Channel Conditions in the 

Lower Toutle and Cowlitz Rivers 

Resulting from the Mudflows 

of May 18, 1980 

By R. E. Lombard, M. B. Miles, L. M. Nelson, 

D. L. Kresch, and P. j. Carpenter 

GEOLOGICAL SURVEY CIRCULAR 850-C 

1981 
United States Department of the Interior 

JAMES G. WATT, Secretary 

Geological Survey 

Doyle G. Frederick, Acting Director 

Library of Congress catalog-card No. 81-600040 

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 conterminous 

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 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 waterresources 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 River; 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. 

Doyle G. Frederick 

Acting Director 

III 
42° 

METRIC CONVERSION FACTORS 

Multiply 

foot (ft) 

cubic yard (yd3 ) 

mile (mi) 

square mile (mi2 ) 

cubic foot per second (ft3 /sec) 

By 

0.3048 

0.7646 

1.609 

2.590 

.02832 

To obtain 

meter (m) 

cubic meter (m3 ) 

kilometer (km) 

square kilometer (km2 ) 

Cubic meter per second (m3 /s) 

120° 105° 

49•) ------l------~r_j'~---------- liO• -----------1 

Seattle Spokane o j \ \ 

WASH I ~ I 

• Olympia INGTON j '\ MONTANA \ 

\ • Mount St. Helens / '! He~ena \ 

1--~-,---------~----~, ( \ 

Portland ) L'\ 1 0 

Salem 

OREGON 

i \ ---------- / ,, ~--l'r ______ _ I ,_r-- I 

I I 

1 0 Boise 1 

I I I IDAHO I 

I I I I 

------- I I ----------~---------------~ 

0 100 200 300 KILOMETERS 

IV 
CONTENTS 

Page 

Foreword --------------------------------------------------------------------------------------------- III 

Abstract ---------------------------------------------------------------------------------------------- Cl 

Introduction ------------------------------------------------------------------------------------------- 1 The mudflows _________________________________________________________________________________________ _ 

Effect of mudflows on channel and flood plain ---------------------------------------------------------------- S 

Effect of mudflows on flood elevations---------------------------------------------------------------------- 10 

Present and planned flood-alleviation measures--------------------------------------------------------------- 12 

Dredging ------------------------------------------------------------------------------------------ 13 

Levees -------------------------------------------------------------------------------------------- 13 

Sediment-retention structures ------------------------------------------------------------------------- 13 

Flood contingency plans ---------------------------------------------------------------------------------- 14 

Evacuation ----------------------------------------------------------------------------------------- 1~ 

Continued surveillance and flood warning---------------------------------------------------------------- 14 

Acknowledgments -------------------------------------------------------------------------------------- In 

Referencescited________________________________________________________________________________________ 16 

ILLUSTRATIONS 

FIGURE 1. Extent of inunduation by mudflows of May 18, H:l80, along the Lower Toutle and Cowlitz Rivers ____________ _ 

2. River stage, Cowlitz River at Castle Rock, Washington, May 18-19, 1980 --------------------------------

3. Channel and valley conditions near Castle Rock, Washington, prior to and after the mudflows of May 18, 1980. 

The cross section is located 1,400 ft downstream of the Castle Rock Bridge ___________________________ _ 

4. After mudflow deposits of 15ft, only the loft area of this barn is above ground level. The barn is located on east 

bank of the Cowlitz River approximately 1 mile south of Castle Rock ---------------------------------

5. Mudflow damage to farmhouse located approximately 11/z miles south of Castle Rock on the west bank of the 

Cowlitz River ------------------------------------------------------------------------------

6. Mudflow destruction along the Cowlitz River at a housing subdivison across the river from Castle Rock. Several 

houses were damaged beyond repair and were subsequently removed --------------------------------

7. Mudflow destruction of farm buildings approximately 1112 miles south of Castle Rock. About 8 ft of debris· covers 

the ground in this area on the west bank of the Cowlitz River ---------------------------------------

8. Remains of the North Fork Toutle River bridge at Weyerhauser Logging Camp 19, Kid Valley ______________ _ 

9. Flooding at fairgrounds across the river from Castle Rock, Washington. From 3-5ft of sediment were deposited 

over the entire fairgrounds, resulting in cancellation of the annual fair. Photograph taken May l!:J, 1980 ____ _ 

10. Relation between river stage and discharge prior to and after the mudflows of May 18, 1980, Cowlitz River at 

Castle Rock, Washington --------------------------------------------------------------------

11. Channel bottom and water surface elevation of the lower Cowlitz River and lower Toutle River prior to and after 

thelnudflowsofMay18,1980 ________________________________________________________________ _ 

12. Dredging operations on the Toutle River just upstream from its confluence with the Cowlitz River. Photograph 

taken September II, 1980 ___________________________________________________________________ _ 

13. Map showing location of river surveillance sites in the Toutle and Cowlitz River systems ____________________ _ 

TABLE 

Page 

C2 ., 

0 

4 

7 

7 

8 

9 

1.0 

11 

14 

15 

Page 

TABLE 1. Peak discharges exceeding 76,000 cubic ft per second and corresponding river stages of Cowlitz River at 

Castle Rock, Washington, during 1927-78 -------------------------------------------------------- C13 

v 

HYDROLOGIC EFFECTS OF THE ERUPTIONS OF MOUNT ST. HELENS, WASHINGTON, 1980 

CHANNEL CONDITIONS IN THE LOWER TOUTLE AND COWLITZ 

RIVERS RESULTING FROM THE MUDFLOWS OF MAY 18, 1980 

By R. E. LOMBARD, M. B. MILES, L. M. NELSON, D. L. KRESCH, and P.]. CARPENTER 

ABSTRACT 

The volcanic eruption of Mount St. Helens on May 18, 1980, 

triggered mudflows on the South and North Forks of the Toutle 

River that deposited up to 15 ft of sediment in the channels of 

the lower Toutle and Cowlitz Rivers. The thickness of the sediment deposited on the flood plain ranged from almost none just 

upstream from Kelso to about 10ft near Castle Rock and the 

mouth of the Toutle River. Existing levees in Castle Rock, 

Kelso, and Longview contained the mudflows and prevented 

flooding of those communities. The mudflow reduced the withinchannel flow capacity, except at Kelso and Longview where the 

effect was minor. The potential exists for unusually high flood 

levels, up to 10-ft higher than normal, from autumn and winter 

precipitation. Planned flood-alleviation measures include dredging, additional levees, and sediment-retention structures. A 

flood-warning system has been established, and the current 

potential for flooding is being monitored through continued 

surveillance of the river system. 

INTRODUCTION 

The explosive volcanic eruption of Mount St. 

Helens on May 18, 1980, triggered mudflows in 

several streams that head on the flanks of the 

volcano. Mudflows that originated in the North and 

South Forks of the Toutle River were especially 

significant because of their immense magnitudes 

and consequent destruction. These two mudflows 

deposited large volumes of sediment in the North 

and South Forks of the Toutle River; and in the 

Toutle, lower Cowlitz, and Columbia Rivers (see 

fig. 1). In the lower Toutle and Cowlitz River 

valleys as much as 15 ft of mud and debris were 

deposited in the river channels, with up to 10ft on 

the adjacent flood plains. These deposits destroyed 

buildings, covered agriculturally productive bottomlands, killed extensive tracts of timber, and 

seriously reduced the flow carrying capacities of 

the channels of the Cowlitz and Toutle Rivers. In 

Cl 

addition, the volcanic eruption provided immense 

volumes of erodible material in the upper Toutle 

River system that potentially could be transported 

into the lower reaches of the Toutle and Cowlitz 

Rivers to further impact the channels. 

The potential for significant flooding was immediately evident. More than 40,000 people live in 

the areas affected by the mudflow deposits and Interstate Highway 5, the major north-south 

highway in western Washington, was at risk. 

Flood mitigation measures were begun immediately and included such activities as channel dredging, 

levee raising, reservoir construction, and flood 

warning systems. 

This report describes: (1) changes in the channels 

and flood plains of the lower Toutle and Cowlitz 

Rivers as a result of the May 18-19, 1980, 

mudflows; (2) the flood hazards that existed in the 

Cowlitz River immediately after the mudflows; (3) 

the flood mitigation system created. Information 

on the morphologic changes resulting from the 

mudflows was obtained by field surveys and observations of May and early June 1980. Potential flood 

stages and inundated limits reported herein were 

defined by the Geological Survey from computer 

models for channel conditions observed following 

the mudflow deposition. Even though the flood 

potential obviously would be altered by the flood 

mitigation activities and possibly by subsequent 

mudflows, they are reported here to document the 

magnitude of the hazard that existed immediately 

after the eruption. 

THE MUDFLOWS 

Mudflows are large masses of water saturated 

rock debris of various sizes that move downslope as 
123° 

LONGVIEW 

Mudflows not mapped 

above this point 

120' 

~~~~: _____ l _____ ! 

WASHINGTON! I 

i ..._/ ___________ \ ,.., 

; 

/ 

i OREGON i 

142' ______________ _i 

_, Gaging station 

b d~{ /iW~\}:j Area inundated by mudflows 

l" ~~ Additional area subject to 

inundation by a flood of 

76,000 cubic feet per second 

under river bed conditions 

of June 1980 

10,000 20,000 FEET 

5000 METERS 

FIGURE L- Extent of inundation by mudflows of May 18, 1980, along the lower Toutle and Cowlitz Rivers. 

C2 
52 ----51.8 ft Dec. 23, 1933 

50 

Highest flood of record NORTH FORK 

MUDFLOW 

......J 48 

<C E 

0 :::J 

~ ~ 46 

w 

> ~ 0 ctS 

- 0> 44 1- 0 w ..... o ..... 0 ..... 

w ~ 42 

(!) § ......J 0 

<( 0 z ..... 0 Q) 40 - Q) 1--

<(N 

~0 1- N 38 

w ..... w 0 

u. ~ 

z t> 36 -.0 

- :::J z(J) 

o-

~ 34 

<( 

> 

w 

......J 

w 32 

28 

0 0 0 0 0 0 

0 0 0 0 0 0 

CX) 0 N "V co CX) 

0 

MAY 18, 1980 

SUNDAY 

0 0 0 0 0 0 0 0 0 0 0 0 

0 0 0 0 0 0 0 0 0 0 0 0 

0 N "V N "V co CX) 0 N "V co CX) 

N N N 0 0 0 0 

MAY 19, 1980 

MONDAY 

FIGURE 2.- River stage, Cowlitz River at Castle Rock, Washington, May 18-19, 1980. 

fluids under the influence of gravity. Stratigraphic 

evidence indicates that mudflows have commonly 

occurred when Mount St. Helens erupts (Crandall 

and Mullineaux, 1978). The explosive eruption of 

May 18 triggered large mudflows in the Toutle and 

Lewis River drainage systems, and small mudflows 

in the Kalama River system (Cummans, 1981). 

Two large mudflows originated in the Toutle 

system, which moved on through the lower Cowlitz 

River into the Columbia River. The first came from 

the South Fork Toutle River; the second from the 

North Fork (Cummans, 1981). Figure 2 shows the 

timing of the mudflows in the Cowlitz River as Castie Rock, Washington. At noon on May 18, the 

river stage at the Castle Rock Bridge was 30.2 ft. 

The peak stage of the South Fork mudflow occurred at about 1:30 p.m. (1330) when the stage 

reached 33.4 ft. The level then quickly dropped, 

returning to 30.2 ft by 2:15p.m. (1415). The South 

Fork mudflow was confined entirely within the 

channel of the Cowlitz River, but the associated 

deposition is unknown, because channel conditions 

were soon altered by the larger North Fork 

mudflow. 

C3 

At about the same time (1:30 p.m. (1330) on May 

18) that the first mudflow peaked at Castle Rock, a 
second larger mudflow was developing on a 

massive debris deposit in the devastated upper 

part of the North Fork Toutle River valley (Cummans, 1981). This massive and highly destructive 

mudflow moved more slowly than the South Fork 

mudflow, with the peak stage at Castle Rock (fig. 2) 

occurring around 12 midnight (2400) on May 18. 

The peak stage of the second mudflow was 49.8 ft, 

16.4 ft higher than the first mudflow. The North 

Fork mudflow spilled over the flood plain, leaving 

deep deposits of sand, volcanic ash, and gravel-size 

pumice on the flood plain and in the channel. The 

aerial extent of the inundation is shown in figure 1. 

80 

1-

w 

w 

a: 

1- Cl 

70 CJ) <( 

0 

I a: ~ 1- w :::> a: w 1- :::> > <( 0 w 

Cl LL ....J 

....J 

<( 60 (.) 

i= 

a: 

w Addition to levee 

> about 2 weeks 

(.) after mudflows 51.6ft 

As shown in figure 2, the shape of the 

hydrograph beginning at about 11:00 a.m. (1100) 

on May 19 is indicative of massive sediment deposition in the channel following the North Fork 

mudflow. The deposition raised the riverbed, and 

reduced the capacity of the channel to convey 

water. For example, at 8:00 a.m. (0800), on May 

18, a flow of 6,160 cubic feet per second occurred 

at a river stage of 30.1 ft. In contrast, at 8:00a.m. 

(0800) on May 20 (not shown), a similar flow of 

6,140 cubic feet per second occurred at a river 

stage of 42.4 ft--12.3 ft higher than before the 

mudflows. 

w 

w 

(ij 

....J 

>- <( 

Ir- ~-C!)r-

-..q 

I Q) 

w- Cl :::J 

CiS~ 1- Q) (/)«; 

w Ci5 ~-

i= .••.••••..•.••.•.•.••••••.•••.•.•.................................•.......... 

w 

Cl 

0 

w 

(!) 

....J 

<( 

z 

0 

i= 

<( 

~ 

1-

w 

w 

LL 

~ 

z 

0 

i= 

<( 

> w 

....J 

w 

50 

40 

30 

20 River stage for 76,000 

cubic feet per second 

- - - Premudflow 

•••••••• Postmudflow 

1QL--------L--------~--------L-------~--------~--------~------~-----

o 500 1 000 1 500 2000 2500 3000 3500 

DISTANCE, IN FEET 

FIGURE 3. -Channel and valley conditions near Castle Rock, Washington, prior to and after the mudflows of May 18, 1980. The 

cross section is located 1,400 ft downstream from the Castle Rock Bridge. 

C4 
EFFECT OF MUDFLOWS ON CHANNEL AND 

FLOOD PLAIN 

Prior to the mudflows, the lower Toutle and 

Cowlitz Rivers had well-defined channels, confined 

in places by artificial levees. Figure 3 compares the 

pre- and postmudflow character of a cross section 

through the Cowlitz River and flood plain, 1,400 ft 

downstream from the Castle Rock Bridge. At this 

location, the North Fork mudflow was contained 

by the east bank (left hand side) levee, but overtopped the west side levee, leaving thick deposits 

of sediment between the levee and the Westside 

highway. Particularly evident in the cross section 

are: (1) the great thickness of deposits within the 

channel, and (2) the smoothing of the overbank 

profile on the west bank. The average depth of 

sediment deposited over the entire cross section 

was 9.3 ft. Within the channel (levee to levee), 

deposition raised the low point (thalweg) by 16.3-ft. 

In the Castle Rock area, the North Fork mudflow 

failed to top the 1.5-mile long east bank levee 

which was built to protect the city; in places, 

however, the flow came within inches of the top. At 

many other locations along the Toutle and lower 

Cowlitz Rivews, the mudflows were not contained 

by levees or roads and, hence, overflowed and left 

considerable deposits on the flood plain (fig. 1}. ln 

general, the thickness of the overbank deposits 

decreased in a downstream direction. 

Downstream from Lexington, few overbank 

deposits occurred, and in the vicinity of Kelso and 

Longview (fig. 1), levees completely confined the 

mudflows to the channel. Part of the North Fork 

mudflow also moved about 2 112 miles upstream in 

the Cowlitz River above the Toutle, River 

,_ 

FIGURE 4. -After mudflow deposits of 15ft, only the loft area of this barn is above ground level. The barn is located on the east bank 

of the Cowlitz River approximately 1 mile south of Castle Rock. 

C5 
FIGU RE 5. - Mudflow damage to farmhouse located approximately 1 112 miles south of Castle Rock on the west bank of the Cowlitz 

River. 

C6 
FIGU RE 6. - Mudflow destruction along the Cowlitz River at a housing subdivision across the river from Castle Rock. Several houses 

were damaged beyond repair and were subsequently removed. 

FIGU RE 7. - Mudflow destruction of farm buildings approximately 1 112 miles south of Castle Rock. About 8ft of debris covers the 

ground in this area on the west bank of the Cowlitz River. 

C7 
FIGURE 8.- Remains of the North Fork Toutle River bridge at Weyerhauser Logging Camp 19, Kid Valley. 

C8 
FIGURE 9.- Flooding at fairgrounds across the river from Castle Rock, Washington. From 3-5ft of sediment were deposited over 

the entire fairgrounds, resulting in cancellation of the annual fair. Photograph taken May 19, 1980. 

C9 
confluence, but overbank deposits were not extensive. Figures 4-9 show the aftermath of overbank 

flow at various places along the lower Toutle and 

Cowlitz Rivers. 

EFFECT OF MUDFLOWS ON FLOOD 

ELEVATIONS 

Hydraulic characteristics of the lower Toutle and 

Cowlitz Rivers were analyzed to compute flood 

stages on both rivers before and after the 

~ 

:J 

f-

<x:: 

0 

_J 

<x:: E 

() ::J 

f= - a:~ w Q) 

> Ol 

() al 

f= Ol 

w .8 

0-

0 Q5 w > 

C) § _J (.) 

<X: .8 z 

0 Q) 

f= 2 

<X: C\J 

z ci 

f- C\J 

ww ~ u.. .._ z15 

-_ ~ 34 

z0 

f= 

<X: 32 

> w 

_J 

w 

30 

28 

Peak elevation of mudf/ow 

---49.8 ft--

mudflows. The water-surface elevations were computed with the aid of the U_S. Geological Survey's 

step-backwater computer program (Shearman, 

1976). 

Cross-sectional data for the backwater analyses 

were obtained from several sources. Stepbackwater analyses for the rivers before the 

mudflows were based on data provided by U_S. Army Corps of Engineers (written commun., 1980), 

Tudor Engineering Company (written commun., 

Bankfull discharge 

76,000 cubic feet 

per second 

Ratings obtained from: 

Stage-discharge measurements 

- - - Stepbackwater hydraulic analysis 

26~--L--L~~~-L~--------L---~--~--~_L_J~_L~------~ 

3 5 1 0 20 50 1 00 200 

DISCHARGE, IN THOUSANDS OF CUBIC FEET PER SECOND 

FIGURE 10. - Relation between river stage and discharge prior to and after the mudflows of May 18, 1980, Cowlitz River at 

Castle Rock, Washington. 

C10 
1980), and Towill, Inc. (written commun., 1980). 

Step-backwater analyses for the rivers after the 

mudflows were based on detailed surveys of the 

channels and flood plains by the U.S. Geological 

Survey and the U.S. Army Corps of Engineers on 

June 19, 1980, and on two channel surveys by the 

U.S. Geological Survey between May 19 and June 

18, 1980. The computed profiles and inundation 

limits for postmudflow conditions were based upon 

the assumptions that the stream channels and 

flood plains would remain in a static and stable condition during the passage of the flood wave, and 

that the flowing fluid would exhibit the same sort 

of friction and viscosity characteristics as normal 

floodwater. 

Prior to the eruption and subsequent mudflows, 

flood stage on the Cowlitz River at the Castle Rock 

gage was 43.2 ft (fig. 10) with a corresponding flow 

of 76,000 cubic feet per second (National Weather 

Service, oral commun., 1980). Floodstage at the 

U.S. Geological Survey gage at Castle Rock after 

the mudflows remained about the same as before, 

but the capacity of the channel to convey water 

was reduced to less than one-tenth (7,300 cubic 

feet per second) of the pre-eruption level. A flow of 

76,000 cubic feet per second occurring after the 

mudflows would produce a stage 9.2 ft higher at 

the Castle Rock gaging station (fig. 10), and a 

stage 9.0 ft higher at the cross section 1,400 ft 

downstream (fig. 3). In the vicinity of Castle Rock, 

lc e illed .t e.a r r,.. r " I ; I II z RIVe 

~ 

::> 

~ 0 

_J 

<( 

() 

i= 

a: 

w 

> 

() 

i= 

w 

0 

0 

w 

(!J 

_J 

<( 

z 

0 

~ ~ 

fw 

w 

LL. 

~ 

z 

0 

~ > w 

_J 

w 

10 

~ w 

5 

(!J 

z 

0 

_J 

"-0 

(/) 

_J 

w 

~ 

~ 

~ Range of backwater effect 

from Columbia River 

20 30 40 50 60 70 80 90 100 110 120 130 140 150 

DISTANCE, IN THOUSANDS OF FEET UPSTREAM OF COLUMBIA RIVER 

FIGCRE 11. - A. Channel bottom and water surface elevation of the lower Cowlitz River prior to and after the mudflows of May 18, 

1980. Pre- and post-eruption water surfaces are based upon flows of 76,000 cubic feet per second below and 38,000 cubic feet per 

second above the Toutle River. 

Cll 
a postmudflow discharge of 76,000 cubic feet per 

second would be contained by the levee on the east 

bank but would inundate the valley to the west. 

Figure 1 shows the area that would be inundated 

by a flood of 76,000 cubic feet per second under 

post-eruption conditions. 

Increases in stage along both the lower Toutle 

and lower Cowlitz Rivers as a result of mudflow 

deposits are shown in figure 11. These stage 

profiles were computed for a flow of 76,000 cubic 

feet per second in the Cowlitz River, and for proportional flows of 38,000 cubic feet per second in 

the Toutle River, and in the Cowlitz River above 

the Toutle River. The post-eruption increases in 

the stage profiles in figure 11 are based on channel 

conditions during June 1980, and do not reflect 

subsequent channel changes that have resulted 

from fill, scour, dredging, or alteration of levees. 

~ 110 

::J 

~ ....J 100 

<{ 

(.) 

i= 

a: 

~ 90 

(.) 

Elevations of mudflows along the lower Toutle 

and Cowlitz Rivers, as determined from mudlines, 

are also shown in figure 11. The peak elevation of 

the North Fork mudflow at the Castle Rock gage 

was 49.8 ft (fig. 10). For comparison, table 1 gives 

the stages and discharges for recorded floods 

which have exceeded 76,000 cubic feet per second 

at this gage. (Compiled from: U.S. Geological 

Survey, 1958, 1963, 1971-79). 

PRESENT AND PLANNED FLOODALLEVIATION MEASURES 

Postmudflow profiles for selected flows were 

computed for the lower Cowlitz River and 

transmitted to the Federal Emergency Management Agency (FEMA) on May 26, 1980. Subsequently, FEMA formally requested the U.S. 

Q) 

.g> 

:§ 

C) 

C) 

~ -~u_RfACE ---- a: I --------- 1 

~ 0 ~ MUDFLOW / 

@ ~ ----- 80 

(!:) 

....J 

<{ 

z 

0 70 

~ ~ 

tu 

UJ 

u. 

~ 

z 

0 

~ ....J 

UJ 

60 

50 

40 

0 

z 

cc ..5~ -------- 1--- WATER SURFACE 

,- POSTMUDFLOW '---;.._.----- .-"' " 

I 

1 2 3 4 5 6 7 8 9 

DISTANCE, IN THOUSANDS OF FEET UPSTREAM OF COWLITZ RIVER 

10 

FIGURE 11. - B. Channel bottom and surface elevation of the lower Toutle River prior to and after the mudflows of May 18, 1980. 

Pre- and post-eruption water surfaces are based upon a flow of 38,000 cubic feet per second. 

C12 
TABLE 1. -Peak discharges exceeding 76,000 cubic feet per 

second and corresponding river stages of Cowlitz River at 

Castle Rock, Washington, during 1927-78 

Date of peak 

Dec. 23, 1933 

Dec. 4, 1975 

Dec. 2, 1977 

Dec. 13, 1946 

Dec. 23, 1964 

Jan. 20, 1972 

Discharge 

(cubic feet per second) 

139,000 

a86,700 

a86,400 

85,100 

b83,900 

a81,600 

River stage 

(feet)c 

51.8 

44.73 

44.68 

44.60 

44.61 

44.00 

aFlow regulated by Mayfield Reservoir and Riffe Lake. Total 

usable capacity, 1,318,800 acre-feet. 

bFlow regulated by Mayfield Reservoir. Usable capacity, 

21,380 acre-feet. 

csubtract 20.2 feet to convert to gage datum. 

Geological Survey to conduct continuing detailed 

field surveys for defining the extent of the hazards. 

Several agencies made extensive efforts to 

reduce the flood hazard before the flood season 

began. The flood season is generally in December. 

Physical steps include dredging of channels and 

the construction of levees and sediment-retention 

structures. N onstructural steps include redesigned 

operating plans for Cowlitz River reservoirs, 

hydrologic surveillance, flood-warning systems, 

and evacuation plans. 

DREDGING 

The Corps of Engineers began dredging operations in the lower Cowlitz River on June 10, 1980 

(see fig. 12). By June 19, 1980, 120,000 cubic yards 

of sediment had been removed from the lower end 

of the river, but no effect was noted on the stagedischarge relation at the Castle Rock gage. As of 

September 30, about 18 million cubic yards of sediment had been removed, and dredging was continuing at the rate of about 1

/s million cubic yards per 

day. By November 1, 1980, the Corps of Engineers 

planned to have removed enough sediment from 

the Cowlitz River channel to increase its bankfull 

(flood stage) capacity to 50,000 cubic feet per 

second- 26,000 cubic feet per second less than the 

bankfull capacity prior to the mudflows. 

LEVEES 

Before the mudflows, the levees constructed 

along the Cowlitz River afforded protection from 

floods of magnitude up to 138,000 cubic feet per 

second for the cities of Castle Rock, Kelso, and 

Longview (Federal Insurance Administration, 

1979a, 1979b, 1979c). The North Fork mudflow 

came within inches of the top, but was contained by 

the east bank levee built to protect the city of Castle Rock (fig. 9). In contrast, the mudflow overtopped the west bank of the river in the vicinity of 

the Castle Rock Bridge where no levee exists (fig. 

9), and farther downstream, topped the west side 

levee illustrated in figure 3. The degree of flood 

protection afforded by the levees after the 

mudflows is unknown. Within 2 weeks after the 

mudflows, the levee protecting Castle Rock was 

raised 2-4 ft to protect against an estimated 

100-year flood under premudflow conditions (fig. 

3). In September 1980, plans were formulated to 

raise the levee an additional 5 ft. The adequacy of 

levees under postmudflow conditions will depend 

on the progress of dredging, and the amount of additional material deposited by future flows, 

especially the high flows of the coming autumn and 

winter. 

SEDIMENT -RETENTION STRUCTURES 

On June 8-9, 1980, the Corps of Engineers let 

contracts for construction of sediment-retention 

structures on both the North and South Forks by 

November 1, 1980. The structure on the North 

Fork is being constructed 20.7 miles upstream 

from the mouth, with a spillway capacity of 8,300 

cubic feet per second and a sediment-storage 

capacity of 2,400 acre feet. As of September 30, 

the structure was about 90 percent completed. The 

structure on the South Fork Toutle River has been 

constructed 7. 0 miles upstream from the mouth, 

with a spillway capacity of 10,000 cubic feet per 

second and a sediment-storage capacity of 210 

acre-feet. 

The potential effectiveness of these structures 

for preventing or slowing the downstream movement of sediment is unknown. Even if the retention reservoirs are effective, some sediment will be 

carried through the structures and additional sediment will be scoured from the downstream channels and flood plains. Sediment deposition will 

definitely continue in the lower Toutle and Cowlitz 

Rivers. 

C13 
FLOOD CONTINGENCY PLANS 

Following the mudflows, local, county, and 

Federal agencies formulated plans for the 

surveillance of the lower Toutle and Cowlitz Rivers 

and for the dissemination of flood warnings. 

EVACUATION 

At a July 8-9, 1980, conference at Washington 

State University entitled "The Aftermath of Mount 

St. Helens," the Corps of Engineers assessed the 

need for possibly evacuating people along the 

lower Cowlitz River. It was noted that evacuation 

might be required prior to autumn and winter high 

flows even after the combined program of dredging, raising levees, and installation of sedimentretention structures. As reported by the Tacoma 

News Tribune on July 10, 1980, the Governor of 

Washington suggested that entire towns located 

along the Cowlitz River may need to be relocated 

to ensure the safety of the residents and their property. No action was taken on this suggestion as of 

September 1980. The time required for major, 

orderly relocation of towns rules out accomplishment of this action before the onset of the high 

river flows anticipated during the autumn and 

winter. Evacuation, therefore, is the only feasible 

course of action if flooding becomes imminent. 

Timely evacuation will require continued 

surveillance of rainfall, snow in storage, and river 

stage and discharge. 

CONTINUED SURVEILLANCE AND FLOOD WARNING 

Within 6 days after the eruption, the U.S. 

Geological Survey installed a river-stage station on 

the North Fork Toutle River at Kid Valley, and 

stages were being transmitted via satellite to the 

National Weather Service River Forecast Center 

in Portland, Oregon; the U.S. Forest Service in 

Vancouver, Washington; the Cowlitz County 

Sheriffs Office; and the Washington District Office, U.S. Geological Survey. Additionally, one 

stage station was installed on the North Fork Toutle River upstream from Kid Valley and two on the 

FIGURE 12. - Dredging operations on the Toutle River just upstream from its confluence with the Cowlitz River. Photograph taken 

September 11, 1980. 

C14 
South Fork Toutle River, all constructed to 

transmit stages via satellite. River stages are also 

being telemetered by radio from gages on the 

Cowlitz River at Castle Rock and the Toutle River 

at the Highway 99 crossing near the mouth, and 

will soon be telemetered from a gage on the Toutle 

River near the town of Toutle Lake. The location of 

the river surveillance sites are shown in figure 13. 

When flooding appears possible, stage data will be 

transmitted every 5 minutes from each gage, 

thereby providing at least several hours of warning 

to residents on the Cowlitz River. 

The U.S. Geological Survey is periodically 

resurveying the lower Toutle and Cowlitz Rivers 

and reanalyzing the river hydraulics. Water 

profiles are being prepared after each resurvey, 

and inundation maps are updated as needed. This 

effort not only provides continued monitoring of 

the flood hazards, but also documents the effects of 

dredging and the movement of sediment. 

ACKNOWLEDGMENTS 

Appreciation is expressed to the U.S. Army 

Corps of Engineers, Portland District; Tudor 

Engineering Company, Seattle, Washington; and 

Towill, Inc., San Francisco, California, who furnished cross-sectional data, aerial photographs, 

and other useful information. 

REFERENCES CITED 

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. C-1 to C-26. 

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

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

Geological Survey Circular 850-B, 16 p. 

Federal Insurance Administration, 1979a, Flood insurance 

study, city of Castle Rock, Cowlitz County, Washington, 

U.S. Department of Housing & Urban Development 

Federal Emergency Management Agency No. 530277, 15 p. 

--1979b, Flood insurance study, city of Kelso, Cowlitz 

County, Washington: U.S. Department of Housing & Urban 

Development, 20 p. 

--1979c, Flood insurance study, city of Longview, Cowlitz 

County, Washington: U.S. Department of Housing & Urban 

Development, 19 p. 

Shearman, J. 0., 1976, Computer applications for step-backwater and floodway analyses, User's manual: U.S. 

Geological Survey Open-File Report 76-499, 103 p. 

U.S. Geological Survey, 1958, 1963, 1971, 1972, Surface water 

supply of the United States, Part 14, Pacific Slope Basins in 

Oregon and Lower Columbia River basin, U.S. Geological 

Survey Water-Supply Papers 1318, 1738, 1935, 2135. 

--1972-75, Water resources data for Washington, Part 1, 

Surface-water records: Tacoma, Washington, 1971-74. 

--1976-79, Water resources data for Washington, 

1975-1978: Tacoma, Washington, report nos. WA75-1, 

WA76-1, WA77-1, WA78-1. 

Cl6 
0 

1-" 

01 

123° 122°45' 122°30' 

46°15' 

WASHINGTON 

122°15' 

I 

8!8 

~I!: 

~~i 81~ I 

I 

I 

Mount St. Helens 

122° 

46°L-----------------~~~------------~----------------------------------~~----------------------~~~-------U----------------------------------~----~ 

0 5 10 15 20 MILES 

EXPLANATION 

0 5 1 0 15 20 KILOMETERS o Town '-'River surveillance site 

_Bridge (stream gage) 

FIGURE 13. - Map showing location of river surveillance sites in the lower Toutle and Cowlitz River systems.