Inventory and initiation zone characterization of debris flows on Mount St. Helens, Washington initiated during a major

storm event in November, 2006 Keith V. Olson (kvo@pdx.edu)

Department of Geology, Portland State University, Portland, OR 97201

Abstract

The heavy precipitation event of November 3-8, 2006 dropped

over 60 cm of rain onto the bare southern slopes of Mount St.

Helens and generated debris flows in eight of the sixteen

drainages outside the 1980 debris avalanche zone. Debris

flows occurred on the upper catchments of the Muddy River,

Shoestring Glacier, Pine Creek, June Lake, Butte Camp Dome,

Blue Lake, Sheep Creek, and South Fork Toutle River. Debris

flows were clustered on the west and south-east sides of the

mountain. Of the eight debris flows, three were initiated by

landslides, while five were initiated by headward or channel

erosion. Six debris flows were initiated in deposits mapped as

Holocene volcaniclastic deposits, while two were in 1980

pyroclastics on andesite flows. The largest (~975,000 m2) and

longest (~8,900 m) debris flow was initiated by landslides in the

upper South Fork Toutle River Drainage. The average debris

flow initiation zone elevation was 1,750 m, with clusters around

1,700 m and 2,000 m elevation. The lower cluster is associated

with basins that host modern or historic glaciers, while the

upper is possibly associated with recent pyroclastic deposits.

Upper drainages with debris flows averaged 41% slopes

steeper than 33 degrees, while those without debris flows

averaged 34%. The geology of upper basins with debris flows

averaged 6% snow and ice cover, 21% consolidated bedrock,

and 74% unconsolidated deposits. Geology of basins without

debris flows averaged 3% snow and ice cover, 27% bedrock,

and 67% unconsolidated deposits. Drainages with debris flows

averaged an 89% loss of glacier area between 1998 and 2009,

while those without debris flows lost 68%. Further comparing

glacier coverage during that period found that only five of ten

glaciers still existed in 2009. On average, the glaciers had

reduced in area by 67%, decreased in length by 36%, and

retreated by an average of 471 m during that period. Basin

attributes were measured or calculated in order to construct a

predictive debris flow model based on that of Pirot (2010) using

multiple logistic regression. The most significant factors were

the percentage of slopes steeper than 33 degrees,

unconsolidated deposits in the upper basin, and average

annual rainfall. These factors predicted the 2006 debris flows

with an accuracy of 94% in a debris flow susceptibility map for

Mount St. Helens.

References Burns, S., Pirot, R., Sobieszczyk, S., and Williams, K., 2009,

Massive debris flow event on Pacific Northwest volcanoes,

November 2006: More to come as the climate changes:

Geological Society of America annual meeting Portland,

Abstracts With Programs, vol. 41, No. 7, 716 p.

Copeland, E. A., 2009, Recent periglacial debris flows from

Mount Rainier, Washington: Oregon State University,

Master’s thesis.

Pirot, R., 2010, Initiation zone characterization of debris flows

in November, 2006 on Mount Hood, Oregon: Portland State

University, Master’s thesis.

Williams, K. J., 2011, Analysis and characterization of debris

flows in November, 2006, Mount Adams, Washington:

Portland State University, Master’s thesis.

Acknowledgements • Advisor: Scott Burns, Portland State University Geology

Department

• Thesis Committee: Thomas C. Pierson (Cascade Volcano

Observatory), Christina L. Hulbe (Portland State University

Geology Department)

• Field assistants: Jon Weatherford, Harold Shields, Kent

Stallings, Matt Gibson, Lina Ma, Rachel Secrest, Craig

Olson, Melissa Carley

Lahar plain, 1986

8/2005

8/2011

Courtesy of David Anderson

Muddy River/Shoestring Glacier

Debris flow inventory map showing defined drainage outlines and 2006 event flow paths.

The largest and longest debris flow occurred in the South Fork Toutle Drainage in the

western quadrant. The depositional area comprised about 975,000 m2 and a total length

of 8,900 m. The next largest debris flow deposit, at 420,000 m2, was from the June Lake

drainage. The smallest of 300,000 m2 occurred in the Butte Camp Dome drainage.

South Toutle

Sheep Creek

June Lake

Butte Camp Dome

Blue Lake

Debris flows

• Channelized flow path

• Initiated by a glacial

outburst, landslide,

erosion, or dam failure

• Transports debris at ~25 mph

• Bulks up during transport with

addition of soil, rock,

organic debris

• Creates a lobate

debris fan

Location

• Southwestern WA

state

• 50 miles (80 km)

northeast of Portland

Background

Geology

• Composite volcano

• ~300,000 years old

• Largely constructed during the past 3,000 years

• Eruptive phases: fallout tephra and pyroclastic events

then a long hiatus

• Three primary river systems drain the volcano: Lewis

River, North and South Toutle Rivers, Kalama River

Initiation event

• Extreme rain event (pineapple express) on Nov. 6-9,

2006

• Over 3x normal daily rainfall within a 24 hour period

• Initiated debris flows on all central Cascade volcanoes:

Rainier (northern-most) to Jefferson (southern-most)

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

3 4 5 6 7 8

Pre

cip

ita

tio

n, cm

Days, November 2006

June Lake

Sheep Canyon

Lone Pine

Spencer Meadow

Spirit Lake

Swift Creek

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

June

Lake

Sheep

Canyon

Lone

Pine

Spencer

Meadow

Spirit

Lake

Swift

Creek

To

tal

Sto

rm P

reci

pit

ati

on

, cm

INITIATION

DEPOSITION

TRANSPORTATION

Map showing the gridded PRISM average annual precipitation data for 2006 in

hundredths of millimeters and as contours. Locations of nearby SNOTEL sites are also

shown. The highest average rainfall values for this area are 430 cm. Crater is denoted

by black circle.

Graph of rainfall measurements during the period of November 3-8 at the SNOTEL

sites June Lake (1,049 m), Sheep Canyon (1,216 m), Lone Pine (1,198 m), Spenser

Meadow (1,036 m), Spirit Lake (1,072 m), and Swift Creek (1,353 m). June Lake and

Swift Creek (see map), experienced the greatest rainfall accumulation (USDA-NRCS,

2007).

Initiation

Type Landslide

Elevation (m) 1,695

Length (m) 8,900

Deposit Area (m2) 975,000

Geology Qvc

Glacier No

Direct Connection NA

Initiation

Type Channel Erosion

Elevation (m) 1,706

Length (m) 4,800

Deposit Area (m2) NA

Geology Qva/Qvc

Glacier No

Direct Connection NA

Initiation

Type Channel Erosion

Elevation (m) 1,986

Length (m) 6,700

Deposit Area (m2) 300,000

Geology Qvc

Glacier No

Direct Connection NA

Initiation

Type Landslide

Elevation (m) 1,706

Length (m) 5,300

Deposit Area (m2) -

Geology Qva/Qvc

Glacier Yes

Direct Connection Yes

Pine Creek Initiation

Type Channel Erosion

Elevation (m) 1,403

Length (m) 3,760

Deposit Area (m2) NA

Geology Qvc

Glacier No

Direct Connection NA

Shoestring Glacier Initiation

Type Channel Erosion

Elevation (m) 1,706

Length (m) 6,400

Deposit Area (m2) 460,000

Geology Qvc

Glacier Yes

Direct Connection No

Muddy River Initiation

Type Channel Erosion

Elevation (m) 1,706

Length (m) 6,600

Deposit Area (m2) 340,000

Geology Qvc

Glacier No

Direct Connection NA

Basin Comparisons

0

1,000,000

2,000,000

3,000,000

4,000,000

5,000,000

6,000,000

7,000,000

8,000,000

9,000,000

10,000,000

Ba

sin

Are

a (

m2)

Upper Basin

Total Basin

0

500

1,000

1,500

2,000

2,500

Ba

sin

Ele

va

tio

ns

(m)

Total Basin Low Upper Basin Low Total Basin High

0

500

1,000

1,500

2,000

2,500

Ba

sin

Hei

gh

t (m

)

Upper Basin

Total Basin

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

Ba

sin

Len

gth

(m

)

Upper Basin

Total Basin

0.00

0.10

0.20

0.30

0.40

0.50

0.60

Ba

sin

Gra

die

nt

Upper Basin

Total Basin

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

Up

per

Ba

sin

Mel

ton

's R

ug

ged

nes

s

Nu

mb

er

Initiation

Type Landslide

Elevation (m) 2,124

Length (m) 5,900

Deposit Area (m2) 420,000

Geology Qvc

Glacier No

Direct Connection NA

View of the Blue Lake Debris Flow deposit covering about 350

meters of Road 8123 and all of the Blue Lake trailhead parking

area by deposits of up to three meters thick (USDA-FS, 2007).

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

Up

per

Ba

sin

Wa

DN

R G

eolo

gy

(%

) Ice Bedrock Unconsolidated

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

Up

per

Ba

sin

Mea

sure

d G

eolo

gy

(%

)

Snowpack & Ice Consolidated Bedrock Unconsolidated

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

Veg

eta

tio

n (

%)

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

Per

cen

t S

teep

Slo

pes

(%

)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

Up

per

Ba

sin

Av

era

ge

An

nu

al

Pre

cip

. (m

)

The 8 of 16 drainages that experienced a debris

flow in November 2006 are described in detail in

the maps and tables below and to the right.

Further comparisons are made below and in the

basin comparison charts in the central panel.

Debris Flows

0

200,000

400,000

600,000

800,000

1,000,000 Deposit (m2)

0

2,000

4,000

6,000

8,000

10,000 Length (m)

Glacial Change

Mount St. Helens edifice with post-May 18, 1980 eruption glacier distribution and extents (left map). Redrawn from Brugman and

Post (1981). On The right are glacier extents and distributions as they existed as of 2009. The glaciers remaining are Nelson,

Ape, Shoestring, June (#1), and Swift. The diagrams show the decrease in glacier area and length between 1982 and 2009.

0 100,000 200,000 300,000

Toutle Glacier

Talus Glacier

Dryer Glacier

Snowfield

Swift Glacier

June Lake

Shoestring Glacier

Ape Glacier

Nelson Glacier

Glacier Area (m2)

2009

1983

0 20 40 60 80 100

Toutle Glacier

Talus Glacier

Dryer Glacier

Snowfield

Swift Glacier

June Lake

Shoestring …

Ape Glacier

Nelson Glacier

Glacier Percent Change 1982-2009 (-%)

Length

Area