45° 

42' 

METRIC CONVERSION FACTORS 

Multiply 

foot (ft) 

cubic yard (yd 3 ) 

mile (mi) 

square mile (mi 2 ) 

cubic foot per second ere /sec) 

By 

0.3048 

0.7646 

1.609 

2.590 

.02832 

To obtain 

meter (m) 

cubic meter (m3 ) 

kilometer (km) 

square kilometer (km 2 ) 

Cubic meter per ser:ond (m3 /s) 

105' 

120' 1100 I 

~ 49 ~ ----- L 115' I ---------~ ~~~ - ------IT_j______________ \ ~-...) , , I 

I ! . Spokane o j \ \ 

' ~ MONTANA \ I '\ . ' ' \ l • Mount St Helens ! 7 He7ena \ 

I __..,...-,---"'-----....\ 1 I o-.------- ' l I 

Portland J '-\ __ J 

I \ ---------------- / '\,_,--,--0~ 

·Seattle 

0 

Salem 

I I 

i ~~ \ 

i IDAHO \ 

OREGON 

I . , I -----------------~---------------~ 

0 100 200 300 MILES 

0 100 300 KILOMETERS 

COVER: North Fork Toutle River, June 30, 1980. Volcanic mud flow breccia 

and debris from the May 18, 1980, eruption of Mount St. Helens (in upper 

right) are as much as several hundred feet thick in the reach shown. 

Photograph by Austin Post, U.S. Geological Survey. 
Sediment Deposition in the Columbia 

and lower Cowlitz Rivers, WashingtonOregon, Caused by the May 18, 1980, 

Eruption of Mount St. Helens 

By F. P. Haeni 

Hydrologic effects of the eruptions of Mount St. Helens, Washington, 1980 

GEOLOGICAL SURVEY CIRCULAR 850-K 

1983 
United States Department of the Interior 

JAMES G. WATT, Secretary 

Geological Survey 

Dallas L. Peck, Director 

Library of Congress Cataloging in Publication Data 

Haeni, F. P. 

Sediment deposition in the Columbia and Lower 

Cowlitz Rivers, Washington-Oregon, caused by the May 

18, 1980 eruption of Mount St. Helens. 

(Geological Survey circular ; 850-K) (Hydrologic 

effects of the eruptions of Mount St. Helens, 

Washington, 1980) 

Bibliography: p. 

Supt. of Docs. no.: 119.4/2:850-K 

1. Sedimentation and deposition. 2 Sediments 

(Geology)-Columbia River. 3. Sediments (Geology) 

-Washington (State)-Cowlitz River. 4. Saint Helens, 

Mount (Wash.)-Eruption, 1980. I. Title. II. Series. 

III. Series: Hydrologic effects of the eruptions of 

Mount St. Helens, Washington, 1980. 

QE75.C5 no. 850-K 557.3s [551.3'54'09797] 82-660386 

[QE571] 

Free on application to Distribution Branch, Text Products Section, 

U. S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 
FOREWORD 

On May 18, 1980, after more than a month of earthquakes and eruptions, 

Mount St. Helens, in southwestern Washington, exploded in a volcanic eruption 

more violent than any in the contenninous United States during the 20th century. A lateral blast of hot gas and rock particles devastated an area of about 

150 square miles on the northern side of the mountain knocking down trees to a 

distance of 15 miles. Several minutes later, a giant ash cloud rose to about 

60,000 feet. Winds then carried the ash cloud across the United States, with 

heavy fallout and deposition in eastern Washington and parts of Idaho and 

Montana Earlier, smaller eruptions deposited ash in western Washington and 

parts of Oregon and Canada 

The hydrologic effects of the May 18 eruption have been both widespread and 

intense. During the eruption, a massive debris avalanche moved down the north 

flank of the volcano depositing about 3 billion cubic yards of rock, ice, and other 

materials in the upper 17 miles of the North Fork Toutle River valley. The 

debris deposits are about 600 feet thick in the upper reaches of the valley. Following the avalanche, runoff from the melted glaciers and snow, and possible 

outflow from Spirit Lake, caused an extraordinary mudflow in the North Fork 

Toutle River. The mudflow shattered and uprooted thousands of trees, 

destroyed most of the local bridges, and deposited an estimated 25,000 acre-feet 

of sediment in the Cowlitz River channel. A considerable amount of additional 

sediment was conveyed through the lower Cowlitz into the Columbia River 

where it was deposited and formed a shoal that blocked the shipping channel. 

Mudflows also occurred in the South Fork Toutle River and in tributaries on the 

east flank of Mount St. Helens which enter Swift Reservoir. 

As part of a concerted Geological Survey effort to study the volcanic event 

and to identify potential hazards, Survey hydrologists have mounted an intensive program to document the hydrologic effects of the eruptions. The major 

initial hydrologic findings are reported in this circular series. Quick, useful 

assessment was made possible only because the Survey has long conducted 

extensive water-resources investigations in the affected areas of Washington, 

Oregon, and Idaho. Hence, there was a well-defined basis for identification and 

documentation of the types and magnitudes of hydrologic changes. 

The Geological Survey Circular 850, "Hydrologic Effects of the Eruptions of 

Mount St. Helens, Washington, 1980," consists of individually published short 

chapters that emphasize data-collection activities, field observations, and initial 

comparisons of pre- and post-eruption conditions. The series will cover hydrologic events occurring on May 18 in the Toutle and Cowlitz Rivers; physical 

alteration of the Toutle River system; the chemical and physical quality of 

precipitation, streams, and lakes affected by volcanic ash fall; ash-leaching 

studies; and Mount St. Helens glaciers. 

III 

Dallas L. Peck 

Director 

CONTENTS 

Page 

Forewoni ------------------------------------------------------------------- Kill 1\bs~t-------------------------------------------------------------------- 1 

Inbroduction ---------------------------------------------------------------- 1 

J\cknowledgments ------------------------------------------------------------ 2 

Configuration of the Columbia and Cowlitz River beds before the May 18, 1980, eruption of 

Mount St. Helens ----------------------------------------------------------- 2 

Mudflows into the Columbia and Cowlitz Rivers on May 18 and 19, 1980 ----------------- 6 

Configuration of the Columbia and Cowlitz River beds after the May 18, 1980, eruption of 

Mount St. Helens ----------------------------------------------------------- 10 

Volume, thickness, and texture of mudflow material deposited in the Columbia and Cowlitz 

Rive~-------------------------------------------------------------------- 12 

Sununmy------------------------------------------------------------------- 14 

References ------------------------------------------------------------------ 14 

ILLUSTRATIONS 

Page 

FIGURE 1. Map showing the location of Mount St. Helens, the North Fork Toutle, the 

Cowlitz, and the Columbia Rive~. Washington-Oregon----------------- K2 

2. Map showing the location and dates of U.S. J\rmy Corps of Engineers bathymetric survey maps, marine seismic reflection profiles, and location of the 

Columbia River Profile shown in figure 8 ----------------------------- 3 

3. Map showing the configuration of the Columbia and Cowlitz River beds near 

Longview, Washington, before the eruption of Mount St. Helens on May 18, 

1980 --------------------------------------------------------- 4 

4. Gage height of the Columbia River at Rainier, Oregon, May 18, 1980 ---------- 7 

5. Map showing the configuration of the Columbia and Cowlitz River beds near 

Longview, Washington, after the eruption of Mount St. Helens on May 18, 

1980 --------------------------------------------------------- 8 

6. Perspective diagrams of the Columbia and Cowlitz River beds near Longview, 

Washington, before and after the May 18, 1980, eruption of Mount St. 

Helens -------------------------------------------------------- 10 

7. Section 1\-J\' of the Columbia River before and after the May 18, 1980, eruption 

of Mount St. Helens --------------------------------------------- 15 

8. Map showing the thickness of sediment deposited in or removed from the 

Columbia and lower Cowlitz Rive~ as a result of the May 18, 1980, eruption 

of Mount St. Helens --------------------------------------------- 16 

9. Perspective diagram showing the thickness of sediment deposited in or removed 

from the lower Cowlitz and Columbia Rive~ -------------------------- 18 

10. Detailed map showing the thickness of sediment deposition in the former scour 

hole near Coffin Rock-------------------------------------------- 19 

11. Detailed map showing the sediment thickness deposited in or removed from 

the lower Cowlitz River------------------------------------------- 20 

12. J\erial photograph of the junction of the Columbia and Cowlitz Rivers on 

May 23, 1980, showing suspended sediment and debris in the Cowlitz River- 21 

v 

HYDROLOGIC EFFECTS OF THE ERUPTIONS OF 

MOUNT ST. HELENS, WASHINGTON, 1980 

Sediment Deposition in the Columbia and 

Lower Cowlitz Rivers, Washington-Oregon, 

Caused by the May 18, 1980, 

Eruption of Mount St. Helens 

By F. P. Haeni 

ABSTRACT 

On May 18, 1980, Mount St. Helens violently erupted, sending billions of cubic yards of mud, ash, rock fragments, and 

debris down the North and South Forks of the Toutle River, 

the Cowlitz and Columbia Rivers, and other streams. A total of 

35.6 million cubic yards of mudflow material was deposited in 

the lower Cowlitz (downstream from the Union Pacific Railroad 

bridge) and Columbia Rivers. 

The arrival of this material at the Columbia River, during a 

period of the daily tidal cycle, when the flow was at a minimum 

or quite probably reversed and flowing upstream resulted in its 

deposition in an area 7 miles upstream and 2 miles downstream 

from the junction of the Cowlitz and Columbia Rivers. The 

thickness of the deposit was at a maximum near Coffin Rock, 

where a hole 140 feet deep was filled with 90 feet of sediment. 

Incomplete filling, compaction, and (or) erosion caused a 

1Q-foot depression to form in the surface of the newly deposited 

material at this location. 

On May 23, 1980, the presence of two scour channels on each 

side of a tonguelike deposit (delta) of very fine sand and silt indicates that the coarse-grained sediment and water surge on 

the morning of May 19 initially scoured the lower 0.5 mile of 

the Cowlitz River bed. A minimum of 0.4 million cubic yard of 

material was scoured from the Cowlitz River bed and subsequently redeposited in the Columbia River before the 

1.3-million-cubic-yard delta was formed. 

INTRODUCTION 

On the morning of May 18, 1980, Mount St. 

Helens, southwest Washington, violently erupted, 

sending billions of cubic yards of rock, ash, snow, 

ice, and debris into the headwaters of the North 

and South Forks of the Toutle River. Subsequent 

K1 

mud and debris flows that developed from this material sent millions of cubic yards of material down 

the North and South Forks of the Toutle River and 

the Cowlitz and Columbia Rivers (fig. 1). Much of 

the mud and debris was deposited in the North 

and South Forks of the Toutle and Cowlitz River 

channels and flood plains and caused extensive 

destruction (Lombard and others, 1981). 

Two major mudflows developed, one each in the 

headwaters of the South and North Forks of the 

Toutle River. The chronology of these mudflows 

and the resulting damage as they proceeded downstream to the Longview Water Treatment Plant is 

documented by Cummans (1981). When they 

reached the mouth of the Cowlitz River and 

entered the Columbia River, they were mostly confined by the river banks or levees. The extent of 

sediment deposition was not immediately known, 

but the 40-ft-deep Columbia River commercial 

shipping channel was only 14 ft deep on the morning of May 19, 1980. 

To document the volume, areal extent, and 

nature of sediment deposition in the Columbia and 

lower Cowlitz Rivers, the U.S. Geological Survey 

obtained detailed bathymetric surveys by the U.S. 

Army Corps of Engineers and conducted seismic 

reflection surveys near their junction. 

This report documents the sediment volumes 

and the nature of the scouring and deposition in 

the Columbia and lower Cowlitz Rivers. It is part 

of a series of reports published by the U.S. Geological Survey documenting the hydrologic effects of 

the May 18, 1980, eruption of Mount St. Helens. 
122°45' 122°30' 

46"15' 

WASHINGTON 

STUDY AREA 

0 5 10 15 

122°15' 

490 120" ' ~: ------l------, ''''·--' I 

I 

WASHINGTON i 

l STUD'!_.~~~J ._.-.__.- / 

( 

OREGON ) 

I 

. ________________ j 

~ 

~ 

Spirit~a~ 

Swift 

20 MILES 

0 5 10 15 20 KILOMETERS 

FIGURE I.-Location of Mount St. Helens, the North Fork Toutle, the South Fork Toutle, the Cowlitz, and the Columbia 

Rivers, Washington-Oregon. 

ACKNOWLEDGMENTS 

Appreciation is expressed to the U.S. Army 

Corps of Engineers, Portland District, who furnished the bathymetric data used in this report. 

Special appreciation is extended to James R. Slack 

of the U.S. -Geological Survey, who developed the 

three-dimensional computer plots. 

CONFIGURATION OF THE COLUMBIA AND 

COWLITZ RIVER BEDS BEFORE THE 

MAY 18, 1980, ERUPTION OF 

MOUNT ST. HELENS 

The U.S. Army Corps of Engineers routinely 

makes detailed bathymetric surveys on the Columbia and Cowlitz Rivers to use in maintaining 

minimum depths and widths of shipping channels. 

The results of these surveys are available from the 

Corps on orthophoto base maps at scales of 1 in 

equals 500 ft for the Columbia River and 1 in 

equals 200 ft for the Cowlitz River. 

Bathymetric surveys on August 30, 1979, 

February 27, 1980, and March 4, 18, 20, and 25, 

1980, were used to determine the shape of the Columbia and lower Cowlitz River beds before the 

May 18 eruption. Figure 2 shows the locations and 

dates of these surveys. The pre-eruption shapes of 

the river beds are shown in figure 3, with contours 

representing the depth of water, in feet, below the 

Columbia River datum. (A computer-generated 

three-dimensional perspective diagram of the same 

surface is shown in figure 6a and b.) 

K2 
D 

A A' 

0 

Lower Dobelbower Bar 

March 18, 1980 

May 19, 21, 1980 

EXPLANATION 

U.S. Army Corps of Engineers bathymetric 

survey maps. Upper date indicates survey 

prior to the eruption of Mount St. Helens, 

lower date indicates survey after the 

eruption of Mount St. Helens 

U.S. Geological Survey Seismic 

Reflection Profiles 

Section Line for Columbia River Profile 

2 

KILOMETERS 

3 MILES 

Kalama Ranges 

March 25, 1980 

June 4, 1980 

Upper Dobelbower Bar 

March 20, 1980 

May 21, 23, 24, 1980 

FIGURE 2.-Locations and dates of the U.S. Army Corps of Engineers bathymetric survey maps and marine seismic 

reflection profiles and the location of the Columbia River Profile shown in figure 7. 

K3 
OREGON 

0 0.5 2 MILES 

0 0.5 2 KILOMETERS 

Datum is Columbia River Datum (0.29 ft above National Geodetic Vertical 

Datum at Columbia River Mile 75.3) 

FIGURE a.-Configuration of the Columbia and Cowlitz River beds near Longview, Washington, before the eruption of Mount 

St. Helens on May 18, 1980. 

One of the most prominent features of the preeruption Columbia River bed was the 140-ft-deep 

hole near Coffin Rock. This hole may have been 

scoured by the increased velocity of the river as it 

changes direction near Coffin Rock. Similar scour 

holes, 70 ft deep, are present north of Sandy Island 

K4 
WASHINGTON 

KALAMA RIVER 

!. 

EXPLANATION 

...._10 _..- LINE OF EQUAL WATER DEPTH-indicates the depth below the Columbia 

River Datum. Dashed where approximately located. IntervallO ft (3. 0 m) 

Based on bathymetric data by U.S. Army Corps of Engineers surveys; August 30, 1979, 

February 27, 1980 and March 4, 18, 20, and 25, 1980 

and south of Rainier Bar. Erosion on the outside 

bend of the Columbia River and deposition on the 

inside bend are typical of streams and rivers and 

generally form a characteristic asymmetrical bottom profile. The actual shape of the Columbia 

River bottom depends upon many factors, but, 

typically, the bottom is gently sloping on the inside bend and has an erosion channel on the outside bend. Straight reaches of the river generally 

have a symmetrical cross section. 

K5 
The bed of the Cowlitz River had the same characteristics as those of the Columbia River. The 

Cowlitz River was much shallower, however, and, 

in the lower 2 mi, the maximum depths ranged 

from 10 to 20ft and exceeded 20ft in two small 

areas. 

In addition to natural forces, dredging affects 

the shape of the riverbeds. A 600-ft-wide and 40-ftdeep channel is maintained on the Columbia River, 

and a 100-ft-wide and 8-ft-deep channel is maintained on the Cowlitz River. 

The complex shape of the Columbia and the 

lower Cowlitz River bottoms, as shown in figures 3 

and 6a and b, before the May 18 eruption was a 

product of many hydrologic, geologic, and manmade forces. 

MUDFLOWS INTO THE COLUMBIA AND 

COWLITZ RIVERS ON MAY 18, 1980 

The eruption of Mount St. Helens on May 18, 

1980, and the subsequent mud and debris flows 

drastically altered the shape of both forks of the 

Toutle, Cowlitz, and Columbia River beds. The effects of these mudflows on the lower Toutle and 

Cowlitz Rivers were massive sediment deposition 

and destruction on the flood plains of these rivers 

(Lombard and others, 1981). The chronology of the 

mudflows is described by Cummans (1981). 

According to this account, two separate mudflows 

moved down the Toutle River system and affected 

the lower Cowlitz and Columbia River beds. 

The first mudflow originated in the headwaters 

of the South Fork Toutle River and proceeded 

downriver. This mudflow was relatively small, 

and, once in the Cowlitz River, it did not overflow 

the banks. 

In the account by Cummans (1981, p. B7), the 

leading edge of the first mudflow passed the Longview Water Treatment Plant (Cowlitz River, Mile 

5.2, fig. 1) at 4:15 p.m. on May 18, moving 4 to 

5 ftJsec. No increase in river height was reported at 

the treatment plant, and the leading edge was 

marked by trees, branches, and floating debris. 

Cummans estimates the peak passed the plant at 

about 5 p.m. or shortly thereafter, and a marked 

increase in turbidity was recorded at 8 p.m. If this 

material continued downstream at this same velocity, the leading edge of trees and debris would 

have reached the Columbia River at about 

6:15p.m.; the peak, at 7 p.m.; and the slug of the 

very turbid water, at 10 p.m. The significance of 

the marked increase in turbidity at the watertreatment plant is not known, but it may have 

been the leading edge of the sediment moving 

downstream. Cummans documents a witness who 

reported a leading edge of trees and debris ahead 

of the actual mudflow as it proceeded down the 

South Fork Toutle River. 

The Columbia and lower Cowlitz Rivers, under 

normal conditions, are affected by tidal action and 

have periods of minimum downstream and possible upstream flow twice each day. The stage of the 

river rises and falls in response to this tidal action, 

and, if this stage is measured at two or more 

gages, the periods of upstream and downstream 

flow can be calculated. The stage of the river at one 

gage and knowledge of the flow characteristics of 

the river can be used in a qualitative way to determine the time of minimum downstream or upstream flow. Figure 5 shows the gage height of the 

Columbia River at Rainier, Oregon, for May 18 

and 19, 1980. As the gage height increases, the 

downstream flow decreases and, for several hours 

near the peak gage height, flow is either downstream at a minimum or upstream. Conversely, as 

the gage height decreases, flow increases downstream. Each change in direction is preceded by a 

short period of slack water-no flow in either direction. The upstream or downstream velocity depends on (1) the rate of change of stage, (2) the 

volume of water or channel geometry, and (3) the 

magnitude of the tidal cycle. 

At 7:30 p.m. on May 18, the Survey gage at 

Rainier, Oregon, reached its maximum for the incoming tide. Shortly thereafter, the direction of 

river flow changed, and the flow of the Columbia 

River was downstream until the next high-tide cycle early on the morning of May 19. Because the 

earliest the leading edge of the debris and the 

water of high turbidity could have arrived at this 

spot was 6:15 p.m. and 10 p.m., respectively, most 

of the material that originated in the mudflow of 

the South Fork Toutle River was either deposited 

at the junction of the two rivers or carried downstream by the Columbia River. The shape and 

areal extent of this deposit is not known because 

the part that remained at the junction of the two 

rivers was subsequently buried by the next mudflow and the part that went downstream seems to 

have been widely distributed. 

The second and significantly larger mudflow 

originated on the North Fork Toutle River; it 

K6 
caused extensive damage and deposited much 

sediment on the flood plains of the Toutle, North 

Fork Toutle, and Cowlitz Rivers (Lombard, 1981). 

In general, the thickness of these deposits decreased downstream, and, in the lower 2 mi of the 

Cowlitz and in the Columbia Rivers, the sediment 

was almost entirely deposited within these stream 

channels. According to Cummans (1981, table 2, 

p. B16), the peak of the second mudflow was reported passing the Longview Water Treatment 

Plant at 4 a.m. Monday, May 19, and moving 

about 4 ftJsec. If it is assumed that the mudflow 

continued down the Cowlitz River at about the 

same velocity, the peak would have reached the 

Columbia River at 6 a.m. The river stage at Castle 

Rock shows that the leading edge of this mudflow 

was approximately 2 hr ahead of the peak stage 

(Cummans, 1981, fig. 11, p. B15). Therefore, the 

earliest that sediment started to enter the Columbia River was estimated to be 4 a.m. on May 19. 

From 4 a.m. to 6 a.m. on May 19, the Columbia 

River flowed minimally downstream and probably 

upstream. Consequently, the major part of the 

mudflow entered the Columbia River and was deposited during upstream flow (fig. 4). 

The timing of these events agrees with reports 

from a ship's pilot on the Columbia River during 

this interval. The pilot noted that logs and debris 

floated in the water as his ship approached Longview, Washington. At 5:05 a.m., his ship was no 

longer making headway; he was, in fact, aground 

on a huge bank of mud. Another ship had successfully made the same passage just 2 hr earlier, at 

3 a.m. (Zonana, 1980). 

The main deposition of mudflow sediment derived from the debris avalanche and caused by the 

May 18, 1980, eruption was, therefore, deposited 

in the lower Cowlitz and Columbia Rivers between 

3 a.m. and 6 a.m. on the morning of May 19, 1980, 

during an incoming tidal cycle (fig. 4). 

U.S. Geological Survey station number 14245299 Columbia River at 

Rainer, Oregon 

River Mile 66.1 

Gage Datum 0.34 ft below NGVD of 1929 

9r-----~------~------r-~--~---r--~------~----~------~ 

8 

1-7 

w 

w 

u. 

~6 

~ 

::I: 

(!) 

jjj 

:::I:5 

w 

(!) 

<( 

(!)4 

3 

6A.M. NOON 

MAY 18, 1980 

9 P.M. 3A.M. 

6 P.M. MIDNIGHT 6 A.M. NOON 6P.M. 

MAY 19, 1980 

PACIFIC DAYLIGHT TIME 

FIGURE 4.-Gage height of Columbia River at Rainier, Oregon, for May 18 and 19, 1980. (NGVD is 

National Geodetic Vertical Datum.) 

K7 
0 

I 

0 

I 

0.5 

OREGON 

0.5 

I 

KILOMETERS 

Datum is Columbia River Datum (0.29 ft above National Geodetic Vertical 

Datum at Columbia River Mile 75.3) 

2 MILES 

I 

FIGURE 5.-Configuration of the Columbia and Cowlitz River beds near 

K8 
WASHINGTON 

EXPLANATION 

'--10 --- LINE OF EQUAL WATER DEPTH-indicates the depth below the Columbia 

River Datum. Dashed where approximately located. IntervallO ft (3. 0 m) 

Based on bathymetric data by U.S. Army Corps of Engineers surveys; May 19, 21, 23, and 

24. 1980. June 4, 1980. and supplemented by U.S. Geological Survey seismic reflection 

profiles; May 23. 24. and 25. 1980 

Longview, Washington, after the eruption of Mount St. Helens on May 18, 1980. 

K9 
FIGURE 6.-Perspective diagrams of the Columbia and Cowlitz River beds near Longview, Washington, before and after 

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

Coffin Rock 

FIGURE SA-Aerial view of the Cowlitz and Columbia Rivers before the May 18, 1980, eruption of Mount St. Helens. 

View is from 45° above the surface of the Earth. 

CONFIGURATION OF THE COLUMBIA AND 

COWLITZ RIVER BEDS AFTER THE 

MAY 18, 1980, ERUPTION OF 

MOUNT ST. HELENS 

The U.S. Anny Corps of Engineers, upon learning that the Columbia River was blocked to shipping, made bathymetric surveys on the Columbia 

and Cowlitz Rivers. The results of surveys on May 

19, 21, 23, and 24 and June 4, 1980, were used in 

this study to determine the configuration of the 

river bottoms after the mudflows of May 18 and 

19. These data were supplemented by marine 

seismic reflection surveys by the Survey from May 

23 to 25. Figure 2 shows the locations of the Corps' 

surveys, their dates, and the locations of the 

marine seismic reflection lines. 

The most noticeable effect of the mudflows on 

the Columbia River bottom was the smoothing of 

K10 
B 

FIGURE 6B.-View of the Cowlitz and Columbia River beds before the May 18, 1980, eruption of Mount St. Helens. View is 

from 40° below the surface of the Earth. 

its topography and deposition of most of the 

mudflow upstream from its entry point at the 

mouth of the Cowlitz River. All depressions in the 

former river bottom, including the dredged channel, the large scour hole near Coffin Rock, and the 

smaller holes north of Sandy Island and south of 

Rainier Bar were filled by the mudflow. Water was 

shallowest at the junction of the two rivers; the 

former 40-ft-deep shipping channel was only 14-ftdeep on the morning of May 19. The contours on 

figure 5 show the depth of water, in feet below 

Columbia River Datum, and depict the shape of 

the river bottom. A computer-generated threedimensional perspective diagram of the same surface is shown in figures 6c and d. 

A longitudinal section of the Columbia River 

bed, A-A' (fig. 7), shows the configuration of the 

river before and after the mudflows of May 18 and 

19. The location of this profile is shown in figure 2. 

The downstream edge of the mudflow is steep, and 

the upstream edge is gradual and gently sloping. 

Its surface is marked by a depression directly 

above the former 140-ft-deep scour hole. This 

depression is due to one or more of the following: 

incomplete filling of the scour hole or both compaction and erosion after the main deposition was 

complete. 

The effect of the mudflows on the lower 2 mi of 

the Cowlitz River was to fill in the navigation 

channel with 10 to 20ft of material and to deposit 

a large pile of stumps and log debris near the confluence with the Columbia River (upper right-hand 

comer of fig. 12). Several sections near the mouth 

of the Cowlitz River were deeper after the mudflow 

than before, indicating that the mudflows eroded 

bottom materials in places. Erosional and depositional events in the lower Cowlitz River (figs. 8, 

11) are explained in the next section. 

Kll 
0 

FIGURE 6C.-Aerial view of the Cowlitz and Columbia Rivers after the May 18, 1980, eruption of Mount St. Helens. View is 

from 45° above the surface of the Earth. 

VOLUME, THICKNESS, AND TEXTURE OF THE 

MUDFLOW MATERIAL IN THE 

COLUMBIA AND COWLITZ RIVERS 

The thickness of mudflow material deposited or 

eroded from the river bottoms is shown in figure 8. 

This map was prepared by comparing the preeruption with post-eruption bathymetric data, 

supplemented by marine seismic reflection profiles. A computer-generated three-dimensional 

perspective diagram representing the total accumulation on the bottom of these rivers is shown 

in figure 9. 

As of June 4, 1980, the total volume deposited in 

the Columbia and the lower Cowlitz Rivers as a 

result of the May 18 eruption was 35.6 million yd3 

(fig. 8). The average thickness deposited in the Columbia near Longview, Washington, was 25ft and 

reached a maximum of 90 ft. The total mudflow 

deposit in the Columbia near Longview as 9 mi 

long, reaching 7 mi upstream from its confluence 

with the Cowlitz. 

K12 
B 

FIGURE 6D.-View of the Cowlitz and Columbia River beds after the May 18, 1980, eruption of Mount St. Helens. View is 

from 40° below the surface of the Earth. 

The mudflow was thickest in the former scour 

hole located near Coffin Rock. This hole, 5 mi 

upstream from the junction of the Columbia and 

Cowlitz Rivers, was filled with 90 ft of material. 

The contours in figure 10 show the thickness of the 

deposited material. 

A detailed thickness map of the mudflow eroded 

or deposited in the lower Cowlitz River is shown in 

figure 11. The deposited material averages 7 ft, 

reaches a maximum of 20 ft, and is confined mainly to the former dredged channel. The patterned 

area shows deeper water after the mudflows than 

before and, consequently, outlines areas of erosion 

caused by the mudflows. A minimum of 0.4 million yd3 of material was eroded from the Cowlitz 

River by the mudflows, but the deposition of this 

eroded material is unknown. Most, however, was 

probably redeposited in the Columbia River near 

Longview, Washington, along with the greater 

part of the mudflow material. 

The mudflow deposits in the lower Columbia and 

Cowlitz Rivers were sampled by the Corps of 

Engineers from May 24 to 31, 1980, with a clamshell dredge. Although the samples are scattered 

in time, distance, and depth, the data provide a 

general idea of the grain-sized distribution of the 

mudflow. The deposit in the Cowlitz River was 

generally fine grained, consisting of silt, very fine 

sand, fine sand, and small pieces of charred wood 

fragments and a trace of coarse angular rock 

fragments. 

Most sediment in the Columbia River was 

coarser than that in the Cowlitz River and consisted of fine to coarse sand and rock fragments 

with some silt and wood fragments. Gravel and 

rock fragments as large as 2 in. in diameter were 

reported in many of the sediment samples. In 

general, the deposit was coarser grained on the 

bottom and finer grained on the top (Mount St. 

Helens debris explorations, explanation and shell 

bucket sample logs, U.S. Army Engineering District, Portland, Oregon, May 1980). More information on bed materials is available in a recent report 

by Hubbell and others (1982). 

K13 
The formation of an elongated deposit (delta) in 

the mouth of the Cowlitz River, the grain-sized 

distribution of the mudflow in the Columbia and 

Cowlitz Rivers, and the shape and extent of the 

area from which material was eroded from the 

Cowlitz River (fig. 11) suggest the following sequence of events on May 19, 1980. 

The second and significantly larger mudflow, 

which moved down the Cowlitz River on the morning of May 19, 1980, initially contained a significant quantity of coarse-grained sediment. When 

this mudflow reached the lower Cowlitz River, it 

scoured up to 10 ft of material from the lower 

0.5 mi of the riverbed and redeposited some of this 

material in the Columbia River. 

The combination of the deposition of material in 

the Columbia River, the rising tidal stage, and the 

widening of the Cowlitz River as it joins the Carrolls Channel, probably formed backwater conditions in the lower Cowlitz River. The backwater 

lowered water velocity and allowed fine-grained 

sediments to be deposited. The 1.3-million-yd3 

delta (fig. 11), which is composed of mostly finegrained sediments, was formed. This sequence is 

supported by stratigraphic evidence throughout 

the Toutle-Cowlitz system which indicates that 

the material from the North Fork Toutle River 

mudflow was initially coarse grained and then 

became fine grained (Richard J ander, written commun., 1981). 

The shape of this deposit is inferred only from 

data collected on May 23, 1980. Sediment movement in both the Columbia and Cowlitz Rivers is a 

dynamic process, and the shape and volume of accumulated material will continue to change for 

months and years to come. Figure 12 is an aerial 

photograph of the junction of the Columbia and 

Cowlitz Rivers taken on May 23, 1980, 5 days 

after the initial mudflow; it shows the large quantity of sediment and debris still moving down the 

Cowlitz River. The size and shape of the river 

channels were changing and will continue to 

change daily because of stream scouring, dredging, and new sediment deposition. 

SUMMARY 

Early on the morning of May 19, 1980, 35.6 million yd3 of mudflow material that originated from 

the debris flows into the Toutle River basin during 

K14 

the eruption of Mount St. Helens was deposited in 

the Columbia and lower Cowlitz Rivers. 

The texture and shape of the mudflow deposits 

indicate that the Cowlitz River bed, near the junction of the Columbia River, was eroded by the 

large mudflow on May 19 and that subsequently 

fine-grained sediment formed an elongated delta 

near the mouth of the river. 

The mudflow material reaching the Columbia 

River was carried upstream 7 mi by tidally influenced upstream flow. The surface of the deposited 

material had a steep downstream slope and a 

gradual upstream slope. The material blocked the 

shipping channel and flattened the river bottom 

by filling in most depressions. The average thickness of this material was 25 ft, reaching a maximum of 90 ft where it filled the scour hole near 

Coffin Rock. Incomplete filling, compaction, and 

(or) erosion caused a depression to form in the surface of the material over the former deep scour 

hole. Mudflow material in the Columbia River consisted mostly of sand and rock fragments and 

small quantities of wood fragments and gravel. 

The material in the lower Cowlitz River was significantly finer than that in the Columbia River and 

consisted of silt and very fine to fine sand. 

The shape and volume of the mudflow deposits 

documented by this report will continue to be 

modified by sediment erosion and deposition as 

well as by dredging. 

REFERENCES 

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

from future eruptions of Mount St. Helens volcano, Washington: U.S. Geological Survey Bulletin 1383-C, p. C1-

C26. 

Cummans, John, 1981, Mudflows resulting from the May 18, 

1980, eruption of Mount St. Helens, Washington: U.S. 

Geological Survey Circular 850-B, p. B1-B16. 

Hubbell, D. W., Laenen, J. M., and McKenzie, S. W., 1982, 

Characteristics of Columbia River sediments following the 

eruption of Mount St. Helens on May 18, 1980: U.S. Geological Survey Circular 850-J. [In press.] 

Lombard, R. E., Miles, M. B., Nelson, L. M., Kresh, D. L., and 

Carpenter, P. J., 1981, Channel conditions in the lower 

Toutle and Cowlitz Rivers resulting from the mudflows of 

May 18, 1980: U.S. Geological Survey Circular 850-C, 

p. C1-C16. 

Zcmana, V. F., 1980, After a volcano, cleaning up means recutting a river: Wall Street Journal, June 24, v. 102 p. 1, 

col. 4. 
~ 

""'"' CJ1 

A A' 

]~_ Water surface 

CQ 

E 

Q5 ~ 

20 •6- ~ t ...... 

~' Q) 

~ C) .~ 

/River bottom after eruption ~ ,;::0:: 

..... 

30 t ~- ~ -......_ :::J 

~ 

C) 8 ~ 

0 40 ~:E al:::> 

~~ 50 

~c soL ~ r-\ "'"""" I "'-"""" "River bottom 

u.a: prior to eruption 

zw 

-> 0 a: a: 70 

w<( ' ~- 80 .~ a: en ex: 

u.:?! ~ 

o=> 90 j ..J 

::t:O 8 ~u ...... a.. 100 C) 

w 

c 'S 

:::J 

110 ~ 

120 

130 

140 

I I I l I I I I I 

66 67 68 69 70 71 72 73 74 75 

DISTANCE, IN RIVER MILES 

VERTICAL EXAGGERATION X158 

Datum is Columbia River Datum (0.29 ft above National Geodetic Vertical 

Datum at Columbia River Mile 75.3) 

FIGURE 7.-Section of the Columbia River bed before and after the May 18, 1980, eruption of Mount St. Helens. (Profile is located along the maximum depth of the 

river before May 18, 1980, and is shown in figure 2.) 
0 

I 

0 

I 

0.5 

0.5 

I 

OREGON 

I 

2 KILOMETERS 

2 MILES 

I 

FIGURE B.-Thickness of sediment deposited in or removed from the Columbia and lower Cowlitz Rivers 

K16 
WASHINGTON 

j KALAMA RNER 

EXPLANATION 

SCOUR AREA-Sediment in this area of the Cowlitz and Columbia Rivers was removed during the mud and debris flow of May 

18, 1980 

LINE OF EQUAL THICKNESS OF SEDIMENT DEPOSITED OR 

--10----... REMOVED. Dashed where approximate~ located. Interval 10ft 

(3.0m) 

near Longview, Washington, as a result of the eruption of Mount St. Helens on May 18, 1980. 

K17 
100 

90 

8 80 

\.l.o 

~ 

10 

0 

Sediment deposited 

in former scour hole 

FIGURE 9.-Perspective diagram representing the accumulation of sediment on the Columbia and Cowlitz River beds after the 

May 18, 1980, eruption of Mount St. Helens. 

K18 
~ ~ 

~ ~ 

A KALAMA RIVER 

WASHINGTON 

RivER 

~ 

OREGON 

EXPLANATION 

--10----... LINES OF EQUAL THICKNESS OF SEDIMENT DEPOSITED 

Interval is 10ft (3.0 m) 

0 1000 2000 3000 4000 5000 FEET I I I I I I I I I I r r r 1 • 1 1 I 

0 500 1000 1500 METERS 

FIGURE 10.-Thickness of sediment deposition in the former scour hole near Coffin Rock. 
WASHINGTON 

~ 

\ 

OREGON 

EXPLANATION 

SCOUR AREA-Sediment in this area of the Cowlitz and Columbia Rivers was removed during the mud and debris flow of May 

18, 1980 

-10~ LINE OF EQUAL THICKNESS OF SEDIMENT DEPOSITED OR 

REMOVED. Dashed where approximately located. Interval 10 ft 

(3.0m) 

1000 

I I 

Based on bathymetric data by U.S. Army Corps of Engmeers surveys; August 30. 1979; 

February 27. 1980; March 4. 18. 20. and 25. 1980; May 19. 21. 23. and 24. 1980; June 

4. 1980. and supplemented by the U.S. Geological Survey seismic reflection profiles; May 

23. 24. and 25. 1980 

3000 

I 

4000 

I 

5000 FEET 

I t r I 

2000 

I 

I 

1000 

I 

500 1500 METERS 

FIGURE 11.-Thickness of sediment deposited in or removed from the lower Cowlitz River. 

K20 
* <:: 

'"' 0 

~ z 

~ z 

~ 

~ 

z 

~ 

z 

'"' 0 

., 

., 

~ 

"' "' w 

..., 

~ 

"' 

~ 

"' "' 0 

[:'2 

....... 

FIGURE 12.-Confluence of the Columbia and Cowlitz Rivers on May 23, 1980, showing suspended sediment and debris in the Cowlitz River .