Fork lahar .................................................. 37

26. Histograms showing distribution of clast roundness in Pine Creek age and 1980 deposits ..................... 38

27. Graph showing clast roundness in sole layer, bar deposits, and flood-plain deposits of 1980 North and South Fork lahars 39

28. Graph showing downstream variations in percentages of andesite, basalt, and pumice in peak-flow deposits of the 1980

North and South Fork lahars ....................................................................... 41

29. Photograph of surface formed by compaction and truncation at contact of two units of the bar deposits of North Fork

lahar ........................................................................................... 44

30. Photograph showing flood-plain facies of three older lahars ............................................... 45

31. Photograph showing sole layer of first lahar of Pine Creek age on valley side slope near Kid Valley Bridge ...... 47

32. Diagrammatic cross section showing distribution of sole layer and inverse grading in North Fork lahar near Tower Bridge 48

33. Photograph showing lateral margin of lahar deposit on surface of the debris avalanche near Elk Rock .......... 51

34. Graph showing discharge and velocity of pyroclastic surge and main lahar in South Fork Toutle River .......... 52

35. Photograph of pyroclastic surge descending southwest flank of Mount St. Helens 2 min after beginning of the May

18, 1980, eruption ................................................................................ 53

36. Diagrammatic sections of depositional units of 1980 South Fork lahar, 1982 lahar, and runout flows evolved from those

lahars .......................................................................................... 58

37. Diagram showing downstream change in texture resulting from transformation of debris flow to hyperconcentrated

streamflow ...................................................................................... 60

38. Photograph of deposits of the main peak of 1982 runout flow at Kid Valley, near downstream end of the transition

interval ......................................................................................... 60

39. Diagram showing textural change in transition facies of 1982 lahar, just upstream from Kid Valley ............ 61

40. Cumulative curves of inversely graded part of transition facies of 1982 lahar ................................ 61

41. Cumulative curves of 1982 lahar and lahar-runout flow deposits, compared with modern and ancient streamflow deposits 63

42. Graphs of discharge aned sediment concentration of 1982 lahar-runout flow at Tower Road .................... 64

43. Sediment transport curve of the 1982 lahar-runout flow at Tower Road ..................................... 65
CONTENTS

TABLES

Page

TABLE 1. Known lahars and lahar-runout flows in the Toutle-Cowlitz River system ................................... A4

2. Interpreted characteristics of at-a-section flow subdivisions in lahars ....................................... 13

3. Dimensions of whaleback bars formed by the 1980 lahar in the South Fork Toutle River ...................... 14

4. Fades characteristics of large lahars and associated flows from Mount St. Helens ........................... 18

5. Textures of selected pre-1980 lahars and lahar-runout flow deposits ........................................ 33

6. Clast roundnessin pre-1980 lahars .................................................................... 38

7. Characteristics of the sole layers of the 1980 North Fork and South Fork lahars ............................. 49

8. Measured peak sediment concentration and adjusted mean size and sorting values for the lahar-runout flow of March

19-20, 1982 ..................................................................................... 67

9. Sequential changes in flow type and particle-support mechanisms during the transformation from lahar, to lahar-runout

flow, to normal streamflow ......................................................................... 68

GLOSSARY

Andesite. Fine-grained volcanic rock characteristically medium dark 

in color and containing 54 to 62 percent silica.

Ash. Fine pyroclastic particles less than 4.0 mm in diameter.

Ash cloud. Turbulent cloud of fine material rising above the main body 

of a pyroclastic flow.

Antidune. A wave of cohesionless sediment that is in phase with the 

overlying water-surface wave (standing wave).

Bagnold effect. The displacement in the direction of least shear of particles within a body of sheared, mainly cohesionless sediment.

Basalt. Fine-grained volcanic rock characteristically dark in color and 

containing less than 54 percent silica.

Base surge. An expanding basal cloud of particles that travels outward from a large explosion.

Bayonet trees. Trees inclined to a low angle and sharpened to smooth 

spikes by lahar erosion.

Bed load. Material moved in traction along the bed of a river.

Benn. A nearly flat-topped, terracelike body of sediment deposited 

along the side of a channel.

Boulder. A sediment particle with a diameter greater than 256 mm.

Bulking. The addition of significant amounts of sediment to a flow 

by erosion.

Celerity. The velocity of a flood wave.

Clast-supported. Having a texture in which the gravel-size particles 

are mainly in contact.

Clay (size). A sediment particle with a diameter less than 0.004 mm.

Cobble. A sediment particle with a diameter of 64-256 mm.

Competence. The transporting ability of a current, expressed in terms 

of the maximum particle size of a given density that can be 

transported.

Critical diameter. In a lahar, the diameter above which most particles 

are dispersed in the finer grained matrix.

Cross strata, beds, or laminae. Sets of strata (beds if >1 cm; laminae 

if <1 cm) that are not parallel to the main layers within which they 

occur.

Dacite. Volcanic rock that characteristically is light in color and contains 62-69 percent silica.

Debris avalanche. A rapid flow of unsorted masses of rock and other 

material including, in the 1980 example in the North Fork Toutle 

River valley, water, snow, ice, and vegetation.

Dispersive stress or pressure. Stress caused by shearing of a static 

grain bed and resulting in dilation or dispersion that selectively 

moves particles upward within the flowing mass.

Dome. A steep-sided mass of viscous lava extruded from a volcanic 

vent.

Drumlin. Hill molded into a streamline form by glacial action.

Fabric. The arrangement of particles in a sedimentary deposit.

Fades. The characteristics of a sedimentary deposit that are imparted 

by the depositional environment.

Flood plain. A low, flat, valley-bottom area inundated by overbook 

flood flow.

Flood surge. Water wave with a steep rising limb. Also called "water 

surge" and "streamflow surge." Types include snowmelt and lakebreakout surges.

Graded layer. A deposit in which the particle sizes show a systematic 

vertical size change. A unit with particles systematically finer upward has normal grading or is normally graded; one with particles 

systematically coarser upward has inverse grading or is inversely 

graded. The terms may be applied to parts of a flow unit.

Grain. In a general sense, a synonym of particle when used as an adjective, as in grain size or coarse-grained deposit.

Grain flow. Flow of particles with little or no cohesion, which are not 

suspended by turbulence but are supported by momentum provided 

by interactions with other particles in the flow.

Granule. A sedimentary particle from 2 to 4 mm in diameter.

Gravel. A collective term for sedimentary particles coarser than 2 mm.

Inverse grading. See graded layer.

Kinematic wave. A wave in which the material forming the crest moves 

bodily forward.

Lahar. Volcanic debris flow and its deposits.

Lahar-abraded pavement. Nearly planar surface cut by a lahar, most 

commonly near channel thalweg.

Lahar bulking factor (LBF). Proportion of sediment in a lahar that 

can be shown to have been introduced by erosion during flow.

Lahar-runout flow. Hyperconcentrated streamflow mainly evolved 

directly from a distal lahar.

T^minpr flow. Fluid flow in which there is sliding within the fluid along 

surfaces nearly parallel to the flow boundary.

Matrix-supported. Containing dispersed coarse clasts.
VI GLOSSARY

Mud. All material in the clay and silt size ranges.

Particle-supported. Having a texture in which particles are mainly 

in contact. A lahar may have a particle-supported matrix, yet as 

a whole may be characterized as matrix-supported because the 

gravel-size particles are dispersed. When the gravel-size particles 

are in contact, the unit is described as clast-supported.

Pebble. A sediment particle having a diameter from 4 to 64 mm.

Phi scale. A geometric grade scale of particle size with class boundaries based on negative logarithms to base 2, 

<£=-log2 size (mm)

Plug. Rigid core of a debris flow, formed in region of flow where yield 

strength of debris exceeds shear stress; velocity distribution in plug 

is nearly constant with depth.

Pumice. Light-colored, low-density, frothy volcanic rock (of dacite composition at Mount St. Helens) formed by the expansion of gas in 

erupting lava.

Pyroclastic. Pertaining to fragmental rock material formed by a 

volcanic explosion or ejection from a vent.

Pyroclastic flow. A flowing mixture of hot gases and pyroclastic 

material.

Pyroclastic surge. A flow that is more dispersed and less likely to approximate steady-state conditions than a pyroclastic flow. Types 

include the ground, base, and ash-cloud surges discussed in the text 

section "Comparison with other pyroclastic surge and flow 

deposits."

Reach. A length of channel with relatively uniform characteristics.

Recession. Falling discharge, or downward trending limb of 

hydrograph.

Rosin's law. A probability distribution produced by crushing of

isotropic material:

y=100(l-ebxn)

where y is weight percent sediment passing a sieve of mesh size x, 

b is the reciprocal of mean size, and 

n is a numerical coefficient analogous to the standard deviation of a normal distribution. 

Sand. Sediment particles having diameters between 2.0 and

0.0625 mm. 

Sediment-transport curve. A plot of sediment discharge and water

discharge. 

Silt. Sediment particles having diameters between 0.0625 and

0.004 mm.

Sole layer. Texturally distinct basal sublayer of a lahar. 

Sorting. The dispersion of particle sizes about the mean. The term

is also used to express the selective transport of different particle

sizes.

Stage. Level above a datum of a lahar or water flood. 

Surge. See pyroclastic surge and flood surge. Used singly, the term

refers to one or the other, in context, or to an ongoing wave of

turbulent flow in the process of transforming from one to the

other. 

Suspended sediment. Sediment in transport whose immersed weight

is supported by the fluid. 

Tephra. Material erupted from a volcanic crater or vent and deposited

from the air.

Thalweg. The deepest line along a stream channel. 

Turbulent flow. Flow that is characterized by vortices and eddies.
LAHARS AND LAHAR-RUNOUT FLOWS IN THE TOUTLE-COWLITZ RIVER SYSTEM,

MOUNT ST. HELENS, WASHINGTON

ORIGINS, BEHAVIOR, AND SEDIMENTOLOGY OF LAHARS AND 

LAHAR-RUNOUT FLOWS IN THE TOUTLE-COWLITZ RIVER SYSTEM

By KEVIN M. SCOTT

ABSTRACT

In the 50,000-yr history of Mount St. Helens more than 35 lahars 

(volcanic debris flows and their deposits) and lahar-runout flows (hyperconcentrated streamflow evolved directly from a distal lahar) have 

inundated flood plains more than 50 km away in the river system draining the northwest sector of the volcano. At least six of the largest 

lahars or their runout phases probably inundated flood plains near 

the Columbia River, more than 100 km downstream. Characteristics 

of the 1980 flow deposits were compared to those of deposits from 

previous eruptive stages and periods to obtain a long-term perspective of lahar origins and behavior.

Several kinds of flow transformations were associated with lahar 

origin and evolution: (1) Pyroclastic surge to lahar. Deflation of a 

catastrophically ejected, lithic pyroclastic surge directly formed the 

1980 lahar in the South Fork Toutle River and is a mode of lahar 

genesis inferred to have occurred in at least one other eruptive period. 

(2) Avalanche to lahar. Catastrophically ejected avalanches, possibly 

also associated with pyroclastic surges or flows to varying degrees, 

yielded downstream lahars in older eruptive periods. (3) Flood surge 

to lahar. The largest two lahars in the history of the river system 

formed by bulking of stream alluvium in flood surges from lake breakouts through volcanically initiated natural dams. Other flows of intermediate size were formed from meltwater surges initiated by 

pumiceous pyroclastic flows in the oldest eruptive stage, and subsequently by lithic pyroclastic flows. The relatively small 1982 lahar 

also formed from a volcanically induced meltwater surge. (4) Lahar 

to hyperconcentrated streamflow. Lahar debulking and change to 

lahar-runout flow occurred in the distal phases of most of the ancient 

and modern flows that could be traced downstream. The most common lahars in the river system were really, therefore, the middle 

segments of flood waves beginning and ending as streamflow 

surges.

The formation of lahar-runout flows involved the progressive incorporation of streamflow overrun by the leading part of the debris flow. 

The change occurred over a channel distance of as much as 19 km, 

producing an inversely graded transition facies in which runout 

deposits appear as a thickening wedge at the base of a flow unit 

and are transitionally overlain by debris flow deposits that thin

progressively and disappear downstream. As water content increased 

downstream, two decrements contributed to the change from poorly 

sorted, mainly matrix-supported debris flow deposits to granular, 

particle-supported runout sand with sorting in the range of 1.1-1.6 0: 

(1) dispersed coarse clasts were deposited because of loss of yield 

strength, and (2) some silt and clay were lost from runout deposits 

by rapid drainage of interstitial fluids as flow receded.

The less pronounced textural change in the few lahars with relatively 

more clay (3 to 5 percent) shows that formation of lahar-runout flows 

was in part a function of low clay content (<3 percent) and a consequent low cohesiveness that facilitated miscibility with streamflow. 

The lahars with more clay are inferred to have formed mainly by slumping of hydrothermally altered material, as was most of the 1980 lahar 

in the North Fork Toutle River. That flow extended more than 100 km 

with gradual textural change tending toward a hyperconcentrated 

runout flow, but it did not complete the transformation.

Most of the lahars in the system are relatively granular, have silty 

sand matrices, and contain more regions of particle support than many 

other reported debris flows. Particle interaction was important during flow, locally to the point of cataclasis. Distinctive flow divisions 

and boundary features were formed, some of which are evidence of 

dilatant fluid behavior. A bed-load-analog basal carpet of interacting 

coarse clasts developed by settling of the coarsest clasts in the lahar 

and by erosion of coarse alluvium from the flow boundary, at which 

shear was locally so concentrated that in reaches with rapid energy 

loss a pavement similar to a glacial pavement was cut. Pulses of basal 

flow formed whaleback bars during a brief period near the lahar peak. 

Other boundary features included a texturally distinct basal subunit 

described as the sole layer, and nearly universal inverse grading. The 

inversely graded lower part of each unit is commonly overlain by an 

ungraded or poorly graded core, which is in turn overlain by a normally graded upper part that was produced by settling within the intralahar "plug." The boundary features reflect variations in the 

intensity of shear-induced particle interactions, as well as migration 

of the boundary of the intralahar "plug" and development of an accretionary, roughness-reducing coating of the boundary during the 

flood wave rise.

Al
A2 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

INTRODUCTION

Modern and ancient labors and their runout phases 

have inundated flood plains for long distances from 

Mount St. Helens periodically throughout the history 

of the volcano. The lahars that formed during the major eruption on May 18,1980, reached down the Tbutle 

and lower Cowlitz Rivers as far as the Columbia River, 

more than 100 km away. The cost of the resulting damage and remedial engineering work in the succeeding 

two years exceeded 300 million dollars. The flows were 

spectacular in their impact on the natural landscape and 

in the diversity of sedimentologic features preserved in 

the deposits (Janda and others, 1981). Deposits from 

older eruptive periods reveal a history of older flows that 

were as extensive as the modern flows and more diverse 

in origin. One ancient flow was substantially more than 

an order of magnitude larger than the largest 1980 flow. 

The purpose of this report is to analyze the 1980 and 

subsequent lahar deposits in the Tbutle-Cowlitz River 

system and relate the character of those flows to their 

observed flow behavior. The results are then applied to 

the older flows to interpret their origin and behavior. A 

companion report (Scott, in press) investigates the size 

and recurrence probabilities of these flows.

Field study began on the east and south sides of the 

volcano (fig. 1) because of the sequence of different flow 

types related to the May 18, 1980, eruption. There the 

largest lahars had originated almost synchronously with 

the lateral blast as the downstream continuations of a 

pyroclastic surge. That behavior has subsequently been 

quantified by Pierson (1985). In the fall of 1980, storm 

runoff rapidly began to erode the flow deposits in the 

Tbutle and Cowlitz Rivers, so study was shifted to that 

drainage. The main lahar in the South Fork of the Tbutle 

River had an origin like that of the large flows on the 

opposite side of the volcano and was the only major 1980 

flow with deposits exposed through the runout phase. 

Although the larger lahar of May 18 in the North Fork 

Tbutle River had a different origin, mainly by slumping 

of surficial parts of the debris avalanche, it shows many 

flow features surprisingly similar to those of the South 

Fork lahar. As the runoff season of 1980-81 progressed, 

each flood flow revealed a new erosional "slice" of the 

1980 lahars and also exposed new stratigraphic sections 

of the older lahars (table 1). The sedimentary features 

of both the modern and ancient flows were then observed 

in three dimensions as erosion progressed.

The use of the term "lahar" for both volcanic debris 

flows and volcanic mudflows, as well as for their deposits, avoids some problems in terminology. The 1980 

lahars have been commonly described in popular and 

scientific reports as mudflows, a category of debris flows 

in which mud (the total of silt- and clay-size sediment)

is a significant part of the deposits. A widely accepted 

practical definition used here is that of Varnes (1978, p. 

18), who defined mudflows as those debris flows with 

deposits containing at least 50 percent sand, silt, and 

clay. Some definitions place greater emphasis on the clay 

content; for example, mudflows commonly contain 25 

percent or more clay according to Friedman and Sanders 

(1978). A mudflow differs generally from other debris 

flows by having enough silt and clay to give the deposit 

a muddy appearance. On this subjective distinction, the 

peak-flow deposits of the 1980 lahars qualify as mudflows. They also qualify on a specific textural basis 

according to Varnes (1978). However, most of the clastsupported bar deposits (having mainly gravel-size clasts 

in contact) do not meet the Varnes criteria for a mudflow 

and also lack the characteristic matrix-supported texture (clasts "float" in matrix) of typical debris flow 

deposits.

Worldwide, lahars had taken lives numbered in the 

tens of thousands by 1976 (Neall, 1976a, b). Then, in 

1985, a single eruption in Colombia produced lahars that 

killed more than 20,000 people. "Lahar" is an Indonesian term for a debris flow containing angular volcanic 

blocks (van Bemmelen, 1949), but is of diverse application there and elsewhere. Some subsequent definitions 

have expanded the term to include torrential waterflows 

(Schieferdecker, 1959) or hyperconcentrated streamflows 

(Fisher and Schmincke, 1984), whereas others have 

limited it to a specific composition (Schmid, 1981) or an 

origin on the flank of a volcano (Crandell, 1971; Bates 

and Jackson, 1980). The term "lahar" is applied here to 

flows and deposits equivalent in most respects to those 

described by D. R. Crandell and D. R. Mullineaux at 

several Cascade Range volcanoes. (See References Cited.) 

Although relatively small debris flows may be a daily 

occurrence on active and some inactive volcanoes, any 

magnitude distinction for a lahar should remain subjective. Usage of the term has generally implied a flow large 

enough to pose a hazard, as well as one that is the direct 

or indirect result of volcanic activity (rather than a product of the alpine environment, as in the case of small, 

snowmelt-induced debris flows). The variety of compositions and textures of lahars documented at Mount St. 

Helens, and the variety of origins of the lahars on and 

beyond the base of the mountain, emphasize that the 

term is most useful if not qualified beyond the definition of a debris flow related (even indirectly) to volcanic 

activity.

DEFINITIONS AND CONCEPTS

Several undescribed lahar features and debris-flow 

behavior modes came to light during the study. These 

are described at appropriate locations in the text, but
123° 00

INTRODUCTION

30°

A3

122°

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Map of study area prior to 1980 eruption

30°

HIGHWAY 99 

BRIDGE O Green Mtn Mill 

£0 Harry Gardner Park

DEBRIS RETENTION 

STRUCTURES

Coldwater 

Lake

BRIDGE 

DESTROYED

EXPLANATION

~" Disappointment 

1 980 CRATER OF MOUNT ST. HELENS Creek

PERIMETER OF LATERAL BLAST ZONE

DEBRIS AVALANCHE Lahars on surface not shown

1980 LAHAR INUNDIATION Note deposits 

in Columbia River

> < BRIDGE DESTROYED

0 5 10 MILES

Map of study area showing locations and extent of 1980 lahars

FIGURE 1. Major streams draining Mount St. Helens. The Toutle-Cowlitz River system drains most of northwest half of the volcano. 

The natural flow of Spirit Lake into the North Fork Toutle River is now blocked by the debris avalanche.
A4 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

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

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

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

of the flows are discussed in Scott (in press)]

Eruptive stages and periods Approximate Tephra overbank 

age Set flows

Occurrence and extent

SPIRIT LAKE ERUPTIVE STAGE: 

Period beginning in 1980 Modern

Goat Rocks eruptive period 150-93

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

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

confluence). Runout phase reached the Cowlitz R.

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

1982 lahar in North Fork extended to Kid Valley. Runout 

phase reached Cowlitz R.

-Dormant interval of 123 yr 

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

Kalama eruptive period

 Dormant interval of about 200 yr 

470-320 W, X 3

Sugar Bowl eruptive period

Avalanches and coarse rubbly lahars buried forests in upper 

South Fork, may correlate with 2 downstream overbank 

lahars that extend to confluence.

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

overbank and extended to confluence.

 ; Dormant interval of about 700 yr 

1,200 0 No lahars extended significant distances past base of cone.

 Dormant interval of about 500 yr 

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

unlikely to be preserved in stratigraphic record because 

of the erosional regime that probably existed after 

emplacement of the voluminous Pine Creek lahar deposits.

 Dormant interval of about 300 yr : 

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

to confluence. 1 huge lahar followed by 3 smaller flows 

in North Fork extended to confluence. At least the first 

and third lahars in this sequence reached Cowlitz R. and 

probably extended to Columbia R.

 Dormant interval of about 300 yr 

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

least to confluence. Youngest 2 lahars originated in 

North Fork. Largest lahar probably reached Columbia R.

 Dormant interval of about 5,000 yr 

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

confluence.

 - Dormant interval of about 5,000 yr 

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

to Columbia R.

COUGAR ERUPTIVE STAGE 20

APE CANYON ERUPTIVE STAGE 50 ,000(?) to 

~36,000

 Dormant interval of about 15,000 yr 

C >8 At least 5 lahars and several overbank runout flows 

extended at least to confluence. At least 2 lahars 

probably extended to Columbia R.

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

historic records) and Kalama period (based on tree rings and radiocarbon dates).
INTRODUCTION A5

an introduction to some concepts and terminology is a 

necessary preliminary to the discussion.

LAHAR-RUNOUT FLOWS

The lahar-runout flows consisted of hyperconcentrated streamflow, defined as flow containing from 40 

to 80 percent sediment by weight (Beverage and 

Culbertson, 1964), which corresponds to 20 to 60 percent by volume or to concentrations of 400,000 to 

800,000 ppm or 530,000 to 1,590,000 mg/L. Hyperconcentrated streamflow of three separate origins has 

been documented by measurements of suspended sediment in the river system: (1) recession streamflow associated with the two largest lahars in 1980, (2) a small 

lake-breakout flood surge in August 1980 on the surface of the 1980 debris avalanche, and (3) the runout 

phase of the 1982 lahar. Further study of the 1980 and 

1982 runout flow deposits and of sediment transport 

in the 1982 flow yields a more specific description. The 

vertical textural sequences in a facies transitional between the lahars and lahar-runout flows show that the 

change occurred through progressive loss of sediment 

concomitant with dilution by streamflow that was overrun by the leading edge of the lahar. Thus most of the 

hyperconcentrated runout flow evolved directly from 

the leading edge and peak of the lahar. Unlike the 

runout flow, the recession hyperconcentrated flow did 

not leave significant deposits. As a practical matter, 

though, beyond the zone of transition, hyperconcentrated flow derived from the recessional limb of the 

lahar flood wave cannot be differentiated from that produced by transformation of the peak and leading edge. 

Therefore, the definition of a lahar-runout flow advanced here embraces all hyperconcentrated streamflow 

downstream from a lahar, regardless of origin. Recognizing that most such streamflow will have evolved 

directly from the distal part of the lahar emphasizes the 

importance of the dilution process.

The hyperconcentrated flows evolved to normal 

streamflow with further dilution downstream. That 

streamflow can be considered the distal part of the 

lahar-runout flow but did not differ significantly from 

normal flood flow in the river system. Because this 

report focuses on the hyperconcentrated flows and the 

characteristics of their deposits, lahar-runout flows are 

described as the hyperconcentrated portion of the 

downstream flow wave. Discovering the unique 

characteristics of modern lahar-runout deposits leads 

to the recognition of ancient lahars and thus to the inclusion of those events in analyses of magnitude and 

frequency.

LAHAR IN THE STRICT AND GENERAL SENSES

The recognition of lahar-runout flows raises the question of whether the hyperconcentrated runout flow and 

the normal streamflow that evolves farther downstream 

should be considered parts of the lahar. The most common usage of "lahar" is for a Theologically obvious 

debris flow. That usage describes a lahar in the strict 

sense. A more general description would include downstream flows and deposits of flows transformed from 

but not part of a debris flow (the usage of Crandell and 

others, 1984). A complication is the fact that, over the 

interval of transformation from lahar to runout flow, 

which may extend for many kilometers, the single 

depositional unit consists of debris-flow deposits overlying streamflow deposits. The front of the single flood 

wave is hyperconcentrated streamflow; the tail consists 

of debris flow. The segment of channel where this occurs is called the transition interval and is included in 

the runout phase; the depositional unit is called the 

transition facies.

FLOW TRANSFORMATIONS

The lahars in the river systems of Mount St. Helens 

differ significantly from most nonvolcanic debris flows. 

(See Costa, 1984.) Major differences are their magnitude, the distance traveled, their very low clay content 

(less than 1 percent in some cases), and the systematic 

downstream textural changes. The textural changes 

reflect the long distances traveled and the flow transformations by which the lahars formed from previously 

sorted materials that had lost much of their fine sediment (Scott, 1984; 1985a). The most common origin, and 

that forming the largest flows (Scott, 1985b), was the 

transformation of a water flood to a lahar through the 

incorporation of eroded sediment. The consequently 

granular, noncohesive texture of most of the lahars was 

the main factor contributing to the downstream transformations to lahar-runout flows by mixing of the 

leading edges of the lahars with perennial streamflow. 

In contrast, many debris flows of alpine and semiarid 

areas rapidly lose water as they move across permeable 

alluvial fans or through ephemeral channels.

Many features of the lahars in the river system are 

similar to the features of sediment gravity flows (fluids 

driven by gravity acting on sediment) in subaqueous 

settings. To emphasize the importance of sediment concentration on the behavior of those flows, Fisher (1983) 

developed the concept of flow transformation and defined it as the change between laminar and turbulent 

behavior involving (1) change without much variation 

in water content (body transformations), (2) separations 

resulting from gravity (gravity transformations), and
A6 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

(3) separations by turbulent mixing with the ambient 

fluid above the flow surface (surface transformations). 

This general concept is extended to the changes by 

which the lahars and their runout flows formed and 

evolved. Their forms of transformations also include 

(1) change from the turbulent behavior of a flood surge 

to the mainly laminar behavior (see Enos, 1977) of a 

debris flow because of sediment added at the flow 

boundary and, conversely, (2) change from a mainly 

laminar debris flow back to turbulent behavior as sediment concentrations were diluted into the lower part 

of the hyperconcentrated range by streamflow that had 

been overrun. The formation of a lahar from a pyroclastic surge and the segregation of basal avalanches 

from the surge represent gravity transformations. The 

continuing flow of avalanches as lahars is included here 

as a category of transformation, although it does not 

involve as radical a change in behavior.

The flows and deposits recording the transformation 

from a lahar to hyperconcentrated flow show a transition in which, although normal debris-flow characteristics and the ability to suspend gravel-size clasts were 

lost, grain contact and interaction were important and 

turbulence was not well developed. This intermediate 

type of behavior is that of a dilatant fluid that has some 

of the characteristics of grain flow (Bagnold, 1954,1956; 

Lowe, 1976, 1979). Sediment support in grain flow is 

in part by particle-to-particle contact that, in a sheared, 

normally distributed mixture, can move larger particles 

in the direction of least shear (generally upward and inward). Dilatant fluid behavior also was characteristic 

of the lahars in the system (Denlinger and others, 1984); 

that behavior is presented as a general model of debris 

flow by Takahashi (1981).

FLOW BULKING

The term "bulking" is used as a general term for the 

incorporation of sediment in a flow by erosion at the 

flow boundary, as in the case of a flood surge adding 

sediment to the point that the flow is transformed to 

a lahar. Although not in common use (see Costa, 1984), 

the term has been applied in the same general sense to 

the incorporation of large amounts of sediment in flood 

flows in mountain watersheds (for example, Flaxman, 

1974), irrespective of whether the bulked flow was 

Newtonian or non-Newtonian. A lahar bulking factor 

(LBF) is defined as the proportion of the sediment in 

a lahar that was demonstrably incorporated through 

bulking of alluvium, leading in combination with other 

evidence to the recognition of lahars formed by transformation of flood surges in channels beyond the base of 

the volcano.

LEVELS OF PARTICLE SUPPORT

Particle or clast support in the lahars may exist at 

three levels: (1) Locally in some lahars, megaclasts form 

an intact framework with normal laharic sediment as 

the matrix. (2) Clasts forming the normal coarse mode 

of the lahar commonly develop an intact framework in 

the bar deposits. (3) The matrices of all types of lahar 

deposits in the river system are commonly supported 

by sand (particles 0.0625-2.0 mm in contact). The view 

of a lahar as consisting of a continuous or matrix phase 

and a dispersed phase (Fisher, 1971) could be replaced 

by a description of the sediment as a matrix phase and 

a coarse-grained phase. The latter phase may or may 

not be dispersed.

BOUNDARY FEATURES

A group of distinctive boundary features provides 

evidence for interpreting flow behavior. A zone or carpet 

of boulder-size clasts near or at the base of the lahars 

was formed by settling of coarse material initially 

dispersed in the flow and by the erosion of coarse 

streambed alluvium. Lahar-abraded pavements beneath 

the flows record intensely concentrated shear and 

boulder impacts on the bed even at flow depths of only 

several meters. The amount of shear has implications 

for dam and spillway design in potentially laharimpacted watersheds. A related feature is the sole layer, 

defined as a texturally distinct basal sublayer in a lahar, 

typically 10-25 cm thick and, in the 1980 examples, 

characterized by compaction and primary foliation. The 

sole layer formed during the rising and peak stages of 

flow and is characteristic of preexisting channels and 

other locations where shear was concentrated at the 

flow boundary. The remarkable "ball-bearing bed" is a 

sole layer of concentrated fine pebbles sorted, abraded, 

and fractured near the flow boundary of the first lahar 

of Pine Creek age, a huge lahar that formed by bulking 

of a flood surge released from an ancestral Spirit Lake.

ACKNOWLEDGMENTS

I am indebted to the staff of the U.S. Geological 

Survey's David A. Johnston Cascades Volcano Observatory for many helpful discussions. Individuals are 

cited in the text. Wayne Steuben performed or supervised the size analyses, solving the analytical problems 

presented by variable particle density and angularity. 

A number of local residents provided important information, among them Ted Conradi, whose observations 

of the 1980 flows as they inundated his property near
LAHARS AND LAHAR-RUNOUT FLOWS OF THE MODERN ERUPTIVE PERIOD A7

Tower verified the identification of the transition fades 

of the South Fork lahar-runout flow in that area. The 

study of the largest lake-breakout lahar of Pine Creek 

age was a cooperative project with the Cowlitz County 

Department of Community Development. S. L. 

Deatherage of that organization greatly facilitated that 

aspect of the work.

Drafts of several sections of the report benefited from 

discussion with or review by R. J. Janda and T. C. Pierson of the U.S. Geological Survey, and R. H. Dott, Jr., 

University of Wisconsin; R. V. Fisher, University of 

California at Santa Barbara; M. F. Sheridan, Arizona 

State University; and J. F. Hubert, University of 

Massachusetts.

LAHARS AND LAHAR-RUNOUT FLOWS OF 

THE MODERN ERUPTIVE PERIOD

LAHARS AND LAHAR-RUNOUT FLOW OF 

MAY 18-19, 1980

The first modern lahar in the Toutle River system was 

observed in the South Fork Toutle River 8 km west of 

the crater of Mount St. Helens less than 10 minutes 

after the eruption at 0832 P.d.t. on May 18, 1980 

(eyewitness account reported by Rosenbaum and Waitt, 

1981). The flow (hereafter called the South Fork lahar) 

began as a pyroclastic surge that deflated at the base 

of the cone and continued as a debris flow, with overbank depths commonly in the range of 4-18 m, to the 

confluence of the forks of the Toutle River. Shortly 

below that point the flow mixed with streamflow of the 

main Toutle River and continued mainly as a runout 

phase that was only locally overbank. The runout flow 

was in turn diluted to normal streamflow at the confluence with the Cowlitz River, causing a rise in stage 

of only 1.0 m at Castle Rock (fig. 2). Peak flow velocity 

(mean velocity at peak stage, in most cases corresponding to peak discharge) of the lahar decreased from more 

than 30 m/s upstream to less than 10 m/s near the confluence of the forks (Fan-child and Wigmosta, 1983).

The largest lahar of the present eruptive period at 

Mount St. Helens occurred in the North Fork Toutle 

River. That flow (hereafter called the North Fork lahar) 

formed mainly by dewatering of the debris avalanche 

deposit (fig. 3) and consequent slumping and erosion of 

its surface, after a relaxation time of about 5 hours on 

May 18 (Janda and others, 1981). Part of the lahar probably formed by bulking of large spring discharges on 

the credible avalanche surface (R. J. Janda and Harry 

Glicken, oral communs., 1982). The devastating, broadly 

peaked flow (fig. 2) covered the flood plains of the North 

Fork and the main Toutle River to depths commonly

Ib

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MAY 18, 1980 I MAY 19, 1980 

HOURS

FIGURE 2. Stage and recession sediment concentrations in the 

Cowlitz River at Castle Rock, May 18-19,1980 (data from Lombard and others, 1981, and Dinehart and others, 1981). The 

earliest measured concentration of recession flow is in the 

hyperconcentrated range.

between 2 and 6 m. The flow inundated flood plains only 

locally in the upstream part of the Cowlitz River. 

Without existing levees, though, overbank areas as far 

as the confluence with the Columbia River would have 

been inundated. Flow velocity, ranging between 6 and 

12 m/s, and peak discharge, ranging between 6,000 and 

8,500 m3/s, declined only gradually downstream in the 

Toutle River system (Fairchild and Wigmosta, 1983). 

Both the North Fork and South Fork lahars caused 

significant channel modification and instability. The 

changes reflect lahar deposition, mainly on flood plains 

but also as bars in the channel, as well as erosion of 

alluvium, mainly from the preexisting active channel. 

The overall change was in most cases one*of net fill and 

significant loss of channel capacity. The reduction in 

channel roughness may have contributed to the more 

peaked discharge of subsequent flows (Orwig and 

Mathison, 1982), but the effects are difficult to separate 

from those of the changed rainfall-runoff relations. In 

the main Toutle River, channel change caused by the 

runout flow of the South Fork lahar appears to have 

been locally pronounced, but could not be separated 

from the larger changes associated with the succeeding 

North Fork lahar because of the thick deposits from 

that flow that covered those of the runout flow. Changes 

at cross sections on the North Fork, South Fork, and 

main Toutle River are shown in Janda and others (1981);
A8 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

FIGURE 3. Surface of the debris avalanche approximately 18 km 

downstream from the 1980 crater. Smooth areas are underlain by 

deposits of the 1980 North Fork lahar. Photograph by Austin Post, 

June 30, 1980.

subsequent adjustments in channel geometry are 

discussed by Lisle and others (1983).

The final lahar of May 18 began about 1400 P.d.t. and 

was a much smaller flow seen in the upper South Fork 

by the observer who reported the initial large flow. This 

lahar formed high on the flanks of the cone and was 

highly pumiceous, suggesting an origin connected with 

the loading and icemelt or snowmelt effects of small 

pumiceous pyroclastic flows like the one shown in figure 

4. As suggested by flow tracks extending continuously 

from the crater rim (fig. 5), the lahar probably formed 

directly from a pyroclastic flow interacting with the

muddy (13-22 percent silt and clay) deposits of the 

earlier pyroclastic surge and the surge-scoured surfaces 

of the Toutle and Talus Glaciers. The second flow 

generally was confined to the channel except (1) in the 

uppermost reaches a short distance from the base of the 

cone (fig. 5), and (2) at its downstream end, where it inundated a large area of flood plain after being dammed 

at Harry Gardner Park by the earlier deposits of the 

North Fork and South Fork lahars.

LAHAR AND LAHAR-RUNOUT FLOW OF 

MARCH 19-20, 1982

A lahar was produced in the North Fork Toutle River 

from a flood surge formed by melting of a thick 

snowpack by hot eruption products during the domebuilding eruption of March 19,1982 (Waitt and others, 

1983). The main flood surge transformed to a lahar by 

sediment bulking as the flow left the crater and crossed 

the debris avalanche surface (Pierson and Scott, 1985). 

An initial peak that could be traced in its runout phase 

for more than 80 km was followed by a second, lower 

peak that could not be traced beyond 50 km from the 

crater. The 1982 lahar is significant because its origin 

by bulking of a flood surge is similar to that of the 

largest older lahar in the watershed and because it 

transformed downstream to a runout flow that extended to the Cowlitz River. The peak velocities of the 

lahar and lahar-runout flow ranged between 8 and 

15 m/s and 4 and 7 m/s, respectively (Pierson and Scott, 

1985).

The progressive changes in the 1982 runout flow and 

deposits and the associated hydrologic and sediment 

data are discussed in the section on flow transformations. Another interpretation of the lahar and laharrunout flow (Harrison and Fritz, 1982) is discussed in 

the section on lahar and lahar-runout facies models.

PHASES, FLOW TYPES AND DIVISIONS, 

AND FACIES MODELS

PHASES OF THE 1980 SOUTH FORK LAHAR

Most ancient and modern lahars in the river system 

changed markedly downstream. The South Fork lahar 

was studied in detail because many of the longitudinal 

changes in the texture and behavior of that flow (fig. 

6) were seen in other 1980 lahars and the March 1982 

lahar. Similar changes in texture and, by inference, in 

behavior were observed in older flows that could be 

traced downstream for a significant distance. In contrast, the damaging North Fork lahar was unusual in 

that its deposits show only a slight yet systematic
PHASES, FLOW TYPES AND DIVISIONS, AND FACIES MODELS A9

FIGURE 4. Pyroclastic flow (on left) formed by marginal collapse of the Plinian eruption column, entering the watershed of the South Fork 

Toutle River. Photograph by Robert M. Krimmel at approximately 1100 P.d.t. on May 18, 1980.

textural change downstream, and the main flood wave 

moved continuously as a mudflow from the debris 

avalanche to the Columbia River. That lahar did not exhibit the pronounced phases of the South Fork lahar, 

a difference that relates to the relatively high clay content of the North Fork lahar.

PROXIMAL PHASE

The South Fork lahar (fig. 6) began as the direct 

transformation of a voluminous macroturbulent flow, 

as shown by the continuity of flow deposits and by the 

accompanying textural and behavioral evidence that is 

described in the section on flow transformations. The

flow originated in the first minutes of the May 18 eruption, was initially highly inflated by air and gas, and had 

many of the characteristics of a pyroclastic surge. Flow 

velocities of the surge measured in confined channels 

on the middle slopes of the cone were at least 60 m/s. 1 

As the flow gradually settled and progressively lost air 

and gas, velocity declined to the range of 30-37 m/s.

A comprehensive study of the dynamics of the 1980 South Fork lahar was not made 

because of other concurrent studies on the subject (for example, Fairchild and Wigmosta, 

1983). Most of the measurements given herein were made upstream from those by Fairchild 

and Wigmosta, to illustrate the dynamic changes of the South Fork flow in the zone of rapid 

deflation and change from an air- and gas-mobilized surge to a water-mobilized lahar. (See 

data in section on flow transformation.) The data are compatible with those of Fairchild and 

Wigmosta in the zone of overlap. See the appendix for an explanation of the methods used 

in calculating flow velocity at peak staga
A10 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON
PHASES, FLOW TYPES AND DIVISIONS, AND FACIES MODELS All

Boundary 

approximate

2500 r

2000 -

£ 1500

LLJ

2

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LLJ

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SEA 

LEVEL

500

RAPID CHANGE IN 

FLOW BEHAVIOR

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Confined bedrock chTnne

DEBRIS FLOW 

PROGRESSIVELY 

EVOLVING TO 

HYPERCONCENTRATED 

STREAM FLOW

^^ ""-*« -

Expanding alluvial Bedrock Alluvial 

channel with bedrock gorge channel 

j valley sides______.________

10 20 30 40 

CHANNEL DISTANCE FROM CRATER, IN KILOMETERS

50 60

FIGURE 6. Profile of the South Fork Toutle River from the crater rim to the confluence with the North Fork, showing longitudinal 

phases of the main lahar of May 18, 1980. Thicknesses of mud coatings on trees are shown in millimeters.

Peak discharge attenuated very rapidly, declining from 

1,100,000 m3/s on the middle slopes to 120,000 m3/s 

near the start of the water-mobilized phase, over a 

distance of 3.3 km.

The deposits of both the pyroclastic surge and the 

proximal lahar are characterized by textural variability. Coarse rubbly deposits settled from the surge in canyon bottoms, whereas thin, silty, locally pebble-rich 

deposits accreted to canyon sides at flow depths that 

exceeded 100 m in some places. As water mobilization 

increased downstream, coarse muddy bars formed in 

channels, and muddy peak-flow deposits with sorting 

as poor as 4.4 0 (graphic standard deviation2) coated 

valley sides. The proximal phase of the South Fork lahar 

can be considered to include the deflating part of the

FIGURE 5 (facing page). Uppermost watershed of the South Fork 

Toutle River and tributary Sheep Canyon. Velocity and discharge 

of first May 18 surge and flow (dark deposits) were determined at 

points indicated by arrows. Note light-colored deposits of the lateral 

blast (a), flow tracks of May 18 flows extending to crater rim (b), 

deposits of the second, pumice-rich lahar (light-colored deposits in 

channel) (c), and confluence with Sheep Canyon (d). Photograph by 

Austin Post, May 19, 1980.

surge or, as in figure 6, to include only the part of the 

flow that shows clear evidence of being dominantly 

water mobilized. The boundary between the surge and 

lahar is transitional and is located in figure 6 at the approximate point where water mobilization clearly 

dominated, although much deflation had occurred 

upstream. Both the surge and the lahar removed trees 

and scoured stumps except near the flow margins or in 

protected areas. The effects on vegetation as they pertain to flow behavior are also discussed in the section 

on flow transformations.

MEDIAL PHASE

The medial phase of the South Fork lahar is 

characterized by only slight changes in texture of the 

peak-flow deposits, in a series of channel reaches extending from slightly beyond the base of the cone, 5.5 km 

from the crater, to a distance of 36 km. Flow velocity 

declined from near 30 m/s near the beginning of the 

phase to 7 m/s just beyond the downstream transition

2The statistics of the grain size distributions are described by the graphically derived 

measures of Folk (1980). See the appendix for a fuller description of these measures.
A12 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

to the distal phase. The celerity of the medial-phase 

lahar wave was approximately 7.0 m/s (Cummans, 1981) 

over most of the channel reach upstream from 

Weyerhaeuser Camp 12. Throughout the 30.5-km 

distance traversed by the medial phase, sorting of the 

peak debris flow deposits remains in the narrow range 

of 2.6-3.3 <£, with a mean grain size of 0.7-1.7 <$> 

(0.31-0.61 mm).

The changing character of the flow is illustrated by 

the thicknesses of mud coatings on trees that remained 

standing on the flood plain and in protected areas 

(fig. 6). The increasing thicknesses of these coatings 

downstream do not correspond to a proportional enrichment of mud content in the peak-flow deposits; silt and 

clay make up a relatively uniform 15-20 percent (5 

samples) of those deposits throughout the medial phase. 

The increase in coating thickness reflects the increased 

duration of the lahar flood wave and the lower velocity 

of flow caused by increased hydraulic roughness 

associated with vegetation on the downstream flood 

plain. Trees on the flood-plain surface locally remained 

standing; those at channel margins and in the mainly 

confined reaches traversed by this phase of the lahar 

were either removed or knocked down and scoured into "bayonet" trees (Janda and others, 1981, fig. 278).

The deposits of the medial phase are divided into a 

channel facies and a flood-plain facies, which are progressively more distinct with greater distance from the 

volcano. Also during this phase a coarse basal carpet 

of flow developed, as evidenced by clast-supported bar 

deposits. The whaleback bars formed from the basal 

flow are most abundant in the downstream part of the 

medial phase but extend into the upstream part of the 

distal phase (fig. 6).

DISTAL PHASE

During the final debris flow phase, the rate of change 

of flow texture accelerated as the peak discharge attenuated. The phase began at the head of a broad, expanding reach in which flow volume was progressively 

lost through deposition and in which the rate of flow 

was consequently reduced. Flow velocity declined downstream to less than 7 m/s, measured 0.3 km below the 

start of the distal phase. The celerity of the distal-phase 

lahar wave was about 3.7 m/s (Cummans, 1981) for the 

channel interval between Weyerhaeuser Camp 12 and 

the Coal Bank Bridge.

Deposits of the distal phase show a rapid downstream 

decline in mean size and an improvement in sorting. 

Upstream from the point where transformation to the 

runout phase began, sorting declines (improves) to 2.2 (j> 

or lower, as mean grain size declines to smaller than 

2.3 $ (0.20 mm). The change from distal phase to runout

phase is defined by the beginning of the transformation 

to hyperconcentrated streamflow, at the upstream start 

of the transition interval.

The differences between the channel and flood-plain 

facies are less distinct than in the medial phase. Whaleback bars, with a single exception near the head of the 

distal reach, are not well developed. The flood-plain 

facies is widely distributed throughout the inundated 

area and thus has a high likelihood of preservation in 

the stratigraphic record. The channel facies, however, 

was not well developed, and has been largely removed 

by subsequent stream erosion. Trees marginal to channels were felled, but high-velocity flow was not of sufficient duration to form well-developed "bayonet" trees.

LAHAR-RUNOUT PHASE

The end of debris flow transport in the distal phase 

marked the end of the lahar in the strict sense and the 

beginning of the runout phase. The debris flow debulked 

over at least 9 km at the beginning of this phase, during which the leading edge of the flow was progressively transformed to hyperconcentrated streamflow. The 

transition interval is included in the lahar as defined in 

the general sense, and the deposits are described as the 

transition facies in the section on facies models. The 

hyperconcentrated flow was identified on the basis of 

eyewitness descriptions, evidence of high sediment concentrations from the deposits, and by analogy and 

deposit identity with the runout phase of the lahar of 

March 19-20,1982. Actual sediment concentrations in 

that flow were measured and are described in the section on flow transformations.

The 1980 runout phase extended from the vicinity of 

Harry Gardner Park, just upstream from the confluence 

of the forks of the Toutle River, to the Cowlitz River 

(fig. 2). Flow velocities of the 1980 runout flow were 

probably comparable to those measured directly in the 

1982 runout flow, which were in the range of 4-7 m/s. 

Celerity of the 1980 flood wave between the Coal Bank 

Bridge and the gaging station at Castle Rock was 

3.3 m/s (Cummans, 1981).

As in the 1982 runout flow, hyperconcentrated flow 

ceased upon dilution with streamflow at the confluence 

of the Toutle and Cowlitz Rivers. The deposits recording that change were buried within hours by the North 

Fork lahar, but observers reported (Cowlitz County 

Sheriff's log) that sediment concentrations in the flow 

in the Cowlitz River did not appear high.

Sorting of the massive or crudely stratified runout 

deposits of the South Fork flow (3 samples), as well as 

the March 1982 runout flow (12 samples), is characteristically in the range of 1.1-1.6 <£. Mean grain size of
PHASES, FLOW TYPES AND DIVISIONS, AND FACIES MODELS A13

the South Fork flow deposits is in the range of 1.7-3.3 0 

(0.10-0.31 mm). Another general characteristic of the 

South Fork runout deposits is a concentration at or near 

the flow boundary of abrasion-rounded wood fragments 

transported from the lateral-blast zone (fig. 1). An analogous feature locally present in the 1982 runout flow 

units and some of the ancient runout deposits is pumice 

concentrated at or near the base and flow boundaries 

of the units (the South Fork lahar deposits contain only 

a small amount of eroded older pumice). These and other 

characteristics of the runout-phase deposits are discussed in detail in the section on flow transformations.

FLOW TYPES AND DIVISIONS 

LONGITUDINAL FLOW TRANSFORMATIONS

The evolution from a lahar to hyperconcentrated 

streamflow is the most commonly observed longitudinal 

change of flow type. Recognition of this process increases the accuracy with which individual flows can 

be traced over long distances, and provides evidence of 

upstream lahars from their downstream runout equivalents. Even the largest lahar in the 50,000-year history 

of the watershed (PC 1; Scott, 1985b, in press) showed 

evidence of this same kind of round-trip transformation: 

initial bulking to form a lahar followed by the beginning of debulking in the direction of a runout flow. The 

second largest lahar in the system (PC 3) formed the 

same way and underwent even more complete 

debulking.

FLOW DIVISIONS AT A SECTION BASAL FLOW AND 

THE WHALEBACK BARS

The deposits and boundary features of the South Fork 

and North Fork lahars show the same evidence of a local 

division of flow mode in a vertical profile. The two divisions, described informally as the peak flow and the 

basal flow (table 2), became more pronounced with increasing distance from the volcano (fig. 7). They are 

comparable in geometry and partly in process to the upper part of normal streamflow, containing suspended 

sediment only, and the basal part, which transmits the 

bed load. Coarse clasts were dispersed in the matrix of 

the peak debris flow, like the grains in streamflow 

suspension. Coarse clasts were in greater concentration 

in the basal debris flow, with the clast-to-clast contact 

and interaction with the flow boundary that characterize bed-load transport. The movement of coarse clasts 

in a debris flow by a different mechanism than that 

transporting the remainder of the flow is considered by 

Daido (1971) and Lowe (1979).

TABLE 2. Interpreted characterisitics of at-a-section flow divisions 

in lahars in the Toutle-Cowlitz River system

Characteristic Peak flow Basal flow

Geometry 

Relation to 

peak.

Main part of lahar 

flood wave.

Composes most of 

lahar flood wave 

except for waves 

of basal flow.

Flow typeShear

distribution.

Debris flow--

Erosive

character.

Deposit

framework.

FaciesLahar-bulking 

factor.

Less concentrated 

than in basal 

flow.

Low; flow commonly 

passive on flood 

plains.

Coarse clasts 

dispersed in 

flow or buoyed 

at flow surface.

Forms flood-plain 

facies.

Low; 15 percent in 

deposits of 

North Fork lahar.

Carpet at base of flow, 

mainly in channels; 

variable in thickness.

Moves as kinematic waves 

during rise of lahar 

and near peak stage! 

discharge. Levels of 

bar crests show deposition began near peak

Basal part of debris 

flow, with some characteristics of grain 

flow or noncohesive 

debris flow.

Local extreme concentration at channel boundary, producing laharabraded pavement, 

grain cataclasis, and 

truncation of size 

distributions.

Locally highly erosive.

Coarse clasts in 

contact.

Forms most of channel 

facies (with sole 

layer formed mainly 

during rising stage).

High; 48 percent in

deposits of North Fork 

lahar.

That the basal carpet of coarse clasts was transported 

during rising and peak debris flow discharge is shown 

by the extension of the crests of the clast-supported bar 

deposits to near the level of the peak stage. Basal concentrations of coarse clasts, moving in waves or pulses, 

are indicated by the periodic intervals of sound reported 

by R. L. Dinehart (written commun., 1980) during the 

rise of the North Fork lahar at the Highway 99 Bridge. 

The sound was the same as that of "boulder movement 

in a glacial outwash river." Additional evidence of the 

basal flow is a high content of alluvium bulked into the 

bar deposits and a relatively small amount of alluvium 

present in the peak-flow deposits. Other aspects of the 

behavior of the basal flow are discussed in the section 

on lahar boundary features.

Peak-flow deposits contain dispersed, matrixsupported clasts with a mode in the pebble size range. 

The basal-flow deposits contain clasts with a mode in 

the cobble size range, are mainly clast supported, and 

have an interstitial matrix like that supporting the 

dispersed clasts in the peak-flow deposits. The matrices
A14 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

f* /^^C:^:^*:^ffe>;;|vfril X7^ >"* -, '-fl^ :' ^s<' ' ^'<i"

j" y ^|#>:; 'y'"I>'4^ v:? "N^ .-.- -'STit W% vv^-iLsisspa^ Ci^f^mSmsJ

'\:;Wv'^-- Slope deposits from .--< \-ii * vrj'b* ...

r^a^-»'^^

lT:'/Ii! /(VA '//feSWo -^x^x Tii'ii/////^^i^Vt- ""v < >i

"I ' i '"(fn Glacier surface^^^VV: o ;v,^

' Vi- '. * scoured . * \^^^+- ^ v

T^;;!!-', Glacier surface ^»v,f^V: < 7>v^^ 

^>

p Media/ phasfk

^ ; ^;^ scoured^ij^^.^ ..^^^o,, ^^ do^" Dis^^

*****,* */* "Basal avalanche deposits «7^l*^ >s«-lj^ °-*'° °V^ ^r1" *__ 1 __

»**»*****,** <* ***.**»** <* v* ** Berms and bars » Basal fi* * * ' ^^TV** ^ 1* * * * ~

(muddy matrix) ^\ »^ 9W » Whaleback bars* */ \*» */ h

* * ' *."*»** ^ * *** * *»"*>*'**»*» *»* »(mitddy matrix)* ,** ** sti

', » *. Hyperconcentrated **,**, 

rix)*^ \ « *, *. streamflovy deposits ^^ * \

FIGURE 7. Diagrammatically portrayed origin of the 1980 South Fork lahar as the transformation of a pyroclastic surge.

of both types of deposits are particle supported. The 

locally preserved boundary between the deposit types 

is a discrete, dynamically shaped surface that forms the 

whaleback bars (figs. 6, 7).

The whaleback bars are as much as 150 m in length 

and resemble drumlins, the bars formed by Pleistocene 

floods from breakouts of glacial lakes (Malde, 1968; 

Baker, 1978), and the bars formed by a flood surge from 

a breached dam (Scott and Gravlee, 1968). The bars 

formed by the South Fork lahar (table 3, figs. 8, 9) have 

shapes generally similar in plan view to that of an airfoil, as do drumlins (Chorley, 1959). A degree of lateral 

asymmetry reflects flow deflected by the channel side. 

The crest and the widest point of the whaleback bars 

occur most commonly downstream of the midpoint, 

unlike the crests and widest points of drumlins, which 

are generally up-glacier of the midpoint. In that respect 

the bars are similar to large gravel dune forms observed 

in the Kenai River in Alaska (Scott, 1982, fig. 12). The 

term "whaleback" has been applied to similarly shaped, 

smaller bedrock features formed by glacial abrasion 

(Flint, 1971, p. 97); use of the term as an adjective with 

"bar" yields an apt description of the lahar depositional 

features.

Although the whaleback bars of the North Fork lahar 

were not measured, because of their rapid erosion, they 

were clearly larger and more irregularly shaped than 

those in the South Fork. The main characteristics of the

TABLE 3. Dimensions of whaleback bars formed by the 1980 lahar 

in the South Fork Toutle River

[Shape measurments were taped in reference to the lowest dosed contour line around the 

features. For example, height is the vertical distance from that contour to the crest of 

the bar. Confidence limits at 95 percent level are shown with mean values]

Number of bars measured................. 6

Mean length............................. 79.3±34.1 m

Mean width ............................. 23.4±11.3 m

Mean length/width ratio................. 3.48±0.70

Mean height............................. 2.5±1.0 m

Range of flow depth at peak stage above

bar crest............................. 1.7-14.6 m

Range of percentages of cross-sectional

area of flow, at peak stage, above

the level of bar crest................ 15-70 pet

Position of crest, in percentage of

length from upstream end of bar ...... 6l±10 pet

features were the same, however; this similarity in the 

bars formed by lahars differing in origin and dynamics 

suggests that the features are normal products of large 

lahars extending beyond the vicinity of the volcano. Incipient whaleback bars were present in other channels 

conveying 1980 lahars, but these features were fully 

developed only in the forks of the Toutle River. In both 

forks, full development occurred only after approximately 25-30 km of flow, the distance apparently
PHASES, FLOW TYPES AND DIVISIONS, AND FACIES MODELS A15

needed for the segregation by settling within the flow, 

and for the introduction of enough eroded material to 

create the basal carpet of coarse clasts from which the 

bars were formed. Lahars in other drainages of the 

volcano did not extend this far because of impoundment 

in reservoirs.

The bars consist typically of poorly stratified gravel, 

and most have well-developed normal grading 

throughout most of the unit. In some, the normally 

graded part overlies a massive ungraded zone of 

variable thickness, or a thin, inversely graded basal 

zone, or both. The deposit is mainly clast supported, and 

both texture and structure generally indicate emplacement in a brief time interval. In this respect the forms ' 

are different from the similarly shaped but larger bars 

produced by glacier outburst floods. Those forms, 

described as longitudinal or pendant bars, are commonly internally stratified (Malde, 1968, fig. 5). They form 

downstream from bedrock projections by the accretion 

of foreset strata from pulses of sediment moving as current ripples (Baker, 1978). Among the whaleback bars 

of the Toutle River drainage, multiple overlapping 

pulses of basal flow were preserved only locally along 

the North Fork.

The level of the crests of the whaleback bars, close 

to peak stage, shows that the bars formed near the lahar 

flood peak and locally were developed at the peak. An 

analogy with bar formation during a water flood suggests that the whaleback bars were forming rapidly during earliest recession flow. The upper, normally graded 

parts of the bars of the North Fork lahar culminate in 

a mud-rich surface layer, the cohesive nature of which 

locally prevented erosion from the overriding recession 

flow. Boulders deposited from recession flow on bar surfaces and crests have a much higher proportion of weak, 

hydrothermally altered rock types than those within the 

bars. The bars of the South Fork lahar, although having upper, normally graded zones, do not have muddy 

surfaces; surficial boulders were also deposited locally 

from recession flow.

The basal flow apparently consisted of kinematic 

waves of coarse sediment at the base of the North Fork 

lahar and did not cause major fluctuations in stage (fig. 

2). Small-scale surface wave movement, which may have 

been related to the basal-flow waves, formed steplike 

terracettes 20-30 cm apart. The terracettes are present 

in the upper 3-4 m of peak-flow deposits upstream from 

the confluence of the forks of the Toutle River. The 

features correspond to regular, successively lower 

oscillations or stillstands in stage, which were also 

shown by mudlines inside inundated houses. These early 

recession fluctuations were best developed near Kid 

Valley but were also present on the surface of the debris

avalanche, where incipient bars and berms formed but 

significant pulses in the basal flow were not evident. 

Thus they may represent either fluctuations resulting 

from the anastomosing of flow on the avalanche, or the 

intermittent surging that has been observed in smaller 

scale debris flows. The most suggestive evidence of 

wave movement is the striking similarity in stage fluctuations shown by the features.

Most of the largest whaleback bars were formed in 

unconfined expanding reaches of both the North and 

South Forks of the Toutle River. The thickest bar 

deposits of the North Fork lahar, where the forms locally consisted of several flow units, are in the constricted 

reach at Coal Bank Bridge, where the flow may have 

been slightly ponded. A common denominator of the 

sites of bar deposition is rapid energy loss.

Within the confined channel, the bars occupy positions strictly analogous to those of bars formed by a 

catastrophic flood flow. The bars downstream from 

lateral bedrock projections resemble lobate pendant 

bars (for example, see Scott and Gravlee, 1968, fig. 15), 

in that they have the same morphology and a generally similar internal structure. This similarity supports 

the analogy of the basal flow in the lahars to the 

boulder-size bed load in a catastrophic flood flow. The 

bed load in rivers is a traction carpet of particles supported, according to Bagnold (1973), by dispersive 

pressure from particle collisions. Although clast interaction was intense in the basal flow, the bars locally contain a few clasts of weak, altered rock types could have 

survived extended transport only in the normal dispersed state. Thus the intense clast interaction in the 

flow was mainly a local phenomenon in areas of low 

energy, with clasts moving in contact as grain flow or 

noncohesive debris flow, probably for discontinuous intervals. The areas of lahar-abraded pavement generally coincide with, or lie just upstream from, the coarsest 

bar deposits (reflecting the "grinding to a stop" of the 

basal flow). This association suggests that extreme 

shearing of the channel boundary was only local. Under 

a constant energy gradient, coarser clasts would normally be moved away from the boundary by sizedependent particle interactions, as probably occurred 

during much of the transport. Thus, the basal flow consists of coarse sediment at or near the base of the lahar, 

in which the degree of clast interaction is highly variable. The coarse sediment apparently moved through 

areas of constant energy gradient at least partly in a 

dispersed state, and this was a more common boundary 

condition than was the intense particle interaction 

responsible for the abraded pavement. This conclusion 

explains why the sole layer was preserved in most 

reaches, but not all.
A16 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

FIGURE 8. Longitudinal view of whaleback bar in the South Fork Toutle River 28.4 km from the crater of Mount St. Helens. Flow direction was from right to left. Although normal grading is present in the deposit, the basal third of the cutbank consists of coarse pre-lahar 

alluvium, thereby exaggerating the amount of apparent grading.

LAHAR AND LAHAR-RUNOUT FACIES MODELS

In the dynamic river regime that accompanies and 

follows volcanic eruptions, accurate process-response 

models can be generated with data gathered over time 

spans much briefer than those that separate the major 

channel-forming events. A facies model is a set of 

generalizations derived from comparison of several 

modern and ancient deposits of a particular process or 

environment. Certain depositional units of lahars and 

their associated flows are sufficiently distinctive to 

serve as facies models representing specific flow processes and environments in the river system. The 

characteristics listed in table 4 and figure 10 are based 

primarily on the 1980 lahars in both forks of the Toutle 

River and on the runout flows of the 1980 South Fork 

lahar and the 1982 lahar. The table includes, however, 

only those characteristics that are likely to be preserved 

and which can be extrapolated to older units, based on

extensive observations of older flows at Mount St. 

Helens and other Cascade Range volcanoes.

With the exception of the transition facies, each facies 

is specific to a single flow process or type. The lahar 

channel facies consists of deposits of the basal debris 

flow and includes, locally, a well-developed sole layer. 

The lahar flood-plain facies represents the extensive 

sheets of peak-flow deposits on the flood-plain surface. 

The lahar-runout facies consists of the deposits of 

hyperconcentrated streamflow that evolved directly 

from debris flow. The lahar-related streamflow 

deposits not distinguished as a facies because of their 

similarity to normal flood deposits were deposited 

from streamflow that clearly had less than 40 percent 

sediment by weight (the lower limit of hyperconcentrated flow), based on comparison with 1982 deposits 

of flows with known sediment concentrations.

Although the 1980 North Fork lahar originated by 

dewatering of an avalanche deposit and did not have 

an identifiable runout phase, its at-a-section flow
PHASES, FLOW TYPES AND DIVISIONS, AND FACIES MODELS A17

FIGURE 9. Transverse view of the whaleback bar in the preceding figure, looking downstream. Apparent horizontal stratification reflects

erosional flow lines formed during lahar recession.

divisions and the resulting channel and flood-plain 

fades were closely similar to those of the medial phase 

of the South Fork lahar. The only significant difference 

was in the presence of mud "skins" on the whaleback 

bars of the North Fork lahar. The more common 

mechanisms for generating lahars in the river system 

are flood surge bulking and pyroclastic surge deflation. 

The sedimentologic evidence for this conclusion, based 

on textures of the complete record of lahars in the river 

system (table 1), is discussed in the Conclusions section 

of this report.

LAHAR CHANNEL FACIES

Initially, the channel facies of the 1980 flows was 

widely present in and adjacent to preexisting stream 

channels, but it was mainly eroded from confined 

reaches by 1985. In broad valley areas the post-eruption 

shifting of channels was extensive as streams adjusted 

to the higher post-eruption flow regime (U.S. Army 

Corps of Engineers, 1984), but the vagaries of the lateral

migration allowed the lahar channel facies to be at least 

temporarily preserved. The greater the valley width the 

more likely is the preservation of the facies in the 

stratigraphic record.

Recognition of ancient channel facies permits the 

determination of locations and relative ages of paleochannels that conveyed significant volumes of flow. The 

lateral transition to the flood-plain facies can be seen 

in some exposures, thereby revealing the paleochannel 

configuration. Flow depths and cross sectional areas can 

be determined where the channel facies can be correlated with the peak-flow deposits of the same older 

lahar (see Scott, 1985a, in press). In the North Fork 

Toutle River, channel facies of the older lahars are present in stratigraphic sections in the valley areas between 

the downstream end of the debris avalanche and the 

community of St. Helens, and from the confluence of 

the forks to a point 6.2 km upstream. A lahar of probable Castle Creek age (about 2,000 years old) locally 

forms the basal part of a channel unit that can be traced 

in the vicinity of Camp Baker for about 2 km.
TABLE 4. Facies characteristics of large lahars and associated flows from Mount St. Helens in the Toutle-Cowlitz River system

[Includes flows extending tens of kilometers from Mount St. Helens]

00

Characteristic Lahar facies Transition facies Lahar-runout facies Lahar channel facies Lahar flood-plain facies Lahar-related streamflow deposits

Agent of Debris flow. Basal part 

deposition. of lahar; locally has

coarse clasts in contact 

and some characteristics 

of grain flow and noncohesive debris flow. Sole 

layer accreted at debris 

flow boundary.

Form of top Locally forms whaleback 

surface. bars.

Particles on Wood debris and normalor near and low-density boulders 

surface. on surface or slightly 

embedded.

Particle Mainly clast-supported 

support. with grain-supported 

silty sand matrix.

Grading Normal in upper 10-80

pet of unit; commonly 

ungraded in central 

zone; inverse in basal 20 pet or less.

Stratifi- Poor, except for contact 

cation. with sole layer and 

local strata within 

sole layer.

Modal size Commonly in cobble or 

class. boulder size range.

Sorting 3.0-4.5 6 (main deposit), 

3.1-4.3 6 (sole layer); 

matrix also poorly 

sorted: 2.5 6.

Wood content Uprooted trees form subhorizontal molds in 

older deposits; wood 

rarely preserved.

Sole layer Commonly forms basal 10-25 

cm; compacted and shows 

primary foliation in 

1980 units.

Erosion at Common; local laharbase. abraded pavement.

Debris flow. Peak flow 

of lahar flood wave.

Combined hyperconcentrated Hyperconcentrated streamstreamflow (front of flow; grain flow and/or 

wave) and debris flow noncohesive debris flow 

(tail of wave); debris in upper part of hyperflow deposits overlie concentrated range, 

streamflow deposits in 

a single unit.

Nearly planar Nearly planar- Nearly planarLocal concentrations of 

wood debris, pumice, 

and low-density clasts.

Mainly matrix-supported; 

local clast support in lower part and at level 

of maximum clast size.

Normal in upper 50-75 pet 

of unit; central ungraded zone; basal 15-50 pet inverse (more pronounced 

than in channel facies).

NoneCommonly in pebble size 

range.

2.0-4.5 6 (1.8 6 is

approximate lower limit 

approached near point of 

transition to hyperconcentrated runout flow).

Standing trees form

vertical molds in older deposits; wood rarely preserved.

Poorly developed or 

absent in 1980 flows, 

but common in older 

units.

Wood debris and pumice 

common on surface; lowdensity clasts embedded.

Clast support at base 

transitional to matrix 

support in upper part.

Commonly inverse in most 

of unit; commonly normal 

in uppermost part.

Mostly massive, except 

for local crude stratification in lower part 

and local pumice lenses 

near flow boundaries.

Like lahar flood-plain 

facies in upper part; 

like lahar-runout 

facies in lower part.

Wood preserved in some 

older deposits, but was 

originally less abundant 

than in lahar facies.

NoneWood debris and pumice common on surface; low- density clasts embedded locally. Granular, noncohesive; mainly has openwork structure. Poorly developed; inverse in subunits or in most of unit; grading may be progressively lost downstream. Appears massive, but may be crudely stratified into inversely graded subunits. Pumice layers seen commonly near lat- eral flow boundary and locally at base of unit. Sand size range, commonly 0.2-2.0 «5. 1.1-1.6 6 Wood commonly preserved in older deposits. NoneRare- Common- CommonStreamflow with sediment concentrations mainly below hyperconcentrated range, formed by debulk- ing of hyperconcentrated flow or by erosion of a lahar. Fluvial bar morphology. Local concentrations of wood fragments. Granular and noncohesive except for surficial silt layer. Not distinct. Good. Cross strata common. Silt layer 5-60 cm thick common at top; silt layer may show convolute lamination. Sand size range. Variable, but commonly better than runout depo- sits of the same mean size; as low as 0.7 6 iu fine-grained strata. Wood abundant and well preserved in older deposits; charred wood locally abundant. None. Common, but usually 

slight.

d 2 

o

i

H
PHASES, FLOW TYPES AND DIVISIONS, AND FACIES MODELS A19

Channel facies

with lahar-cut b) with sole 

pavement layer

Whaleback bar

Flood plain facies Transition facies Lahar-runout facies Lahar-related

streamflow deposits

> igb 

Pavement

igb

 Poorly 

developed 

sole layer

igb

igb

Silt with pumice

and charred wood 

Convolute lamination 

Cross-lamination

Angularity in 

fine pebble class

Underlying lahar

lOvJ-'t'V^ Alluvium

FIGURE 10. Portrayal of the facies types described in table 4. igb=Inversely graded bedding.

LAHAR FLOOD-PLAIN FACIES

The lahar flood-plain facies is the most widespread 

of the lahar facies and is formed by relatively uniform 

deposition of the peak debris flow on the modern active 

flood plain. The broad distribution of this facies obviously gives it a high probability of preservation in the 

stratigraphic record. At the Coal Bank Bridge section 

(Scott, in press), in which 15 flows from 4 eruptive 

stages or periods are recorded, 1 flow deposit represents 

the lahar channel facies, 1 is present as both a channel 

and flood-plain facies, and 11 are examples of the floodplain facies. The remaining 2 deposits represent the 

runout facies.

LAHAR-RUNOUT FACIES

Runout flows associated with the 1980 and 1982 

lahars resulted in little overbank deposition. Older 

deposits with the same characteristics are interbedded 

with units of the flood-plain facies in the stratigraphic 

record, as at the Coal Bank Bridge section, showing that 

the runout phases of some older lahars were large 

enough to inundate flood plains more than 80 km from 

the volcano. The characteristics of the modern flows 

that were most definitive for identification of the facies 

in previous eruptive periods were (1) a massive or crudely stratified structure, (2) inverse grading in most of the 

depositional unit, (3) the local occurrence of pumice or 

wood fragments at or near the flow boundaries (contrasting with the concentrations of pumice commonly 

found near the tops of other units), and (4) a texture of 

granular, openwork sand with sorting in the range of 

1.1-1.6 0.

TRANSITION FACIES

The intervals of transition of the 1980 South Fork 

lahar and the 1982 lahar to lahar-runout flows were 

characterized by a basal granular deposit consisting of 

sand and fine gravel, like that of lahar-runout facies, 

commonly grading (inversely) upward to muddier, 

coarser, more poorly sorted debris flow deposits. Similar 

inversely graded transitional units were seen in the 

deposits of previous eruptive periods at Mount St. 

Helens and at other Cascade Range volcanoes. This vertical sequence within a single flow unit apparently 

represents a common, but not exclusive, transition 

mode from lahar to lahar-runout flow in confined channels. The relation of the transition to sedimenttransport dynamics is discussed in the section on flow 

transformations.

A generalized three-part sequence was presented as 

a model of the 1982 sediment flows at Mount St. Helens 

by Harrison and Fritz (1982), but it mainly represents 

the transition facies. Their model consisted of a lower 

unit of gravel, a middle unit of crudely laminated sand 

or granules, and a thinner upper, muddy unit supporting boulder-size clasts. Extensive observations of the 

1982 lahar and runout deposits indicate that the upper 

two units describe the 19-km interval in which the first, 

higher peak of the lahar was transitional to lahar-runout 

flow, and the 9-km transition interval of the second peak 

of the flow. At the farthest upstream locality of Harrison and Fritz, the coarse basal unit consists of gravel 

transported by the March 19-20 flow but artificially 

concentrated behind a sediment-retention dam on the 

North Fork. At their intermediate localities, the basal 

unit of the model sequence commonly consists either
A20 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

of deposits of the channel facies of the 1980 North Fork 

lahar or of normal pre-1980 channel alluvium. Both 

types of deposits underlie the 1982 flows near their 

study sites, commonly with a sharp contact separating 

units 1 and 2, as they observed. Although rare gravel 

bars were formed downstream from channel obstructions and bridge piers during the runout of the 1982 

lahar, the forms are highly localized and do not constitute a uniform basal unit of the flow deposit. The bars 

and isolated coarse clasts correlate with reaches where 

coarse material was derived from highly credible 

dredge-spoil piles. Finally, farther downstream, the 

deposits at the Harrison and Fritz locality on the lower 

Cowlitz River consist of flood deposits formed during 

February 1982.

LAHAR-RELATED STREAMFLOW DEPOSITS

Streamflow deposits directly associated with a lahar, 

other than the hyperconcentrated lahar-runout flows, 

include (1) those of flood surges in the process of 

undergoing bulking to lahars, as in the formation of the 

1982 lahar; (2) those resulting from downstream dilution of a lahar-runout flow, as occurred in the 1980 and 

1982 runout flows; and (3) those formed from 

streamflow during or shortly after lahar recession.

A flow undergoing bulking is by definition mainly erosional, and thus its deposits are unlikely to be widely 

preserved in the stratigraphic record. At Mount St. 

Helens the bulking process generally has been rapid 

throughout the mountain's history because of the abundance of erodible pyroclastic debris. At some other 

Cascade Range volcanoes pyroclastic detritus is relatively scarce, and some flood surges, particularly those 

originating from glacier outbursts or moramal-lake 

breakouts, have bulked only to the point of hyperconcentrated flow. These flows constitute a locally 

significant hazard, and their deposits are preserved in 

downstream valley fills. An example of a flow at Mount 

St. Helens that may not have completed the transformation to a lahar but did attain a sediment content in 

the range of hyperconcentration was the relatively small 

flood surge that resulted from failure of a natural dam 

on the avalanche surface on August 27,1980. Deposits 

of that flow were not preserved. Because no other incompletely bulked flows have been observed at Mount 

St. Helens, a corresponding facies model is not shown 

in table 4.

The downstream deposits representing diluted runout 

flows are like the deposits of normal flood flows, but 

may contain angular particles from the associated lahar, 

charred wood, or uncommonly large amounts of pumice 

(table 4; fig. 10).

ALTITUDE, IN METERS 

O Ui 01I

1

-V

V»*J*

Pre-eruption profile /

I

I I

Streamflow deposits \ / 

Bar Post-eruption profilev.^-

Streamflow deposits ....5 / ^»*.^*/ 

- -x -jlrC* ';« ^ 

Sample K-29// 

Lahar deposits /

I I

) 50 100 150 

HORIZONTAL DISTANCE, IN METERS

I 

_ a

-d 

- c

I

200

FIGURE 11. Cross section showing deposits of the lahar originating 

in the North Fork Toutle River on May 18, 1980, and subsequent 

streamflow. Location is in the lower Cowlitz River near Rocky Point, 

103 km downstream from the 1980 crater. Preemption profile from 

Tudor Engineering; posteruption profile by U.S. Geological Survey. 

The stratigraphic position of a sample discussed in the text is projected to this section, 

a. Peak stage of lahar.

b. Level representing the "runback" of debris flow that initially 

flowed upstream in the Cowlitz River above the confluence with 

the Toutle River during the rising stage, and then during recession reversed direction to flow back down the Cowlitz River, 

c. Level of lahar surface in center of channel when movement

ceased (overriding streamflow continued), 

d. Level of post-lahar fill.

Deposits of streamflow that was either transitional 

with the receding lahar or followed the lahar and contained high concentrations of sediment derived from it, 

were inset against the lateral berms of the lahar 

throughout the lower Toutle and Cowlitz River system 

(fig. 11). Consequently these deposits were largely 

removed by lateral erosion during succeeding storms, 

a process that was accelerated by dredging but would 

have occurred naturally over a period of months. 

Although concentrations in the range of hyperconcentration were measured in streamflow after the North 

Fork lahar (initial measurement in fig. 2) and also after 

the South Fork lahar (streamflow 3 hours after the peak 

stage had a concentration of 1,160,000 mg/L), most of 

the resulting deposits were not like those of hyperconcentrated runout flows. Well-stratified sand, having 

distinctly better sorting than that of the runout 

deposits, composed most of the post-lahar streamflow 

deposits (table 4; fig. 10).

Figure 11 illustrates the geometry of the streamflow 

deposits relative to those of the North Fork lahar in the 

Cowlitz River. Level b represents a recessional stabilization of flow (not recorded by the sporadic stage 

measurements, fig. 2) during which as much as 1 m of 

well-laminated silt and fine sand was deposited. The 

level probably represents the "runback" of the debris 

flow that, during the rising stage, flowed upstream in
SIZE DISTRIBUTIONS OF THE LAHARS A21

the Cowlitz River above its confluence with the Toutle 

River. Reversal of the flow and its addition to the recession stage temporarily stabilized flow at this level 

downstream. Charred wood fragments are abundant in 

the surficial part of the lahar at this level and in the 

sediment deposited from streamflow on the surface of 

the moving lahar. The debris flow flowing upstream during the nearly 2-hour rise (fig. 2) of the lahar would have 

ponded a significant volume of Cowlitz River streamflow that, as the debris flow reversed, would have been 

released to flow across the receding lahar. Velocity of 

the water surge may have exceeded that of the receding 

debris flow. Level d represents the surface of the main 

post-lahar streamflow deposit, which consists mainly 

of crossbedded sand. Level c was the surface of the lahar 

at the time movement ceased, as interpreted from the 

texture of a sample from the top of the lahar, which is 

discussed in the section on peak-flow deposits of the 

North Fork lahar.

None of the above types of streamflow deposits can 

be distinguished texturally as a useful facies model, 

although features such as clast angularity and charred 

wood can identify their association with a lahar. The 

characteristics are shown in table 4 and figure 10 to emphasize differences with the lahar-runout facies.

FACIES OF THE LARGEST LAHAR IN THE RIVER SYSTEM

The channel and flood-plain facies of the older lahars 

are closely similar to the corresponding facies of the 

1980 flows. An exception is the first lahar of Pine Creek 

age (PC 1; Scott, in press), which was so large that the 

broad valley area along more than 10 km of the lower 

North Fork Toutle River was temporarily aggraded to 

a depth of 7-8 m in channels and 2.5 to 5.0 m on flood 

plains. The aggradation was caused by the channel constriction at the head of the gorge beginning just 

downstream from the Coal Bank Bridge. Similar valleywide aggradation by the same flow occurred upstream 

of a constricting gorge near Tower (fig. 1).

Because the large lahar aggraded both the channel 

and flood-plain areas upstream from constrictions, the 

characteristic channel facies resulting from flow 

through and out of channels (leaving only residual bar 

deposits and a sole layer) did not develop at those locations. Channels filled, flow continued, but at least some 

of the channels remained filled as a result of the extensive aggradation. These paleochannel fills are thus a 

variant of the normal channel facies. Figure 12 illustrates the facies of the first and third Pine Creek lahars 

(PC 1 and PC 3) and the 1980 North Fork lahar. The 

similarity of the large channel fills to the typical channel facies is shown by extensive regions of clast support

in the older deposits and the unusually well-developed 

sole layer described here as the "ball-bearing bed." Exposures in other reaches, however, show the normal 

facies distinctions and some extremely coarse graded 

bars that can be correlated with flow PC 1.

SIZE DISTRIBUTIONS OF THE LAHARS

The most widely studied aspect of lahar texture is particle size distribution. It is therefore the best means of 

comparing the flows in the river system with those in 

other areas, but only if the comparison is with the same 

phase and facies. Applications of textural study include, 

for example, inferring the dynamic behavior or origin 

of an ancient flow from the known behavior and origin 

of a modern textural analog.

Lahar deposits containing pebble-size and finer sediment were analyzed by wet sieving of the sand-size and 

coarser parts of the sample and by pipet analysis of the 

silt- and clay-size fractions. Some samples were sieved 

both dry and wet and, except where organic material 

was a significant part of the sample, the results of the 

two methods were generally coincident within 1 percent. 

During 1980 and most of 1981, the 1980 lahar deposits 

remained moist; only in late summer of 1981 did drying begin. To minimize aggregation of fine particles, the 

samples were maintained in a moist state by sealing in 

plastic bags. A sample of the peak flow of the North 

Fork lahar, which contained an amount of silt and clay 

typical of that flow, was split and analyzed both with 

and without a chemical dispersing agent. The resulting 

analyses were identical, indicating both that no significant postdepositional or post-sampling aggregation had 

occurred, and also that the clay was deposited in an 

unaggregated state.

The graphic standard sorting measure of Folk (1966, 

1980) is used to represent sorting, because of its near 

equivalence to the widely reported a^ of Inman (1952) 

and because 016 and 084 can be measured or accurately extrapolated for size distributions analyzed with 

Wentworth class intervals (unit phi intervals). The inclusive graphic standard deviation was calculated for 

comparison whenever possible, but the value of 095 

cannot be readily extrapolated for distributions in which 

much of the sediment is in the coarsest interval. The 

same comments apply to graphic skewness, which was 

used for the reported skewness values, and to the 

calculation of inclusive graphic skewness for comparison. The lahars are so poorly sorted that their particle size distribution commonly ranges over 12 to 14 

classes, which are sufficient to define the size distributions accurately without the use of fractional class 

intervals.
A22 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

FLOOD PLAIN FACIES

DEPOSITS OF 

MAY 18, 1980

Forest litter

CHANNEL FACIES

Crest of whaleback bar 

'Boulderon bar

-Mud-rich "skin"-upper 

part of graded unit

.gb

 : ~- Poorly developed 

\ sole layer 

l\ Lahar-cut pavement

}igb

 Sole layer of North 

Fork lahar

Second basal flow deposits of 

North Fork lahar

First basal flow deposits of 

North Fork Lahar

South Fork lahar and runout flow 

(absent on flood plain)

DEPOSITS OF

PINE CREEK

ERUPTIVE PERIOD

VERTICAL SCALE

IN METERS

3-

EXPLANATION

gb Normal graded bedding 

igb Inverse graded bedding

gb

Trunk molds 

}'gb

gb

Megaclasts of hydrothermally altered 

dacite breccia (da)

igb

"Ball-bearing bed" 

Runout sand

Runout sand 

Buried stumps: 

Date 2510±40 

radiocarbon years

PC4 final lahar in sequence 

0.0-1.5m

PC3 second largest lahar of 

Pine Creek age

PC2 locally pumice-rich lahar, 

0.0-2.0m

PC1 largest lahar of Pine Creek age

Alluvial sand with cross-bedding, 

interbedded with runout sands 

surrounding stumps that were 

the source of limb and trunk 

molds in the overlying lahar

FIGURE 12. Channel and flood-plain fades of the 1980 North Fork lahar at the Coal Bank Bridge, and of the lahars of Pine Creek age near 

the Green Mountain Mill, based on sections exposed between 1980 and 1983, 3.8 km upstream from confluence with the South Fork. 

Stratigraphy of associated deposits is also shown. Plotting of Pine Creek unit thicknesses as seen over the time interval of exposure 

leads to a total thickness greater than that present at any point and an apparent exaggeration of the paleochannel depth. The relative 

magnitude of the ancient and modern flows is not apparent because of the influence of differing valley width on deposit thickness.

Except for separate analyses of the matrix in coarse, 

clast-supported deposits, all the size distributions are 

from samples large enough to be representative of the 

specific deposit. The size distributions of deposits too 

coarse for laboratory analysis were measured in the field 

using grid sampling and the point-counting technique 

of Wolman (1954). Although this procedure yields frequency by number in a given class, the results are 

directly comparable with frequencies by weight as 

determined in normal laboratory analyses (Kellerhals

and Bray, 1971). A point count and a matrix analysis 

thus can be combined to obtain the total distribution 

of a very coarse deposit.3

3Procedure used for obtaining a complete cumulative curve for coarse samples: During 

the field point count, the frequency of particles larger than -1 <t> (2 mm) was recorded, and 

a grab sample of matrix was collected that needed only to be large enough to yield the accurate relative proportions of particles smaller than -1 <t>. Point-count frequencies were plotted 

first; then the remaining distribution in the classes smaller than -1 <t> was proportioned to 

accommodate the relative quantities determined from the sieve and pipet analyses. The 

assumption of near uniformity of matrix over the area of grid sampling is probably valid. 

(See subsequent discussion of critical diameter.)
SIZE DISTRIBUTIONS OF THE LAHARS A23

NORTH FORK LAHAR PEAK-FLOW DEPOSITS OF 

THE FLOOD-PLAIN FACIES

The bulk of the flood wave of the large North Fork 

lahar consisted of a mudflow whose deposits ranged in 

silt and clay content from 19 percent in upstream 

reaches to 28 percent in downstream reaches. Total 

sand, silt, and clay content meets the Varnes (1978) 

criteria for a mudflow and ranges from 62 percent on 

the surface of the debris avalanche to 88 percent in the 

Cowlitz River near its confluence with the Columbia 

River. Although many dispersed, mainly pebble-size 

clasts are clearly visible in the deposit, sand and silt are 

the dominant constituents. The deposits look muddy 

but are only slightly cohesive, as documented by a low 

clay content of 3-5 percent. Even this amount of clay 

is high in comparison with the content of nearly all other 

lahars in the Toutle River system.

The North Fork lahar did not transform to a laharrunout flow, but it did undergo gradual, progressive textural changes downstream (fig. 13). These correspond 

in trend to the more rapid changes in other lahars, such 

as the South Fork and 1982 lahars, as they approached 

the transformation to lahar-runout flows. During the 

more-than-90-km course of the flow from its origin on 

the surface of the debris avalanche, mean grain size 

decreased from approximately 0.3 0 (0.81 mm) to 2.3 0 

(0.21 mm). Correspondingly, sorting improved progressively from approximately 4.4 to 2.9 0, a range 

typical of mudflow deposits reported in the literature. 

The other lahars in the river system transformed to 

runout flows as sorting reached values of 2.0 0 or less.

Skewness showed a consistent downstream trend 

from slight negative skewness (excess coarse material) 

to slight positive skewness (excess fine material). This 

change corresponds with the gradual downstream loss 

of bimodality in the distribution; the coarse mode drops 

out, as shown in the histograms in figure 14. Positive 

skewness and a unimodal size distribution in the floodplain facies can thus be regarded as potential indicators 

of distance from source in lahars of this textural type.

Kurtosis showed no pronounced trends; values were 

in the range of 0.95 to 1.09, indicating general Gaussian normality of the size-distribution curve in terms 

of peakedness the relative amount of sorting in the 

tails of the curve to that in the central part. A tendency toward slightly higher values of kurtosis downstream reflects the loss of the coarse peak in the bimodal 

distribution. The histograms in figure 14 illustrate this 

gradual downstream change.

The downstream textural change (fig. 13) is an example of the minimum amount of change expectable in 

older lahars in the river system; greater rates of change 

are expectable in flows with a lower clay content. The

K29

1.0

0.25 g

0.125

5.0

. 4.0 

O

^ 3.5

EC

o on <o J-°

2.5 

0.5

3 n 

<o 0

*

10

K29

K29

K29

-K29

Uce 40 

OT £ 

z 30

(-£ 10

5555 0

-K29

o o oooooooo o 

r- CM m 9 10 oa r»ooo»o o

CHANNEL DISTANCE FROM CRATER, IN KILOMETERS

FIGURE 13. Particle-size distribution characteristics of peak-flow 

deposits of the 1980 North Fork lahar: downstream changes in 

mean size (graphic mean, Mz), sorting (graphic standard deviation, ffG), skewness (graphic mean, SkG), percent silt, percent 

clay, and ratio (in percent) of silt-clay to sand-silt-clay. Sample 

K 29 is discussed in the text.

changes occurred at a relatively constant depth in the 

flow; samples were collected 1.0-1.5 m below peak stage. 

The three histograms inset in figure 14 illustrate the 

size distributions of deposits at varying levels of flow 

in a partly destroyed house on the flood plain. The 

samples represent three levels within a single, generally synchronous deposit rather than separate deposits 

formed successively during a rising stage or a recession 

sequence. They indicate that the coarser fractions are 

concentrated with depth to form the characteristic 

bimodal size distribution. Mean size changed from 

2.18 0 (0.22 mm) at a depth of 0.1 m, to 2.00 0 (0.25 mm) 

at 1.9 m, and to 1.21 0 (0.44 mm) at 3.8 m. Concomitantly, sorting worsened with depth from 3.06 0 to 3.26 0
0.01

0.05 

0.1 

0.2

0.5

1

2

5

10

20

30

40

50

60

70

80

90

95

98

99

99.8

99.9

99.99

I I I T 

Vertical change at km 62.4

DEPTH 15 

10

0.1 m

1.9m

15r

-Pebbles-

-Sand- -Silt-

-5 

32

-4 

16

-3 

8

-2

4

1 2

0.5 0.25

SIZE

345

0.125 0.0625 0.031

6

0.016

km 103.3

-Clay

10 

5 

0

7

0.008

8

0.004

15

10

5

0

15

1

5

0

15

10

H 5

* »

o 15

K 10

UJ

Q. 

0

15

1

1

1

1

1

90 

0.002mm

FIGURE 14. Cumulative curves and histograms of particle sizes in the peak-flow deposits of the North Fork lahar. Phi classes 9 an

clude sediment finer than phi class 8 proportioned by projecting cumulative curves. Channel distance is in kilometers from 
SIZE DISTRIBUTIONS OF THE LAHARS A25

and to 3.68 0. The changes in a vertical profile of flow 

are thus as pronounced as the changes over many tens 

of kilometers of flow at the same depth. The change in 

size distribution with distance between km 47.8 and 

km 105.6 (at a depth of 1.0-1.5 m) and the change 

with depth, between 3.8 and 0.1 m, at a point, are similar 

(fig. 14).

The histograms (fig. 14) show that both the vertical 

and longitudinal changes involve primarily the gravitational settling of sediment coarser than a critical 

diameter, which corresponds approximately to the upper boundary of the sand size range (-1 0; 2 mm). This 

settling of gravel in a sandy matrix is reflected in welldeveloped coarse-tail grading in the upper portions of 

the flood-plain facies. The flood-plain facies of the older 

lahars commonly also show normal grading reflecting 

the same critical diameter. Crandell (1971, p. 6-7) likewise has documented coarse-tail grading that mainly 

reflects variation in the amount of sediment coarser 

than sand.

The successive cumulative curves also show the settling of the coarser fractions (fig. 14). The steepening 

slope of the successive curves reflects the gradual but 

continuous improvement in sorting as sediment sinks 

in the flow. The temporary loss of a coarse size fraction, 

such as the -4.0 to -5.0 0 (16-32 mm) class in the sample at km 47.8, reflects the randomness of sampling 

larger clasts that may be very widely dispersed in the 

flood-plain facies and may consist of low-density rock 

types.

The downstream size decline in the main part of the 

flow is thus almost entirely due to the debris flow 

equivalent of hydraulic or selective sorting the cause 

of most downstream size decline in coarse fluvial 

sediment moved by a single catastrophic flood flow 

(Scott, 1967). The coarse material settled from the 

peak debris flow to incorporate in the basal flow and 

then was deposited in bars. Conversely, little eroded 

material was introduced to the peak flow from the basal 

flow, as indicated by the lahar bulking factors discussed in the section on clast roundness. Substantial 

volumes of eroded alluvium were introduced to the basal 

flow but were not further dispersed into the peak flow. 

Part of the longitudinal size decline in the basal flow 

was due to cataclasis, as discussed in the roundness 

section.

During dredging in the lower Cowlitz River near Kelso 

the contact between the mudflow deposits and the 

overlying streamflow deposits was temporarily exposed, and a sample (K 29) of the mudflow was obtained 

from the center of the channel (fig. 11). The texture is 

essentially the same as that of the peak-flow deposits 

along the channel boundary in the same reach, with

most size classes containing the same amount of sediment and no class differing by more than 2 percent by 

weight. This near identity in texture indicates that the 

mudflow in the center of the channel was part of the 

peak flood wave that continued for a short distance 

downstream during recession and was deposited at a 

level 3.6 m lower than the peak stage.

NORTH FORK LAHAR BASAL-FLOW DEPOSITS 

OF THE CHANNEL FACIES

The bars in or adjacent to preexisting channels are 

coarser than the flood-plain deposits and commonly 

have a clast-supported framework. Where not clearly 

whaleback in shape, their geometry is nevertheless 

similar, with rounded forms predominating over flattopped forms; they have formed generally on the 

downstream side of obstacles and in point-bar and 

backwater areas. Although the bars are highly variable in texture, both laterally and vertically, 

measurements at the coarsest part of each bar show 

that grain sizes do not decline between the debris 

avalanche and the vicinity of the confluence of the 

forks of the Toutle River. This finding concurs with 

similar observations by Gilkey (1983). In the main 

Toutle River below the gorge beginning at the Coal 

Bank Bridge the bar deposits were significantly finer 

than in upstream reaches and were not measured. Below 

the junction of the Toutle and Cowlitz Rivers the lahar 

was confined to the main river channel (fig. 11) and the 

basal deposits could not be observed except as dredge 

spoils.

Clasts in the size fraction of -10 to -11 0 (1024- 

2048 mm) were the coarsest observed. Mean size determined by field measurements at seven locations ranged 

between -3.1 and -4.4 0 (9-21 mm). Sorting ranged between 3.1 and 4.5 0, and values for five of the seven 

samples were between 3.1 and 3.8 0.

The upper 10 to 80 percent of each bar deposit, and 

of the analogous deposits in the stratigraphic record, 

is normally graded (fig. 15). The upward rate of size 

decline commonly increases sharply in the upper 0.2- 

0.5 m, culminating in a muddy surface layer in bars of 

the North Fork lahar. Bars of the South Fork lahar exhibit normal grading but do not have this surficial layer. 

Analysis of the layer near Kid Valley at a depth of 0.1 m 

reveals a silt and clay content of 15 percent, a mean size 

of 0.01 0 (1.0 mm), and sorting of 3.64 0. By comparing 

a separate matrix analysis with a field measurement of 

the coarsest part of the same bar, the silt and clay content of the entire bar can be accurately estimated as 

5 percent.
A26 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

FIGURE 15. Bar deposits of the North Fork lahar at the confluence of the forks of the Toutle River. The light-colored clast to the right 

of the shovel and the large clast to the immediate left of the shovel base consist of hydrothermally altered lithologies. The thickness 

of the section is 2.1 m. Note the high degree of rounding in several boulders of introduced stream alluvium.

NORTH FORK LAHAR SOLE LAYER OF 

THE CHANNEL FACIES

Unlike the overlying clast-supported deposits of the 

channel fades, the coarse clasts in the sole layer are 

dispersed. Pebble-size clasts protrude from the eroding 

surfaces of the semiconsolidated matrix to give a 

characteristic "clast-studded" appearance (fig. 16).

Mean size in nine longitudinally spaced samples 

ranges between 1.4 and -2.1 <$> (0.38-4.4 mm); sorting 

from 3.2 to 4.3 <f>. No significant downstream trend is 

present in either measure, as is the case for the overlying bar deposits. This lack of trend is in sharp contrast 

with the trends in the peak-flow deposits (fig. 13). No 

significant downstream change is evident in the total 

silt and clay (mud) content, which ranges from 11 to 21 

percent, or in the clay content, which ranges from 2 to 

5 percent. Compared with the peak-flow deposits at or 

near the same location, the sole layer is most commonly 

coarser but is generally similar in sorting. The difference

between the respective cumulative curves shown in 

figure 17 is typical.

The sole layer locally has crude stratification and color 

variations that correspond with variations in texture. 

In the lower of the two units forming the thick sole layer 

in figure 18, the central part has a mean size of -0.47 <j> 

(1.38 mm) and sorting of 4.29 </>; color of the band is a 

pale bluish gray. The middle part of the overlying unit 

has a mean size of 0.05 <f> (0.98 mm) and sorting of 3.22 </>; 

color is a light grayish brown. Each unit of the sole layer 

at that point shows symmetrical grading a normally 

graded upper part that is nearly a mirror image of the 

inversely graded bedding in the lower part.

All samples of the sole layer have a size distribution 

with at least two modal classes, and in all but one sample the coarsest mode is dominant (fig. 17). The coarse 

mode is mainly in the -3 <f> (8-16 mm) or -4 <f> (16- 

32 mm) size classes; the finer mode is mainly in the 3 <t> 

(0.125-0.25 mm) or 2 </> (0.25-0.5 mm) intervals. Compared with the distributions of the peak flow shown in
SIZE DISTRIBUTIONS OF THE LAHARS A27

FIGURE 16. Channel fades of the North Fork lahar below Kid Valley. The resistant, light-colored sole layer overlies rounded pre-lahar 

alluvium and is overlain by darker, clast-supported bar deposits that are the fine-grained tapering edge of a whaleback bar to the left 

of the photograph. The sole layer averages 20 cm in thickness and surrounds protruding alluvial boulders at this locality.

figure 14, the variation from class to class is more 

irregular.

The histograms in figure 17 illustrate the typical 

"truncated" size distributions of the sole layer, in which 

the coarse tail is poorly developed and skewness is 

positive. Unlike the downstream trend toward positive 

skewness in the peak-flow deposits, skewness in the sole 

layer does not correlate with distance. The skewness of 

the size distributions shown as histograms in figure 17 

ranges from -0.08 to +0.84. Skewness of the sole layer 

size distribution shown as a cumulative curve is +0.10.

SOUTH FORK LAHAR PEAK-FLOW DEPOSITS OF 

THE FLOOD-PLAIN FACIES

The peak flow of the South Fork lahar did not undergo 

the gradual changes described for the North Fork 

flow, because of the uniform confined, bedrock channel 

that conveyed the medial phase of the lahar at high 

velocities. Nevertheless, the basal flow did segregate

and form bars, including whalebacks near the downstream end of the medial phase. Textural change in the 

peak-flow deposits was largely concentrated in the proximal and distal phases of the lahar and in the rapid 

transition to lahar-runout flow at the end of the distal 

phase (fig. 19). In gross appearance the deposits of the 

peak flow of the lahars in each fork are very similar, 

however.

The South Fork peak-flow deposits were initially 

highly variable, inheriting the turbulent character of the 

deflating pyroclastic surge. As the water-mobilized 

phase dominated, the rate of change decreased. 

Throughout the medial phase the mean size ranged between 0.7 0 (0.61 mm) and 1.7 <£ (0.31 mm) and sorting 

varied between 2.6 and 3.3 <f>. The mean size is comparable to that of the North Fork lahar after a similar 

interval of flow, but the sorting is about 1.0 <£ better. 

The better sorting is in part a function of a lower silt 

and clay content relative to the North Fork flow 11-20 

percent silt and 2-3 percent clay in the South Fork
A28 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

0.05 

0.1

0.2

0.5

1

H 2

2 5

tu

0 10 

cc

uj 20

°- 30

40

> 5° 

- 60

^ 80

S 90

3

<-> 95 

98

99.8

99.9

99.99

I I \ \ \ \ I I \ I I I I

Bar deposits

Peak flow deposits

Cobbles- -Pebbles- Sand- Silt- -Clay

-9 

512

-8 

256

-7 

128

-6 

64

-5 

32

-4 

16

-3 

8

52

30

20

10

r

Longitudinal change in sole layer

km 27.7

-2

4

km 63.4

0 

1 

I Z E

1 

0.5

2

0.25

345

0.125 0.06250.031

6 7

0.016 0.008

8

0.004

90 

0.002 mm

km 81.4

(upper

part)

0 5 100

1 0.031 0.001 mm

-505 100 

32 1 0.031 0.001 mm

FIGURE 17. Cumulative curves and histograms of particle sizes in 

the sole layer, bar, and peak-flow deposits of the North Fork lahar 

near Kid Valley. The size distribution of the bar deposits is based 

on field measurement of clasts larger than -1 </> (2 mm) and a separate 

matrix analysis of smaller particles. The histograms illustrate 

downstream variation in the size distributions of the sole layer of 

the North Fork lahar.

versus 15-24 percent silt and 3-5 percent clay in the 

North Fork.

With the accelerated deposition caused by the entrance of flow into the expanding valley of the lower 

South Fork, mean size decreased and sorting improved 

significantly in the distal phase. Mean size dropped 

from approximately 1.0 0 (0.5 mm) to 2.2 0 (0.22 mm), 

and sorting approached 2.0 0. Cumulative curves in 

figure 20 show these changes reflected in progressive 

loss of the pebble and sand size fractions. The continuation of this trend triggered the transformation to laharrunout flow, seen in both the sequential cumulative 

curves and the histograms (fig. 20).

Skewness oscillated without a notable trend from 

small negative values to small positive values, in the 

range of -0.10 to +0.36 (fig. 19). Kurtosis also showed 

no trend, with values ranging from 0.93 to 1.25.

The consecutive histograms in figure 20 illustrate the 

overall textural change from deflating pyroclastic surge
SIZE DISTRIBUTIONS OF THE LAHARS A29

FIGURE 18. Sole layer of the North Fork lahar at the Highway 99 Bridge, showing the layer composed of two subunits. The top of the 

shovel handle marks the contact between the 52-cm-thick sole layer and the underlying runout deposits of the South Fork lahar. Shredded 

wood from the blast zone is concentrated at the base of the lahar-runout deposits, overlying well-stratified pre-lahar alluvium. Deposits 

overlying the sole layer have been eroded. (Side of bridge pier forms background at top.)

to lahar to lahar-runout flow. Coarse material that was 

locally introduced to the peak flow in upstream reaches 

subsequently settled out as flow competence lessened 

during the distal phase. A noteworthy characteristic of 

the entire flow is the consistency of the modal class in 

the 2 0 (0.25-0.5 mm) interval. Only in downstream 

reaches does a shift to the 3 <j> (0.125-0.25 mm) class 

begin. Where the distribution is bimodal, the finer mode 

is represented by the 2 0 class, even in the pyroclastic 

surge deposits of Sheep Canyon shown in the three inset histograms in figure 20.

SOUTH FORK LAHAR AVALANCHE DEPOSITS

OF DEFLATING PYROCLASTIC SURGE AND 

BASAL-FLOW DEPOSITS OF THE CHANNEL FACIES

The avalanche and bar deposits both represent basal 

segregations of coarse material, from the deflating 

pyroclastic surge for the avalanches, and from the 

downstream water-mobilized lahar, with additions from

eroded alluvium, for the bars. The avalanche deposits 

differ from the lahar bar deposits in having a rubbly 

character and a smaller amount of muddy matrix, in 

spite of similar mean size and sorting (fig. 21) as determined from pebble counts. The field relations of the two 

types of 1980 deposits show them to be distinct, 

separate types and not the result of a continuous basal 

zone of flow. Much larger older avalanches that may 

have been analogs of the 1980 avalanches did continue 

downstream as lahars.

The avalanche deposits show no obvious trends in 

grain size or sorting, in part because of the difficulty 

in correlating separate flow units; the bar deposits show 

a general downstream size decline and improvement in 

sorting (fig. 21). The avalanche deposits lack a welldeveloped muddy matrix and consequently show only 

moderately positive skewness, in the range of -0.09 to 

+0.42. The bar deposits have a muddy matrix and show 

positive skewness; skewness of the bar deposits in 

figure 21 is in the range of +0.35 to +0.55.
A30 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

+0.5

0

-0.5

Distal 

phase,

0.125

i i i i r

i I i i I

5 

n

i i i i i i i i i i i i i i i i i 

CL r^ -*"i <"*- __ r> no

1 1 I 1 1 1 I I 1 I 1 1 1 I I I I

3 456789 10 20

DISTANCE FROM CRATER, IN KILOMETERS

30 40 50 60 708090100

FIGURE 19. Downstream changes in mean size (graphic mean, Mz), sorting (graphic standard deviation, CTG), skewness (graphic skewness, 

SkG), and percent clay in the peak-flow deposits of the 1980 South Fork lahar.

The lahar bulking factor discussed in the roundness 

section indicates that significant amounts of stream 

alluvium were eroded throughout the medial phase of 

the South Fork lahar. Incorporation of alluvium is also 

indicated by the local coarsening seen in the peak-flow 

deposits in the medial phase (fig. 20) and in bar deposits 

that are coarser than upstream avalanche deposits 

(fig. 21).

SOUTH FORK LAHAR SOLE LAYER OF 

THE CHANNEL FACIES

The sole layer at the base of the South Fork lahar is 

closely similar to that of the North Fork lahar. It is a 

feature of the water-mobilized part of the flow; it was 

absent in the proximal phase and developed gradually 

over the initial 10 km of the medial phase. It was not 

observed as a discrete layer closer than about 12 km 

to the crater.

Mean size in seven longitudinally spaced samples 

ranges between 1.6 and -1.3 </> (0.33-2.4 mm); sorting 

from 3.1 to 3.9 </>. These values are similar to those of 

the sole layer of the North Fork lahar, and likewise show 

no significant downstream trends. In terms of mean 

size, the sole layer is commonly coarser than the peakflow deposits at the same point, as in most of the North 

Fork lahar deposits. The South Fork sole layer is, 

however, significantly more poorly sorted than the 

peak-flow deposits at the same point, by an average of 

nearly 1 </>.

All the South Fork sole-layer size distributions, like 

those of the North Fork, have at least two modes. The 

two dominant modes represent similar proportions of 

the sample; only in the farthest downstream sample 

does the coarse mode dominate, as is typical of the 

North Fork sole layer. (Compare fig. 17.) The modal 

classes of the South Fork sole layer are the same as
0.01

0.05 

0.1 

0.2

0.5

1

2

5

10

20

30

40

50

60

70

80

90-

3

0 95

98

99

99.9

99.99

Sheep Canyon 

40

Lahar-runout flow at km 63.4

Lahar at km 43.4

-Pebbles -

-Sand- -Siltkm 37.2

-5 0 5 100 

32 1 0.031 0.001 mm

South Fork

km 3.5

Pyroclastic 

surge

Lahar

-5 0 5 100 

32 1 0.031 0.001 mm

-6-5-4-3-2-101 2 3 4 5 6 7 8 90 

64 32 16 8 4 2 1 0.5 0.25 0.1250.0625 0.031 0.016 0.008 0.004 0.002mm

SIZE

FIGURE 20. Cumulative curves and histograms of particle sizes in the distal phase of the South Fork lahar and the lahar-runout flow derived from t

successive size distributions in the South Fork and Sheep Canyon.
A32 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

nIiiiirr

Avalanches

128

64

32

16

8

D G 

ODD

Avalanches Bars

2 3 4 5 6 7 8910 20 30 40 50 

CHANNEL DISTANCE FROM CRATER, IN KILOMETERS

FIGURE 21. Downstream changes in mean size (graphic mean, Mz) 

and sorting (graphic standard deviation, <t>G) of the avalanche and 

bar deposits of the South Fork pyroclastic surge and lahar.

those typical of the distributions of the North Fork sole 

layer. The coarse mode is either the -3 <£ (8-16 mm) or 

-4 <£ (16-32 mm) class; the finer mode is either the 3 <j> 

(0.125-0.25 mm) or 2 0 (0.25-0.5 mm) fraction. Skewness 

is in the neutral range of -0.22 to +0.16, lacking the 

tendency toward positive values seen in the size distributions of the North Fork sole layer.

LAHARS OF PREVIOUS ERUPTIVE STAGES 

AND PERIODS

The stratigraphic record of older lahars (table 1) is 

dominated by the flood-plain facies. The size distributions (table 5) are similar to those of the 1980 flows in 

the dominance of sand and the relatively small amount 

of silt and clay, as compared with most mudflows in 

alpine and semiarid areas and with some large lahars 

such as the Osceola and Electron mudflows at Mount 

Rainier (Crandell, 1971, tables 2 and 10). The range in 

silt content is 3.3 to 15 percent; the range in clay content is 0.5 to 3.3 percent. Only three lahars, however, 

have a total silt and clay content of clearly more than 

10 percent in the main body of the flow unit; the others 

exceed 10 percent only in the uppermost, muddier parts 

of some graded units.

A large older lahar that is exceptional in both silt and 

clay content is the lahar of Smith Creek age (unit 1 at 

Outlet Creek, unit 16 at the Coal Bank Bridge; Scott, 

in press). The unit contains 14.4 percent silt and 3.3 percent clay at Outlet Creek. This lahar is the only large 

flow that could be traced down the Toutle River and

that is texturally similar to the 1980 North Fork lahar. 

(A small lahar, also of Smith Creek age, has the same 

texture.) The similarity extends to the size, rounding, 

and proportion of coarse clasts. Like the 1980 North 

Fork flow, the large Smith Creek lahar does not show 

the rapid textural change associated with the transformation to runout flow.

The ratios and proportions of sand and finer sediment 

in older lahars (table 5) are quite consistent, whereas 

the proportions of gravel-size clasts differ greatly. The 

critical diameter separating these phases is, as in the 

1980 lahars, -1 <£ (2 mm). The matrices in the clastsupported bar deposits and in flood-plain deposits with 

widely dispersed clasts are nearly identical.

The flood-plain facies of the older flows (table 5) are 

compared here to 1980 peak-flow deposits (figs. 13 and 

19) collected from valley side slopes, not directly from 

the flood plain, because the modern flood-plain deposits 

were poorly exposed in the early days of this study. The 

validity of this comparison was investigated by comparing the valley-side deposits with the well-developed 

flood-plain facies of the 1980 flows once exposures 

became available. At six sites, three in each fork of the 

Toutle River, the amount of clay in the 1980 flood-plain 

deposits was slightly less than in the valley-side 

deposits. The difference, in each case a fraction of 1 percent in clay content, generally validates the comparisons of the two types of peak-flow samples. Relative 

to most of the older lahars, the 1980 North Fork flow 

is unusual in its content of fine sediment, and the 1980 

South Fork flow has a content of fine sediment near the 

upper limit typical of the older flows.

LARGE LAHAR OF THE PINE CREEK ERUPTIVE PERIOD

This huge lahar (PC 1; Scott, in press) is composed 

mainly of rounded cobbles and pebbles in a silty sand 

matrix and contains from about 1 to less than 2 percent clay. Mean size of the channel facies shown in 

figure 20 is -3.70 <£ (12.8 mm), sorting is 3.62 <£, and 

skewness is +0.55. In the flood-plain facies at the same 

location, mean size is -0.25 <t> (1.2 mm), sorting is 3.60 <£ 

and skewness is -0.25. The relations of the two facies 

of this flow are shown in figure 12.

"BALL-BEARING BED"

The layer of concentrated marble-size pebbles at the 

base of the first Pine Creek lahar (fig. 12) contains less 

than 1 percent clay. It is distinguished by a mode in 

either the -4 to -5 <£ (16-32 mm) or -3 to -4 <£ (8-16 mm) 

class; the coarser class is more common where the layer 

forms the base of the channel facies, and the finer class
TABLE 5. Textures of selected pre-1980 lahars and lahar-runout flows in the Toutle-Cowlitz River system

[See Scott (in press) for stratigraphic sections at localities listed here. Analyses based on sieve analysis of sand and gravel and pipet analysis of silt and clay unless otherwise indicated. Leade

Thickness c n ... 

Flow and locality Unit 1 of unit SamP^ [£fj tlon 

(m)

Third lahar of Pine Creek age 

Green Mountain Mill 

(flood plain facies).

Outlet Creek 

First lahar of Pine Creek age 

Green Mountain Mill:

Outlet Creek 

Runout flow of Smith Creek 

or Pine Creek age at 

Outlet Creek.

Lahar of probable Smith 

Creek age:

Outlet Creek 

Runout flow of Smith Creek 

age at Coal Bank Bridge.

Lahars of Swift Creek age 

at Coal Bank Bridge.

(PC 3): 

4 2.2

7 3.0 

(PC 1):

2 2.3 

4b 6.0 

4a 1 .1

5b 4.8

4b 

0.5 

4a

1 >1 .2 

16 3.2

15 1 .0

12 1 .4 

9 1 .2

Middle 

0.2 m

Middle 

Basal 

Middle

0.4 m 

Coarse 

0.15 m

Silt-r 

part 

0.2 m

0.9 m 

0.6 m

Middle

0.5 m 

0.4 m

FLOWS

above base

2.5 m 

below top -- 

medial part 

above base

FLOWS

ich upper 

above base 

above base 

FLOWS

above base 

above base 

Mean size, M,, ,_, 

z Sorting

<f> (mm)

OF PINE CREEK AGE

-0.31 (1.2)

-2.49 (5.6)

-0.25 (1.2) 

2 -3.70 (13) 

-1.92 (3.7)

-1.29 (2.4) 

3 -3.67 (13) 

-0.75 (1.6)

OF SMITH CREEK AGE

5.35 (0.024) 

2.10 (0.23)

1.22 (0.42) 

2 -2.95 (7.6)

1.13 (0.45)

OF SWIFT CREEK AGE

0.71 (0.60) 

-1.85 (3.5)

ffG

3.10 

3.33

3.60 

3.62 

2.67

3.18 

3.60 

3.97

1 .46 

1 .60

3.10 

4.10

1.51

2.28 

3.00

Skewness 

SkG

0

1 Q

+0.58

-0.25 

+0.55 

+0.61

+ 0.67 

+0.55 

-0.13

+0.08 

+0.39

+0.11 

+0.25

+0.29

+0.17 

+0.63

Content in

Sand

55.0 

26.0

51 .2 

17.0 

23.7

29.8 

43.2

16.2 

80.7

58.2 

52.1

91.5

62.6 

24.7

Silt

5.7 

3.3

8.4 

4.4 

3.5

7.7 

7.5

76.0 

15.3

14.4 

6.9

6.0

9.7 

5.2

pet

Clay 

0.8 

0.8

1 .2 

0.6 

0.5

1 .8 

1.3

7.8 

2.7

3-3 

0.9

0.8

0.9 

0.5

Rubbly lahar of Swift Creek 

age at Coal Bank Bridge. 2

Lahar of probable Swift 

Creek age at Coal Bank 

Bridge (channel facies). 3

1 .0

>3.5

0.3 m below top

Upper half-

-1.06 (2.1) 4.38 +0.36 52.0 13-8 1.4

-3.64 (12) 4.20 +0.45

'As designated in Scott (in press). 

Combination of field pebble count of 

3 Field pebble count.

coarse fractions (>~1 <t>, 2 mm) and sieve and pipet analyses of finer fraction
A34 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

2

5

10

20

30

40

50

60

70

80

90

95

98

99

99.8

99.9

99.99

-10 -505 100 

102432 1 0.031 0.001 mm

I I I l

-10 -505 100 

1024 32 1 0.0310.001 mm

-9 -8-7-6-5-4-3-2-10 1 2 3456 7 8<P 

512 256 128 64 32 16 8 4 2 1 0.5 0.25 0.1250.0625 0.031 0.016 0.008 0.004mm

SIZE

FIGURE 22. Cumulative curves and histograms of particle sizes in the channel and flood-plain facies of the first lahar of Pine Creek 

age (PC 1), the "ball-bearing bed" at the base of that lahar, and alluvium in a megaclast (see fig. 24) included in that lahar. The 

size distributions of the lahar channel facies and the alluvium are based on field measurements of clasts larger than -1 <j> (2 mm) 

and separate matrix analyses of smaller particles. The megaclast is located near Pullen Creek; the lahar deposits are near the Green 

Mountain Mill.

is more common where the layer thins on valley side 

slopes. More than 50 percent (and as much as 67 percent) of the subunit is characteristically concentrated 

in one (the finer) or in both of these classes, which explains its striking appearance. The similarity of the 

layer to the sole layers of both main 1980 lahars in the 

watershed lies in the fact that the coarsest class present may also constitute the modal class and that the 

modal class is one of two classes forming the mode of 

the 1980 sole layers. The chief textural difference is that 

the clasts of the coarse mode in the "ball-bearing bed" 

are commonly in contact; whereas the coarse clasts of 

the sole layer of both 1980 lahars are dispersed.

The size distribution in figure 22 is from the locality 

illustrated in figure 23. Mean size is -1.92 0 (3.7 mm), 

sorting is a surprisingly high 2.67 0, reflecting the 

presence of a matrix like that of the rest of the unit, 

and skewness is +0.61. The cumulative curve of the 

"ball bearing bed" (fig. 22) and those of the 1980 sole 

layers (fig. 17) have similar shapes and similar relationships to the cumulative curves of the overlying channel facies.

The prevalence of broken and abraded clasts in the 

"ball-bearing bed" indicates an origin partly by 

crushing. The distributions do not conform to Rosin's 

law, thereby suggesting that shear-induced sorting also 

played a major role in the unusual size distribution. The 

truncated, one-tailed distribution seen in the histogram 

in figure 20 is discussed in the section on boundary 

features.

ALLUVIUM SOURCE OF SEDIMENT IN LAHARS OF THE 

PINE CREEK ERUPTIVE PERIOD

A megaclast incorporated in the first lahar of Pine 

Creek age near Pullen Creek contains five stratigraphic 

units of which the second (A in fig. 24) consists of 

coarse, undisturbed alluvium that was probably the 

source of most of the sediment in the lahar. The 

megaclast itself is rounded, and the roundness of the 

alluvium is compared with that of the clasts in the 

enclosing lahar in the section on roundness. The 

stratigraphy of the mass is discussed in the companion 

volume (Scott, in press).
CLAST ROUNDNESS AND THE LAHAR BULKING FACTOR A35

FIGURE 23. The "ball-bearing bed" at the base of the first lahar of Pine Creek age (PC 1). The bed is transitional to the overlying channel 

facies of the lahar at this locality, near the Green Mountain Mill. The flow line near the top of the shovel handle reflects peak 1980-81 

streamflow in the adjacent channel.

The size distribution of the coarsest alluvium in the 

megaclast is generally similar to that of the sediment 

in the lahar (fig. 22). The alluvium became mobilized 

with the addition of only 4 percent silt and clay: the 

alluvium contains 1 percent silt and clay; the lahar channel facies contains 5 percent. The visual distinction 

resulting from the differing percentage of fine sediment 

is pronounced in the flood-plain facies but is not obvious 

in clast-supported parts of the channel facies.

CLAST ROUNDNESS AND THE 

LAHAR BULKING FACTOR

Particle roundness (r, the ratio of the average radius 

of curvature of particle edges to the radius of curvature 

of the largest inscribed sphere) is a useful means of 

distinguishing lahars from other gravel deposits.

Mullineaux and Crandell (1962) noted that the clasts in 

lahars of the Toutle River system are more angular than 

those in alluvium at a comparable distance from the 

volcano. Their figure 1, however, mainly compares 

originally rounded, broken clasts of stream alluvium 

bulked into lahar PC 3, with similarly rounded but unbroken clasts of stream alluvium. Abrasion processes 

in lahars normally are reduced through the partial 

cushioning of particle-to-particle contact by the viscous 

debris flow matrix; in the granular lahars of the ToutleCowlitz River system abrasion locally proceeded to the 

point of cataclasis.

Roundness varies significantly with size. Large particles become round with the least amount of transport; 

boulders can be abraded from angular (r<0.15) to 

subrounded (r=0.25-0.40) and even rounded (rX).40) in 

less than 3 km of macroturbulent transport by a flood 

surge in a mountain stream (Scott, 1967). The same
A36 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

FIGURE 24. The large egg-shaped unit is a megaclast, 5X12 m, in the first lahar of Pine Creek age (PC 1). The mass protrudes about 1 m 

above the surrounding upper contact of the flow, which is overlain by 2 other lahars of Pine Creek age. The coarse, immediately overlying lahar (PC 3) is the second largest of the eruptive period. The depositional surface in the foreground is the lahar of March 19-20, 

1982. The Pine Creek-age terrace surface is approximately 9-10 m above the flood-plain surface at this locality in the North Fork Toutle River just upstream from Pullen Creek. The stratigraphic units in the megaclast include alluvium (labeled A).

sizes and types of particles should be measured for a 

valid comparison. The pebble-size fractions of -3 to -4 0 

(8-16 mm) and -4 to -5 <j> (16-32 mm) were selected 

because those size classes are well represented in both 

the finer grained parts of the lahars, such as the peakflow and sole-layer deposits, and the coarser grained bar 

deposits. Each of the reported roundness values is the 

mean of at least 50 measurements made by comparison 

with outlines of particles of known roundness.

Because scoriaceous particles round more rapidly 

than associated rock types, they were excluded from the 

analyses of the 1980 flows to make the values more comparable to those of the older flows. Scoriaceous andesite 

and basalt are present in the modern cone of Mount St. 

Helens, but are absent from rocks, and thus from lahars, 

of Pine Creek and pre-Pine Creek age. Pumice was also 

excluded. It is present in the 1980 North Fork lahar and 

the second South Fork lahar but is essentially absent

from the main South Fork lahar and from many of the 

older flows.

ROUNDNESS IN THE 1980 LAHARS

Clasts with a significant degree of angularity are 

typical of most of the 1980 lahar deposits. The exceptions are the basal-flow deposits of the channel facies 

of both the North Fork and South Fork lahars. They 

include clasts that are better rounded than those in the 

peak-flow deposits of the flood-plain facies. In the North 

Fork lahar near the confluence of the forks (fig. 25), the 

difference is between subangular pebbles in the floodplain deposits and subrounded pebbles in the bars, 

based on the particles in the -5 to -6 $ (32-64 mm) 

range. The contrast is less pronounced in the finer sizes.

Particles with roundness greater than 0.25 are stream 

polished over at least part of their surfaces and represent
CLAST ROUNDNESS AND THE LAHAR BULKING FACTOR A37

0.8

0.7

0.6

0.5

Q 0.4

0.3

0.2

0.1

Pre-lahar alluvium

Largest lahar of Pine Creek eruptive period

Flood plain deposits of 1 980 North Fork lahar

WELL ROUNDED

SUB- ANGULAR

-1 to -2 

2-4

-2 to -3 

4-8

-3 to -4 

8-16

-4 to -5 

16-32

-5 to -6 

32-64

-6 to -7 

64-128

-7 to -8 

128-256

-8 to -9 

256-512

-9 to -10 

512-1024

-10 to -110 

1024-2048 mm

FIGURE 25. Mean roundness in pebble, cobble, and boulder size classes of the first lahar of Pine Creek age (PC 1), in alluvium from 

a megaclast included in that lahar, and in bar and flood-plain deposits of the North Fork lahar. Lahar localities are near the confluence of the forks of the Toutle River; the megaclast is that shown in figure 24. Confidence limits are at the 95 percent level.

stream gravel bulked into the flow. The channel fades 

at the confluence of the forks (fig. 26E) contains 20 percent particles with roundness of 0.25 or more in the -3 

to -5 0 (8-32 mm) range. Only 6 percent of the corresponding sizes in the flood-plain facies at that location 

are that rounded (fig. 26D). Such particles expectably* 

increase downstream in each type of deposit but remain 

significantly more abundant in the bar deposits.

BULKING FACTORS OF THE 1980 LAHARS

The lahar bulking factor (LBF) is the proportion of 

sediment that can be shown to have been introduced 

by erosion during flow, as opposed to that present in 

a lahar at its point of origin. The LBF was determined

from roundness data as follows: All sediment in the -3 

to -5 0 (8-32 mm) interval with roundness above 0.25 

was assumed to have been derived from the stream 

channel. To this quantity were added (1) the obviously 

broken particles (with partial surfaces of stream rounding) LQ the 0.0-0.15 and 0.15-0.25 roundness categories 

and (2) some remaining clasts in those categories with 

surfaces produced entirely by cataclasis during flowconservatively estimated as equal to the percentage of 

obviously broken alluvial clasts. Applying this reasoning to the North Fork lahar, LBF values are 48 percent 

for the channel deposits (fig. 26E) and 15 percent for 

the flood-plain deposits (fig. 26Z)). These values apply 

only to the -3 to -5 <j> (8-32 mm) interval, representing 

the coarsest material that is common in the flood-plain
A38 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

fades but an intermediate component of the channel 

facies. Based on roundness variation with size, an LBF 

of more than 48 percent would apply to the entire channel deposit, and a value of less than 15 percent would 

apply to the flood-plain facies.

40

30

o 20

10

m

40 r

o d o o d o o 

A Alluvium in mass in largest 

lahar of Pine Creek eruptive 

period

32.3 percent 

broken

79.4

50

40

o 30

20

10

0

8.0 percent 

broken

b o

B. Channel deposits of largest 

lahar of Pine Creek eruptive 

period 

40 r

o o o d 

D. Flood plain deposits 

of 1980 North Fork 

lahar 

50 r

21.9 percent 

broken

o o o o o 

C "Ball-bearing bed" £. Channel facies of 1980 

North Fork lahar

FIGURE 26. Distribution of clast roundness in the -3 to -5 </> 

(8-32 mm) size interval. Locality A is at the confluence of the 

North Fork and Pullen Creek, B and C are near the Green 

Mountain Mill, and D and E are at the confluence of the forks 

of the Toutle River. The proportion of broken particles in each 

roundness class is shaded.

The flood-plain deposits of the South Fork lahar 

record a more rapid increase in roundness of the constituent particles than in the North Fork lahar (fig. 27). 

This difference indicates a greater LBF in the peak flow 

of the South Fork lahar, probably due to a less distinct 

segregation of the basal-flow and peak-flow divisions 

and a consequent greater mixing of eroded clasts 

throughout the South Fork flow, which had a greater 

velocity than the North Fork flow. In the North Fork 

lahar, moreover, the mud "skin" on the surfaces of the 

whaleback bars may have acted as a protective cohesive 

surface during recession flow. The lack of such a surface on bars of the South Fork lahar may have permitted erosion of early bar deposits, thus introducing 

a greater proportion of alluvial clasts into the peak flow.

ROUNDNESS IN THE PRE-1980 LAHARS

The two largest lahars of Pine Creek age (PC 1 and 

PC 3) contain pebbles and cobbles that are better 

rounded than clasts in the coarse mode of other pre-1980 

lahars. The degree of roundness is striking but is 

somewhat less pronounced when the expectable variations in roundness with particle size are considered. 

Table 6 compares roundness in the two flows of Pine 

Creek age with roundness in flows of similar texture in

TABLE 6. Clast roundness in pre-1980 lahars exposed near the confluence of the forks of the Toutle River

[Roundness=ratio of average radius of curvature of particle edges to radius of curvature 

of the largest inscribed sphere. Letters in parentheses denote locations of sections, as 

follows: a, Harry Gardner Park; b, Coal Bank Bridge; and c. Outlet Creek. Confidence limits 

are at 95 percent level]

Size class

in i 

(and mm)

Lahars of Pine Creek age

Largest flow 

(PC 1)

Second largest 

flow (PC 3)

Lahars of 

Swift Creek

and Smi th 

Creek ages

-6 to -7 0.61±.12 (a) Unit contains 

(64-128) .56±.12 (c) few cobbles.

-5 to -6 

(32-64)

-4 to -5 

(16-32)

-3 to -4 

(8-16)

-2 to -3 

(4-8)

.511.15 (a) 

.491.13 (c)

.301.076 (a) 

.28±.074 (c)

0.521.10 ( 

.471.069 C 1 )

.401.054 (c)

.21±.034 (a) .191.037 (c) 

.17±.029 (c)

.18±.025 (a) .13±.022 (c) 

.12±.016 (c)

0.33±.046 (b) 

.41±.062 (b)

.291.042 (b)

.331.052 (b)

.341.050 (c)

.211.035 (b) 

.251.045 (b) 

.241.041 (c) 

.231.030 (!)

 14±.018 (b) 

.121.015 (b) 

.151.024 (c)

.12±.015 (b)

.111.013 (b)

.121.015 (c)

Larger sample from locality c, above.
CLAST ROUNDNESS AND THE LAHAR BULKING FACTOR A39

0.3

0.2

0.1

f

Bars

Flood plain layer

South Fork lahar

10 20 30 40 50 60 70

CHANNEL DISTANCE FROM CRATER, IN KILOMETERS

80 90 100 110

FIGURE 27. Clast roundness in the -3 to -5 <j> (8-32 mm) size interval of the sole layer, bar deposits, and flood-plain deposits of the 

1980 North and South Fork lahars. Confidence limits are at the 95 percent level.

the Swift Creek eruptive stage and Smith Creek eruptive period. All the flows in these two intervals that 

have a cobble or coarse-pebble modal class are included. 

The coarser the size fraction that is compared, the more 

statistically valid are the roundness differences. The 

clasts in the first Pine Creek lahar (PC 1) are significantly better rounded than those in either the channel 

or flood-plain facies of the 1980 North Fork lahar 

(fig. 25).

BULKING FACTORS OF THE PRE-1980 LAHARS

Roundness data show that at least 69 percent of the 

-3 to -5 0 (8-32 mm) size range in the main body of the 

first lahar of Pine Creek age originated as eroded stream 

alluvium (fig. 2QB). A similarly high degree of rounding 

is present in the "ball-bearing bed" (fig. 26C), but some 

of this rounding reflects grinding action during lahar 

transport. The resultant high LBF's are minimum 

values; it is probable that many of the particles in the 

0.0-0.15 roundness class were produced by the complete 

crushing of alluvial clasts during transport and thus 

that more than 90 percent of the sediment was added 

to the flow by erosion.

The dominance of introduced particles in the main 

body (channel facies) of the first Pine Creek lahar contrasts somewhat with the intermediate percentage of 

such particles in the bar deposits (channel facies) of the 

North Fork lahar, but a more apt comparison would be 

with the low proportion of stream alluvium in the North 

Fork flood-plain deposits, since they represent the main 

body of that flow. The huge older lahar is thus composed of sediment bulked into a flood surge from the 

stream channel, whereas the North Fork lahar consists 

mainly of sediment already in the lahar at its point of 

origin, or eroded from the debris avalanche surface by 

the lahar. Although LBF's were not calculated for the 

second largest Pine Creek lahar (PC 3), the roundness 

values (table 6) indicate an alluvium content similar to 

that of the larger flow, as do qualitative observations 

of roundness in the other two flows in the series (PC 

2 and PC 4).

A high LBF defined by rounding is not a feature of 

all older lahars formed by the bulking of eroded sediment in streamflow; it applies only to those in which the 

bulking occurred in a stream channel beyond the base 

of the volcano. Lahars transformed from streamflow on 

the slopes of the volcano or on the surface of a debris 

avalanche cannot be readily identified in this way.
A40 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

ROUNDNESS IN THE SOLE LAYERS

The coarse clasts dispersed in the sole layers of the 

1980 lahars were a population of angular particles to 

which quantities of rounded particles were sporadically introduced from the stream channel. Mean roundness 

fluctuated downstream without an overall trend (fig. 27) 

as alluvial clasts were incorporated, broken, or dispersed 

upward in the flow. The particles in the sole layers of 

the 1980 lahars are more angular than those in the 

overlying bar deposits (fig. 27), reflecting the greater 

textural affinity of the sole layer to the peak-flow 

deposits than to the bar deposits.

The near-boundary concentration of shear during 

deposition of the sole layer is expressed by a higher 

percentage of broken clasts in the sole layer than in the 

peak-flow deposits. Such clasts typically have broken 

faces with roundness less than 0.15 but also have a part 

of the earlier unbroken surface, which has a roundness 

of 0.40 or better. Sole layers of both the North Fork and 

South Fork lahars contain 8-36 percent broken clasts 

in the -3 to -5 <£ (8-32 mm) range, as compared to the 

flood-plain deposits which have a relatively uniform 0-9 

percent broken particles in that size range. The bar 

deposits contain 5-25 percent broken clasts.

The "ball-bearing bed" at the base of the first lahar 

of Pine Creek age likewise contains a greater proportion of broken clasts than does the main part of the flow 

(fig. 26). The highest proportion of broken clasts in any 

size class of any lahar was observed in this unit (50 percent of the -4 to -5 <£, or 16-32 mm, class). This size 

class is commonly the modal class of the bed and is the 

class above which the coarse tail of the size distribution is truncated. The high percentage of broken clasts 

of this size suggests that destruction of larger clasts 

by cataclasis may explain the unusual size distribution.

ABRASION AND CATACLASIS IN LAHARS

The roundness of lahar sediment is determined by the 

roundness of the original constituent particles, the bulking of rounded stream gravel, and two simultaneous 

processes within the flow that yield opposing results. 

These processes are intergranular abrasion especially 

in the basal-flow carpet and in clast-supported sole 

layers like the "ball-bearing bed" and the breakage of 

particles at and near the flow boundary. Shear stress 

is concentrated in a narrow vertical zone, in which the 

particles grind, rotate, or slide against each other rather 

than roll or saltate as in normal bed load.

The longitudinal trends in roundness show that values 

in the body of the lahar generally are more sensitive

to the introduction of alluvium than to the intralahar 

processes. Exclusive of the effect of particle introduction, the net effect of the debris flow process, in which 

dilatant fluid behavior is evident, may be to decrease 

roundness by intense cataclasis near the base of the flow. 

Roundness probably decreases in bedrock reaches where 

little bulking occurs, as the rounded clasts already in 

the flow are fractured and crushed.

A degree of rounding caused by abrasion and cataclasis was evident from roundness measurements, 

especially on particles from the "ball-bearing bed." 

Hackly surfaces on some particles were formed both by 

abrasion and by the fracturing of angular corners, with 

the net result of increasing sphericity and thus, to some 

extent, of increasing roundness. In such cases, a roundness measurement became a judgment based on the 

scale of the observations the crude overall rounding of 

the particle versus the hackly, smaller scale relief. The 

higher the magnification at which the particles were 

viewed, the more the angular surface relief influenced 

the roundness determinations. Intralahar abrasion was 

concomitant with continuing clast breakage. The "ballbearing bed" recorded the extremes in both effects, and 

the uniformity and roundness of its clasts are partly due 

to abrasion and cataclasis during lahar transport. This 

applies particularly where the unit is best developed and 

exceeds 1 m in thickness. The effects of these processes 

combine with those of the shear-induced sorting that 

was the primary cause of the truncated size distribution and the concentration of marble-size clasts, as 

described in the section on boundary features.

The identification of lahars in stratigraphic sections 

away from the volcano was generally unequivocal, even 

if based only on the contrast in rounding with associated alluvial units. Cobble-size clasts in the lahars of 

Pine Creek age that consist mainly of eroded alluvium 

are as well rounded as those in alluvium (fig. 25), and 

those units could be misidentified at incomplete exposures. Clast support and poor grading in the lahar 

would add to the difficulty. However, a distinctive 

feature of the lahars, which is apparent in even a small 

sample, even for lahars formed mainly of stream 

alluvium, is the roundness variation with size; specifically, the relative angularity of the granules and the fine 

pebbles. Despite a high degree of roundness in coarser 

fractions (fig. 25), clasts in the finer pebble size classes 

are much more angular in the main bodies of the lahars 

formed from alluvium than in any fluvial deposit in the 

river system, including lahar-related streamflow 

deposits (table 4). A distinctive degree of angularity in 

that size range probably is produced in granular lahars 

by cataclasis even if the sediment is derived entirely 

from alluvium.
CLAST LITHOLOGY A41

40

30

20

10

40

30

20

10

\ I \ i i

South Fork lahar Andesite plus basalt

i i

i i i i r

North Fork lahar

i i i i i

8 9 10 20 30 40 50 60 70 80 90 100

CHANNEL DISTANCE FROM CRATER, IN KILOMETERS

200

FIGURE 28. Downstream variations in the combined percentage of pyroxene andesite and olivine basalt in the peak-flow deposits of 

the South Fork and North Fork lahars and in the percentage of pumice in the peak-flow deposits of the North Fork lahar.

CLAST LITHOLOGY

The degree to which the 1980 lahars incorporated 

stream alluvium during flow also can be estimated 

from downstream changes in clast lithology. The construction of the modern cone of Mount St. Helens, 

which began about 2500 years ago, was marked mainly by a change from domes of hornblende-rich dacite 

composing the older volcanic center to cone-building 

lava flows of pyroxene andesite and olivine basalt 

(C. A. Hopson, Univ. California Santa Barbara, unpub. 

mapping, 1980; Crandell, 1987). Rock types abundant 

in the modern cone thus are absent from the lahars and 

alluvium of Pine Creek age and older. These older 

deposits were locally eroded by the 1980 lahars, thereby 

diluting the quantity of andesite and basalt in the 1980 

flows by an amount proportional to the amount of erosion. Moreover, with the exception of some lahars of 

Castle Creek and Kalama age, the deposits of post-Pine 

Creek age in the river system away from the volcano 

also contain fewer rock types from the modern cone 

than were present in most of the 1980 lahars at their 

origin. Therefore, the erosion of these deposits also 

reduced the amount of andesite and basalt in the 

1980 flows.

PYROXENE ANDESITE AND OLIVINE BASALT

, Both dark rock types (andesite is more common) contrast with the fine-grained gray dacite composing the 

Kalama-age summit dome of the pre-1980 cone and with 

the porphyritic pink and gray hornblende-bearing 

dacites and andesites of the older volcanic center. The 

rock types are readily distinguished in the pebble size 

fractions (fig. 28). The trends indicate little bulking of 

the North Fork lahar but substantial bulking of the 

South Fork flow. Erosion that occurred during passage 

of the North Fork lahar was not enough to decrease the 

percentage of pyroxene andesite and olivine basalt 

significantly in the peak-flow deposits. The roundness 

data indicate that the North Fork basal flow eroded 

substantial sediment but that it was not dispersed into the peak flow in significant amounts. In contrast, the 

addition of sediment to the peak flow of the South fork 

lahar reduced the andesite-basalt percentage by approximately 42 percent (from 35 to 20 percent) between the 

base of the cone and the confluence of the forks. A 

similar trend in the peak-flow LBF is reflected in the 

roundness data.

Measurements of downstream change in composition 

and roundness by Gilkey (1983) showed no significant
A42 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

trends in the South Fork bar deposits, possibly because 

the data were obtained from the entire bar deposit and 

thus may have been influenced by downstream change 

in particle size. Major (1984) distinguished significant 

compositional differences according to particle size in 

the 1980 lahars in watersheds south of Sheep Canyon. 

Data in figure 28 reflect the compositional changes in 

a constant, intermediate size range (-3 to -5 <£; 

8-32 mm), but these changes could be influenced by differential rates of clast breakage related to composition.

PUMICE

Pumice content provides a logical means of identifying lahars formed in association with pumiceous 

pyroclastic flows. The main South Fork lahar originated 

with the initial explosion on May 18 and contains no 

pumice from the 1980 eruption. The second South Fork 

lahar was the result of interaction of pyroclastic flows 

with snow and glacial ice; it contained, before its erosion, 65 percent pumice by weight in the -3 to -5 <£ 

(8-32 mm) interval at the base of the cone and more 

than 90 percent pumice in that size range at the confluence of the forks. The downstream increase in pumice 

reflects the progressive deposition of the normal-density 

constituents. More than 99 percent of the pumice in the 

flow at the base of the cone was from the 1980 eruption, as was 97 percent of the pumice in the flow 

downstream.

The North Fork lahar also formed after the Plinian 

eruptive phase began on May 18, but the areas of lahar 

formation on the debris avalanche were mainly outside 

those affected by the northeast-directed air fall. Of the 

small amount of pumice in the flow, more than 50 percent is of 1980 origin. The remainder was derived by 

erosion of older tephra units. The changes in pumice 

content of the peak-flow deposits are plotted in figure 

28. The downstream increase can be ascribed to gradual 

upward flotation of the lighter particles. In a house inundated by the North Fork lahar near Tower, a sample 

0.1 m below the peak stage contained 19 percent 

pumice, and samples 1.9 and 3.8 m below the peak stage 

contained negligible amounts.

LOW-DENSITY ROCK TYPES FROM DOMES

Some dome rock types older than that forming the 

Kalama-age summit dome consist of dacite lava or breccia with a density sufficiently low for clasts to have been 

concentrated at the lahar surface. Boulders of dome rock 

as large as 1.6X0.8X0.6 m protruded from the berm surfaces of the North Fork lahar in the Cowlitz River channel. The most distinctive dome rock is that of the

cryptodome intruded in early 1980. The juvenile 

microvesicular dacite has a mean density of 1.66 (Hoblitt 

and others, 1981). It is present in all the lahars of May 

18, including those formed outside the zone of the lateral 

blast, with the exception of the second South Fork lahar 

and its analogs in other watersheds. The distinctive 

bluish-gray, prismatically jointed rock type is obvious 

as cobbles and boulders on berm surfaces, even though 

it composes no more than 2 percent of the peak-flow 

deposits of both the North Fork and South Fork lahars. 

The 1982 lahar contained a larger amount of modern 

dacite, which was mainly concentrated in the coarser 

pebble fractions. In the upper, debris flow portion of the 

transition facies above Kid Valley, the dacite locally composes 45 percent of the pebble size class. The dacite 

locally forms most of the clasts embedded in the surface of the flow deposit, as opposed to the pumice 

deposited on the surface by dilute recession flow. The 

dacite was derived from erosion of the 1980 lateral-blast 

deposits and of the 1982 dome; the pumice had been 

eroded from 1980-81 pyroclastic flow deposits.

JUVENILE DACITE DERIVED FROM FACIES OF 

THE DEBRIS AVALANCHE

The color variations in subunits of the sole layer of 

the North Fork lahar (fig. 18) reflect materials derived 

from different parts of the debris avalanche. The initial slumping of material is believed by Harry Glicken 

(oral commun., 1985) to have occurred in an area of 

the avalanche in which a block facies is overlain by a 

matrix facies and, in turn, by a surficial unit consisting 

of as much as 5 m of lateral-blast deposits rich in 

juvenile dacite. In one of the first flights over the 

avalanche, Glicken observed lahars ponding (but not 

flowing downstream) in that area before noon on May 

18,1980. The bluish-gray color of the lower unit of the 

sole layer is derived from comminuted juvenile dacite 

mixed in that earliest part of the lahar. The subsequent 

and main source area of the lahar is interpreted by 

Glicken to have been a more distal part of the avalanche, which consisted of mixed block and matrix facies 

and no surficial blast deposits. The upper, brownishgray unit of the downstream sole layer was accreted 

from the later portion of the lahar wave, derived from 

that source.

ERODED SOIL

The finer grained parts of the main South Fork lahar 

have a more brownish cast than those of the North Fork
LAHAR BOUNDARY FEATURES A43

lahar. The runout deposits of the South Fork flow are 

also distinctively tan or light brown. The brown coloration is present in the proximal-phase slope deposits on 

the flanks of the volcano and thus may represent the 

scour of thin alpine soils by the macroturbulent 

pyroclastic surge. Soil was eroded locally along the 

course of the North Fork lahar, and clasts of soil were 

locally deposited on bar surfaces. The amount of included soil was, however, insufficient to color the main 

deposits of that flow.

LAHAR BOUNDARY FEATURES

The amount of shear stress concentrated at or near 

the boundaries of the lahars appears to vary greatly. 

Surfaces underlying the flood-plain facies generally 

show little erosion; undisturbed forest litter is locally 

covered by the matrix-supported deposits. The channels 

that conveyed sustained flow show a highly variable 

amount of boundary shear. Locally the sole layer, which 

is a characteristic of the channel facies where it is best 

developed, rests on easily credible sand; the sole layer 

of the North Fork lahar, for instance, locally overlies 

deposits of the South Fork lahar-runout flow (fig. 18), 

which were freshly deposited about nine hours before. 

Little erosion of the runout sand occurred. At other 

localities, however, boulders in the preexisting stream 

alluvium were truncated in place by the action of clasts 

in the overriding flow.

LAHAR-ABRADED PAVEMENT

In places, dense volcanic boulders were cut at the flow 

boundary of the North Fork lahar, producing a surface 

similar to a glacial pavement cut in conglomerate. 

Although the surface was not grooved or polished, the 

surfaces of individual boulders were sharply truncated, 

apparently by grinding and impacts from boulders in 

the basal-flow carpet. A similar surface was formed between two pulses of the basal flow of the North Fork 

lahar at the confluence of the forks of the Toutle River 

(fig. 29). The lahar deposits in which that surface was 

formed are finer grained than the alluvium but contain 

some boulders, which were truncated like those locally 

forming most of the main flow boundary.

The lahar-abraded pavement and the sole layer were 

observed in close lateral association at the base of the 

bar deposits but were not seen stratigraphically superimposed. The pavement directly underlies lahar bar 

deposits without a sole layer, and at those points the

compaction that is characteristic of the sole layer has 

been imposed on the preexisting deposits. The deposits 

above and beneath the surface shown in figure 29 are 

texturally similar; the difference in credibility, which 

allowed the deposits overlying the surface to be stripped 

away, is due to the compaction of the underlying 

deposits by the flow that formed the surface.

The lahar-abraded surface occurs only (1) where it is 

directly overlain by bar deposits that have a coarse 

mode in the boulder size range, or (2) at localities that 

are less than several hundred meters upstream from 

bars containing boulders. This correlation indicates that 

cutting occurred beneath the basal flow wherever intergranular stress was applied as a grinding action on 

the flow boundary. Pressure compacting the substrate 

was sufficient to hold boulders in the unconsolidated 

alluvium while they were being truncated.

The sheared surfaces at the base of the North Fork 

lahar were observed within 1 to 2 m of the altitude of 

the channel thalweg in reaches where the peak flow was 

in the range of 8 to 12 m in depth and the slope was 

0.006. Where the surface was formed between two 

pulses of the basal flow of that lahar, maximum flow 

depth above the surface was only 3.5 m at a slope of 

0.0045. The relatively low values of boundary shear at 

the more shallow depth indicate that the pavement cutting is not just a function of shear, but also is related 

to the coarseness of the basal sediment load under conditions of rapid energy loss. This is evident from the 

more common boundary condition (a sole layer overlying credible sediment) where the calculated boundary 

shear was an order of magnitude higher than in some 

reaches having pavement. Where the sole layer was 

preserved, in channel reaches of approximately constant 

energy gradient, dispersive stress in the inversely 

graded boundary layer kept the coarse material away 

from the boundary.

As indicated by its spatial distribution, stratigraphy, 

and compaction, the sole layer generally was formed 

early in the flow and then was locally eroded by the 

pulses of basal flow that cut the pavements. That the 

concentration of shear and the degree of basal-flow clast 

interaction were extremely variable is also shown by the 

local inclusion of weak clasts near the boundary (fig. 15) 

and by the fact that, at most points, a substantial 

amount of flow traversed the surface of the accreted sole 

layer with little erosion.

Similar intense shear would have been focused on the 

spillways of any impoundment structures present in the 

drainage at the time of the eruption. Upstream energy 

losses in a filled debris basin (associated with expanding flow and reduced slope) would probably create basal 

flow with shear focused directly on the flow boundary
A44 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

FIGURE 29. Surface formed by compaction and truncation at the contact of two units of the bar deposits of the North Fork lahar. The overlying bar deposits have been removed by erosion. The shovel points in the direction of flow. Location is the confluence of the forks of 

the Toutle River.

by the interaction and settling of boulder-size clasts. 

The effects of the 1982 lahar on the spillway of the 

debris basin upstream from Camp Baker, constructed 

after the 1980 eruption, illustrate this point. That flow 

passed over several parts of the structure, where the 

shear associated with relatively shallow depths of flow, 

in the range of 1-3 m, was sufficient to abrade steelreinforced, roller-compacted concrete and locally to 

remove emplaced boulder-size riprap. The exposed ends 

of reinforcing rods were sharpened to points analogous 

to those on the bayonet trees.

SOLE LAYER

The term "sole layer" is applied to the variety of texturally distinct basal subunits that are present in many 

of the modern and ancient lahars. The features can be 

subdivided into three general textural types, designated

according to the amount of near-boundary shear with 

which they were associated.

TYPE I SANDY SOLE LAYERS LACKING 

DISPERSED COARSE CLASTS

Basal layers consisting mainly of sand are relatively 

common in flood-plain facies of the older lahars and are 

the product of relatively passive flow. They rarely show 

the compaction evident in the Type II sole layers. They 

also differ from Type II layers, in lacking dispersed 

coarse clasts and thus do not have a texture characteristic of the overlying lahar. The Type I layer and the 

overlying bed were parts of a single flow, however, as 

shown by the similarity in texture of the layer and the 

matrix of the overlying part of the lahar, and by the 

transition between the parts of the unit (unit 9 in fig. 

30, for example).
LAHAR BOUNDARY FEATURES A45

FIGURE 30. Flood-plain facies of three older lahars, units 7, 8, and 9 of the Coal Bank Bridge section (Scott, in press). The lower half of 

the oldest unit is inversely graded. Only the basal 10-20 percent of the middle rubbly unit is inversely graded. The basal 15-20 cm 

of the overlying unit is a poorly developed Type I sole layer transitionally overlain by an inversely graded zone forming 0.2 m of the 

1.2-m-thick bed. Total thickness exposed is 2.8 m.

The frequency of Type I layers in the pre-1980 lahars, 

and their minor part of the unit thickness (typically 

10-20 percent), indicate that sole layers cannot all be 

explained as exact analogs of the basal, sandy parts of 

the transition facies. The analog explanation applies 

where the downstream thickening of the sole layer is 

progressive and the upper, debris flow part of the unit 

is replaced. A different explanation applies to many 

flows that have a Type I layer. Even where these flows 

encountered a large roughness element, the sole layer 

varies little in thickness. The sole layer was thus formed 

by a boundary effect within the flow rather than as the 

product of a distinct and Theologically different earlier 

part of the flow. Both explanations can thus fit this type 

of sole layer, but the evidence indicates that most Type 

I layers were created as boundary effects.

TYPE II SOLE LAYERS WITH DISPERSED COARSE CLASTS

The best-developed sole layers of both the 1980 South 

Fork and North Fork lahars are Type II; they are finer 

grained, texturally more uniform, and strikingly more 

compacted than the generally coarser overlying bar 

deposits in the channel facies. As described in the section on texture, the layers commonly have bimodal size 

distributions and consist of silty sand containing 

dispersed pebbles that rarely exceed -5 $ (32 mm). Textural affinity to the peak-flow deposits is greater than 

to the bar deposits that commonly overlie the layer; 

particle-size distributions are similar to those of the 

peak-flow deposits in the sand and finer grades, but 

show a greater development of the coarse mode in the 

-3 and -4 0 (8-16 and 16-32 mm) classes. The peak-flow
A46 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

deposits show the progressive development of a coarse 

mode in the -3 <£ (8-16 mm) class with depth (fig. 14). 

The size distributions of the sole layer also are more likely to be truncated (no coarse tail) than those of the peakflow deposits (compare figs. 14 and 17).

The freshly deposited sole layers of the 1980 channel 

facies, which tended to fracture like a partly consolidated rock, have locally resisted erosion (figs. 16 and 

18). They were apparently consolidated by a simple 

physical process, and their cohesion cannot be attributed to clay content, which is no higher than in other 

lahar deposits, as shown by the relative amounts of clay 

in the sole layer and peak-flow deposits (compare figs. 

14 and 17). The "clast-studded" appearance of the 

weathered surfaces of the layer reflects retention of the 

dispersed large clasts in the compacted matrix. The 

layers locally exhibit a primary foliation that is distinct 

from stratification and is developed parallel to the flow 

boundary. This fabric is particularly evident where the 

layer forms upward-tapering, cone-shaped pedestals 

around the bases of trees but is not common in the sole 

layers of older lahars.

The Type II layer ranges from 8 to 52 cm in thickness 

in the 1980 lahar deposits and commonly forms less 

than 10-15 percent of the thickness of a flow unit. Contact with overlying deposits is sharp in some places and 

gradational over several centimeters elsewhere. Various 

types of grading are present, but inverse grading, in at 

least the lower part of the layer, is most common. Where 

Type II layers are transitional to overlying bar deposits, 

inverse grading in the sole layer may continue into the 

overlying unit. Locally the sole layer is crudely stratified but shows little textural variation between strata. 

Symmetrically graded units of differing color (fig. 18) 

were seen only in the lower Toutle River. As discussed 

in the section on composition, there is convincing 

evidence for deposition of these subunits from different 

segments of the single lahar flood wave.

TYPE III SOLE LAYERS OF CLAST-SUPPORTED GRAVEL; 

THE "BALL-BEARING BED"

The "ball-bearing bed" is an unusual sole layer at the 

base of the first lahar of Pine Creek age (PC 1). This 

remarkable concentration of fine pebbles in a silty sand 

matrix constitutes the basal 0.45 to 1.10 m of the total 

thickness (5.9 to 7.6 m) of the lahar channel facies or 

of channel fills (fig. 23). Pebbles in the -3 <j> (8-16 mm) 

and -4 <f> (16-32 mm) classes form the mainly clastsupported framework of the bed. This is in contrast with 

the Type II sole layers of the 1980 lahars, which have 

no intact framework, even though their modes are commonly in those same size classes.

Other features of the bed indicate that it is the sole 

layer of the huge overlying lahar: It is commonly present at the base of the channel facies (fig. 12) and is absent from the flood-plain facies, in which vertical tree 

molds define the flow as generally passive and of short 

duration. The upper contact of the bed is sharp in many 

places; elsewhere it is transitional over an interval that 

is commonly about 1 cm but locally greater.

A thinner, finer grained version of the "ball-bearing 

bed" that is less completely clast supported is common 

where the lahar deposits are on valley side slopes. At 

the locality shown in figure 31, the flow depth of the 

lahar approached 30 m on the flood-plain surface, and 

the equivalent of the "ball-bearing bed" is present 

within 5 m of the level of maximum stage on a 

reconstructed valley side slope of 8-10°. This relation 

shows that the bed was not the product of a separate 

flow, nor was it derived from an earlier part of the lahar 

that was subsequently overrun by the main flood wave. 

The bed would be present only at lower levels in either 

case. The presence of the unit on slopes and in channels, but not in association with passive deposition on 

flood plains, indicates that it is the product of boundaryrelated dynamics.

INVERSE GRADING

In their study of older lahars in the Toutle River 

system, Mullineaux and Crandell (1962) recognized normal grading as a useful criterion for recognizing a 

laharic debris flow beyond the base of the volcano. Inverse grading in the basal parts of the 1980, 1982, and 

older lahars (fig. 30) is as common. Although the inverse 

grading involves a smaller proportion of flow thickness 

(table 4), it is especially diagnostic because of its comparative rarity in deposits of other origins. Schmincke 

(1967) described inverse grading in lahars, and Fisher 

(1971) observed the feature in a variety of debris flow 

deposits. The following vertical sequence is variable but 

is clearly the typical pattern for lahars in the ToutleCowlitz River system, as well as those at most other 

Cascade Range volcanoes: (1) inverse grading beginning 

at the flow boundary or in or above a sole layer, (2) an 

ungraded or poorly graded central core (which may be 

absent), and (3) normal grading in the remaining upper 

part of the unit. The degree of grading development and 

the relative thicknesses of the sequential subdivisions 

vary between lahars.

Much of the normal grading and part of the inverse 

grading is coarse-tail grading, in which particles below 

a critical diameter are ungraded or very poorly graded. 

The obvious normal grading mainly involves sediment 

coarser than -1 <£ (2 mm). This is shown by the vertical
LAHAR BOUNDARY FEATURES A47

FIGURE 31. Sole layer at the base of the first lahar of Pine Creek age 

on a valley side slope 310 m WNW. of right-bank abutment of the 

Kid Valley Bridge. The knife marks the top of the bed, which is 

10-12 cm thick below 1.4 m of lahar deposits. This sequence correlates with that seen at the present channel thalweg in figure 23. 

Note the fluvial Founding of the clasts in the lahar.

changes in size distributions (fig. 14, insets) and by the 

longitudinal consistency in the relative proportion of silt 

and clay to sand, silt, and clay (fig. 13) as coarser sediment settled in the flow. Crandell (1971, table 2) describes normal grading that qualifies as coarse-tail 

grading in a group of lahars, including two in the Toutle 

River system, where the critical diameter is also -10 

(2 mm). The inverse grading, however, may be a function of overall size changes distribution grading or 

shifting-mode grading especially in the clastsupported deposits.

The basal 0 to 20 percent of bed thickness of the 1980 

channel facies is commonly inversely graded (table 4), 

and the difference in mean size between the bottom and 

top of the zone is highly variable. From 15 to 50 percent of the flood-plain facies of the 1980 and older lahars 

is inversely graded, and the mean size difference between 

the lowest and coarsest parts of the bed is typically 

at least 2 phi classes. The size change in the inverse 

grading of both facies types is commonly an exponential

decrease as the boundary is approached. The same relation occurs in lahar deposits on steep valley side slopes 

and was seen in Type I and II sole layers preserved on 

vertical channel walls. Inverse grading thus is also, like 

most of the sole layers, the product of a boundary effect.

ORIGIN AND RELATIONS OF 

THE BOUNDARY FEATURES

Several partly related processes are recorded by the 

boundary features. They include (1) shear stress concentrated near the flow boundary and the consequent 

sorting and inverse grading produced by coarse particles preferentially moving up in the flow; (2) shear concentrated at the flow boundary at sites of rapid energy 

loss, where coarse clasts directly abrade channel 

alluvium rather than being supported away from the 

boundary by particle interactions; (3) the suppression 

of dilatancy so that frictional forces dominate and 

cataclasis occurs within the boundary layer of the flow;

(4) variations in grading type and intensity due to 

migration of the boundary of the intralahar plug; and

(5) accretionary smoothing of the boundary during the 

rise of the lahar wave. Also locally influencing the 

boundary features are alluvium bulked into the flow and 

water incorporated from streamflow overrun by the 

lahar. The latter process is discussed in the section on 

flow transformations; in the transition interval from 

lahar to lahar-mnout flow it produces inverse grading 

and a basal layer locally like a Type II sole layer.

The boundary features record the degree to which 

shear was concentrated, sustained, and modified at the 

channel boundary. The following depositional sequence 

was observed in passing from areas where intense shear 

occurred near the channel thalweg to areas of passive 

deposition on the flood plain:

1. A lahar-abraded pavement overlain by clastsupported bar deposits with a mode in the boulder 

size range, which commonly have poorly 

developed basal inverse grading. The Type II sole 

layer is mainly absent, probably having been 

eroded.

2. A highly compacted Type II sole layer in sharp contact with overlying bar deposits that have basal 

inverse grading and a mode in the pebble or cobble range.

3. A less distinct, less compacted sole layer transitional 

to deposits that have basal inverse grading affecting as much as half the unit and a mode in the pebble range.

4. Sole layer absent; basal inverse grading.

5. Less pronounced inverse grading.

The close relation of the Type II sole layer to inverse 

grading is indicated by the distributions of both
A48 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

features in a reach without a flood plain (fig. 32). Both 

are less well developed near the flow margin, though inverse grading is the more persistent. The thicknesses 

of both features vary with flow depth at this locality, 

where the deposits are intermediate between channel 

and flood-plain facies. The Type II sole layer in particular occurs where the channel was hydraulically 

smooth and boundary shear was maximal; it dies out 

laterally where the velocity distribution was disturbed 

by vegetation. Near the main channel, the sole layer laps 

up the bases of the remaining trees to form the pedestal 

bases studded with dispersed pebbles. The thickness of 

the pedestals shows that deposition was more rapid in 

the "dead" region of flow at the apex of the angle 

formed by the tree and ground. The pedestals are in turn 

continuous upward with the layers of mud coating the 

trunks. Both pedestals and coatings show the compaction and foliation characteristic of the sole layer.

The stratigraphic relations of the Type I and Type 

II sole layers show that the first are features of deposition on flood plains, the second of deposition in channels. The amount and duration of shear stress 

determined which type was formed. Nevertheless, the 

flood-plain facies of the 1980 North Fork and South

Fork lahars generally show only inverse grading, not 

a well-developed Type I sole layer. The flood-plain facies 

of lahar PC 1 also does not have a pronounced sole layer. 

Each of these flows was the first major flow of an eruptive period, and consequently the downstream flood 

plains were densely vegetated. Roughness was so extreme and flow consequently so passive that the Type 

I layer did not form. The flood-plain facies in each of 

these three cases includes a large volume of vegetation 

(removed by weathering from lahar PC 1). The Pine 

Creek-age lahars following PC 1 flowed across relatively 

smooth surfaces and developed Type I sole layers, 

especially in flows PC 3 and PC 4.

Pyroclastic flow units commonly have a fine-grained, 

inversely graded basal zone known as the basal layer 

(Walker, 1971; Sparks and others, 1973; and Sparks, 

1976). As described by Sparks (1976), it is not a discrete 

layer and also differs from the sole layer in that the inverse grading is always transitional to the main body 

of the flow unit. The feature occurs irrespective of flow 

depth and channel side slope. Because pyroclastic flows 

represent partly fluidized debris flow (Sparks, 1976), the 

boundary forces producing both features are probably 

similar.

15

> 5

A gradual horizontal velocity profile is 

indicated by variations in mud run-up on 

upstream side of tree trunks

EXPLANATION

INVERSE GRADING BELOW THIS LEVEL

aSrms iar//'' '

:^^pPi^^// / I

^^^^/ J___[_

Runout deposits of 

1980 South Fork lahar

SOLE LAYER CONTINUOUS UP BASE OF TREES 

FORMING PEDESTALS REMAINING AFTER 

EROSION OF OTHER DEPOSITS

PRE-LAHAR SURFACE

VfWKZ&fSyr^S&P

THALWEG

______I

10 20 30 40 50 

HORIZONTAL SCALE, IN METERS

60 70 80 90

FIGURE 32. Distribution of sole layer and inverse grading in the North Fork lahar shown in diagrammatic cross section on the left

side of reach (viewed from upstream) near Tower Bridge.
LAHAR BOUNDARY FEATURES A49

The based, finer grained parts of debris flow 

deposits including both the discrete fine-grained zones 

like the sole layer and the inversely graded zones have 

been subject to many interpretations. Naylor (1980) 

reviewed a group of hypotheses based on the premise 

that larger clasts tend to be concentrated upward within 

the flow. In explaining inverse grading he favored a 

reverse concept, based on the behavior of clays, in which 

the larger clasts are dropped out of the flow as a result 

of loss of strength of the clay matrix. This mechanism 

can be excluded as the origin of the sole layer or of the 

inverse grading in the 1980 lahars, both because of the 

relatively low clay content of those flows and because 

the hypothesis fails what Naylor describes as the critical 

test: the necessary coarse lag deposits, representing the 

material dropped from the flow, are absent. The bar 

deposits represent coarse material concentrated and 

deposited from the base of the flow, but they occur 

above, or have eroded, the sole layer, which clearly 

formed earlier in the lahar flood wave.

The Type I sole layers are close analogs of the basal 

zones of a series of Tertiary lahars described from central Washington by Schmincke (1967). The sole layers 

of those deposits are mainly composed of poorly graded 

sand, are less than 20 cm thick, and form the bases of 

lahars from 2 to 5 m thick. The lahars contain a nongraded, clast-supported central zone of coarser sediment 

that, like the basal zone, has less than 10 percent silt 

and clay. Schmincke was essentially correct in describing the basal zones as a type of inverse grading possibly 

related to shear distribution in the granular flows.

In commenting on the basal zones of Schmincke, 

Crandell (1971) noted that beds of sand underlie many 

lahars but that most he observed were part of underlying fluvial sequences rather than of the lahar. The observation raises the possibility that sole layers consist 

mainly of sandy alluvium from the channel that was incorporated into the lahar. The lack of textural correlation of the Type II sole layer with easily erodible 

underlying sediment is illustrated both in figure 16, 

which shows the North Fork sole layer overlying coarse 

stream alluvium, and figure 18, which shows it overlying 

runout sand from the South Fork lahar. The relation 

shown in figure 18 prevailed continuously downstream from the confluence of the forks of the Tbutle 

River. Even the large change in boundary sediment 

from coarse alluvium to fine sand induced no corresponding changes in the texture of the sole layer (fig. 

17); its texture remained related to that of the peak-flow 

deposits of the lahar. The same textural comparison applies to localities where the base of the "ball-bearing 

bed" is visible; that Type III sole layer also overlies texturally dissimilar fluvial and runout sand and gravel as 

well as soils and clay-rich beds.

The features of the Type II sole layer that are relevant to its origin are summarized in table 7. Where well 

developed, the layer reflects an interval of accretion 

from a probably inversely graded boundary layer during rising and peak flow. Once deposited, the layer was 

subject to intense compaction in channels but generally was not eroded, except locally by the coarse basalflow carpet, because of its function in hydraulically

TABLE 7. Characteristics of the sole layers of the 1980 North Fork and South Fork lahars

Characteristics Interpretation

Sole layer overlain by bars with crests locally extending to

within 1.7 m of peak flow stage. Texture of sole layer resembles 

that of peak flow deposits more than that of overlying bars.

Sole layer locally compacted and semiconsolidated, though 

overlying deposits in sharp contact are not. Sole layer has 

primary foliation; overlying deposits do not.

Color banding and stratification present locally in North Fork 

sole layer.

Development of sole layer is proportional to flow depth and lack 

of roughness elements.

Sole layer better developed in broadly peaked North Fork lahar 

than in smaller South Fork lahar, even where depth and slope 

were similar.

Sole-layer size distributions lack coarse tails, are

characteristically bimodal, and have dominant, narrow coarse 

mode in -3 or -4 i> (8-16 or 16-32 mm) classes, regardless of 

coarseness of overlying sediment.

Character of sole layer not affected by the highly variable 

deposits that form the flow boundary.

Sole layer formed before or during flow peak; 

bar deposition was most rapid during 

earliest part of recession stage.

Sole layer underwent intense shear during 

significant time interval between its deposition and that of the overlying deposits.

Sole layer was locally accretionary over a 

significant time interval. Color 

differences record flow from different 

facies of 1980 debris avalanche.

Development of sole layer was related to 

intensity of boundary shear.

Thickness and compaction of sole layer were in 

part a function of flow duration.

Size distribution was truncated at coarse end; 

coarser clasts moved away from flow 

boundary.

Sole layer resulted mainly from intraflow 

processes, rather than the introduction of 

eroded sediment.
A50 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

smoothing the flow boundary. The primary foliation 

seen in the 1980 sole layers, best developed in finer 

grained, relatively clay-rich equivalents such as the 

pedestal deposits around trees, is probably similar to 

the foliation induced by experimental shearing and compaction of the matrices of clay-rich debris flows. The 

degree of compaction corresponds with flow depth and 

thus with the amount of boundary shear.

The locally sharp upper contact of the sole layer 

represents an interruption in the processes that otherwise might form only an inversely graded basal zone, 

like that of pyroclastic flow deposits. That interruption 

is the interval of increased pressure and sustained shear 

that compacts the layer and produces the foliation. The 

sharpness of the contact correlates with the amount of 

compaction. The hiatus represents the passage of the 

flood peak and transport of the pulses of basal flow, the 

deposition of which initiates the formation of the whaleback bars.

The inverse grading is primarily the result of coarse 

clasts moving away from the boundary during flow as 

a result of particle-to-particle interaction. The effect is 

clearly seen at steep or vertical flow boundaries. This 

clast movement is also indicated by the positioning, in 

lahar PC 1, of clasts eroded from the bedrock gorge 

below the Coal Bank Bridge. The lahar eroded boulders 

of pre-Mount St. Helens volcanic rocks that are not a 

significant part of the flow deposits upstream from the 

gorge. In the first lahar exposures at the distal end of 

the gorge, the locally derived boulders are present only 

in the coarsest medial part of the flow unit. They moved 

several meters from the flow boundary to associate with 

clasts of similar size near the center of the flow, in an 

obviously brief interval of transport. The usual hypothesis of a lack of particle interactions in debris flows is 

based on the assumption of clast dispersal in a clay-rich 

matrix with a consequent lack of clast collisions. The 

relatively granular character of the Toutle River lahars 

evident in their particle-supported silty sand matrices, 

compared to some other debris flows with a higher clay 

content, is a logical explanation of behavioral differences involving more intense particle interaction.

The inverse grading is most readily explained by 

dilatant flow behavior in the sense of Bagnold (see Denlinger and others, 1984). The mechanism of kinetic sieving (Middleton, 1970), whereby small grains pass 

downward through the interstices of larger particles 

during dilation, may explain part of the inverse grading 

in clast-supported units such as the bar deposits. The 

process has been shown to be effective in experimentally produced shear zones in clast-supported materials 

(Mandl and others, 1977).

The textures of the Type II and Type III sole layers 

also show a response conforming to the behavior of a 

dilatant fluid (with coarse clasts preferentially moved

away from the boundary by particle interactions, and 

consequent truncation of the size distributions). The 

coarse mode of both types is remarkably consistent in 

the -3 </> (8-16 mm) and -4 </> (16-32 mm) fractions, and 

larger clasts are almost entirely absent. More than half 

of the "ball-bearing bed" is concentrated in those 

classes (fig. 22), suggesting that the mainly clastsupported unit represents the extreme product of shearinduced sorting. The high percentage of broken clasts 

in the unit (fig. 26) and a degree of rounding by 

cataclasis indicate that the response to shear stress of 

clast movement was replaced by one of abrasion and 

breakage. This response probably changed after clast 

support and the accompanying truncation of the size 

distribution in the boundary layer were achieved. In experimental shearing of granular material, total shear 

stress can be separated into (1) that acting to dilate the 

shear zone and (2) frictional stress acting to dissipate 

energy by intergranular friction and grain breakage 

(Mandl and others, 1977). Suppression of dilatancy and 

consequent grain cataclasis occurs with increased 

normal load. The flow producing the "ball-bearing bed" 

was the fastest and deepest flow in the system and also 

had a clay content as low as any flow, ancient or 

modern. This combination of characteristics yields the 

greatest intergranular stresses as well as the greatest 

potential for suppressed dilatancy and consequent grain 

cataclasis.

Some of the variation in shear concentration recorded 

by the boundary features is probably explained by fluctuations in the internal boundary between the rigid core 

(or "plug") of the lahar and the zone of laminar flow 

around its edges. The intraflow boundary forms at the 

level at which internal shear stress exceeds yield 

strength (see Johnson, 1970, 1979) and will occur in 

either plastico-viscous or dilatant fluid behavior. The 

boundary migrates continuously during flow due to 

changes in slope and roughness (Hampton, 1975). The 

frozen flow lines preserved in the deposits of anastomosing distributary flows of the North Fork lahar on the 

debris avalanche (fig. 33) and in logged valley flats (Lipman and Mullineaux, 1981, photograph on p. 460) indicate plug flow like that of both experimental (Johnson 

and Hampton, 1969, fig. 3.9) and natural debris flows 

(Johnson, 1970, fig. 14.9). After the lahar entered a 

single confined channel downstream, shear was more 

widely distributed in the horizontal plane. Figure 32 illustrates a gradual horizontal velocity profile shown by 

runup heights on trees. The distribution of shear in the 

1980 lahars on forested flood plains and in confined 

channels was apparently distorted by vegetation and 

bedrock roughness elements.

The lahar-abraded pavement occurs at locations 

where changes in depth and velocity would have caused 

the plug boundary to approach the flow boundary,
FLOW TRANSFORMATIONS A51

FIGURE 33. Lateral margin of lahar deposit on the surface of the debris avalanche near Elk Rock. Flow lines asymptotically approach 

parallelism with flow margin near backpack and are transverse to flow throughout most of the central area of plug flow. Flow is toward 

left foreground. Earlier position of flow margin preserved as an island in right foreground.

thereby concentrating the shear. The variation in 

thickness of either the inversely graded basal part or 

the coarse central part of a typical lahar unit may be 

a function of the amount of plug boundary migration. 

Hampton (1975) noted that the migration would produce a grain size variation but did not describe the 

variation. Although the plug is rigid with respect to flow 

behavior, large clasts are free to settle within the plug, 

concentrating progressively with depth. (See vertical 

changes in size distributions in figure 14.) The typical 

upper zone of normal, mainly coarse-tail grading is 

ascribed to this settling.

FLOW TRANSFORMATIONS

The following sections describe the main types of flow 

transformation that have recurred in the river system: 

the formative transformations, from pyroclastic surge, 

avalanche, and flood surge to debris flow, and the distal 

transformation from debris flow to hyperconcentrated 

streamflow.

PYROCLASTIC SURGE AND AVALANCHE 

TO DEBRIS FLOW

Evidence that the 1980 South Fork lahar began as 

a highly inflated pyroclastic surge includes (1) a rapid

decline in discharge at the base of the cone, (2) deposits 

in the same interval that record the hydraulically continuous transformation from a highly dispersed, air- and 

gas-mobilized state to a water-mobilized state, (3) 

photographs documenting this change in flow behavior, 

and (4) the effects of the flow on overrun vegetation and 

bedrock roughness elements. Because of the originally 

continuous exposures of the flow deposits, the evidence 

for the 1980 transformation is unequivocal. The same 

transformation probably also produced lahars from 

pyroclastic surges in pre-1980 eruptive periods; the conditions that provided the water content of the 1980 

surge cone saturation and snow and ice cover would 

have been present at the time of any similar catastrophic ejection of material.

The solids dispersed in the inflated flow ejected into 

the South Fork watershed were mainly accessory lithic 

material from the modern cone. They formed a thin, 

uniform, noncohesive coating locally more like a 

"dusting" on valley side slopes at flow depths of over 

100 m. On the middle and lower slopes of the cone the 

deposits are associated with only small amounts of erosion. Dynamically consistent peak-flow marks document the continuity of this inflated flow as it moved 

downslope to continue as a lahar with water as the continuous phase. The deflation is reflected by the rapid 

decrease in flow velocity and discharge (fig. 34). The collapse of the flow reflects the loss of ingested air and
A52 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

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Deflating pyroclastic surge | Mainly water-mobilized

and basal avalanches

I

debris flowI

0 5 10 

CHANNEL DISTANCE FROM CRATER. IN KILOMETERS

FIGURE 34. Discharge and velocity of the pyroclastic surge and 

main lahar in the South Fork Toutle River on May 18, 1980. The 

measurements upstream of km 5.5 apply to the pyroclastic surge 

after its channelization in canyons cut into the slopes of the cone.

probably the condensation of steam, as well as the expectable attenuation of the sharply peaked flow wave. 

Only a small proportion of the total flow volume was 

lost through deposition from basal avalanches. The 

avalanches were coarse material that concentrated at 

the base of the surge and moved as segregated, rubbly 

flows.

The pyroclastic surge deposits consist of both the thin 

deposits extending high on valley side slopes, and the 

rubbly berms of the basal avalanches low in the channel. The transition to the downstream, water-mobilized 

lahar is shown in figures 6 and 34 at the approximate 

point where debris flow berms (distinguished by a muddy matrix and surfaces with water-deposited sediment 

and buoyed low-density clasts) completely replace both 

types of surge deposits. Much of the deflation had 

already occurred upstream from that point, however, 

as seen in the drop in discharge (fig. 34). Downstream 

from the transition, the limits of the peak-flow lahar 

deposits, rather than the limits of the slope deposits of 

the pyroclastic surge, define the peak stage.

Stratigraphic relations show that the inflated flow 

that transformed into the South Fork lahar was closely related to the directed volcanic blast that devastated 

the 600-km2 area north of the mountain. That

phenomenon is variously described as a lateral blast 

(Hoblitt and others, 1981), a pyroclastic surge (Moore 

and Sisson, 1981), a pyroclastic density flow (Waitt, 

1982), or a high-velocity ash flow (Walker, 1983). Locally 

that flow swirled across the northern drainage divide 

of the South Fork at velocities probably near those of 

the main part of the flow, estimated by Rice and Watson 

(1981) at 150-250 m/s. The resulting deposits layer Al 

of the blast deposits (Waitt, 1981) mainly underlie, or 

were eroded by, those of the slower South Fork flow, 

which moved at about 60 m/s in the areas reached by 

the blast (fig. 34). Layer Al locally overlies the deposits 

of the South Fork flow, however. Aerial photographs 

(fig. 5) show a distinct (at this scale) contact between 

the two flows and thus indicate a definite (but probably 

small) time interval between the main part of each. 

Although air-fall layer A3 of the blast deposits was 

poorly developed on the southwest side of the volcano 

and was rapidly eroded, it was observed on top of the 

South Fork lahar deposits beyond the base of the cone 

(R. J. Janda, oral commun., 1982). That relation establishes deposition of the lahar within approximately 15 

minutes of the 0832 eruption. The actual time difference 

was much less, as shown by the relationship of the 

deposits to layer Al.

Ground photographs (fig. 35; and fig. 24 of Foxworthy 

and Hill, 1982) show that the South Fork inflated flow 

was created by the first and largest of the explosive 

events recognized by Moore and Rice (1981) and by Rice 

and Watson (1981). The resulting cloud circled the 

mountain outside the blast zone. The photographs show 

only the surficial cloud rising above the main body of 

the surge; the deposits and the effects on vegetation 

record the passage of the main part of the flow at depth.

That the basal avalanches settled from the surge and 

continued to flow in channels (fig. 7) is indicated by the 

Stratigraphic relations and the geometry of the deposits. 

Moreover, the lack of pumice in these deposits verifies 

their origin before the second South Fork lahar in the 

afternoon of May 18. Sorting of the avalanche deposits 

is poor, from 2.8 to 4.2 0 (fig. 19), and mean size is in 

the coarse pebble and cobble ranges. Unlike the basal 

bar deposits of the downstream lahar, the basal avalanche deposits contain little fine material; in a relatively muddy sample, total silt and clay content is less 

than 4 percent. The basal avalanches were clearly subordinate in flow volume to the main part of the surge 

that yielded the downstream lahar (fig. 7). As revealed 

by the relative heights of the avalanche berms and the 

peak-flow stage, the flow depth of the avalanches was 

2 to 10 percent of the flow depth of the surge. The 

segregation of the avalanches from the deflating part 

of the surge and the subsequent segregation of the bar 

deposits from the deflated, water-mobilized lahar are
FLOW TRANSFORMATIONS A53

FIGURE 35. The leading edge of the pyroclastic surge descending the southwest flank of Mount St. Helens, 2 min, 20-25 s after the beginning of the 0832 P.d.t. eruption of May 18, 1980. The watershed of the South Fork Toutle River and tributary Sheep Canyon forms 

the left (west) margin of the volcano. Note the lighter colored layer that is best developed in the headwaters of the South Fork. The 

faint cloud above the west flank of the volcano is an atmospheric effect of the eruption. Photograph by Harold Fosterman. Time of 

photograph estimated by S. D. Malone (Univ. Washington, written commun., 1984).

analogous in sequence to the initial segregation and 

subsequent resegregation of block and ash flows from 

a primary and evolved secondary ash cloud surge as 

described by Fisher and Heiken (1982).

The thin deposits coating valley side slopes high 

above the berms of the basal avalanches have a 

geometry and texture showing that the upper, nonavalanche part of the surge was air and gas mobilized. 

Although the boundaries of both the pyroclastic surge 

and the downstream lahar appear sharp in figure 5, the 

surge boundary was not obvious in the field once the 

deposits had dried. The thin (several centimeters or 

less), uniformly distributed surge deposits feathered to 

a peak-flow level that was indistinct over a distance of 

several tens of centimeters, in contrast to the precisely 

defined peak-flow mark of the downstream lahar. The 

slope deposits are indistinctly stratified and are variable 

but mainly noncohesive mixtures of silty sand with 

scattered small pebbles. (Sediment size terms are applied to the surge eject a for comparison with the 

downstream lahar.) Sorting in the slope deposits is 

equivalent to or poorer than that of the avalanches and 

ranges between 2.8 and 4.4 </> (fig. 21), but the mean size 

is distinctly finer, in the sand size range. The similar 

sorting in a much finer deposit mainly reflects the

relative lack of fine sediment in the rubbly avalanche 

deposits. Although the silt and clay content of the slope 

deposits is between 13 and 22 percent nearly the same 

as in the downstream lahar the vegetation overrun by 

the surge lacked significant mud coatings. Laminated, 

surficially vesicle-rich coatings as much as several centimeters in thickness were characteristic of vegetation 

inundated by the medial and distal phases of the lahar 

(fig. 6). The coatings reflect the progressively greater 

duration of the lahar flood wave as it moved downstream; they thin again throughout the increasingly 

dilute distal phase.

South of Sheep Canyon, on the southwest flank, the 

distinction between slope and basal avalanche deposits 

was not observed. There, Major (1984) described mainly coarse rubbly lahars that originated as catastrophically ejected avalanches high on the mountain. Those 

flows did not transform from a highly dispersed state 

and did not extend significantly beyond the base of the 

cone.

The dynamics of the highly dispersed surge, which 

deflated downstream to form the lahar, are revealed by 

the effects on vegetation. The upstream surfaces of 

trees that remained standing were shredded as if hit by 

grape shot, a coarser version of the sandblast effects
A54 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

of a base surge described by Moore (1967). Angular pebbles penetrated the shredded surfaces like projectiles. 

Variation in the depth of trunk abrasion indicates that 

part of the coarser gravel fraction other than that in 

the basal avalanches moved in traction and that the 

maximum flux of particles was from 0.5 to 2.0 m above 

the flow boundary. Pebble-size gravel of normal-density 

rock types was supported by turbulence at the peak 

flow levels, and many small conifers were stripped only 

of their bark and needles by a sandblast effect that left 

small branches and twigs intact. This was in striking 

contrast to the "bayonet" trees, whose trunks were 

sharpened to a point by the downstream lahar. Locally 

the surge appeared to have jetted past rather than 

flowed around bedrock projections, as inferred from patterns of stripped vegetation.

The deposits and vegetation effects show that at 

depth, as the cloudlike leading edge seen in the photographs (fig. 35, for example) passed, the surge was settling and deflating. The bulk of the flow was rapidly 

channelized and behaved as a dense fluid without the 

ability of the more energetic, northward-directed blast 

to move across large topographic obstacles. The highly 

dispersed fine material seen in the ground photos continued to boil above the deflating surge like an ash 

cloud. The probably steam-rich cloud within the surge 

(fig. 35) did not yield significant deposits, nor did it 

obscure the peak-flow marks of the main, deflating body 

of the surge.

The flow dynamics, peak-flow marks, and sedimentology clearly define a single major flow that transformed downstream. Alternative hypotheses involve 

separate events, such as a hot pyroclastic surge closely followed by a meltwater flood that bulked with sediment to form the downstream lahar. A flood hypothesis 

cannot explain, by melting of ice and snow in a few 

seconds, an instantaneous peak water discharge that 

is even a small fraction of the measured value for the 

surge of 1,100,000 m3/s (fig. 34). The peak-flow marks 

clearly reflect a canyon-filling discharge. Flow volume 

near the base of the cone is estimated at 13,000,000 m3 

by Fairchild and Wigmosta (1983), and this volume 

would have passed in just a few seconds at the stated 

discharge rate. Moreover, the passage of a flood would 

have caused profound erosion and sediment transport 

on the middle and lower slopes, and these effects have 

not been observed.

TEMPERATURE OF THE PYROCLASTIC SURGE

The juvenile dacite from the 1980 cryptodome is present in the surge deposits in small amounts, generally 

less than 2 percent in the South Fork and less than 1

percent in Sheep Canyon. However, this and other lowdensity rock types are locally abundant on the surfaces 

of the lahar berms. The deposits of the lateral blast contain about 50 percent juvenile dacite (Hoblitt and 

others, 1981; Moore and Sisson, 1981; and Waitt, 1981), 

and the greatest amounts correlate with the areas of 

highest temperatures evident from wood charring 

(Moore and Sisson, 1981). Although most dacite blocks 

were broken during transport, some in the pyroclastic 

surge deposits and in the downstream lahar have 

prismatic cooling joints. Prismatic jointing can be 

evidence that the deposit was emplaced while hot if confirmed by consistent directions of thermoremanent 

magnetism (Crandell, 1980). Although thermoremanent 

magnetism was not measured, the occurrence of the 

jointed dacite blocks adjacent to uncharred wood shows 

that emplacement temperatures of both the surge and 

lahar deposits were not high.

A seared (but not burnt) zone near tree line on the side 

of the cone (Major, 1984) establishes that the temperature of the cloudlike leading edge of the flow was relatively low at that point. Even lightly charred vegetation 

was not seen in place below that point on the middle 

slopes of the cone. A small amount of macerated and 

charred vegetation is present in the main deposits of 

the surge but may have been derived from erosion of 

deposits of the much hotter lateral blast. Similar 

fragments are extremely rare at the timberline outside 

the blast zone.

The original temperature of the surge entering the 

South Fork and Sheep Canyon must have been lower 

than the initial temperature of the lateral blast, which 

was estimated as 327 °C (Davis and Graeber, 1980). 

Less juvenile dacite, more of the surficial parts of the 

cone, and a greater proportion of snow and ice were incorporated in the material ejected into those watersheds 

than into the blast zone. A ground observer 4 km from 

the base of the cone described the immediate lahar 

deposits as resembling warm concrete (Rosenbaum and 

Waitt, 1981). The appearance of fir needles remaining 

on trees generally suggested temperatures less intense 

than in the marginal seared zone of the lateral blast; 

needles used as thermometers at two locations in the 

seared zone indicated temperatures of 50 and 250 °C 

(Winner and Casadevall, 1981). Blistered and peeled 

vegetation at the tree line was observed by Major (1984) 

in a zone extending around the southwest flank of the 

volcano from Sheep Canyon and, by the same analogy, 

suggested temperatures in the 50-250 °C range.

The lack of muddy tree coatings from a pyroclastic 

surge was interpreted as evidence of temperatures over 

100 °C by Moore (1967), in contrast to the cooler base 

surges. The lack of significant coatings from the South 

Fork pyroclastic surge, however, can also be ascribed
FLOW TRANSFORMATIONS A55

to the brief duration of the surge and to its dispersed 

state and coarser sediment, even if wet.

Evidence of a wet condition, and therefore of an 

emplacement temperature less than 100 °C, is the dark 

color and distinct definition of the pyroclastic surge 

deposits shown in figure 5, as contrasted with their light 

color and indistinct definition after drying. The wet 

lahar deposits downstream showed an identical color 

change; the initially dry deposits of the lateral blast in 

the watershed did not.

ORIGIN OF WATER IN THE LAHAR

The initial and probably main source of the water that 

mobilized the collapsed flow was water and steam that 

were derived from the preexisting rocks of the volcano 

and were initially mixed in the main body of the surge. 

The upper part of the mountain was apparently near 

or at saturation at the time of the eruption (Moore and 

Sisson, 1981; Voight and others, 1981), and elevated 

pore pressures from an artesian system may have contributed to the slope failure that initiated the debris 

avalanche (Voight and others, 1983). Subsurface water 

or derived steam would have been mixed throughout 

the surge. The largest lahars at Mount Rainier were 

derived from transformations of huge sector-collapse 

avalanches, and some of these were not associated with 

known volcanism, indicating that mainly interstitial 

water mobilized the material.

Additional moisture was derived from ice and snow 

incorporated in the surge. About half the combined 

volume of the Toutle and Talus Glaciers was removed 

on May 18, a total of 7 million m3 (Brugman and 

Meier, 1981). Assuming (1) the estimated volume of the 

South Fork lahar of 13 million m3 at a point just 

downstream from the point where deflation was complete and (2) the volume proportion of solids typical of 

debris flow (Sharp and Nobles, 1953; Pierson, 1980; 

1985) of 0.60-0.78, a minimum of 3.25 million m3 of 

water could have mobilized the flow. Half of the lost 

glacial ice, if incorporated in the pyroclastic surge, 

would have been sufficient to mobilize the deflated flow. 

Such incorporation is unlikely, however. Part of the ice 

and the associated snowpack was scoured by the surge 

entering the South Fork and Sheep Canyon; a majority of it probably a large majority was melted by the 

heat from tephra and numerous pyroclastic flows later 

on May 18 to form streamflow and the second lahar. 

The dynamics of the surge on the lower slopes suggest 

that the few seconds of scour and melting at the base 

of the surge would have yielded a basal concentration 

of ice fragments and water or steam that may not have 

mixed throughout the entire depth of the surge. Only

negligible parts of the Toutle and Talus Glaciers extended above the present crater rim and thus were explosively incorporated directly in the surge.

Most of the watersheds outside the zone of the lateral 

blast yielded lahars formed from catastrophically 

ejected material on May 18 (Major, 1984; Pierson, 1985). 

Those on the southwest flank of the volcano formed 

in a quadrant with little glacier ice, leading Major 

(1984) also to favor water of geothermal origin as the 

primary source. Streamflow is an additional source of 

incorporated water, but the volume in channels on the 

steep sides of the volcano in the early morning of May 

18 would have been negligible relative to that of 

the lahars.

COMPARISON WITH OTHER PYROCLASTIC SURGE AND 

FLOW DEPOSITS

Pyroclastic surges differ from pyroclastic flows in being less dense, less continuous, and less likely to approximate steady-state conditions (Wohletz and Sheridan, 

1979). They are turbulent, low-concentration gas-solid 

dispersions (Sheridan, 1979; Wright and others, 1980; 

Malin and Sheridan, 1982; Walker, 1983; and Fisher and 

Schmincke, 1984). Types include the ground surge 

(Sparks and Walker, 1973), the deposits of which underlie those of an associated pyroclastic flow; the base 

surge described by Moore (1967) and Waters and Fisher 

(1971) as the low-temperature density flow sweeping 

outward from the base of a weak phreatomagmatic 

eruption column; and the ash cloud surge formed by 

separation of fine material from the surface of a moving pyroclastic flow (Fisher, 1979).

The South Fork pyroclastic surge has in common with 

base surges a low temperature and an initially ringshaped leading edge (fig. 35; see also fig. 4 in Waters 

and Fisher, 1971). The body of the South Fork 

pyroclastic surge that transformed to a lahar was clearly not a base surge, however. The slope deposits are 

significantly different from described base-surge 

deposits, and the differences suggest why no base surge 

has been observed to transform to a lahar. The better 

sorting and particularly the crossbedding of base surge 

deposits indicate a relatively low sediment concentration and suggest that those surges, upon deflation and 

collapse in their wet distal phases, lacked both the finegrained matrix and the water content necessary to continue flow as a lahar. The South Fork pyroclastic surge 

at depth had a much higher sediment concentration. 

The derivation of lahars from a variety of upstream 

pyroclastic flows and surges in other eruptions is 

reported by, among others, Moore and Melson (1969), 

Neall (1976b), and Rose and others (1977). The direct
A56 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

transformation of an inflated flow to a nonpumiceous 

lahar has not been documented previously, however. 

Sheridan and Wohletz (1981) consider the possibility of 

direct transformation and describe how either a lahar 

or a water flood may arise indirectly from the wet 

distal stage of water-pyroclastic interaction in a cooling ash cloud model.

The initial steam-rich phase of the pyroclastic surge 

in the main canyon of the South Fork was both overrun and followed by a colder and wetter part of the 

surge flow wave. No evidence of the early phase as a 

distinct ground surge deposit, the basal unit in a 

pyroclastic surge model applied to Bandelier and other 

Plinian deposits by Fisher (1979), was observed beneath 

the avalanche deposits. The lack of ground surge 

deposits may be due to the low height of the eruption 

column, as proposed by Fisher and others (1980) for 

Mount Pelee. Much of the material in the South Fork 

surge was probably ejected with a distinct lateral 

component.

The proximal flow characteristics are those of a 

pyroclastic surge. The deposits, however, have some of 

the characteristics of deposits of pyroclastic flows: 

heterogeneity, variability in sorting and grain size, and 

occurrence in thin deposits mantling steep slopes. They 

differ from most previously described surge deposits, 

including ash clouds or ash cloud surges (Crandell and 

Mullineaux, 1973; Crandell, 1987; and Fisher, 1979), 

because they are coarser and only indistinctly bedded. 

In these respects, however, they are similar to the ash 

cloud deposits at Mount Pelee described by Fisher and 

Heiken (1982). Sorting of the slope deposits is poorer 

than that of most surge deposits of equivalent mean 

size. (Compare Fisher and Waters, 1969, table 1; Waters 

and Fisher, 1971, table 1; Sparks, 1976, fig. 12; Wohletz 

and Sheridan, 1979, table 2; and Sheridan and Wohletz, 

1983, fig. 3.) The crossbedding typical of nearly all 

previously described surge deposits (Wohletz and 

Sheridan, 1979), including a unit of the 1980 lateral blast 

deposits (Waitt, 1981), is absent from the 1980 South 

Fork surge deposits.

The basal avalanches are interpreted to have formed 

by gravity segregation like the block and ash flows at 

the base of the ash cloud surge at Mount Pelee (Fisher 

and Heiken, 1982). They are similar to some deposits 

described as lithic pyroclastic flows, such as parts of 

the Pine Creek volcanic assemblage on the south side 

of Mount St. Helens (Crandell and Mullineaux, 1973) 

units not known to be associated with a pyroclastic 

surge other than an ash cloud. Lithic pyroclastic flows 

generally were emplaced at higher temperatures than 

the avalanches, as a result of their formation by dome 

collapse or explosion.

COMPARISON WITH KALAMA- AND 

POST-KALAMA-AGE DEPOSITS

The avalanche deposits are also similar to parts of a 

Kalama- and post-Kalama-age sequence of rubbly avalanches and lahars in the South Fork channel in a reach 

extending from the base of the cone to more than 12 km 

from the crater (dating and stratigraphy in Scott, in 

press). The sequence includes an avalanche and lahar 

associated with a pyroclastic surge deposit that is strikingly similar to the deposit of the 1980 lateral blast.

The effects of the mainly Kalama-age flows on vegetation were similar to those of the 1980 flow. The upstream sides of a few exhumed standing trees near the 

center of the largest flow show shredding and highvelocity rock impacts like those of the overrun modern 

trees. Evidence of temperatures high enough to singe 

wood is mainly lacking, but trees extending into the 

surge were charred on their upstream sides, like many 

of the trees exposed to the 1980 lateral blast. The depth 

of charring on trunks increased with distance into the 

flow. Fine twigs were preserved on smaller trees overrun by other flows, near the point where the 1980 surge 

had collapsed into a lahar and had begun to form the 

"bayonet" trees characteristic of the water-mobilized 

flow. This evidence, combined with the essential textural identity of the modern, Kalama-age, and postKalama-age avalanche and lahar deposits, suggests that 

the older flows also formed by the deflation of 

pyroclastic surges and by continuation of the basal 

avalanches.

The continuing flow of the avalanches, the erosion of 

soil, and the addition of water contributed to the downstream evolution of debris flows with muddy matrices. 

The Kalama- and post-Kalama-age avalanches were 

much larger than those in 1980 and probably constituted a greater proportion of the catastrophically 

ejected mass. Lahars forming directly from the smaller 

1980 avalanches were not significant. In 1980. transformation of the upper, more dispersed part of the surge 

(but not the initial cloudlike phase) produced the main 

South Fork lahar. In the history of the volcano, many 

types of catastrophic-ejection behavior probably have 

occurred, from nearly pure pyroclastic surge, as in the 

South Fork in 1980, to explosively initiated avalanches 

with little or no associated surge. Lahars may have 

originated from any combination of these.

The direct transformation of a lithic pyroclastic surge 

or flow to a lahar is probably not unique to the 1980 

eruption at Mount St. Helens. The conditions existing 

at the time of the 1980 explosive eruption, including 

saturation of the cone, would have yielded water- or 

steam-rich proximal surges, flows, and avalanches with 

any comparable previous eruption. Even avalanches not
FLOW TRANSFORMATIONS A57

associated with volcanism could have transformed to 

lahars under these conditions. Saturation conditions 

also probably exist presently at other Cascade Range 

volcanoes (Janda, 1981). The evolution of lahars at 

Mount Rainier from huge avalanches, some of which 

were probably not associated with volcanism, confirms 

the importance of that lahar-forming mechanism.

FLOOD SURGE TO DEBRIS FLOW

The most common way lahars have formed and probably will continue to form during the modern eruptive 

period is by transformation of flood surges. The flood 

surges have been produced by the interaction of heat 

and pyroclasts with snowpack and, less importantly, 

glacial ice. Meltwater discharge from the major explosive eruption of May 18, 1980, produced the flood 

surge that transformed into the second lahar in the 

South Fork Toutle River. Surges also resulted from 

subsequent dome-building events in 1982-1984. In addition, part of the 1980 North Fork lahar may have been 

formed by transformation of spring discharge on the 

surface of the debris avalanche.

The bulking of flood surges with sediment occurs 

rapidly on steep and commonly denuded volcanic slopes 

underlain by fragmental pyroclastic debris. Even small 

discharges pass through the range of hyperconcentrated 

flow and transform to debris flow in such a setting. An 

example of rapid bulking is the small flood surge released on the surface of the debris avalanche in August 

1980. The peak sediment concentration observed in this 

flow at Kid Valley was 568,000 mg/L at a discharge of 

only 50 m3/s (Dinehart and others, 1981), but it had 

probably debulked continuously below the end of the 

debris avalancha On the avalanche surface, the small 

flow may have traveled briefly as a debris flow. The 

deposits were rapidly eroded and were not observed.

The snowmelt surge producing the 1982 lahar was 

amplified by brief impoundment and subsequent lake 

breaching in the crater (Waitt and others, 1983). The 

much larger Pine Creek-age lahars formed from flood 

surges originating entirely as lake breakouts. The 

evidence that their formative surges were derived from 

ancestral Spirit Lake includes a comparison of the 

amount of water required to produce at least the largest 

of the flows (PC 1), with the amount likely from other 

sources (Scott, in press). Mapping of the area inundated 

by that huge flow (Scott and Janda, 1982) established 

that the transformation to debris flow was not complete 

until the surge had traveled 20 km from the northern 

base of the mountain. Harrys Ridge is the site of the 

1980 blockage and possibly also of the ancient blockages. The characteristic peak-flow deposits of the lahar

appear gradually over a channel interval of several 

kilometers near the downstream end of the broad valley 

from which the alluvium was eroded to form the lahar. 

The transformation to debris flow in a valley beyond 

the base of the volcano was more gradual than with 

smaller flows, because of the vast amount of alluvium 

required.

A large debris flow (but one smaller than PC 1) was 

formed by bulking of a lake-breakout flood surge in the 

Bol'shaya Almatinka River in Russia in 1977 (Yesenov 

and Degovets, 1979). That flow began as a relatively 

small surge (maximum discharge of 210 m3/s) from 

breakout of a moraine-impounded lake. The flood surge 

transformed to a debris flow immediately by bulking, 

and the flow continued to enlarge by mobilization of 

channel alluvium and contributions from slope failures, 

evolving into a succession of waves. Although the 

dynamics of flow at the peak were similar to those of 

the PC 1 lahar, the continuing large expansion of 

discharge and volume are not seen in the behavior of 

the lahar at Mount St. Helens. The progressive attenuation of the lahar peak, once bulking was complete, is 

well documented by the mapping of the inundation area 

(peak stage) of flow PC 1 (Scott and Janda, 1982).

The probable volume of alluvium bulked into the PC 

1 lahar does not require unrealistic amounts of 

upstream erosion. The area of pre-1980 flood plain in 

the channel interval between Spirit Lake and a point 

near Camp Baker is 31.6 km2. A minimum of 0.03 km3 

of uneroded PC 1 deposits remain in the river system, 

a volume derived from 1 m of net, flood-plain-wide erosion. The total volume of the flow can only be crudely 

estimated. Based on the assumption that the flood wave 

was sharply peaked, a total volume of the lahar flood 

wave of about 1 km3 is possible. With adjustments for 

water content and for sediment derived from the 

blockage deposits, flood-plain-wide erosion of 10-20 m 

could have produced the flow. Erosion of 10-15 m was 

recorded during passage of the Russian debris flow cited 

above. Alluvial megaclasts in the PC 1 deposit document at least 5 m of erosion at a point.

DEBRIS FLOW TO 

HYPERCONCENTRATED STREAMFLOW

EVIDENCE FOR TRANSFORMATION

The evidence for the evolutionary transformation 

from lahars to hyperconcentrated lahar-runout flows 

comes from the vertical and longitudinal changes in texture and structure in the flow units. The transition 

facies a vertical sequence of hyperconcentrated flow 

deposits overlain by debris flow deposits (fig. 10) is
A58 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

characteristic of a 19-km interval of the first peak of 

the 1982 lahar, and less than a 9-km interval of the 

second peak (fig. 36). Upstream from these intervals 

each flow unit consists mainly of debris flow deposits; 

downstream from these intervals, the units are composed of hyperconcentrated flow deposits. The same 

textural sequence occurs throughout a 9-km interval of 

the 1980 South Fork lahar and runout flow (fig. 36), but 

the changes could not be observed as continuously 

because of the thick overlying deposits of the North 

Fork lahar. Deposits of the 1982 lahar and its runout

flow were observed continuously from the end of debris 

avalanche to the Toutle-Cowlitz River confluence within 

days of their emplacement.

The transition facies occurs in the deposits of flows 

large enough to have inundated flood plains in previous 

eruptive periods, so it is not just the product of flow 

confined to channels. I have also seen it in association 

with relatively granular lahars at several other Cascade 

Range volcanoes. It may be evidence of a general mode 

of behavior of lahars with small amounts of cohesive 

sediment and particle-supported matrices.

-1980 South Fork laharDeposits overlain by second 

1980 South Fork lahar 

upstream of confluence

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EXPLANATION

UNSTRATIFIED DEPOSITS WITH DISPERSED PEBBLEAND COBBLE-SIZE CLASTS

MASSIVE AND CRUDELY STRATIFIED DEPOSITS 

WITHOUT DISPERSED COARSE CLASTS

BOUNDARY BETWEEN THE ABOVE TYPES OF 

DEPOSITS

CLASTS OF LOW-DENSITY LITHOLOGIES Pumice, 

juvenile microvesicular dacite (1980), and older 

hydrothermally altered dacite

CLASTS OF NORMAL-DENSITY LITHOLOGIES Dacite, 

andesite, basalt, etc.

gb NORMAL GRADED BEDDING 

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p HIGH PUMICE CONTENT 

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DEVIATION (SORTING)
FLOW TRANSFORMATIONS A59

If the deposits of the transition intervals of the 

modern (fig. 36) and ancient flows were exposed continuously, they would look generally like the diagram 

of figure 37. The overlying debris flow deposits thin

progressively and disappear at the point that flow transformation was complete. The vertical contact between 

the two deposit types in each flow unit is transitional. 

The rate of transformation depends on the volume of

123° 00' 15° 122° 00'

EXPLANATION

1980 CRATER OF 

MOUNT ST. HELENS

PERIMETER OF LATERAL BLAST ZONE

DEBRIS AVALANCHELahars on surface 

not shown 

1980 LAHAR INUNDATION Note deposits in Columbia 

River 

© STUDY SITE

® GAGING STATION

46° 

15'

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FIGURE 36. Depositional units of the 1980 South Fork lahar, the 1982 lahar, and the runout flows evolved from those lahars. Variable 

thicknesses of the units reflect local changes in flow depth due to channel configuration, as well as downstream changes in discharge.
AGO LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

Entire flow 

is debris 

flow Transitional interval

Entire flow is 

hypercon centrated 

streamflow

Confluence with 

^____^^ major tributary

Flow

FIGURE 37. Downstream change in texture resulting from 

transformation of debris flow to hyperconcentrated streamflow. 

Acceleration of the change occurs at the confluence with large 

tributaries, reflecting an increased volume of water to dilute the 

debris flow. Diagrammatic hydrographs show accompanying 

changes in character of the flow wave.

overrun streamflow in the channel, as shown by the increased rate of replacement of debris flow deposits by 

hyperconcentrated flow deposits at and downstream 

from major tributaries. The final part of the change in 

the 1980 South Fork lahar to its runout phase occurred 

in response to mixing with North Fork streamflow in 

the reach downstream from the confluence of the forks. 

Similarly, the final part of the conversion of the 1982 

lahar to runout flow began with the addition of Green 

River streamflow (at km 39.7 in fig. 36). Both the 1980 

and 1982 runout flows transformed to normal streamflow upon mixing with the Cowlitz River flow.

CHARACTERISTICS OF DEPOSITS OF 

HYPERCONCENTRATED LAHAR-RUNOUT FLOWS

The lahar-runout flow deposits are distinct from both 

the lahar deposits, which occur upstream, and the

FIGURE 38. Deposits of the main peak of the 1982 runout flow at Kid Valley, near the downstream end of the transition interval. Note 

cobbles partly of low-density lithologies protruding from the flow surface. Point C is the level of maximum clast size at the top of the 

inversely graded part of the flow unit. Thickness of the deposits (overlying alluvium) is 1.4 m.
FLOW TRANSFORMATIONS A61

deposits of normal streamflow farther downstream. As 

a group, the following characteristics define laharrunout flow deposits and can be used to identify that 

type of flow in the stratigraphic record where the 

complete transition from a lahar is not visible (tables 

1 and 5). These characteristics apply to the hyperconcentrated runout deposits in the transition interval, 

where they form the basal part of the flow units, as well 

as downstream where they form the entire unit.

LACK OF STRATIFICATION IN PARTICLE- AND 

C LAST-SUPPORTED DEPOSITS

The most obvious characteristics of lahar-runout 

deposits are their generally massive or crudely stratified 

nature (fig. 36) and their particle- or clast-supported texture. Except for their massive appearance, both the 

modern and ancient depositional units, which range up 

to 2.5 m thick, might be mistaken for units of normal 

fluvial sand. Throughout the river system the latter 

show either crossbedding or well-developed planar 

stratification. The poor or nonexistent stratification in 

the runout deposits reflects the rapid and generally 

uninterrupted deposition from the runout flood waves.

The degree to which stratification may be developed 

depends on channel geometry. The deposits near the 

main channel thalweg, preserved by overlying lahar 

deposits (1980 runout flow) or observed before erosion 

(1982 runout flows), are more likely to be more completely massive. The 1982 runout flow eroded many spoil 

piles, triggering periodic fluxes of introduced sediment 

that locally created stratification downstream from and 

on the same side of the channel as the source. Stratification thus appeared slightly better developed in lateral 

and backwater areas of both the 1980 and 1982 flows. 

A thin (<1 cm), silt-rich stratum locally occurred near 

the middle of the 1982 runout-flow units.

INVERSE GRADING

The dominant feature of the 1980 and 1982 transition 

units is inverse grading in the basal part of each depositional unit (which is different in origin from the inverse 

grading in the lahars). It commonly extends continuously from near the base of the deposit in the portion 

representing the earlier, hyperconcentrated part of the 

flow wave, to the level of maximum clast size in the upper, debris flow portion of the bed (figs. 36, 38, and 39). 

The level of maximum clast size rises progressively 

downstream (fig. 37) as the debris flow subunit becomes 

thinner. The grading is interrupted locally by finer 

grained intervals or discontinuities, especially near 

lateral channel boundaries, but the continuous upward

increase in particle size is the more general case (figs. 

39 and 40). After the complete transition to hyperconcentrated flow downstream, inverse grading is present but may be most pronounced in subunits (fig. 36). 

Generally, inverse grading disappears downstream. In 

the most distal runout-flow deposits a small degree of 

overall normal grading may even be present.

High water mark v 

Coating of silt and clay 

0.5m

Mean size

Mz 

<£> mm

*H- 1.63 0.32

 -0.20 1.1

-1.52 2.8

-0.95 1.9

-0.28 1.2

0.04 0.96

0.93 0.52

1.23 0.42 1.45 

1.40 0.37 1.10 

1.32 0.40 1.25 

Debris flow texture

Silt and clay 

(percent)

9.5 

5.7 

4.1

* 4.2 

4.6 

5.4 

5.6

5.0 

4.1 

4.1

FIGURE 39. Textural change in the transition fades of the 1982 

lahar, 0.4 km upstream from Kid Valley (at locality shown in 

fig. 38). Points labeled A represent the flow boundary and the 

stage at the time of peak discharge, B represents the inversely graded central part of the unit, and C is the level of maximum clast size.

EXPLANATION

Debris flow 

Hyperconcentrated

1234

0.5 0.25 0.1250.0625

FIGURE 40. Cumulative curves of particle sizes in the inversely 

graded part of the transition facies unit shown in figure 39. 

Arrows point stratigraphically upward in unit and indicate the 

progressive changes in flow deposits at a point. Points A, B, and 

C are shown in figure 39.
A62 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

COARSE-TAIL GRADING OF LOW-DENSITY C LASTS

Low-density lithic and organic fragments are expectable in the upper parts and on the surfaces of lahars 

and lahar-runout deposits. The concentration of wood 

fragments at the base of the 1980 runout flow units and 

of pumice near the base of some 1982 and pre-1980 

units reflects the extremely rapid deposition from a 

hyperconcentrated flood surge. The front of the 1980 

runout flow wave was marked by a moving mass of logs. 

This churning framework could have caused extreme 

but transitory turbulence producing the mainly 

framework-supported deposit of wood fragments that 

was buried by subsequent rapid deposition that approached mass emplacement in character. Many of the 

wood fragments are rounded, and the matrix is sand like 

that of the overlying deposit, as is the matrix in the 

units where pumice is concentrated at the base. The 

resulting grading is coarse-tail grading in which the 

coarse part of the distribution consists of low-density 

particles that in normal flows would be buoyed to the 

surface. This type of grading i& attributed primarily to 

local extreme turbulence at the flow front, unlike the 

inverse grading in the transition interval that relates 

to the transformation of debris flow to runout flow. Intense turbulence is characteristic of the fronts of large, 

high-velocity debris flows in Yunnan Province, China 

(Hua Guo-xiang, Chengdu Univ. of Science and 

Technology, oral commun., 1985).

CONCENTRATION OF LOW-DENSITY C LASTS 

NEAR LATERAL BOUNDARIES

Pumice and wood fragments concentrated near the 

sides of the channel or on the inundated flood plain are 

distinctive of the modern runout flows and some of their 

ancient analogs. Where pumice is present in any runout 

flow unit, it is abundant near lateral flow boundaries, 

where it may occur in stringers that thicken toward the 

banks or valley side slopes. It accumulated in backwater 

areas where it then was incorporated in the deposit by 

rapid deposition from hyperconcentrated flow.

SORTING

The most definitive characteristic of hyperconcentrated runout-flow deposits is sorting in a range intermediate between that of debris flow deposits and 

most normal streamflow deposits. The 1982 debris flow 

deposits both upstream from the transition (fig. 36 

and curve a, fig. 41) and in the upper parts of the flow 

units in the transition interval (fig. 36 and curve b, fig.

41) have values of sorting that approach 1.8-2.0 4> as 

an approximate lower value. Although the deposits 

have low values of total silt and clay (typically 4-10 percent), relative to the 1980 lahars, the degree of clast 

dispersal is characteristic of debris flow, albeit toward 

the low-strength and noncohesive end of the spectrum. 

Similarly, the sorting of the distal 1980 South Fork flow 

declines (improves) downstream to at least 2.2 <£ (fig. 

20). The 1980 and 1982 hyperconcentrated flow deposits 

in the transition facies (fig. 36 and curve c, fig. 41) and 

downstream from the transition interval (fig. 36 and 

curve d, fig. 41) have values of sorting in the range of 

1.1-1.6 4>. This improvement in sorting is reflected in 

grain and clast support, greater presence of void space, 

and lack of dispersed coarse clasts.

Judging by the dispersion of sediment sizes, the 

difference between hyperconcentrated runout-flow 

deposits and normal streamflow deposits is more subtle than the difference between debris flow and runoutflow deposits. Nevertheless, the difference is apparent 

where the deposits are juxtaposed as, for example, 

where the 1982 runout flow deposits were emplaced in 

bars and berms in contact with normal streamflow 

deposits of similar mean size. Like the 1981 and 1982 

streamflow deposits illustrated in figure 41, most fluvial 

sand has sorting in the range of 0.5-0.8 <£ (Friedman, 

1962). However, the sorting of some, mainly fine-grained 

streamflow deposits in the river system overlaps the 

1.1-1.6 4> range characteristic of the runout deposits. 

Runout deposits can be distinguished from streamflow 

deposits in the river system if they have sorting in the 

1.1-1.6 <£ range, have a mean size greater than approximately 2.5 4> (0.17 mm), and occur in units that have 

the other characteristics described here.

TRANSITIONAL CUMULATIVE CURVES

The progressive steepening of the cumulative grainsize curves representing a distal lahar and the runout 

flow derived from it (figs. 40 and 41) is similar to the 

pattern of vertical changes observed in the depositional 

unit of a subaqueous sediment gravity flow (Visher, 

1969, fig. 20A). That pattern records the progressively 

more dilute nature of a single flow, both laterally and 

vertically. The curves representing the successive, 

longitudinal parts of flows in the river system likewise 

appear to record gradual, evolutionary change. A 

reverse pattern of sequential changes is seen in ascending a vertical section of the inversely graded part of the 

transition facies (figs. 39 and 40) for reasons described 

in the following section on sediment transport.
FLOW TRANSFORMATIONS A63

2

5

10

20 iz 30

LU

o 40

* 50

°- 60

70

LLJ

> 80

l-

"* 90

l>

D

o 98 

99

99.8

99.9

99.99

/ Lower part of 

Upper part of 1982x7 , 982 transition 

transition facies / farie« ,=2.390 / ^^

Peak flood berm of

1980-81 runoff season

^=0.670

j

/" Peak flood berm of

/ 1981-82 runoff season 

o-=0.750

1982 lahar 

runout flow 

°-=1.150

f 9

s Cross-bedded sand 

of Pine Creek age 

o- =0.940

EXPLANATION

 Debris flow

 Lahar-runout flow

 Streamflow

-Pebbles- -Sand- -Silt- --Clay-

-6 

64

-5 

32

-4 

16

-3 

8

-2

4

1 2

0.5 0.25

SIZE

3

0.125

4

0.0625

5

0.031

6 7

0.016 0.008

8 0 

.004 mm

FIGURE 41. Cumulative curves of particle sizes in representative samples of the 1982 lahar and lahar-runout flow deposits, compared 

with those of modern and ancient streamflow deposits. Localities: a, 1.5 km upstream from the North Fork debris basin; b and 

c, St. Helens (community); d, Coal Bank Bridge; e, Green Mountain Mill; f and g, near the confluence of the forks of the Toutle River.

LACK OF COARSE INFLECTION POINT 

IN CUMULATIVE CURVES

The cumulative grain-size curves of 18 samples of the 

runout deposits lack a coarse inflection point, illustrating the generally positive skewness of the 

distributions. The S-shape of curve g in figure 41, which 

shows both a coarse and a fine inflection, is typical of 

most fluvial deposits. The inflections are commonly interpreted to be the result of subpopulations of grains 

moved by different transport mechanisms, with the 

coarser inflection believed to be the point at which most 

coarser sediment that represented by the part of the 

curve with lower slope forms a subpopulation moved 

by rolling or sliding (see review by Visher, 1969) or traction, equivalent to bed load (Middleton, 1976; Bridge, 

1981). The lack of this subpopulation in the runout 

deposits implies that the coarser sediment was not 

transported in a mode separate from that moving the 

remainder of the deposit.

SEDIMENT TRANSPORT IN THE LAHAR-RUNOUT FLOW

OF MARCH 19-20, 1982, AND CORRELATION WITH

DEPOSIT CHARACTERISTICS

Direct measurements of sediment concentrations in 

a lahar-runout flow, downstream from the transition interval, were obtained around midnight on March 19-20, 

1982, by U.S. Geological Survey personnel at the three 

gaging stations shown on the Toutle River in figure 36. 

Samples were collected from the upper parts of the flow 

with heavily weighted open-mouthed bottles (mouth 

diameter 36 mm) lowered into the center of the channel 

from bridges and cableways. Consequently they do not 

represent integrated samples of the complete depth of 

flow. The concentration data indicate the same generally uninterrupted wave of sediment recorded in the 

deposits.

The most complete record of the 1982 runout flow, 

collected at Tower Road (fig. 42), shows that sediment 

concentrations characteristic of hyperconcentrated flow
A64 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

700

600

500

400

300

200

100

0

1000

900

800

700

600

500

400

300

200

100

0

1200

u. 1000

800

- 600

2

g

^ 400

i i i i i i i r 

A ./Lag between A and B=34 minutes

% S-Q.5SS8<oSinc

11 Hyperconcentrated I 

streamflow

EXPLANATION

 Total concentration 

A Silt and clay (<40, 0.0625 mm) _ 

o Sand (4 to 10, 0.0625 2.0 mm 

+ Granules and pebbles ( 1 to 

 60, 2-64 mm)

Lower limit of 

hyperconcentrated 

streamflow

200

2300 2400 0100 0200 0300 0400 0500 0600 0700 0800 0900

March 19, 

1982

March 20, 

1982

TIME, IN HOURS

FIGURE 42. Discharge and sediment concentration of the 1982 runout flow at Tower Road.

(530,000-1,590,000 mg/L) were reached after peak 

discharge and then continued throughout much of the 

recession. As at the upstream station at Kid Valley, concentrations at Tower Road approached or exceeded 

1,000,000 mg/L for at least an hour. Only slightly lower 

concentrations were present for a similar duration at 

the downstream station at Highway 99, 1.6 km above 

the confluence with the Cowlitz River. Peak discharge 

declined from 960 m3/s at Kid Valley, to 650 m3/s at 

Tower Road, and to 450 m3/s at Highway 99 (Pierson 

and Scott, 1985). Dilution of the runout flow to normal 

streamflow by mixing with Cowlitz River water is 

shown by the data from the Castle Rock gaging station, 

3.9 km below the confluence, where the river stage rose 

only 0.3 m and the maximum concentration was only 

158,000 mg/L.

The runout flood wave at Tower Road consisted of 

water during the rise and of water and sediment during the recession (fig. 42). At the peak streamflow 

discharge (point A on fig. 42), 94 percent of the flow 

volume was water. At the time of maximum sediment 

discharge (point B), 34 minutes into the recession, 44 

percent of the flow volume (67 percent by weight) consisted of sediment. The conversion to a clear-water 

equivalent for the peak streamflow discharge thus is 

minor (a reduction of 6 percent), yet a corresponding correction of the total runout flow volume, reflected by the 

shaded area under the streamflow hydrograph in figure 

42, is substantial (a reduction of 39 percent).

The common relation in a flood wave is for peak sediment discharge to precede the streamflow peak and for 

the resulting deposits to be normally graded. After flow 

competence has peaked and bar deposition has begun, 

both the concentration and the size of the suspended 

sediment in the flow typically decline. Rates of bed load 

discharge normally also fall throughout flow recession. 

Yet, in the 1982 lahar-runout flow and in some normal 

flood flows in the Toutle-Cowlitz system, concentration 

initially increased as streamflow declined (fig. 42). This 

lagging concentration peak, its high level, and the 

coarseness of sediment in the runout flows cause the 

dominant grading to be inverse. Other instances in 

which peak sediment discharge lags behind the flow 

peak may result from the contribution of sediment from 

a downstream subwatershed during flow recession or 

from a long travel time during which wave celerity exceeds mean water velocity. A long lag in the concentration peak generally reflects a significant fine-material 

load (Heidel, 1956).

The sediment transport curve of the 1982 runout flow 

(fig. 43) is a hysteresis loop reversed from the normal 

direction. (Compare Leopold and others, 1964, fig. 7-15, 

for instance.) Sediment load (discharge by weight) on 

the "falling" limb of the hydrograph is more than an
FLOW TRANSFORMATIONS A65

order of magnitude greater than that at the same height 

on the "rising" limb. This dramatic increase in sediment 

concentration, discharge, and size (fig. 42) as streamflow 

discharge initially falls illustrates the dynamics that 

produce the inverse grading. The lag between peak 

discharge and peak concentration was 34 minutes at 

Tower Road and increased downstream, to 50 minutes 

at Highway 99 and to 1 hour at Castle Rock on the 

Cowlitz River. The lag intervals are approximate in that 

the sediment sampling was periodic.

The variations in the proportions of water and sediment and in sediment size in the runout flood wave correlate with the texture of the lateral berms. The 

correlation is shown by comparing sediment transport 

in the lahar-runout flow at Tower Road (fig. 42) and the 

textural sequence in the transition facies near Kid 

Valley (fig. 39). Deposit grading and geometry are 

similar at Tower Road and Kid Valley; the deposits at 

Tower Road, however, are downstream from the transition interval and do not record the transition from 

debris flow to lahar-runout flow. Although only the flow 

recession was sampled at Kid Valley, beginning about 

90 minutes after the streamflow peak, the complete pattern of transport there was probably similar to that at 

Tower Road, but with a shorter lag period. Extrapolating from the lag at downstream stations to the 

point of no lag (the upstream end of the transition interval) shows the lag at Kid Valley was approximately 

15 minutes.

Deposition can be assumed to have begun after peak 

streamflow discharge (point A in figs. 39 and 42). Maximum scour depth normally coincides with peak 

streamflow discharge and, given the low proportion of 

sediment in the peak runout flow, almost certainly did 

in this case. During recession, however, the higher sediment content increased the density of the flow and partially offset the effect of falling depth on boundary 

shear. Nevertheless, the point of maximum sediment 

concentration and sediment discharge (synchronous in 

this case; point B) was probably near the point of most 

rapid deposition. It probably corresponds approximately with the middle of the inversely graded part of 

the unit (figs. 39 and 42). Continuing the correlation, 

the coarsest upper part of the inversely graded berm 

deposits corresponds with the maximum flux of gravel 

(point C in figs. 39 and 42). The uppermost, normally 

graded part of the berms correlates with the initial part 

of the subsequent decline in both sediment and water 

discharge and the accompanying fining of sediment. As 

recession continued, deposits formed earlier during the 

flow were incised, and the concentration of silt and clay 

peaked (point D in fig. 42).

Thus the peak flow preceded the deposition of the 

berm surface; the surface formed well into flow

100,000

go 

I- cc

10

LU O

5

Q 

LU

10,000

1000

.$7 -si

100 1

10 100 1000 10,000 

WATER DISCHARGE, IN CUBIC METERS PER SECOND

FIGURE 43. Sediment transport curve of the lahar-runout flow of 

March 19-20, 1982, at Tower Road. Water discharge was 

calculated by subtracting sediment discharge from the streamflow 

discharge.

recession. This conclusion is supported by the character of the thin, fine-grained coating of the peak-flow 

boundary (fig. 39), which evidences the dilute streamflow at peak streamflow discharge rather than the 

sediment-charged flow during berm emplacement. The 

streamflow hydrographs (such as fig. 42) show that the 

coating must have come from the peak and could not 

have come from a dilute water flow overriding or following a sediment gravity flow, as I had proposed earlier 

for flows in steep, semiarid watershed (Scott, 1971). The 

berm surface probably formed when the entire channel 

was filled with deposits and flowing sediment to the 

level of that surface or to a dynamically related level. 

The variation in concentrations of various size fractions (fig. 42) shows that the sediment component of 

the flow at Tower Road was virtually a wave of sand. 

At the peak concentration (point B), 73 percent of the 

sediment was sand. Only then did larger clasts appear 

in the flow, reaching a peak (point C) that lagged the 

peak concentration by approximately another half hour. 

The further lag of the peak in silt and clay content 

(point D) can be ascribed in part to volumetric displacement of the fine-material load by coarser sediment 

earlier in the flow and in part to the addition during 

recession of fine sediment draining from incised, earlier 

deposits. It may also reflect the downstream effect of 

a second, lower peak in the flow, which was recorded 

by upstream deposits (fig. 36) but did not extend 

tolbwer Road, based on the gage height record,
A66 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

eyewitnesses, and the deposits. The general case, as in 

this instance, is for the fine-material load in hyperconcentrated flows in the river system to be highly persistent (Dinehart, 1983).

EFFECT OF POSTDEPOSITIONAL DRAINAGE ON 

DEPOSIT TEXTURE

During the field study of the 1982 deposits, large 

areas of both the runout and transition facies were as 

firm as sand on a beach; only local areas of the upper 

debris flow deposits of the transition facies retained 

pore fluid and remained quick. Observers of the 1982 

runout flow (R. L. Dinehart, written commun., 1982) 

noted that the deposits were firm when they first appeared above the receding flow. The firm deposits were 

located near berm cuthanks; the quick deposits remained only locally in subordinate backwater areas 

away from channels and in which water was replenished 

from local runoff. The firmness therefore was largely 

the result of interstitial drainage in hydraulic continuity with the recession flow. As the upper, debris flow 

parts of the transition units also drained, they probably 

lost some fines that were retained in most of the 

upstream, more poorly sorted distal debris flow 

deposits. (See Pierson and Scott, 1985.) Subsequent 

observations showed both types of transition-fades 

deposits to function as permeable aquifers. The amount 

of silt and clay remaining in the transition debris flow 

deposits was similar to the low values in the runout 

deposits, but more variable (fig. 36). Drainage from the 

debris flow parts of the transition facies was facilitated 

by the inversely graded transition from the underlying 

runout sand.

At time of deposition the interstitial space in the 

rapidly deposited, framework-supported units was filled 

by fluid with high concentrations (but probably not 

hyperconcentrations) of silt and clay, much of which 

probably drained as the flow receded. If a similar proportion of fine sediment is subtracted from the concentrations measured in the flow, the adjusted distribution 

statistics closely resemble those of the deposited sediment. The sediment in the flow samples with concentrations over 1,000,000 mg/L consisted of 28-31 percent 

silt and clay at Kid Valley (4 samples) and 23-29 percent silt and clay at Tower Road (5 samples). These 

values are higher than the general range of 4-6 percent 

(one fine-grained sample has 9.4 percent silt and clay 

at Highway 99) in the deposits (fig. 36). By assuming 

the loss of all but 5 percent of the silt and clay from 

the sediment in the flow samples, the sorting is reduced 

from more than 2.0 <£ in cases where the distribution 

was not skewed by a single large clast, to the 1.1-1.6 

0 interval that is characteristic of the modern and

ancient runout deposits (fig. 42, table 8). Some fine sediment was, of course, regained by the flow directly from 

the accreting flow boundary, but a significant part 

would have been incorporated in the deposit and then 

would have accompanied the draining interstitial fluid.

ORIGIN AND EVOLUTION OF LAHAR-RUNOUT FLOWS

The transformation of a lahar to a lahar-runout flow 

involved the fluid component of the flow separating 

from and outrunning the sediment in a manner more 

complex than normal, progressive deposition as flow 

competency fell. The downstream increase in lag time 

between the streamflow and concentration peaks illustrates the continuous separation of the two phases 

once transformation began. The lag developed because 

the close juxtaposition of particles in the hyperconcentrated flow inhibited the ability of the fluid to move the 

sediment at the same speed.

The transition facies (fig. 10) indicates that the 

transformation was triggered by the mixing of the 

leading part of the lahar with streamflow in the channel. Successive parts of the debris flow wave continued 

for progressively greater distances in the transition interval (fig. 34) before increased water content caused 

a significant decline in yield strength and conversion 

to hyperconcentrated streamflow. The debris flow flood 

wave was transformed progressively from leading edge 

to tail. Thus were formed the inversely graded units consisting of downstream-thinning debris flow deposits 

overlying downstream-thickening hyperconcentrated 

flow deposits.

An expected mode of formation of the runout flows, 

given the common behavior of debris flows in alpine and 

semiarid settings (Costa, 1984), was for the debris-flow 

front to stop and be overrun by the more fluid recession flow. However, the progressive changes in the transition facies, as well as the systematic change in texture 

of the distal lahar flow, establish the direct evolution 

of one flow type from the other. The evolution is 

facilitated by the miscibility of the grain- and clastsupported lahars with streamflow, which results from 

their low clay content and consequent low cohesiveness. 

The large majority of lahars in the river system have 

such characteristics, as a result of their formative flow 

transformations from flood and pyroclastic surges. The 

formation of lahar-runout flows was facilitated by the 

occurrence of lahars in channels with substantial 

streamflow volumes. Based on the large size of some 

of the lahars transformed this way, however, the process can require only a relatively small volume of water. 

The older overbank runout flows show that, once the 

transformation began, textural change continued with 

little additional water.
FLOW TRANSFORMATIONS A67

TABLE 8. Measured peak sediment concentration and adjusted mean size and sorting values for the lahar-runout flow of March 19-20, 1982,

at Kid Valley and Highway 99

[Mean size and sorting adjusted as described in text. Values at Tower Road are shown in figure 42]

Time on 

20 March

Sediment

concentration

(mg/L)

Silt and clay 

(percent)

Sand 

(percent)

Granules and

pebbles

(percent)

Adjusted 

mean size, M

Adjusted 

z_ sorting, OQ

(mm)

Kid Valley gaging station 1

0001

0003

0010

0015

1 ,140,000

1,150,000

1 ,160,000

1,090,000

28

29

29

31

69

69

69

67

3

2

2

2

1.77

1.69

1.78

1.86

(0.29)

(0.3D

(0.29)

(0.27)

1.46

1.52

1.51

1.44

Highway 99 gaging station2

0135

0150

0158

0220

0225

882,000

834,000

968,000

977,000

929,000

30

39

31

32

35

69

60

68

67

64

1 1.97

1 2.30

1 1.96

1 2.02

1 2.10

(0.25)

(0.20)

(0.26)

(0.25)

(0.23)

1.31

1 .22

1.32

1.32

1 .29

*Kid Valley peak discharge = .960 mVs at 2230 hours, 19 March. 

2 Highway 99 peak discharge = 450 mVs at 0030 hours, 20 March.

The physical threshold in sediment content between 

debris flow and hyperconcentrated flow can be defined 

because of the rapid decline in strength with small increases in water content near the threshold (Pierson and 

Scott, 1985). The continuous spectrum of textural 

change (figs. 38-41) records the approach to the transition and the continued dilution beyond it, suggesting 

that the multiple physical support mechanisms in debris 

flow and hyperconcentrated streamflow (table 9) are 

part of a continuum. Transitional behavior has been 

hypothesized by Hampton (1975), Carter (1975), and 

Middleton and Hampton (1976), and Lowe (1979 and 

1982). Multiple support mechanisms have been 

postulated by Daido (1971) and Lowe (1979), and were 

observed by Lawson (1982) in small-scale flows.

Dilatant fluid flow and turbulent flow both occur 

within the hyperconcentrated range, but their relative 

importance is controversial. Although end-member 

grain flow (Lowe, 1976, 1979) probably did not occur, 

the question is whether momentum was transfered 

mainly by the collisions or near-collisions of particles 

(dilatant flow) or by the entraining fluid (turbulence). 

The importance of turbulence in some hyperconcentrated flows has been doubted. Nordin (1963) noted that 

in streams of the arid Southwest, turbulence probably 

was not an effective support mechanism above concentrations of approximately 370,000 mg/L. Beverage and 

Culbertson (1964) considered the effects of viscosity,

temperature, buoyancy, and Bagnold dispersive stress 

from grain collisions as possible substitutes for the role 

of turbulence as a support mechanism in hyperconcentrated flow. They did not define the relative importance 

of these other factors, but did note that the increased 

viscosity and density of the flow result in the need for 

less turbulence to support the coarser fractions of the 

load. The hyperconcentrated flow in the arid Southwest 

(Beverage and Culbertson, 1964), the 1980 runout flow 

(Ted Conradi, local resident, oral commun., 1980) and 

the 1982 runout flow (R. L. Dinehart, written commun., 

1982) all had a smooth appearance that reflected a 

significant damping of large-scale turbulence.

Turbulence was well developed at the fronts of the 

1980 and 1982 runout flows, as indicated by the coarsetail grading of low-density clasts. Reported observations of standing waves the surface expressions of the bed forms known as antidunes, which indicate an upper regime flow show that turbulence also 

could have been effective during later parts of the 1982 

runout flood wave (observations by R. L. Dinehart, S. 

A. Gustafson, K. K. Lee, and W. W. Larson, all U.S. 

Geological Survey, written communs., 1982). At Kid 

Valley, waves developed approximately an hour after 

the streamflow peak. About 45 minutes later, large 

rocks appeared to be rolling on the water surface, which 

was probably shallow flow on the berm surface. At 

Tower Road, large standing waves (over 1.8 m in height)
A68 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

TABLE 9. Sequential changes in flow type and particle support mechanisms during the transformation from lahar, to lahar-runout flow,

to normal streamflow

[Flow direction and decreasing sediment concentration are left to right]

TRANSITION INTERVAL 

(progressive transformation of 

non-Newtonian toward

Newtonian flow)

WTYPE

O

U-

^

(/)

Z

I 

0 111

tE 

QQ.

Q.

V)

X

X

DEBRIS FLOW X

X 

May be noncohesive (more X

LAHAR-RUNOUT FLOW

HYPERCONCENTRATED

STREAMFLOW

rapid transformation), or X 

cohesive (less rapid ^^

transformation / 

X With attributes of gram flow ^ ?

x (Bagnold

X

, 1954, 1956; Lowe, 1976, 1979)

-25

Debris flow, including grain flow (Takahashi, 1 981 )

X

Buoyancy X

Yield strength X Particle interaction ^^

Particle interaction X

Turbulence (at front of flow) X

^~~~^~~» Possible -^^'

^~~ alternations ^^_

LAHAR-RUNOUT FLOW 

DILUTED TO

STREAMFLOW

NORMAL 

STREAMFLOW

^^-^ Turbulence

, , . , / / Turbulence """^ ^-^^ ^^^1 (relative importance of / 

these factors depends ^ Residual effects of:

on sediment concentration/ Yielc 

size, and composition) x Buo^

X

strength

fancy -?

_^ Particle interaction 

(bed load only)

|19 15

Mechanisms other than turbulence

Early part of flow wave

<~60

Later part of flow wave 

>~60 J M >~bO

_ ^ R _ (Beverage and Culbertson, 1 964; 

<~ bU Pierson and Scott, 1 985 )

I

I Turbulence (Qian

"* T~and others, 1 980)

(Beverage and 

Culbertson, 1 964)

DECREASING PERCENT SEDIMENT (VOLUME)

were observed at 0020 to 0030 hours and at 0044 hours 

on March 20, during the lag between peak streamflow 

discharge and peak concentration (fig. 42). The forms 

were sporadically present throughout the remainder of 

the streamflow recession at that station. At Highway 

99, flow near the peak was very smooth and velocity 

appeared lower than that of comparable storm runoff. 

Standing waves developed late in the recession. At Castle Rock, where concentrations did not approach hyperconcentrated levels, flow was continually turbulent, 

"with the look of boils" in contrast to the slick, oily appearance of the upstream flow.

The nature of the deposits, compared to the planebedded or crossbedded berms resulting from normal 

floods, is the strongest evidence that turbulence was 

of subordinate importance during the main wave of

sediment transport. Scour structures that reflect turbulent erosion of the bed are rare within the runout 

deposits. Bedding inclined to the flow, which would 

record dune forms developed during the main flux of 

sediment, is absent. Massive berms like the one shown 

in figure 38 are generally typical. Antidunes were sufficiently common throughout the passage of the flood 

wave, however, to suggest that at least local transformation to turbulent flow may have occurred. Alternating body transformations (Fisher, 1983) may account 

for the dichotomy of evidence; such an alternation in 

laminar and turbulent behavior has been hypothesized 

in submarine sediment gravity flows (Middleton, 1970). 

At least, variations in support mechanisms, such as 

increased turbulence through bar crossings in the generally braided post-1980 channel, are probabla
CONCLUSIONS ON LAHAR TEXTURE AS RELATED TO ORIGIN AND BEHAVIOR A69

Evidence from the deposits of the transition interval 

suggests that the waning debris flow graded transformed to flow resembling grain flow through much of 

the interval, and that, as a sediment-support mechanism, particle collisions were progressively less effective as water content increased and a residual yield 

strength derived from the fine-sediment content declined. Hampton (1975) reported that the grain flow 

mechanism is operative in fine-grained debris flows, in 

addition to particle support by strength and buoyancy. 

The concentrations in the runout flow imply the 

presence of an interstitial fluid within the sand that had 

a high concentration of silt and clay and thus an enhanced viscosity and yield strength. In fact, some 

Chinese investigators establish the lower boundary of 

hyperconcentrated flow at the point where an incipient 

yield strength first appears (Hua Guo-xiang, Chengdu 

Univ. of Science and Technology, oral commun., 1985). 

The interval of high (>250,000 mg/L) but relatively constant fine-material concentration during the peak in 

sand concentration (fig. 42) is probably the period during which grain flow was most effective as a sandtransport mechanism. During that interval the coarse 

phase increased greatly without an accompanying large increase in silt and clay, and dispersive stress 

was an important support mechanism supplementing 

the yield strength provided by the finer phase.

CONCLUSIONS ON LAHAR TEXTURE 

AS RELATED TO ORIGIN AND BEHAVIOR

The interpretation of a stratigraphic record of lahar 

deposits far from a volcano is fundamental to the longterm analysis of lahar magnitude and frequency. These 

aspects are discussed in the companion volume to this 

report (Scott, in press). Less obvious is the extent to 

which lahar origin and behavior are revealed by the 

stratigraphic record. For example, the most common 

lahar in the watershed proves to have been the middle 

segment of a lahar flood wave beginning and ending as 

a streamflow surge.

Two general categories of lahars that inundated flood 

plains many tens of kilometers from Mount St. Helens 

are apparent from the textural data: (1) common lahars 

that have a clay content of clearly less than 3 percent, 

and (2) rare lahars that have a clay content of about 3 

percent or more. The common lahars formed mainly, if 

not entirely, from flow transformations; consequently 

they consisted mainly of volcanic materials previously 

subject to progressive or selective sorting, which had 

removed some or most of the clay in the hydrothermally altered volcanic rocks. Hydraulic sorting in stream 

channels is the most efficient sorting process. Sorting

also can be explosion induced, as by the winnowing of 

elutriated fines from a deflating, air-mobilized pyroclastic surge.

Significant lahars of meteorologic origin probably 

have not occurred since Ape Canyon time. Intense rainfall acting on ash-mantled slopes is a common way 

lahars are produced and may have been an effective 

mechanism during that eruptive stage, when large 

volumes of pumice were produced. The series of lahars 

of that age in the river system contains much pumice, 

but the larger flows in that sequence probably 

originated through the transformation of streamflow 

surges produced from the melting of snow and ice by 

pyroclastic flows.

LAHARS FORMED BY FLOW TRANSFORMATIONS

The three types of lahar-forming flow transformations 

recognized in the modern eruptive period have probably 

operated to produce many of the large lahars originating 

from the northwest half of Mount St. Helens. This conclusion is strengthened by good geologic evidence of 

older flood surges and by necessarily less certain 

evidence of air- or gas-mobilized surges and their 

associated avalanches, and of avalanches without 

associated dispersed phases.

FLOOD SURGES TRANSFORMED TO LAHARS

The location and length of channels where bulking occurred can be inferred from the textures of lahars 

formed from water surges. Particle shapes indicate that 

the two largest lahars in the system were formed of 

stream alluvium well beyond the base of the volcano. 

Other evidence, discussed in the companion volume 

(Scott, in press) and including the character of megaclasts in the deposits, shows that the two large flows 

formed by breakouts through natural dams of volcanic 

origin. Mapping of the flow deposits (Scott and Janda, 

1982) shows that the largest single flow completed the 

transformation to a lahar only after more than 20 km 

of flow from ancestral Spirit Lake, the most probable 

source of the water surge.

The most likely way for smaller lahars to be generated 

in the present eruptive period, as inferred from the 

observed lahar behavior to date, particularly that of the 

second South Fork lahar and the March 1982 flow, is 

by transformation of eruption-induced snowmelt surges. 

Such surges include meltwater surges produced by heat, 

tephra, pumiceous pyroclastic flows, and hot avalanches 

from the dome. These surges may undergo much of their 

bulking on the slopes or shortly beyond the base of the
A70 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

volcano and, with the exception of those associated with 

tephra or pumiceous pyroclastic flows, cannot be readily 

identified from particle shape or composition. Lahars 

generated in this way doubtless have been common in 

previous periods, but few of these have inundated flood 

plains (and thus have been preserved in the stratigraphic record) more than 50 km from the crater. No 

large lahars of a post-Ape Canyon age have a pumice 

content clearly indicating an origin associated with 

snowmelt by tephra or a pumiceous pyroclastic flow. 

The relatively small and locally pumiceous second lahar 

(PC 2) in the Pine Creek sequence is a possible example 

of that flow type, but the content of rounded clasts 

shows that most bulking occurred beyond the base of 

the volcano.

Between the size extremes of lake-breakout lahars and 

those formed from most other types of water surges are 

lahars probably resulting from large lithic pyroclastic 

flows formed from major dome disruptions, and their 

associated snowmelt. Observations at other volcanoes 

show that lahars of significant size have formed this 

way, in addition to those resulting elsewhere from 

pumiceous pyroclastic flows (C, G. Newhall, U.S. Geological Survey, oral commun., 1985). The bulking factors of these flows may be similar to those of flows 

formed by catastrophic ejection, and bulking will occur 

at least partly on the cone. A large proportion of 

prismatically jointed dome rocks (and pieces of disaggregated blocks) may be diagnostic, and the flow may 

be nearly monolithologic or at least have a dominant 

lithology near the volcano. Lahars in this category include a thick unit of probable Swift Creek age in the 

upper South Fork Toutle River (unit 6 of the Disappointment Creek section; Scott, in press), and numerous 

flow units in the sequence of Pine Creek age south of 

the volcano. (See Crandell and Mullineaux, 1973.)

Why, considering the vast volume of lahar deposits 

in 100 km of the Toutle-Cowlitz River system, was the 

pre-1980 Spirit Lake not a lahar-aggraded marsh? The 

lake formed during Smith Creek time as the expanding 

alluvial apron of the cone impinged against Harrys 

Ridge (Crandell, 1987). The radial flow pattern and the 

probability of channel shifting on alluvial surfaces lead 

to the inference that many flows have entered the lake. 

Most of these were probably only partly bulked with 

sediment, and major flows did not transform to lahars 

through bulking until they reached the lower slopes of 

the cone or beyond. Lahars issuing fully formed from 

a crater, like those formed as extensions of block-andash flows from dome collapse, have contributed only 

minor volumes beyond the mountain. Thus, Spirit Lake 

probably has impounded many water surges and dilute 

muddy flows, but has not been filled by lahars. It has

been a source of flood surges, either through water 

displacement or through outlet blockage (by flows with 

higher yield strengths than lahars) and subsequent 

breaching. The blockage provided by the alluvial apron 

of the volcano would normally have been broad, stable, 

and armored with coarse sediment.

CATASTROPHICALLY EJECTED SURGES AND AVALANCHES 

TRANSFORMED TO LAHARS

In texture and in composition and shape of component clasts, lahars formed by pyroclastic surge transformation may be like lahars formed from snowmelt surges. Textural likenesses reflect the similar 

results of sorting by hydraulic and explosion-induced 

mechanisms. The lahars originating with the initial May 

18 eruption, including the 1980 South Fork lahar, illustrate the potential of catastrophic ejection to produce an approximately symmetrical ring of large lahars 

around the volcano. This is a likely origin for the larger 

lahars in the system that have relatively low bulking 

factors near the volcano and that may have a low content of prismatically jointed dome rocks.

This origin is more probable for older lahars that are 

associated with evidence of major explosive events. As 

in 1980, the volcano would probably have been 

saturated with water at the time of any previous major explosive eruption. Also as in 1980, relatively cool, 

water-saturated material may have been ejected 

peripheral to hot lateral blasts. The evidence for lahars 

of this origin in the Kalama eruptive period includes 

their association with deposits closely analogous to 

those of the 1980 lateral blast. Depending on the nature 

and energy of the catastrophic ejection, lahars may have 

formed from surge deflation or avalanche continuation. 

The presence of texturally similar, nonpumiceous lahars 

in aU or most drainages implies a major, central explosive eruption. The Kalama-age lahars seem to be 

symmetrical; texturally similar flows of that age occur 

in all major drainages of the volcano, but uniform timing of the flows has not been demonstrated and the symmetry may reflect the central position of the dome of 

that period.

LAHARS FORMED FROM SLOPE FAILURES OF 

VOLCANIC DEPOSITS NOT SELECTIVELY SORTED

The lahars with a clay content of approximately 3 percent or more obtained much of that clay directly from 

hydrothermally altered volcanic rocks. This alteration 

implies a nearly continuously operating hydrothermal
CONCLUSIONS ON LAHAR TEXTURE AS RELATED TO ORIGIN AND BEHAVIOR A71

system that would also have provided much of the 

water in any catastrophically ejected material. In the 

lower part of the modern crater are exposed 

predominantly dacite domes of pre-Castle Creek age (C. 

A. Hopson, U. Calif., Santa Barbara, written commun., 

1980) that are altered to varying degrees.

The 1980 North Fork lahar was produced directly 

from the surface of the debris avalanche, representing 

the failed north flank of the mountain, and contains 

from 3 to 5 percent clay. The clay content of the lahar 

corresponds closely with the average clay content of the 

surfical debris avalanche, measured by Voight and 

others (1981) as an average of 4 percent. A similar relation between lahar origin and clay content was described by Crandell (1971) at Mount Rainier. There, 

however, the critical clay content was approximately 

5 percent, but this higher value is partly due to sampling procedure. Lahars with more clay were ascribed 

by Crandell to slides of hydrothermally altered parts 

of the volcano; those with less were ascribed to eruptions producing block-and-ash flows or to interactions 

of a snowpack with pyroclastic and lava flows. Possible reasons for the somewhat lower clay content of 

lahars formed directly from hydrothermally altered 

material at Mount St. Helens include the relative 

youthfulness of the volcano and the occurrence of less 

intense or less continuous alteration.

Analogs of the 1980 North Fork lahar are relatively 

rare in the record of flows in the Toutle River system. 

The two flows of Smith Creek age are texturally comparable in clay content (where their mean grain size and 

facies are similar). All other flows of post-Ape Canyon 

age that extend far from the volcano's base fall clearly 

in the low-clay category. The relatively high-clay Smith 

Creek flows are logically interpreted as the result of a 

slope failure on the cone, with possible derivation via 

the intermediate stage of a debris avalanche. The flows 

may record explosive behavior (see discussion on origin 

of the Osceola Mudflow in Crandell, 1971), but more 

passive origins are also possible.

SUMMARY OF THE GENERAL RELATION OF 

LAHAR ORIGIN TO MAGNITUDE

Lahar magnitude is analyzed quantitatively in Scott 

(in press). Of three general sizes of lahars originating 

after Ape Canyon time in drainages of the northwest 

half of Mount St. Helens, (1) the largest lahars have 

formed from lake-breakout flood surges bulked by 

stream alluvium; (2) most intermediate-size lahars have 

probably originated (a) from meltwater floods produced 

by lithic pyroclastic flows derived from major dome

explosions, and (b) from catastrophic ejection of 

saturated cone material, either by transformation of 

voluminous but dilute pyroclastic surges, gravity 

transformations of large volumes of basal avalanches 

that continued as lahars, or the continuation of explosively induced avalanches; and (3) a probably large 

number of smaller lahars have resulted from eruptioninduced snowmelt. Pumice-rich lahars of intermediate 

size were associated with large pumiceous pyroclastic 

flows during Ape Canyon time and probably formed by 

bulking of their associated meltwater surges. The larger 

of the two flows ascribed to possible slope failure in the 

Smith Creek period also falls in the intermediate 

category.

INTERPRETATIONS OF LAHAR DYNAMICS 

BASED ON TEXTURE

The rate of downstream transformation to a laharrunout flow is strongly influenced by the amount of clay 

in a lahar, partly because the more granular, less cohesive low-clay debris flows have greater permeability and 

greater miscibility with overrun streamflow. The 1980 

North Fork lahar, with 3 to 5 percent clay, traveled 

more than 100 km to the Columbia River without transforming but nevertheless showed continuous, gradual 

textural change in the direction of transformation.

The modern lahars that transformed attenuated more 

rapidly in peak discharge than did the 1980 North Fork 

lahar, and Scott (in press) showed that the older flow 

PC 1, which transformed, also attenuated rapidly. 

Because the typical lahar in the river system is of the 

low-clay variety and transformed to a lahar-runout flow, 

lahar flood routing and flow models should incorporate 

this behavior. The lahars are not simply waves of a constant mixture of mud, sand, and gravel moving through 

the system. At Mount St. Helens, they may be visualized more accurately as a flow wave beginning and ending as a streamflow surge, with only the middle segment 

consisting of debris flow.

A corollary, that the relatively high-clay lahars will 

move the longest distances, is true at Mount St. Helens 

if lahars are defined in the strict sense (that is, as debris 

flow). Lahars of equivalent initial magnitudes will move 

untransformed as debris flows for distances roughly 

proportional to their cohesiveness. The two largest 

lahars from the northwestern sector of the mountain 

were of the low-clay variety and did transform; even 

with the consequently more rapid attenuation in peak 

discharge, however, their distal runout flows probably 

exceeded, in magnitude and extent, the distal phase of 

the largest high-clay flow.
A72 LAHARS AND LAHAR-RUNOUT FLOWS, TOUTLE RIVER, WASHINGTON

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APPENDIX
EQUATIONS USED IN CALCULATING LAHAR VELOCITY 

AND STATISTICS OF GRAIN SIZE DISTRIBUTIONS

VELOCITY

The mean velocity of the 1980 South Fork lahar at peak stage was 

calculated locally from the tilt of the superelevated flow surface at 

a channel bend and the radius of curvature of the bend (Johnson, 1979, 

p. 27):

u = (Rg cos S tan B)v*

where: u = mean velocity

R = radius of curvature of bend 

g = acceleration of gravity 

S = angle of channel slope

B = angle of tilt of flow surface measured perpendicular 

to flow direction

Velocity was also determined by measuring the vertical distance of 

runup against valley walls and bedrock projections where flow impinged at a high angle:

where: u = mean velocity 

h height of runup

The uncertainties and qualifications of the results of these techniques 

are discussed by Fairchild and Wigmosta (1983) and Pierson (1985). 

Regardless of whether the derived velocity determinations are absolutely accurate, they clearly document the abrupt downstream decline 

in relative velocity and discharge that accompanied flow deflation.

GRAIN SIZE STATISTICS

The statistics of the grain size distributions are described by the 

graphically derived measures of Folk (1980). With cumulative frequency 

plotted on probability paper as percentage of grains finer than the limit 

of each phi class, mean grain size is defined as the graphic mean,

Standard deviation is defined as the graphic standard deviation,

Standard deviation can also be defined as the inclusive graphic standard deviation,

Skewness is defined as the graphic skewness,

Skewness can also be defined as the inclusive graphic skewness,

Kurtosis is defined as the graphic kurtosis,

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