45° 

42° 

METRIC CONVERSION FACTORS 

Multiply 

foot (ft) 

cubic yard (yd3 ) 

mile (mi) 

square mile (mi 2 ) 

cubic foot per second (fe /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 second (m3 /s) 

105' 

49' -----~f' 115' 

1

\

00 

~ -----------L; ------IT_L___________ \ , , I I ! , Seattle Spekane o j \ \ 

Olympia WASHINGTON j ~'\ MONTANA \ 

' ' \ 

\ • Mount St Helens ! '1{, He~ en a '\ 

I ,...------~ ';--....._.,..-.. __....-_....-- ' , I 

~ortland ) l,~ _J 0 

Salem 

OREGON 

I \ ---------------- / ,, -f'~ 1 ... ~ I 

I I 

J 0 Boise \ 

j IDAHO \ 

j \ ---- _______________ _j ____________________ ~ 

0 100 200 300 MILES 

0 100 200 300 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. 
Mudflows Resulting from 

the May 18, 1980, Eruption of 

Mount St. Helens, Washington 

By John Cummans 

Hydrologic Effects of the Eruptions 

of Mount St. Helens, Washington, 1980 

GEOLOGICAL SURVEY CIRCULAR 850-B 

1981 
United States Department of the Interior 

CECIL D. ANDRUS, Secretary 

Geological Survey 

H. William Menard, Director 

Library of Congress Catalog-card No. 81-600007 

Free on application to Branch of Distribution, 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 ofhot 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 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. 

H. William Menard 

Director 

Ill 

CONTENTS 

Page 

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

Abstract --------------------------------------------------------~------------ B1 

Introduction ------------------------------------------------------------------ 1 

Description of drainage systems and chronology of the mudflows ------------------------ 4 

Lewis River --------------------------------------------------------------- 4 

Drainage system -------------------------------------------------------- 4 

]dudflows -------------------------------------------------------------- 4 

Toutle and Cowlitz Rivers ---------------------------------------------------- 4 

Drainage system -------------------------------------------------------- 4 

]dudflows -------------------------------------------------------------- 5 

South Fork Toutle River ----------------------------------------------- 5 

North Fork Toutle River----------------------------------------------- 7 

Discussion -------------------------------------------------------------------- 12 

]dudflow velocities ---------------------------------------------------------- 12 

Post-mudflow channel changes------------------------------------------------ 13 

Acknowledgments ------------------------------------------------------------- 15 

References ------------------------------------------------------------------- 16 

ILLUSTRATIONS 

Page 

FIGURE 1. ]dap of ]dount St. Helens area ------------------------------------------ B 2 

2. Aerial view of the east end of Swift Reservoir ----------------------------- 5 

3. Aerial view of valley mudflow deposits in the upper reaches of the South Fork 

Toutle River ----------------------------------------------------- 6 

4. Aerial view looking east across the thickest part of the debris avalanche deposit in 

the North Fork Toutle River---------------------------------------- 8 

5. Surface of the debris avalanche deposit in the North Fork Toutle River downstream from Elk Rock --------------------------------------------- 9 

6. Debris avalanche deposit along right bank of the North Fork Toutle River 

downstream from Elk Rock---------------------------------------- 10 

7. Characteristic mudflow marks ~long the Toutle River just downstream from 

Highway 504 Bridge---------------------------------------------- 11 

8. Scoured channel and mudflow marks on trees downstream from the site of the 

Toutle River gage ------------------------------------------------ 12 

9. Flooded area at destroyed Highway 504 Bridge just below the confluence of the 

North and South Forks Toutle River--------------------------------- 13 

10. View of mudflow near Highway 99 Bridge on the Toutle River--------------- 14 

11. Stage from recorder and wire weight gage sounding. Cowlitz River at Castle 

Rock, ]day 18-19, 1980-------------------------------------------- 15 

TABLES 

Page 

TABLE 1. Stage, storage, and inflow to Swift Reservoir during the mudflow of ]day 18, 

1980 ----------------------------------------------------------- B5 

2. Approximate time, location, and average velocity of the peak stages of mudflows 

originating in the South and North Forks Toutle River, ]day 18, 1980 ------- 16 

v 

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

MUDFLOWS RESULTING FROM THE MAY 18, 1980, ERUPTION OF 

MOUNT ST. HELENS, WASHINGTON 

By jOHN CUMMANS 

ABSTRACT 

On May 18, 1980, Mount St. Helens, in southwestern 

Washington, erupted violently, setting off a chain of 

devastating hydrologic event. During the eruption, a massive 

debris avalanche moving down the north side of Mount St. 

Helens was deposited in the North Fork Toutle River valley. 

Approximately 3 billion cubic yards of material, including rock, 

ash, pumice, snow, and ice was deposited directly north of the 

mountain in the upper 17 miles of the valley. 

Following the eruption, mudflows quickly developed in the 

South Fork Toutle River, and in the Lewis River tributaries of 

Smith Creek, Muddy River, and Pine Creek. Several hours later 

a massive mudflow originated from the debris avalanche deposit 

in the North Fork Toutle River valley. This North Fork 

mudflow caused widespread destruction as it moved 

downstream through the lower Toutle and Cowlitz Rivers. 

This report describes the location and chronology of the 

mudflows which followed the May 18 eruption. Average 

velocities are presented for the mudflows in the South and 

North Fork Toutle Rivers, and photographs illustrate the 

character of the debris and mud deposits. 

INTRODUCTION 

On March 27, 1980, Mount St. Helens, 

Washington erupted for the first time in more than 

100 years. On May 18, following 7 weeks of earthquakes and minor eruptions, the volcano exploded 

violently, setting off a chain of devastating 

geologic and hydrologic events. The major components of the eruption included: (1) a directed 

blast of hot gases and ash which leveled the forest 

to the north for a distance of up to 15 miles, (2) a 

massive debris avalanche that deposited the remnants of the detached north flank of the volcano 

(approximately 3 billion cubic yards of rock, ash, 

pumice, snow, and ice) in the upper 17 miles of the 

North Fork Toutle River valley (Youd and Wilson, 

1980), (3) a northward flow of fragmented volcanic 

materials that dammed the outlet of Spirit Lake, 

B1 

and (4) an ash column 10 miles high that traveled 

eastward across the United States as a giant ash 

cloud. 

Previous geologic studies had determined that 

mudflows were common in past volcanic eruptions 

at Mount St. Helens (Crandell and Mullineaux, 

1978). Thus, following the initial activity on March 

27, mudflows were anticipated as part of a large 

eruption, although the streams most likely to be affected and the general magnitudes of flows were 

matters of conjecture. In preparation for potential 

flooding hazards, a flash flood warning system was 

devised by Cowlitz and Skamania county officials 

(see fig. 1). 

Within minutes of the May 18 eruption, county 

personnel were ordered to previously-designated 

observation posts as part of the flood warning 

system. Simultaneously, residents along the major 

rivers that drain the volcano were instructed to 

prepare for hasty evacuation. The eruption quickly 

led to mudflows in the South Fork Toutle River 

and in the Lewis River tributaries of Smith Creek, 

Muddy River, and Pine Creek. Several hours later, 

a large mudflow came down the North Fork Toutle 

River after originating from the massive debris 

avalanche deposit. 

This report describes the chronology and 

magnitude of mudflows on May 18 in tributaries of 

the Lewis River, and in the South and North Forks 

of the Toutle River. Particular attention is given to 

the chronology of events in the Toutle River 

system, with the North Fork mudflow given the 

greatest attention because of its relatively large 

magnitude and the consequent destruction. 

Mudlfows are masses of water-saturated debris 

of various sizes that flow downslope as a result of 

gravity. Mudflows commonly resemble masses of 
123° 

0 

0 

Castle 

Rock 

Bridge 

5 

5 

10 

WASHINGTON 

Scale 1 :250,000 

10 15 

15 20 KILOMETERS 

FIGURE 1. -Map of Mount St. Helens area. 

B2 

20 MILES 
Crockett's Car 

. ·:'·):,.,,,,,,::::,,~,,,,.. . Debris 

{!!i=!!!'!!=!l·!!!!!!l·!!i!!i!!i!!!!)!!!!!Jt:::-avalan che ··.· .. ·· deposit 

B3 

Mount St Helens 

EXPLANATION 

o Town 

>-< Bridge 

~ Stream gage 

o Lumber camp 
wet concrete or mortar and, indeed, these terms 

were used by several observers to characterize the 

mudflows in the Toutle River system. 

DESCRIPTION OF DRAINAGE SYSTEMS AND 

CHRONOLOGY OF THE MUDFLOWS 

Water originating on Mount St. Helens drains into four river systems: the North Fork Toutle River, 

the South Fork Toutle River, the Kalama River, 

and several tributaries of the Lewis River (see fig. 

1). Except for the Kalama River, large mudflows 

occurred in each of the systems following the May 

18 eruption. Aerial observations in the Kalama 

system revealed that small mudflows did occur at 

high elevations in South side tributaries. However, 

the mudflows were comparatively so small, that all 

further work was conducted in the three other 

systems. 

LEWIS RIVER 

DRAINAGE SYSTEM 

The sizes of drainage areas listed in the following 

descriptions of the Lewis River tributaries, South 

Fork Toutle, and North Fork Toutle are those 

prior to the May 18 eruption. Post-eruption 

drainage areas are slightly different because the 

drainage divides between the headwaters of the 

streams were altered by the blast which lowered 

the mountain top by about 1,300 feet. The drainage 

systems and the geographic and cultural locations 

mentioned in the text are shown in figure 1. This 

map is designed to aid the reader in following the 

location and chronology of the mudflows. 

South of Mount St. Helens, the Lewis River 

flows in a westerly direction into Swift Reservoir 

(fig. 1). The Muddy River system, which includes 

Smith Creek, drains most of the eastern slope of 

Mount St. Helens before flowing into the Lewis 

River 1 mile upstream from the reservoir. Pine 

Creek heads on the southeast summit of Mount St. 

Helens, and flows southward into the Lewis River, 

entering about 1/2 mile upstream from Swift 

Reservoir. Together, the Muddy River system, 

Pine Creek, and other tributaries to Swift Reservoir drain about 10 square miles directly from the 

higher slopes (above 4,000 feet) of Mount St. 

Helens. At Swift Reservoir, the Lewis River drains 

a total of 480 square miles. 

MUDFLOWS 

Immediately following the eruption, mudflows 

moved rapidly down Smith Creek, Muddy River, 

and Pine Creek, destroying the bridge near the 

mouth of Pine Creek and the Eagle Cliff bridge at 

the head of Swift Reservoir. The mudflows scoured 

the stream valleys except along the lower reaches 

of the Muddy River where scattered trees remained. The lowermost reach of the Muddy River valley 

was completely choked with logs for a distance of 1 

mile. 

Between 9:00 a.m. and noon on May 18, the 

mudflows from the streams draining Mount St. 

Helens deposited about 11,009 acre-feet of water, 

mud, and debris in Swift Reservoir. Records of 

water level changes in the reservoir (see table 1). 

indicate average inflows of about 60,000 cubic feet 

per second between 9 and 10 a.m., 55,000 cubic 

feet per second between 10 and 11 a.m., and 12,000 

cubic feet per second between 11 a.m. and noon. In 

total, the water surface of the reservoir rose 2.6 

feet during this period. 

Most of the incoming sediment appears to have 

settled near the head of the reservoir. On the morning of May 20, mud and debris extended only 

about 1 mile below the upstream end of the reservoir, down to an area of floating logs (fig. 2). 

TOUTLE AND COWLITZ RIVERS 

DRAINAGE SYSTEM 

Both the North and South Forks of the Toutle 

River originate on the slopes of Mount St. Helens 

(see fig. 1). Prior to the eruption, the two streams 

drained a combined area of about 15 square miles 

from the slopes above 4000-feet elevation. 

The North Fork flows in a westward direction 

away from Mount St. Helens. At its confluence 

with the South Fork, the North Fork drains 303 

square miles, of which 132 square miles is drained 

by the Green River, a major tributary to the north. 

The South Fork of the Toutle River originates on 

the western slopes of Mount St. Helens and drains 

129 square miles above its junction with the North 

Fork. 

The Toutle River, formed by the junction of the 

North and South Forks, flows in a generally 

westward direction into the Cowlitz River. At its 

mouth, the Toutle drains 512 square miles. The 

Cowlitz River flows south into the Columbia River, 

and several miles of its lower reach, including the 

location of the Longview Water Treatment Plant, 

are affected by tides in the Columbia River (fig. 1.). 

B4 
TABLE 1. -Stage, storage, and inflow to Swift Reservoir during the mudflow of May 18, 1980 

Accumu- Event 

Water- Estimated lative Hourly inflow 

surface storage Actual runoff event rate in Time elevation without storage from gain preceding 

(feet) mudflow (acre-ft) event (acre-ft) hour (acre-ft) (acre-ft) (ft3 /s) 

9a.m. 973.02 637,150 637,610 460 460 • • " ••• 0. 0 ••• 

10a.m. 974.24 637,320 642,740 5,420 4,960 60,000 

11 a.m. 975.37 637,530 647,480 9,950 4,530 54,800 

12 noon 975.63 637,660 648,580 10,920 970 11,700 

1 p.m. 975.68 637,910 648,790 10,880 • • • • • • 0 ••• 0 • •••• 0 •• 0 0 0 • 

FIGURE 2. -Aerial view of east end of Swift Reservoir. The mud and debris originated in the upper reaches of Smith Creek, Muddy 

River, and Pine Creek. Photograph taken May 20, 1980. 

MUDFLOWS 

SOUTH FORK TOUTLE RIVER 

The mudflow on the South Fork Toutle was first 

observed by David C. Crockett, KOMO TV News, 

Seattle, who later related his experiences to 

Richard B. Waitt, Jr., of the U. S. Geological 

Survey. Mr. Crockett reported that he was on the 

south side of the river about 4112 miles west of the 

· mountain when the eruption occurred. He took 

several photographs and then drove rapidly down 

the valley. Mr. Crockett said, "Soon a huge mass of 

water, mud, and trees crashed through in front of 

me ... snapping off trees." He stopped and was 

B5 
quickly trapped by the flow. The rate of flow 

decreased rapidly, but it was still sufficient to 

carry large quantities of trees and other large 

debris. Mr. Crockett quickly gathered his cameras 

and other gear, abandoned his car, and hiked to 

safety. He reported crossing "warm peat, or warm, 

fluid, brown concrete" during his escape. These 

events occurred about 8:50a.m. The approximate 

location of Crockett's abandoned vehicle is shown 

in Figure 1. 

Mr. Crockett also said that a second mudflow 

came down the South Fork at about 2:00 p.m., 

which he could hear before it came into view. He 

reported that the river had earlier returned to its 

channel, but during the afternoon event, it 

overflowed again. The second mudflow was 

reportedly much smaller than the first, lasting 

about 20 minutes. 

A photograph (fig. 3) taken May 23, five days 

later, shows the mudflow deposits in these upper 

reaches of the South Fork valley. 

The first direct report on the South Fork 

mudflow occurred at 10:03 a.m. when a flood 

watch deputy radioed the Cowlitz County communications center that "a 5-foot rise of South Toutle River" was passing Weyerhaeuser's Camp 12. 

The camp is 42.2 river miles upstream from the 

mouth of the Cowlitz River. At 10:05 a.m., a second radio report indicated the flow was "knocking 

down trees," and at 10:07 a.m., it was reported 

that a "12-foot flash flood" was coming down the 

South Fork from Camp 12. The South Fork flow 

was intense, but short in duration. The mudflow 

generated on the South Fork Toutle River traveled 

27 river miles, from the mountain to Camp 12, in 

90 minutes. 

FIGURE 3.- Aerial view of valley mudflow deposits in the upper reaches of the South Fork Toutle River. Photograph taken May 23, 

1980. 

B6 
Farther downstream, a Mr. Roald Reitan was 

camped along the South Fork just upstream from 

the town of Toutle. He was awakened by warning 

sirens nearby and noted that "the river was already 

half-full of logs." According to Reitan, over the 

next few minutes the river's height rose fast. At 

first, the water was clear with a large number of 

logs, but the water quickly muddied, and Reitan 

observed a portion of trestle (probably part of 

Weyerhaeuser's railroad bridge) carried along in a 

mass of logs. The flowing mass eventually turned 

to chocolate color and became thicker immediately 

after the trestle and logs passed, knocking down 

trees along the edges of flow. Reitan later stated 

that the mass "seemed to be moving at 30 to 35 

miles an hour," and he estimated that only 3 to 5 

minutes elapsed between the "half-full channel" 

and the overbank-flow conditions. During this 

time, the channel was completely filled with logs. 

At 10:13 a.m., a river watch deputy radioed the 

Cowlitz County communications center that mud 

and debris were moving toward the Harry Gardner 

Bridge. At 10:14 a.m., the Deputy described the 

flow as a 12-foot wall of water containing logs, 

debris, and buildings. 

At 10:20 a.m., the leading edge of the mudflow 

arrived at the South Fork's confluence with the 

North Fork, just upstream from the Highway 504 

Bridge (locally called Coal Banks Bridge or 

Morgan Bridge.) A Deputy reported that the 

mudflow was close to the underside of the Highway 

504 Bridge at 10:48 a.m., which was probably 

about the time of the peak-stage at that location. 

Both the Harry Gardner Bridge and Highway 504 

Bridge survived the South Fork mudflow. 

At the Toutle River gaging station at Silver Lake 

(located 1/4 mile below the Highway 504 Bridge), 

the river stage at 10:20 a.m. was 2.54 feet, a normal level. By 10:25 a.m., the river stage was rising, 

but the heavy mudflow quickly rendered the 

automatic continuous stage recorder inoperative 

and the actual timing of the peak was not recorded. 

At 2:50 p.m., Geological Survey field personnel 

determined that the high water mark at the gage 

was about 23.5 feet. This is the highest stage ever 

observed at the site, which has been maintained 

since 1909. The previous highest stage, 22.56 feet, 

occurred on December 2, 1977. 

After the peak, the stage receded rapidly in this 

reach. At 11:12 a.m., sheriffs deputies reported 

that the river level at the Route 504 Bridge had 

dropped 6 feet; at 11:16 a.m., 8 feet. They also 

reported that the entire surface of the river there 

was covered with floating logs for about 25 

minutes. 

At 11:51 a.m., a log jam was reported to have 

just broken up on the Toutle River near Tower 

Bridge (26. 5 river miles upstream from the mouth 

of the Cowlitz River), as the peak passed that area. 

By this time, the river had returned to "just a 

trickle" 10 miles upstream at the Highway 504 

Bridge. 

By 1:00 p.m., the mudflow had entered the 

Cowlitz River. At 1:30 p.m., the stage at the Castle 

Rock Bridge (Cowlitz River mile 17.3 and 2. 7 miles 

below the mouth of the Toutle River) had risen to 

13.15 feet, as recorded at the Geological Survey 

gage: Prior to the approach of the mudflow, the 

river was at a normallO.O feet stage, and it returned to 10.0 feet by 2:15p.m. The mudflow, with its 

logs and debris, continued within banks down the 

Cowlitz River. 

According to Mr. Leland F. Edtl, Longview 

Utilities Engineer, the leading edge of the South 

Fork mudflow, "with logs, trees, limbs, and bark," 

arrived at the intake of the Longview Water Treatment Plant (Cowlitz River mile 5.2) at 4:15 p.m. 

The peak probably occurred about 5 p.m. or shortly 

thereafter; however, Mr. Edtl observed logs in the 

river for 3 to 3 112 hours. During this period, Mr. 

Edtl observed no increase in water levels nor 

deterioration of the water quality. Water 

temperature was 51 °F, and turbidity remained at 

its normal level of 4 Jackson Turbidity Units. 

However, between 8:00 and 8:30p.m., the turbidity abruptly increased to 420 Jackson Turbidity 

Units, and the treatment-plant water intake was 

closed. 

On May 20, U. S. Geological Survey personnel 

observed the South Fork Toutle River and found 

clear evidence of the mudflow. The flow had been 

confined within steep valley walls except for a 5 to 

6-mile reach above the confluence with North 

Fork, where some overbank flow had occurred. 

NORTH FORK TOUTLE RIVER 

The massive debris avalanche which accompanied the May 18 eruption blocked State Highway 

504 along the North Fork Toutle River with a 

17-mile plug of rock, ash, ice, snow, and other 

debris. At Spirit Lake, at the upper end of the plug, 

the debris depth reached 400 feet; near Elk Rock, 

at the lower end, about 150 feet. The thickest 

debris, up to 600 feet, was deposited about 1 mile 

below Spirit Lake (fig. 4). 

B7 
FIGURE 4.- Aerial view looking east across the thickest part of the debris avalanche deposit in the North Fork Toutle River. This 

location is about 1 mile downstream from Spirit Lake. Ph?tograph taken May 23, 1980. 

Large volumes of material from the debris 

avalanche deposit entered Spirit Lake. High-water 

marks later observed above the northeast arm of 

the lake suggest that the lake water may have been 

temporarily displaced as the debris came in, subsequently flowing back over the newly-deposited 

materials. The new lake surface is about 200 feet 

higher than prior to the eruption and about 200 

feet lower than the surface of the massive debris 

deposit. 

The first post-eruption observations of the upper 

North Fork Toutle River were made by Harry 

Glicken of the Geological Survey during attempted 

helicopter rescue missions for personnel at the 

Coldwater field camps (fig. 1). Between 12-noon 

and 1 p.m., Glicken observed small plumes of ash 

and steam emanating from numerous mudflows on 

top of the debris avalanche deposit. Below the toe 

of the deposit, Glicken observed downed trees, ash 

cover, and flowing mud; the mud was shallow, failing to innundate 2-foot boulders on the river bed. 

At about the same time, a Weyerhaeuser Co. 

team reconnoitering the area by helicopter noted 

that the stage of the North Fork Toutle River was 

low, presenting no problems at either Camp Baker 

or Camp 19 (fig. 1). About 2 miles upstream from 

Camp Baker, the team noted blown-down timber 

across both Highway 504 and the main 

Weyerhaeuser logging road. They also saw the 

debris deposit, and a few miles farther upstream 

(probably near Elk Rock), observed the surface 

character of the debris which filled the valley floor. 

At 1:04 p.m., while flying over Camp Baker, the 

team received a radio report that water and debris 

were moving down the North Fork. The team 

headed upstream to check the report and observed 

streams of muddy water flowing down both sides 

of the valley on the surfa~.:e of the debris avalanche 

deposit. The two flows were heavy and visible for 

some distance as their leading edges kicked up 

large plumes of ash. At 1:25 p.m., the mudflows arrived near Elk Rock, about 63 miles upstream from 

the mouth of the Cowlitz River. Figures 5 and 6 

show two areas of the debris deposit downstream 

B8 
from Elk Rock; each figure clearly shows the dark 

remains of the mudflows on top of the lighter surface of the debris avalanche deposit. 

Additional observations of the mudflows were 

made by Mr. Jack Schoening, a member of the 

Weyerhaeuser Co. team. According to Mr. Schoening, the team was over Camp Baker at 1:57 p.m. 

when the flows along the north side of the valley 

were reaching that area. A minute later, the north 

side flow hit the camp, moving into the equipment 

and shop areas. At this time, the mudflow on the 

south side of the valley had yet to reach Camp 

Baker. By 2:30 p.m., the camp was reported as 

destroyed. The maximum flow in the area probably 

occurred around 3:00 p.m., when a log jam near 

the camp reportedly broke loose. At 5:00p.m., Mr. 

Schoening was again over the camp, and stated the 

devastation was worse than he observed at 2:30 

p.m. 

Some subsequent observations in the Camp 

Baker area are of interest. On May 18, Mr. Ray 

Pleasant, a pilot for Weyerhaeuser Co. observed 

the edge of the mudflow at one shop building. 

Several days later he noted that the mud around 

the building covered a wider area than before, and 

was about 3 feet deeper. Another Weyerhaeuser 

employee who noted a "little" mud near his camp 

equipment at about 6:00 p.m. on May 18, observed 

that the mud was deeper when he returned on May 

19. These observations indicate that either (1) later 

mud flowed on top of the earlier deposits, or (2) the 

consistency of the mudflow was so thick that flow 

in the center was higher than at the edges, thus requiring time for the edges to stabilize at a final, 

higher depth. 

At 2:40 p.m., the Cowlitz County Communications Center received a report that the mudflow 

was passing Alder Creek. At 3:01p.m., the bridge 

FIGURE 5. -Surface of debris avalanche deposit in the North Fork Toutle River downstream from Elk Rock. The lighter areas are 

the exposed surface of the debris deposit; the darker areas are brown sediment deposited by the subsequent mudflows. 

Photograph taken May 19, 1980. 

B9 
FIGURE 6.-Debris avalanche deposit along right bank of the North Fork Toutle River downstream from Elk Rock. Note edge of the 

blast area near center of photo where trees were blown down. Photograph taken May 19, 1980. 

on Highway 504, 0.6 mile upstream from the 

mouth of Alder Creek, was reported as destroyed 

by the flow; by 3:30p.m., the mudflow was passing 

the mouth of the Green River. 

Mr. and Mrs. Dan Calvert, residents of Kid 

Valley, observed the mudflow at the Weyerhaeuser 

Camp 19 Bridge from about 3:00p.m. until about 

4:30 p.m., at which time they were rescued by 

helicopter. They stated that during this period, the 

flow kept rising and had the consistency of fresh 

mortar. At one point, they saw a fully-loaded logging truck moving downstream submerged only to 

the level of the lower-most tier of logs. Mr. Calvert 

also observed many buildings and ice chunks being 

carried along in the flow. The surface of the 

mudflow appeared to be smooth; Mr. Calvert did 

not recall seeing waves. Upon his return several 

days later, Mr. Calvert observed that the mud lines 

along the channel appeared to be higher than the 

stage at the time of his rescue. 

At 4:55p.m., Weyerhaeuser personnel observed 

Camp 19 by helicopter and noted that the mudflow 

was over the wheels of railroad cars; this level was 

later determined to be about the maximum height 

of the flow. At about 5:20p.m., they reported the 

flow was continuing and that more logs had been 

swept from the storage area. Based on the descriptions, the peak flow at Camp 19 probably occurred 

at about 5 p.m. 

The Cowlitz County Communications Center 

received a report of the mudflow reaching the 

Toutle Boys Ranch at about 5:45 p.m. Ray Pleasant, of Weyerhaeuser Co., observed the site shortly after 6:45 p.m., and reported that the mudflow 

depth appeared to have reached its maximum. As 

much as 3 to 4 feet of mud had been deposited in 

some of the buildings. The peak flow at this location probably occurred at about 6 p.m. 

The North Fork mudflow was just upstream 

from the Highway 504 Bridge near Toutle at 5:48 

p.m. After uprooting trees at Harry Gardner Park, 

the mudflow destroyed the Highway 504 Bridge, 

B10 
FIGURE 7.-Characteristic mudflow marks along the Toutle River just downstream from Highway 504 Bridge. The material when 

dry, has a coarse, abrasive texture. Photograph taken May 19, 1980. 

apparently between 6:10 and 6:15 p.m. At the 

Geological Survey gage, 1/4 mile downstream, the 

automatic radio stage transmitter was functioning 

at 6:00 p.m., but failed to report at 6:15p.m. The 

peak flow at the gage probably occurred at about 

7:00p.m. 

Sheriffs deputies reported high stages at the 

confluence of the two forks until8:55 p.m., when it 

was reported that the water level was falling as 

though something broke loose. On May 19, mudflow marks at the gaging site (figs. 7 and 8) just 

below the Route 504 Bridge were measured as 53 

feet above gage datum, about 30 feet higher than 

for the South Fork mudflow. A surveying team 

then ran levels from marks at the gaging site to a 

narrow gorge 800 feet downstream. The measurements revealed only a slight downstream slope of 

the mudflow surface, indicating that ponding had 

occurred in this reach. The ponding was largely 

responsible for the extraordinary depths in this 

reach, and apparently was caused by a massive log 

jam at the entrance to the constricting gorge, 

which is located 0.8 mile downstream from the confluence of the North and South Forks. The blockage and ponding may have been aided by debris arriving from the secondary mudflow observed by 

David Crockett on the upper South Fork at 2:00 

p.m. Figure 9 shows the resultant devastation just 

downstream from the confluence, in the area of the 

destroyed Highway 504 Bridge. 

At 8:29 p.m., the mudflow covered the Tower 

Bridge, 10.3 miles downstream from the Route 504 

Bridge; at 8:41 p.m., a report said the bridge had 

been destroyed. The peak at Tower Bridge probably occurred at about 9:15p.m. 

The mudflow arrived at the mouth of the Toutle 

River at about 8:30p.m., where it was described as 

having a homogeneous, mortar-like consistency 

from bank to bank (fig. 10). At this location, the 

early part of the flow carried logs and buildings, 

Bll 
FIGURE 8.-Scoured channel and mudflow marks on trees downstream from the site of the Toutle River gage. Photograph taken 

May 19, 1980. 

and a pickup truck was also observed. As the 

massive mudflow entered the Cowlitz River, the 

main mass moved downstream , but part moved 

upstream into the Cowlitz for 21

12 miles. 

The floodwave continued downstream and the 

automatic gage at Castle Rock was quickly covered 

with mud and rendered inoperative. However, 

water levels were measured periodically at this site 

by Geological Survey personnel. As shown in Figure 11, water levels rose to a crest of 29.3 feet at 

midnight. Stages only 0.1 to 0.2 foot lower were 

observed from 11:55 p.m. (May 18) to 12:45 a.m. 

(May 19). 

At the Castle Rock Bridge, the temperature of 

mixed mudflow and Cowlitz River water was 85 ° F 

at 1:15 a.m. On May 19, at 6:30a.m., the stage was 

down to 25.0 feet, but the temperature remained at 

85° F. For comparison, at 9:45a.m., the mudflow 

temperature in the Toutle River at the Highway 99 

Bridge was 91 o F. 

At 3:00 a.m., on May 19, the stage at the 

Longview Water Treatment Plant (Cowlitz River 

mile 5.2) was reported to be 21 feet, which was at 

or near the maximum. Mr. Edtl, Longview 

Utilities Engineer, reported that the flood had 

receded 4 feet by 8:00 a.m., when the water 

temperature was 90° F, and turbidity was 

astronomically high and unmeasurable. 

DISCUSSION 

MUDFLOW VELOCITIES 

The movement of the mudflows in the North and 

South Forks Toutle River is summarized in table 2. 

It should be noted that the computed travel 

velocities shown in the table are averages between 

successive locations. Velocities at any particular 

point may have varied considerably from that 

B12 
FIGURE 9.-- Flooded area at destroyed Highway 504 Bridge just below the confluence of the North and South Forks Toutle River. 

Photographs taken May 19, 1980. 

shown because of local differences in channel configuration and stream-bed slope. The computed 

velocities are based upon the time-of-travel of the 

peak mudflow stage. The reported timing of the 

peak stages at the designated locations are believed to be accurate to within one-half hour, except 

for that of the Cowlitz River at Castle Rock. 

Large differences occurred in the velocities of 

the mudflows in the North Fork and South Fork. 

Above the point of confluence (near Highway 504 

Bridge), the weighted average velocity for the 

peak stage of the North Fork mudflow is about 7 

feet per second, whereas that for the South Fork 

mudflow is about 22 feet per second. The three-fold 

higher velocity in the South Fork possibly resulted 

from a combination of steeper stream-bed slopes, 

smaller channel capacities, and a higher water content of the mudflow. 

The South Fork mudflow moved through the 

Toutle river down to the Castle Rock Bridge at a 

greater velocity than the North Fork mudflow. 

Downstream from Castle Rock, under the influence of the Cowlitz River, the velocities of the 

two mudflows were very similar. 

POST-MUDFLOW CHANNEL CHANGES 

Comparison of pre- and post-eruption profiles indicate that the mudflow significantly altered the 

channel configuration and, hence the flow capacity 

of the lower Cowlitz River. Cross-sectional 

measurements of the Cowlitz River channels at 

both Kelso and Castle Rock reveal that deposits of 

sediment and debris raised the average elevation 

of the channel bottom about 15 feet at both locations. Prior to the mudflows, the maximum carrying capacity of the Cowlitz River at Castle Rock (at 

the flood stage of 23.0 feet) was 76,000 cubic feet 

per second. As of July 1980, the channel capacity 

was about 7,300 cubic feet per second. At that 

time, the river could carry less than 10 percent of 

its former capacity without flooding the surrounding valley. 

B13 
FIGURE 10.-View of mudflow near Highway 99 Bridge on the Toutle River. The flow had a consistency of wet mortar. Photograph 

taken at 8:45 p.m., May 18, 1980. 

B14 
I l I I I 1 

30 f---

l 

j......- High Water Mark -

~ 29.26 

I \ 

~ \ f- \ -

' \ 1- ' \ A;>--c l.U 

l.U 

LL. ' ~ 20 f---

"~ -

1- 1 I 

(.!} 

jjj 

I I l.U t- ' -

(.!} 

<( 

(.!} 

10 __AJ -

May 18, 1980 May 19, 1980 

o~~~--~~--~~--~--~~--~~---~1~ 6 a.m. NOON 6 p.m. MID- 6 a.m. NOON 6 p.m. 

NIGHT 

PACIFIC DAYLIGHT TIME 

FIGURE 11.--Stage from recorder and wire weight gage 

readings, Cowlitz River at Castle Rock, May 18-19, 1980. 

ACKNOWLEDGMENTS 

Information contained in this report was compiled from numerous sources. The author gratefully 

acknowledges the cooperation and contributions of 

the many private individuals cited in the text and 

the following organizations and their representatives. 

City of Longview·-Leland F. Edtl, 

Utilities Engineer 

Cowlitz County-Sheriff Les Nelson 

Cowlitz Communications Center--AI Gervenack, 

Brenda Hostetter 

Pacific Power and Light CompanyStan DeSousa 

Weyerhaeuser Company of Longview--

Jack Schoening, Ray Pleassant 

U.S. Geological Survey, Menlo Park, 

California--Harry Glicken, 

Dick Janda, Mike Nolan, Richard B. Waitt, Jr. 

B15 
TABLE 2.- Approximate time, location, and average velocity of the peak stages of the mudflows originating in the 

South and North Forks Toutle River, May 18, 1980. 

Time of 

peak stage 

8:32a.m. 

8:50a.m. 

!0:10a.m. 

10:50 a.m. 

11:50 a.m. 

1:30 p.m. 

5:00p.m. 

8:32a.m. 

1:30 p.m. 

3:00p.m. 

5:00p.m. 

6:00p.m. 

7:00p.m. 

9:15p.m. 

12 midnight 

4:00a.m. 

(May 19) 

Site 

Location (see fig. I) 

South Fork Mudflow 

Eruption at source of South Fork. 

David Crockett's car .. 

Weyerhaeuser Camp i 2. 

Highway 504 Bridge 

Tower Bridge . . . . 

Castle Rock Bridge (gage) . 

Longview Water Treatment Plant .. 

North Fork Mudflow 

Eruption (dam below Spirit Lake) . 

Elk Rock area ........ 

Weyerhaeuser Camp Baker. 

Weyerhaeuser Camp 19 (1900 Bridge) . 

Toutle Boys Ranch ........ 

Toutle River gage (Ii4 mile below 

Highway 5 04 Bridge) . . . 

Tower Bridge ........ 

Castle Rock Bridge (gage) . 

Longview Water Treatment Plant . 

Average velocity 

River miles up- between locations 

stream from mouth (feet per second) 

of Cowlitz River 

69.4 

} 27.4 

63.8 } 23.5 

42.4 } 12.3 

36.8 

} 15.1 

26.5 } 8.1 

17.3 } 5.1 

5.2 

74 . ........... 

63 } 7.8 

55.0 

} 6.9 

45.6 

} 7.9 

40.2 

} 5.6 

36.4 

} 6.5 

26.5 } 4.9 

17.3 } 4.4 

5.2 

--··-------------------------------------------

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. C-1 

to C-26. 

B16 

Youd, T. L., and Wilson, R. C., 1980, Stability of Toutle River 

Blockage, Mount St. Helens Hazards Investigations: U.S. 

Geological Survey Open-File Report 80-898, 14 p. 

*U.S. GOVERNMENT PRINTING OFFICE: 1981- 341-614/103