Prepared in cooperation with the U.S. Department of Agriculture, U.S. Forest Service, Gifford 

Pinchot National Forest

A Multidecade Analysis of Fluvial Geomorphic Evolution of 

the Spirit Lake Blockage, Mount St. Helens, Washington

Scientific Investigations Report 2020–5027

U.S. Department of the Interior

U.S. Geological Survey
Cover. Oblique aerial view to south of Mount St. Helens and Spirit Lake blockage. The blockage occupies the 

center and left sides of the image below the volcano. Channels on the left side of image drain northeast toward 

Spirit Lake, those on far right of image drain northwest toward North Fork Toutle River. Photograph by Jon Major 

taken July 16, 2018.
A Multidecade Analysis of Fluvial 

Geomorphic Evolution of the Spirit Lake 

Blockage, Mount St. Helens, Washington

By Jon J. Major, Gordon E. Grant, Kristin Sweeney, and Adam R. Mosbrucker

Prepared in cooperation with the U.S. Department of Agriculture, U.S. Forest 

Service, Gifford Pinchot National Forest

Scientific Investigations Report 2020–5027

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

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Suggested citation:

Major, J.J., Grant, G.E., Sweeney, K., and Mosbrucker, A.R., 2020, A multidecade analysis of fluvial geomorphic 

evolution of the Spirit Lake blockage, Mount St. Helens, Washington: U.S. Geological Survey Scientific Investigations 

Report 2020-5027, 54 p., https://doi.org/10.3133/sir20205027.

ISSN 2328-0328 (online)
iii

Contents

Executive Summary.......................................................................................................................................1

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

Hydrogeomorphic Disturbance by Eruptions and Landscape

Responses—General Context...............................................................................................3

Landscape Disturbance by the 1980 Mount St. Helens Eruption—Context for Spirit

Lake Blockage..........................................................................................................................5

Mitigation of the Blockage Hazard ....................................................................................................7

Scope and Objectives of this Report .................................................................................................8

Geomorphic and Stratigraphic Context of Upper North Fork Toutle River Basin................................9

Hydrologic Setting of Upper North Fork Toutle River Basin .................................................................10

Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin...............................................14

Methodology........................................................................................................................................14

Broad Evolution of Upper North Fork Toutle Basin and Relation to Hydrologic Events..........16

Fine-Time-Scale Landscape Evolution from 1980 to 2018 and Linkages to Hydrology and 

Hydrologic Events .................................................................................................................28

Pre-1980 to 1980 .........................................................................................................................28

1980–81 ........................................................................................................................................29

1981–82 ........................................................................................................................................29

1982–84 ........................................................................................................................................30

1984–85 ........................................................................................................................................32

1985–87 ........................................................................................................................................32

1987–96 ........................................................................................................................................34

1996–99 ........................................................................................................................................36

1999–2003 ....................................................................................................................................37

2003–07 ........................................................................................................................................38

2007–09 ........................................................................................................................................39

2009–15 ........................................................................................................................................40

2015–17 ........................................................................................................................................40

2017–18 ........................................................................................................................................42

Geomorphic Processes and Relations with Topography and Surface Geology ......................43

Implications for Future Geomorphic Development in Response to Management Options.............46

Closed-Conduit Outlet ........................................................................................................................46

Open-Channel Outlet..........................................................................................................................47

Tradeoffs Among Outlet Alternatives ..............................................................................................48

Summary and Conclusions.........................................................................................................................49

Acknowledgments.......................................................................................................................................50

References Cited..........................................................................................................................................50
iv

Figures

1. Diagram illustrating explosive eruption processes and subsequent landscape 

responses.......................................................................................................................................4

2. Distribution of volcanic disturbance zones of 1980 Mount St. Helens eruption 

 and locations of gaging stations ................................................................................................5

3. Topographic map of the Mount St. Helens area before the 1980 eruption .........................6

4. Simplified surface geologic map of 1980 eruption deposits in upper North

 Fork Toutle River basin draped over digital terrain.................................................................7

5. Oblique aerial view of Spirit Lake blockage looking north from vantage over 

 northeast side of volcano............................................................................................................9

6. Topographic profiles across the Spirit Lake blockage.........................................................10

7. Time series of daily mean streamflow and annual peak streamflow of North 

 Fork Toutle River below the sediment retention structure near Kid Valley.......................11

8. Views of sediment retention structure on North Fork Toutle River

 looking upstream ........................................................................................................................13

9. Exceedance probability plots for streamflow at North Fork Toutle River below

 the sediment retention structure and at Toutle River at Tower Road ................................14

10. Plots of repeat cross-section surveys along Loowit Creek and mainstem 

 North Fork Toutle River channels.............................................................................................17

11. Plots of repeat cross-section surveys along Truman channel ...........................................23

12. Digital terrain models of topographic difference based on digital terrain models

 derived from aerial photography or airborne lidar...............................................................25

13. Schematic diagram and oblique aerial view of Step-Loowit fan .......................................27

14. Digital terrain model of topographic difference of uppermost North Fork Toutle 

 River basin from pre-1980 topography to September 5, 1980..............................................28

15. Digital terrain model of topographic difference of uppermost North Fork Toutle

 River basin from September 5, 1980, to July 27, 1981, ..........................................................29

16. Digital terrain model of topographic difference of uppermost North Fork Toutle 

 River basin from July 27, 1981, to September 22, 1982, ........................................................30

17. Photograph of lahar generated by melting of crater snow during a small explosion

 on March 19, 1982.......................................................................................................................31

18. Digital terrain model of topographic difference of uppermost North Fork Toutle 

 River basin from September 22, 1982, to July 7, 1984 ...........................................................32

19. Digital terrain model of topographic difference of uppermost North Fork Toutle 

 River basin from July 7, 1984, to July 25, 1985........................................................................33

20. Digital terrain model of topographic difference of uppermost North Fork Toutle

 River basin from July 25, 1985, to mid-June 1987 ..................................................................33

21. Digital terrain model of topographic difference of uppermost North Fork Toutle

 River basin from mid-June 1987 to August 29, 1996..............................................................34

22. Digital terrain model of topographic difference of uppermost North Fork Toutle 

 River basin from August 29, 1996, to September 3, 1999 ......................................................36
v

23. Digital terrain model of topographic difference of uppermost North Fork Toutle 

 River basin from September 3, 1999, to late September 2003 .............................................37

 24. Digital terrain model of topographic difference of uppermost North Fork Toutle 

 River basin from late September 2003 to late October 2007................................................38

25. Digital terrain model of topographic difference of uppermost North Fork Toutle 

 River basin from late October 2007 to late September 2009................................................39

26. Digital terrain model of topographic difference of uppermost North Fork Toutle 

 River basin from late September 2009 to September 27, 2015 ............................................40

27. Digital terrain model of topographic difference of uppermost North Fork Toutle 

 River basin from September 27, 2015, to late September 2017. ..........................................41

28. Digital terrain model of topographic difference of uppermost North Fork Toutle

 River basin from late September 2017 to September 26, 2018 ............................................42

29. Simplified surface geology of the Spirit Lake blockage.......................................................43

30. Example of topographic influence on location of channel development by the 

 rugged surface texture of the debris-avalanche deposit....................................................44

31. Examples of initial topographic influence on location of channel development by

 a hummock and depression, followed by lateral erosion in the 

 debris-avalanche deposit..........................................................................................................44

32. Example of how channel development may be influenced by location of 

 geologic contacts between the blast pyroclastic density current deposit and 

 pyroclastic-flow deposits..........................................................................................................45
vi

Tables

1. Key hydrogeomorphic events in post-eruption channel development of upper 

 North Fork Toutle River ..............................................................................................................12

2. Available digital terrain models (DTMs), source of topography, and method of 

 creation ........................................................................................................................................15

3. Annual mean streamflow and total runoff of North Fork Toutle River below

 sediment retention structure near Kid Valley. .......................................................................35

4. Summary of scientific, engineering, and societal tradeoffs among 

 outlet alternatives.......................................................................................................................48

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Area

square meter (m2

) 0.0002471 acre 

square kilometer (km2

) 247.1 acre

square meter (m2

) 10.76 square foot (ft2) 

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

cubic meter (m3

) 264.2 gallon (gal) 

cubic meter (m3

) 0.0002642 million gallons (Mgal) 

cubic meter (m3

) 35.31 cubic foot (ft3)

cubic meter (m3

) 1.308 cubic yard (yd3) 

cubic kilometer (km3

) 0.2399 cubic mile (mi3) 

cubic meter (m3

) 0.0008107 acre-foot (acre-ft) 

Flow rate

cubic meter per second (m3

/s) 70.07 acre-foot per day (acre-ft/d) 

cubic meter per year (m3

/yr) 0.000811 acre-foot per year (acre-ft/yr) 

meter per second (m/s) 3.281 foot per second (ft/s) 

meter per minute (m/min) 3.281 foot per minute (ft/min) 

meter per hour (m/h) 3.281 foot per hour (ft/h)
vii

Multiply By To obtain

meter per day (m/d) 3.281 foot per day (ft/d)

meter per year (m/yr) 3.281 foot per year ft/yr) 

cubic meter per second (m3

/s) 35.31 cubic foot per second (ft3/s)

cubic meter per day (m3

/d) 35.31 cubic foot per day (ft3/d) 

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

metric ton (t) 1.102 ton, short [2,000 lb]

metric ton (t) 0.9842 ton, long [2,240 lb]

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

A Multidecade Analysis of Fluvial Geomorphic 

Evolution of the Spirit Lake Blockage, Mount St. 

Helens, Washington

By Jon J. Major,1 Gordon E. Grant,2 Kristin Sweeney,3 Adam R. Mosbrucker1

Executive Summary

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

began with an immense landslide (debris avalanche) followed 

by a catastrophic, laterally directed pyroclastic density current 

(commonly called the “lateral blast” but herein called the 

“blast PDC”), and by subsequent pyroclastic flows. This suite 

of volcanic events reconfigured the Spirit Lake basin, blocked 

its outlet, and reset the fluvial landscape of upper North Fork 

Toutle River (NFTR) valley. Consequently, Spirit Lake basin 

effectively became a tub with no drain. To mitigate a potentially catastrophic breaching of the lake blockage, the U.S. 

Army Corps of Engineers bored a 2.5-km-long tunnel through 

bedrock to provide an outlet. The tunnel has performed its 

intended function and kept the lake at a safe level since 

becoming operational in 1985. However, episodic maintenance 

and repairs have required extended tunnel closures that have 

allowed the lake to rise occasionally to potentially problematic 

levels. Within this context, alternative lake outlets are being 

considered and evaluated. Here, we discuss characteristics of 

the blockage and geomorphic evolution of the drainage network on and near the blockage to provide additional landscape 

context for potential outlet options.

The Spirit Lake blockage is a composite of volcanic 

deposits that have variable thicknesses and degrees of 

resistance to erosion. The 1980 debris avalanche filled 

upper NFTR valley with more than 150 meters (m) of a 

heterogeneous sand-and-gravel mixture of shattered volcanic 

rock. Few particles in this deposit are larger in diameter than 

a few meters. Thus, the deposit is not strongly resistant to 

erosion if sufficient stream capacity to transport sediment 

is applied. The part of the debris-avalanche deposit that 

impounds Spirit Lake is overlain by a veneer of pyroclastic 

deposits that range in composition from rocky bits of sand 

and gravel to pumiceous gravel and silt. The sequence 

of pyroclastic deposits, emplaced during the May 18 and 

subsequent eruptions in 1980 and which ranges from 

individual deposits less than 1 m thick to a stratigraphic 

sequence nearly 40 m thick, has physical compositions that 

vary both laterally and vertically owing to overlapping of 

units. During emplacement of the pyroclastic flows, large 

billowing clouds of ash lofted upward and subsequently 

settled. These “ashcloud” deposits are very fine grained 

(mostly very fine sand and silt) and are interspersed 

stratigraphically within the pyroclastic-deposit sequence.

The profound landscape change caused by the 

cataclysmic eruption hydrologically disconnected upper 

NFTR basin on several scales. It severed hydrologic 

connection between the upper and lower parts of the basin, 

between hillsides and the valley floor, and between the 

volcano and the valley, and it isolated Spirit Lake. Landscape 

changes also temporarily altered streamflow magnitude and 

frequency within upper NFTR Basin. Basin reconnection 

required establishment of a new drainage network. Drainage 

development in upper NFTR basin began within hours after 

emplacement of the debris-avalanche deposit when several 

ponds on its surface, formed by groundwater seepage into 

depressions, enlarged and breached. Though channel initiation 

began within hours of deposit emplacement, it took nearly 

3years to fully integrate a new drainage network. By the mid1980s, the drainage system was defined and well-established

since then, channels have enlarged, and their beds have 

coarsened. However, this drainage network lacks an overland 

outlet for Spirit Lake, as outflow passes through the tunnel, 

bypasses the blockage, and returns to NFTR downstream from 

Coldwater Lake.

Initial channel development on the blockage and farther 

downstream occurred during a period of enhanced runoff 

when the hydrologic regime that translates precipitation input 

to streamflow output was radically changed compared to 

pre-eruption conditions. For several years after the eruption, 

post-eruption streamflow peaks for a given precipitation input 

were larger than those before the eruption by a few percent to 

a few tens percent. Initially, channels upstream from Elk Rock, 

20 kilometers (km) downstream from the volcano, incised 

swiftly by as much as tens of meters and widened by hundreds 

of meters. By the mid-1980s, rates and magnitudes of channel 

erosion had diminished, but since then channel widening has 

persisted more than channel incision. Furthermore, by the 

mid- to late 1980s geomorphic evolution across upper NFTR 

basin became more event-driven, requiring moderate- to 

1

U.S. Geological Survey

2

U.S. Forest Service, Pacific Northwest Research Station

3

University of Portland
2 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

large-magnitude daily mean streamflow (greater than 

150 cubic meters per second, m3/s, as measured below the 

U.S. Army Corps of Engineers’ sediment retention structure 

40 km downstream from the volcano) to do much geomorphic 

work beyond nibbling channel margins.

Overall channel positions appear to have been influenced 

primarily by basin topography and secondarily by geologic 

constraints. The overall position of the post-eruption drainage 

system bears strong resemblance to the pre-eruption drainage 

network. Despite thick valley fill, an overall topographic low 

abuts the base of the south face of Johnston Ridge—and both 

the pre-eruption and post-eruption courses of upper NFTR 

occupy that part of the landscape. Truman channel, an informal name for the watercourse that abuts Johnston Ridge closer 

to Spirit Lake and lies along the pre-eruption course of NFTR, 

was formed chiefly by water pumped from Spirit Lake from 

1982–85. Its location was dictated largely by the location of 

the pumped outflow. Location of Loowit Creek channel, which 

drains the Mount St. Helens crater and cuts across the Pumice 

Plain, has been influenced largely by topography, but also by 

both pre- and post-eruption geology. Differential resistance 

to erosion among debris-avalanche and assorted pyroclasticflow deposits, as well as the location of a pre-1980 lava flow, 

appears to have influenced channel position. Overall evolution 

of Loowit Creek channel has also been influenced by development of a fan of flood and debris-flow deposits formed at the 

base of the volcano’s lower north flank. Near the apex of that 

fan, which lies along the blockage drainage divide, Loowit 

Creek channel and Step creek channel, another (informally 

named) drainage from the crater, are shallow (about 1 m), and 

Loowit Creek has switched its course from one side of the 

drainage divide to the other on several occasions. It presently 

(2020) flows westward toward NFTR, but with only modest 

sediment deposition on that fan it might easily switch and flow 

again toward Spirit Lake.

In most mountainous landscapes, channel adjustments 

typically occur in response to sediment transport during 

rainfall- or snowmelt-driven hydrologic events, such as 

floods, debris flows, and landslides. At Mount St. Helens, 

major hydrologic events also can be, and have been, driven by 

volcanic eruptions. With extensive glacier ice now covering 

much of the volcano’s crater floor, there is potentially a readily 

available source of water during future eruptions. If liberated, 

a large volume of meltwater could cause extensive erosion of 

the Spirit Lake blockage and potentially initiate geomorphic 

instabilities along both perennial and ephemeral channels, 

which could have significant impact on channel stabilities.

Although the location of the drainage network in upper 

NFTR basin is largely settled, and the network now evolves 

mainly through persistent, low-magnitude erosion of channel banks, significant event-driven channel incision, particularly along Loowit Creek channel, still occurs. Single-storm 

incision of up to many meters occurred as recently as 2006. 

Furthermore, the swift and substantial incision of Truman 

channel (20 m of incision over a span of weeks to months), 

when pumping from Spirit Lake introduced a 5.1 m3

/s flow, 

vividly illustrates the exceptional mobility of sediment in 

this landscape and that even modest streamflow has abundant 

transport capacity under the right conditions. Our documentation of substantial erosion, channel avulsion, and patterns of 

channel evolution following emplacement of the Spirit Lake 

blockage illustrates the geomorphically dynamic character of 

upper NFTR basin. The considerable mobility of sediment in 

this landscape can establish geomorphic instabilities such as 

knickpoints or knickzones (abrupt steps in channel bottom 

profile) that can migrate rapidly headward and potentially 

trigger additional instabilities in smaller channels tributary 

to main-stem channels. These instabilities are most likely to 

occur when moderate to large amounts of concentrated surface 

runoff are introduced to parts of the landscape not adjusted to 

such flows.

The erosional susceptibility documented here has significant implications—and cautionary ramifications—for an 

open-channel outlet for Spirit Lake. The most logical location 

for an open-channel outlet is along or near the present alignment of Truman channel. But that channel has equilibrated to 

a mean streamflow of 5.1 m3

/s. Streamflow of substantially 

greater magnitude, almost certain because discharge would be 

unregulated and vary with lake level, is very likely to induce 

additional channel incision and widening unless the channel is heavily armored, especially given the overall 3-percent 

gradient (125-m elevation drop over 4 km) from Spirit Lake 

to the Truman channel-NFTR confluence. Furthermore, if 

water somehow escaped such an open channel and breached 

across the blockage drainage divide, it would largely flow over 

landscape lacking channels adjusted to such flow. Because this 

landscape is very susceptible to erosion, such a situation could 

generate geomorphic instabilities with possibly catastrophic 

consequences. A rigorous analysis of this risk is difficult 

owing to many uncertainties, chief among them the actual 

location and design of an open channel. Nevertheless, such 

risk could be reduced by careful design and engineering that 

accounts for the hydrogeomorphic behavior of this landscape.

Introduction

Volcanic eruptions can affect landscapes in many ways 

(Pierson and Major, 2014) and consequently alter erosion and 

the fluxes of water and sediment. Hydrologic and geomorphic 

responses to volcanic disturbances are varied in both space 

and time, and, in some instances, can persist for decades to 

centuries. Understanding the broad context of how landscapes 

respond to eruptions can help inform how they may evolve, 

and therefore provides context for managing and mitigating 

hazards associated with future volcanic and hydrologic events. 

To provide a lens through which to view the landscape of 

upper North Fork Toutle River (NFTR) basin as it exists today 

(2020)—the basin most heavily affected by the cataclysmic 

May 18 and later 1980s eruptions of Mount St. Helens—we 

first briefly discuss landscape effects and responses to volcanism broadly, then focus more sharply on specific geomorphic 

changes caused by the 1980s Mount St. Helens eruptions. 
Introduction 3

In this way, we provide context for the landscape changes 

caused by the eruptions as they relate to potential hydrological hazards associated with Spirit Lake, an iconic landform 

at the northern foot of the volcano, which was transformed 

by the cataclysmic 1980 eruption and had its outlet blocked. 

Here, we provide detailed analyses of geomorphic evolution of 

upper NFTR valley immediately north of the volcano. These 

analyses are presented to provide context for considerations 

of potential outlets for Spirit Lake, a landform which might be 

viewed as a “sleeping giant” on this landscape: a giant capable 

of causing catastrophic downstream consequences if water is 

released uncontrollably from the lake.

Hydrogeomorphic Disturbance by Eruptions and 

Landscape Responses—General Context

Volcanic eruptions can fundamentally alter the hydrologic 

and geomorphic (hydrogeomorphic) conditions of a landscape 

(Pierson and Major, 2014). Changes in hydrology and sediment delivery from watersheds following volcanic eruptions 

owe to many factors. For example, eruptions can remove or 

damage vegetation allowing more water to fall directly on the 

ground surface, and complex sequences of volcanic activity—

pyroclastic flows, debris avalanches, tephra falls, and lahars 

(volcanic debris flows)—can bury areas surrounding volcanoes with loose, unconsolidated sediment. Commonly after 

explosive eruptions, hillsides blanketed by tephra-fall deposits 

have fine-grained (silt to fine sand) surfaces that inhibit infiltration, channels can be deeply filled with sediment, and drainages can be blocked by thick sediment accumulations (fig. 1). 

Such fundamental alteration of the hydrogeomorphic conditions of the landscape commonly enhances runoff of rainfall 

and snowmelt for several years (for example, Major and Mark, 

2006; Pierson and Major, 2014) and spawns exceptional sediment delivery. As a result, landscapes surrounding volcanoes 

are subject to some of the highest erosion rates and sediment 

yields on Earth following eruptions. Sediment yields after 

eruptions can exceed those from nonvolcanic river systems 

by several orders of magnitude (Gran and Montgomery, 2005; 

Gran and others, 2011; Pierson and Major, 2014).

Typically, erosion rates and consequent sediment delivery 

peak in the months to years following an eruption and then 

decline (Collins and Dunne, 1986; Major and others 2000, 

2016; Hayes and others 2002; Gran and others, 2011). Initially, 

extraordinary sediment delivery results from runoff erosion 

of hillside tephra and erosion of new channel fill. Commonly, 

erosion of hillside tephra diminishes within a few years after 

rills and gullies stabilize (for example, Collins and Dunne, 

1986, 2019), but channel erosion persists (Gran and others, 

2011; Pierson and Major, 2014; Major and others, 2019). After 

the most easily erodible channel sediment is depleted, sediment delivery diminishes greatly from peak levels, but it can 

remain higher than pre-eruption levels because of persistent, 

but lower magnitude, channel modification. In some instances, 

extraordinary sediment delivery can be rejuvenated when 

landslides that lag eruptions increase delivery of hillside sediment (Swanson and Major, 2005; Korup and others, 2019), if 

lakes impounded by volcanic sediment breach and reinvigorate channel erosion (for example, White and others, 1997), or 

when substantial hydrologic events (floods and debris flows) 

perturb quasi-stable channel conditions and effectively reset 

the recovery clock (for example, Tunnicliffe and others, 2018). 

As a result, the hydrogeomorphic legacy of a large eruption 

(one that deposits perhaps more than 1 km3

 of sediment on 

a landscape) can last for decades, and possibly centuries, 

creating exceptionally long-term channel- and sediment-management challenges (Gran and others, 2011; Major and others, 

2000, 2018, 2019; Sclafani and others, 2018).

Major factors driving sediment-delivery rates and 

temporal patterns are the establishment and equilibration of 

the drainage network. Establishment of the drainage network 

in a landscape substantially reset by an eruption occurs in 

both a downstream direction, as overland flow incises tephramantled hillsides and creates rills and gullies that coalesce into 

larger channels, and an upstream direction as channels initiate 

and develop knickpoints or knickzones that migrate upstream. 

Once channels are established, they can further degrade, 

aggrade, or widen as a function of the imposed flow regime 

and pattern of upstream sediment delivery. Hence, evolving 

channels can undergo complex patterns and durations of 

response depending on the relations between sediment supply 

and sediment-transport capacity (for example, Schumm, 

1999; Simon and Rinaldi, 2006; Nicholas, 2013; Major and 

others, 2019; Renshaw and others, 2019). When this relation 

is in balance, channels are in a state of equilibrium and 

change slowly; when this relation is out of balance, channels 

adjust, and geometric adjustments can be swift, dramatic, and 

persistent. Furthermore, channel evolution rarely follows the 

simple, sequential trajectory commonly portrayed in channel 

and stream evolution models (for example, Schumm and 

others, 1984; Simon and Hupp, 1986, 1987; Cluer and Thorne, 

2014). Rather, channel evolution can follow nonsequential 

trajectories that involve varying combinations of degradation, 

aggradation, widening, and narrowing (Cluer and Thorne, 

2014; Major and others, 2019; Renshaw and others, 2019).

Although channel adjustments typically occur in response 

to rainfall and snowmelt events, channel evolution can be 

accelerated dramatically if water stored on the landscape in 

lakes or ponds is released suddenly (Janda and others, 1984; 

White and others, 1997; Manville and others, 2007). Under 

these circumstances, channel adjustments that might ordinarily 

have taken months to decades to occur may occur over the 

course of hours to days. Such lake-breakout events can lead 

to floods and debris flows (lahars) that can be extremely 

destructive (for example, Janda and others, 1984; Manville 

and others, 2007; Gudmundsson, 2015). A breakout of an 

ancestral Spirit Lake, Washington, following an eruption of 

Mount St. Helens about 2,600 years ago, spawned the largest 

lahar documented in the Toutle River basin geologic record 

(Scott, 1988). That lahar, about 10 times greater in magnitude 

than the largest one triggered by the 1980 eruption (Janda and 
4 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

men19-7369_fig01

Wind

direction

Stream

channel

Lahar

Eruption cloud

A

B

Pyroclastic

density

current

Debris

avalanche

Eruption

column

Blocked

tributary Lake

Former

river channel

Rapid erosion of

fresh, unconsolidated

volcanic deposits

Aggrading

river

Channel

development

Perimeter of ash on ground

Landslide

Rills

Tephra fall

Figure 1. Diagram illustrating explosive eruption processes and subsequent landscape responses. A, Volcanic 

processes from or related to an explosive eruption, including tephra fall, pyroclastic density current, debris 

avalanche, and lahar. B, Landscape changes and responses to disturbances caused by eruption processes 

shown in panel A include rapid erosion of fresh tephra by rills and landslides, erosion of channel fill and 

development of new channel networks, channel aggradation and formation of braided channel patterns, and 

impoundments of channel-margin lakes. From Pierson and Major (2014).
Introduction 5

men19-7369_fig02

Kelso

Yale

Lake

Swift

Reservoir

Coweeman River

Cowlitz

River

Castle

Rock

Kid

Valley

Cougar

Silver

Lake

Le wis

Riffe La ke

14240800

(GRE)

SRS

14242580

(TOW)

14241100

(KID)

14241500

(SFT)

MOUNT

ST. HELENS

Clearwater C reek

Swift Creek

Pine

Creek

South

Toutle River

Kalama River

Toutle River

Toutle

Clear Creek

River

Fork

Spirit

Lake

Muddy

River

G

River

reen

Map area

WASHINGTON

0 5 10 KILOMETERS

Lake Merwin 0 5 10 MILES

Longview

Columbia River

45' 30' 15'

46°

30'

46°

15'

46°

Elk Rock

14240525

(FTP)

North

Fork

Toutle

River

123°00' 122°00'

Smith Creek

WASHINGTON

EXPLANATION

Area swept by ‘blast’ PDC

USGS streamgaging station

Debris-avalanche deposit

Lahar deposits

Pyroclastic-flow deposits

OREGON

Area of figure 4

Figure 2. Distribution of volcanic disturbance zones of 1980 Mount St. Helens eruption and locations of gaging 

stations. PDC, pyroclastic density current; SRS, sediment retention structure. Gaging stations: GRE, Green River above 

Beaver Creek near Kid Valley (USGS streamgage 14240800); FTP, North Fork Toutle River below SRS near Kid Valley 

(USGS streamgage 14240525); KID, North Fork Toutle River at Kid Valley (USGS streamgage 14241100); SFT, South 

Fork Toutle River at Toutle (USGS streamgage 14241500); TOW, Toutle River at Tower Road near Silver Lake (USGS 

streamgage 14242580).

Helens, as we see it, is a youthful volcano that sits on rugged, older volcanic terrain that has been modified by glacial, 

volcanic, and fluvial processes. Much of the area immediately 

around the volcano has been filled and smoothed by volcanic 

debris shed over millennia (fig. 3; Clynne and others, 2005, 

2008). The distribution of sediment shed by the volcano shows 

clearly that Spirit Lake is a landform modulated by eruptive 

disturbance and volcanic deposits (Mullineaux and Crandell, 

1962; Crandell, 1987; Hausback and Swanson, 1990). Before 

the May 18, 1980, eruption, upper NFTR channel, the outlet 

for Spirit Lake, formed along the edge of volcanic fill that 

abuts Johnston Ridge (fig. 3). The 1980 eruption added to this 

volcanic fill, and once again affected Spirit Lake (see Lipman 

and Mullineaux, 1981; W. Meyer and others, 1986; Grant and 

others, 2017).

The May 18, 1980, eruption consisted of a suite of volcanic processes (see papers in Lipman and Mullineaux, 1981). 

It began with catastrophic failure of the volcano’s north flank, 

producing a massive landslide known as a debris avalanche. 

That failure simultaneously unroofed a magmatic intrusion 

high within the edifice, generating a devastating, laterally 

directed pyroclastic density current (herein called the “blast 

others, 1981; Fairchild, 1987), inundated the present sites of 

Castle Rock, Kelso, and Longview, Washington (fig. 2).

Catchments affected by severe volcanic disturbance 

may never recover to pre-eruption geomorphic conditions 

(that is, with regard to hydrologic and sediment fluxes)—or 

at least not for many human generations. Rather, they can 

attain a degree of quasi-stability and ecological function under 

different equilibrium conditions, and they can remain on the 

verge of precarious threshold (tipping point) conditions having 

prolonged societal consequences. One important lesson that 

has been learned from studies at many volcanoes over the 

past several decades is that volcanically disturbed channels 

are highly sensitive to erosion and can remain so for decades 

depending on the magnitude of initial disturbance.

Landscape Disturbance by the 1980 Mount 

St. Helens Eruption—Context for Spirit Lake 

Blockage

The landscape around Mount St. Helens has been 

disturbed and modified repeatedly for millennia. Mount St. 
6 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

men19-7369_fig03

Pine-Muddy fan

Ape Canyon

Smith

Creek

Muddy

River

Clearwater

Creek

Bean

Creek

Clear

Creek

Muddy

River

Pine

Creek

Lewis

River

Swift

Creek

June

Lake

North

Fork Toutle River

South Fork Toutle River

Kalama River

Creek

Clearwater

Johnston Ridge

Coldwater Creek

South Coldwater Creek

Spirit

 Lake

MOUNT ST. HELENS

122 ̊15’ 122 ̊10’ 122 ̊05’

46 ̊15’

46 ̊10’

46 ̊05’

122 ̊00’

0 1 2 3 4

1 2 3 4

5 KILOMETERS

0 5 MILES

Figure 3. Topographic map of the Mount St. Helens area before the 1980 eruption. Dashed lines denote the approximate limit 

of the volcanic sediment apron that surrounds the volcano. Fragmental deposits derived from Mount St. Helens extend down 

all drainages (adapted from Clynne and others, 2008). Note the location of Spirit Lake, the pre-1980 location of North Fork Toutle 

River, and the pre-1980 locations of channels draining the north flank of the volcano.
Introduction 7

PDC”). These events were followed by lahars (volcanic debris 

flows), a billowing eruption column that delivered widespread 

tephra fall (mostly east of the volcano), and pyroclastic flows 

when parts of that eruption column collapsed. Subsequent 

eruptions through summer 1980 produced additional pyroclastic flows that added to the volcanic fill in upper NFTR valley.

These volcanic processes transformed Spirit Lake 

basin and blocked the lake’s outlet (W. Meyer and others, 

1986; Glicken and others, 1989). The Spirit Lake blockage 

is a composite of volcanic deposits (fig. 4). It is composed 

principally of an extraordinarily thick (locally more than 150 

m deep) mass of sediment emplaced by the debris avalanche, 

sediment that consists of shattered parts of the volcanic 

edifice, a deposit from the consequent gravel-and-sand-rich, 

laterally directed blast PDC (herein called the “blast deposit”), 

and deposits from subsequent pumiceous pyroclastic flows 

(Lipman and Mullineaux, 1981; Glicken and others, 1989). 

During emplacement of the pyroclastic flows, large billowing 

clouds of ash lofted from the basal flows and subsequently 

settled. These ashcloud deposits are very fine grained (mostly 

silt to fine sand), of variable thickness, and are interspersed 

stratigraphically within the pyroclastic-flow-deposit sequence 

(Glicken and others, 1989). Composition and stratigraphy of 

the blockage is described in greater detail in the section on 

geomorphic and stratigraphic context of upper NFTR basin. 

Greater details of the respective deposits can be found in 

Lipman and Mullineaux (1981), Glicken (1996), and Brand 

and others (2014, 2016).

Mitigation of the Blockage Hazard

Transformation of Spirit Lake basin, and blockage of its 

outlet, caused the lake level to rise. Uncontrolled rise would 

have eventually allowed the lake to breach the blockage, 

likely leading to a catastrophic flood and lahar downstream 

as happened during an eruptive period about 2,600 years ago 

(Scott, 1988). To prevent such a catastrophe and to modulate 

and manage the level of Spirit Lake, water was pumped from 

Spirit Lake over the crest of the blockage while a suitable 

outlet was designed and constructed. Pumping to stabilize 

and lower the lake level began in late 1982, was continuous 

through August 1983, and then intermittent through early 

Sediment retention

structure

Step creek

Loowit Creek

Johnston Ridge

Carbonate

springs creek Truman channel

Tunnel

Windy Ridge

NF100

LO033

LO040

NF105

NF120 TR070

TR065 TR060B

LO030

NF110

Step-Loowit fan

North Fork Toutle River

South Coldwater Creek

Spirit Lake

Castle

Lake

Coldwater Lake

MOUNT ST. HELENS

Elk

Rock

Pumice Plain

N

Harry’s Ridge

0 1 2 3 4

1 2 3 4

5 KILOMETERS

0 5 MILES

EXPLANATION

Debris avalanche

Blast PDC

Pyroclastic flow

Lahar

Eruption deposit

Cross section

Figure 4. Simplified surface geologic map of 1980 eruption deposits in upper North Fork Toutle River basin (adapted from Lipman 

and Mullineaux, 1981) draped over digital terrain model derived from a 2009 airborne lidar survey. Locations of cross sections used 

for channel-evolution analysis are shown. The Spirit Lake blockage is defined as the region bounded on the east by Spirit Lake, on the 

west by Loowit Creek, on the north by the base of Johnston Ridge, and on the south by Step-Loowit fan. Dashed line highlights drainage 

divide along the blockage crest.
8 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

1985 (Paine, 1984). To provide an outlet for the lake, the U.S. 

Army Corps of Engineers bored a tunnel through bedrock 

that bounds the western shore of Spirit Lake (Sager and 

Chambers, 1986). The tunnel began full operation in May 

1985. Water from the lake presently flows through the tunnel, 

into South Coldwater Creek valley, and re-enters NFTR 

downstream from Coldwater Lake (fig. 4). Since 1985, the 

tunnel has maintained the lake at a safe operating level, though 

it has required episodic closures to repair sections that have 

deteriorated (Britton and others, 2016).

Episodic closures of the Spirit Lake tunnel have caused 

the water level to rise to elevations near that having the 

potential to compromise tunnel integrity and functionality 

(Grant and others, 2017). At times of such elevated lake 

levels, the potential for breaching the blockage and causing 

an uncontrolled release of lake water is heightened. Thus, the 

U.S. Forest Service (the agency that owns the tunnel) and U.S. 

Army Corps of Engineers (the agency responsible for tunnel 

inspections and conducting repairs) are evaluating alternative 

long-term options for managing the security of Spirit Lake. 

Within that context, a risk assessment of various alternative 

outlet options was conducted (Grant and others, 2017).

Scope and Objectives of this Report

As part of a re-evaluation of options for managing the 

long-term security of Spirit Lake, alternative strategies for 

maintaining safe lake levels are being considered by the U.S. 

Forest Service and other interested parties (Grant and others, 

2017; National Academies of Sciences, Engineering, and 

Medicine, 2017). This report documents hydrogeomorphic 

evolution and functioning of upper NFTR basin, focusing 

on development and evolution of the drainage network, to 

provide additional information for understanding potential 

ramifications of various alternative outlet strategies, especially 

of an open channel across the Spirit Lake blockage. The 

analyses herein provide additional insights that complement 

the more formal, semi-quantitative risk assessment of outlet 

alternatives for Spirit Lake provided by Grant and others 

(2017).

Post-eruption evolution of the drainage network in 

upper NFTR basin has been mediated by the size and 

sequencing of hydrologic events driven by storms, eruptions, 

and anthropogenic manipulations, and by the composition, 

structure, and architecture of the underlying volcanic 

deposits. Erosion that established and integrated the drainage 

network was driven mainly by runoff from typical winter 

precipitation, episodic large storms (especially those caused by 

intense atmospheric rivers of moisture aimed directly at, and 

sometimes stalled over, the Pacific Northwest [Neiman and 

others, 2008, 2011]), lake breakouts (Janda and others, 1984; 

Simon, 1999), controlled releases of water from Castle and 

Coldwater lakes, and water pumped from Spirit Lake (Sager 

and Chambers, 1986). In addition, small volcanic eruptions 

and large storms during the 1980s generated debris flows and 

floods that issued from the crater of Mount St. Helens (Waitt 

and others, 1983; Major and others, 2005; Mosbrucker and 

others, 2019) and modified the evolving channel network in 

varied ways.

Although there has been ongoing research on channel 

response to the 1980 eruption over the past four decades, 

primarily using repeat surveys of cross sections established 

by USGS scientists and analyses of aerial photographs 

(Rosenfeld and Beach, 1983; Janda and others, 1984; Parsons, 

1985; Meyer and Martinson, 1989; Simon, 1999; Simon and 

Klimetz, 2012; Zheng and others, 2014; Major and others, 

2019), post-eruption evolution of the channel network at the 

landscape scale has not been well-addressed. This gap in 

understanding was highlighted by Grant and others (2017, 

page 122):

One of the least well-understood dynamics of the 

Mount St. Helens landscape is the propensity for 

substantial geomorphic change along channels 

draining the north flank of the volcano. Recent 

surveys document as much as 15 m (50 ft) of 

vertical incision along some reaches of the Loowit 

channel during single storm events... Yet, other 

reaches of the same channel are comparatively 

stable. Understanding controls on both vertical and 

lateral stability of channels in this area will provide 

information essential to determining the feasibility 

of establishing and maintaining an alternative outlet 

across the debris blockage without exacerbating 

the risk of an uncontrolled lake release owing to 

geomorphic instability.

In this report we provide the most comprehensive 

examination to date of the geomorphic evolution of upper 

NFTR basin and its linkages to hydrology, topography, and 

geology. Our analysis is based primarily on interpretations and 

measurements from remote sensing, including orthorectified 

stereoscopic photographs from 1980 to 2015, high-resolution 

lidar (airborne laser scanning) data from 2003, 2007, 2009, 

and 2017, and oblique aerial photographs from 2018. We complement these remote-sensing analyses with repeat surveys of 

cross sections made over the past nearly 40 years. Our goals 

are to: (1) analyze fluvial processes in the upstream reaches 

of NFTR and across the Spirit Lake blockage to evaluate the 

susceptibility of the blockage to fluvial erosion and related 

rapid geomorphic changes that could affect an open-channel 

outlet or other management actions; (2) develop a processbased understanding of how the geometry and location of 

the channel network evolved; and (3) relate channel network 

development to hydrological events, overall topography, and 

the surface geology of the blockage.
Geomorphic and Stratigraphic Context of Upper North Fork Toutle River Basin 9

men19-7369_fig05

Loowit Creek

Step-Loowit fan

Pumice Plain

(PF deposit)

Debris-avalanche

deposit

Johnston Ridge

Spirit Lake

Mount Rainier

Figure 5. Oblique aerial view of Spirit Lake blockage looking north from vantage over northeast side of 

volcano (just east of crater mouth). Tan, subdued topography of Pumice Plain is comprised of pyroclasticflow (PF) deposits that overlie debris-avalanche deposit. Eroded terrain in foreground is covered mainly by 

blast pyroclastic density current deposit that overlies thin (less than 15 m) debris-avalanche deposit and 

pre-1980 deposits and bedrock. U.S. Geological Survey photograph by J. Vallance, June 26, 2007.

Geomorphic and Stratigraphic Context 

of Upper North Fork Toutle River Basin

The Spirit Lake blockage is a composite of multiple 

deposits (figs. 4, 5)—it is not a single, homogenous deposit. 

It is composed of a thick, heterogeneous mixture of shattered 

rock from the volcano—the debris-avalanche deposit—

overlain by a veneer of pyroclastic deposits ranging in 

composition from rocky bits of sand and gravel to pumiceous 

gravel and silt with pumice boulders. Field observations 

indicate there are few, if any, large, coherent, erosion-resistant 

masses within the blockage. Nearly all rock in the debrisavalanche deposit was shattered at the volcano as the giant 

rockslide-debris avalanche detached and slid into NFTR 

valley (Glicken, 1996). Near the surface, few clasts in that 

deposit are larger in diameter than a couple of meters, which, 

while large, are unlikely to impede erosion at the scale of 

the blockage (Grant and others, 2017, p. 112–113). The 

overlying pyroclastic deposits, including the blast deposit and 

subsequent pyroclastic-flow deposits (fig. 5), are composed 

largely of particles ranging in size from cobble gravel to 

silt, have physical compositions that vary both laterally and 

vertically owing to overlapping of multiple units, and are 

of variable thickness. As described by Glicken and others 

(1989), the blast deposit consists of a lower unit of angular, 

unstratified, clast-supported rock debris larger than coarse 

sand, overlain by an upper unit of silt- to sand-sized sediment 

composed of shattered bits of rock. Its thickness within the 

Spirit Lake blockage ranges from a few centimeters to as 

much as 13 m. The overlying pyroclastic-flow and ashcloud 

deposits are composed mainly of pumice gravel, sand, and silt. 

Across the blockage, individual deposits range from less than 

1 m to as much as 12 m thick, and the stratigraphic sequence 

of deposits is as much as 40 m thick (Rowley and others, 

1981). Cohesion and likely compaction of the pyroclastic-flow 

deposits (Poland and Lu, 2008; Welch and Schmidt, 2017) 

to some extent allow them to support nearly vertical walls 

when eroded. Nevertheless, these pyroclastic deposits are very 

erodible as shown by rapid channel erosion and by evidence 

of subsurface piping during very high stands of Spirit Lake 

before the tunnel was constructed (Glicken and others, 1989).

The surface of the blockage is irregular. Near its northern 

end at the base of Johnston Ridge, it is particularly irregular 

and exhibits rugged mounds and depressions where the 

debris-avalanche deposit is at the surface (figs. 4, 5). Closer 

to the volcano and along the southern and western parts of the 

blockage, the topography is smoother, reflective of both the 

contiguous mantle of pyroclastic sediment that overlies the 

debris-avalanche deposit there (figs. 4, 5) and a fan of alluvial 

and debris-flow deposits formed by erosion of the lower 

northern flank of the volcano.
10 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

Both the debris-avalanche and overlying pyroclastic 

deposits are highly erodible. As we will show, channels near 

the northern and western edges of the blockage incised tens 

of meters through the pyroclastic-flow deposits and into the 

debris-avalanche deposit and widened hundreds of meters 

within months to a few years. Channels on the Spirit Lake side 

of the crest are less deeply incised than those to the west. This 

is because there has been only modest-magnitude streamflow 

east of the blockage crest, owing largely to runoff from local 

springs and occasional discharge from the crater following 

episodic channel avulsions, and because the base level for 

erosion east of the crest (Spirit Lake) is substantially higher 

than that to the west (fig. 6). The elevation difference between 

Spirit Lake and the confluence between Truman channel (an 

informally named channel that traverses the south base of 

Johnston Ridge) and NFTR is 125 m (figs. 4, 6); the elevation difference between Spirit Lake and the sediment retention 

structure (see “Hydrologic Setting of Upper North Fork Toutle 

River Basin” section)—which sets local base-level control 

west of the blockage—is 760 m (fig. 4). No coherent channels 

cross the blockage crest.

Hydrologic Setting of Upper North Fork 

Toutle River Basin

The hydrologic regime of upper NFTR basin was fundamentally altered by the 1980 eruption. Alteration was caused 

by razing or removal of vegetation and by extensive sediment 

deposition on hillsides and within the upper NFTR channel. 

Like other types of landscape disturbance (such as wildfire), 

the hydrologic, geomorphic, and ecologic impacts of the 

eruption affected key components of the hydrologic cycle and 

influenced the character, magnitude, duration, and timing of 

runoff (Lettenmaier and Burges, 1981; Major and Mark, 2006; 

Pierson and Major, 2014).

The 1980 eruption razed (and locally removed) mature 

forest over hundreds of square kilometers (km2

), broadly 

deposited tephra that created a fine-grained, nearly impervious surface over more than 1,000 km2

, deposited extensive 

sediment in river channels, and greatly altered the character 

of major channels that drain the volcano (Lipman and Mullineaux, 1981; Janda and others, 1984; Simon, 1999; Swanson 

and Major, 2005; Lisle and others, 2018). In many basins, 

these effects led to enhanced runoff, greater flood-conveyance 

efficiency, and peak flows (for a given precipitation input) that 

were a few percent to many tens percent larger after the 1980 

eruption than before it (Leavesley and others, 1989; Major and 

Mark, 2006). Disruption of upper NFTR valley by the colossal 

debris-avalanche deposit blocked several channels tributary 

to NFTR, and the deposit’s irregular, hummocky topography 

and many depressions initially disrupted coherent streamflow. 

Such profound landscape perturbation severed hydrologic connection between the upper and lower parts of the watershed 

as well as between the volcano and the valley, and it isolated 

Spirit Lake. Reconnection required establishment of a new 

drainage network.

Drainage development on the debris-avalanche deposit 

began hours after emplacement when groundwater seepage 

C C'

B B'

Spirit Lake

MOUNT ST.

 HELENS

Pumice Plain

Johnston Ridge

Loowit

Creek

Step-Loowit fan

Drainage divide

Truman

channel

A

A'

N

800

1,200

1,600

2,000

2,400

2,800

Crater rim

Lava dome

Johnston Ridge

Truman channel

A

Pumice Plain

Distance, in meters

900

1,000

1,100

1,200

B

C

Spirit Lake shore

Loowit Creek

North Fork

Toutle River

Pumice Plain

WEST EAST

SOUTH NORTH

Elevation, in meters above NAVD 88

0 2,000 4,000 6,000 8,000 10,000 12,000

0 1,000 2,000 3,000 4,000 5,000

A'

B'

C'

Figure 6. Topographic profiles across the Spirit Lake blockage. A–A’ is a south to north profile, B–B’ is a west to east profile across 

the approximate location of Truman channel (see figure 4), and C–C’ is a west to east profile across the central part of blockage. 

Spirit Lake blockage defined as the region bounded on the east by Spirit Lake, on the west by Loowit Creek, on the north by the base 

of Johnston Ridge, and on the south by Step-Loowit fan. Dashed line depicts blockage drainage-divide. Base map derived from a 

2009 airborne lidar survey.
Hydrologic Setting of Upper North Fork Toutle River Basin 11

and melting glacier ice in the deposit formed ponds that 

subsequently enlarged and breached (Janda and others, 1984). 

As water (and muddy slurry) flowed from one depression to 

another, it eroded sediment and formed connecting channels. 

This nascent channel initiation was subsequently augmented in 

several ways by (1) breakouts of lakes impounded adjacent to 

the avalanche deposit; (2) controlled releases from the largest 

impounded lakes (Coldwater and Castle Lakes); (3) pumping water from Spirit Lake across the deposit surface during 

tunnel construction; (4) meltwater floods and debris flows 

issuing from the crater; and (5) runoff erosion (Dunne and 

Leopold, 1981; Rosenfeld and Beach, 1983; Janda and others, 

1984; Sager and Chambers, 1986; Paine and others, 1987; 

Pierson, 1999; Simon, 1999). Although channel initiation on 

the debris-avalanche deposit began within hours of emplacement, it took nearly 3 years to fully integrate a new drainage 

network across the deposit (Janda and others, 1984; Meyer, 

1995; Simon, 1999). Nevertheless, once sufficient continuous 

drainage pathways developed, they conveyed enhanced runoff 

from the basin.

The record of post-eruption daily mean streamflow 

on NFTR below the U.S. Army Corps of Engineers’ large 

sediment retention structure (SRS) (USGS streamgage 

14240525, labeled FTP in fig. 2) illustrates an early phase of 

enhanced runoff from the basin and highlights significant, 

episodic hydrologic events (fig. 7; table 1). NFTR streamflow 

was unregulated through 1988, except for restricted inflow 

from Spirit Lake through its tunnel. From 1988 to 1998, 

streamflow was regulated because it passed through a series 

of culverts in the SRS—a structure designed to trap sediment 

but pass water (fig. 8). Since 1998, flow has passed over the 

SRS spillway in run-of-river fashion and is unregulated—

there is no deep pool of water impounded. Frequency of 

moderate to large daily mean streamflow (greater than 

150 m3/s) was greater from 1980–83 than after. From 

1981–83, daily mean streamflow of NFTR below the SRS 

(as recorded at streamgage 14240525, site FTP) exceeded 

150 m3/s seven times. In contrast, from 1984 through 2018, 

daily mean streamflow exceeded that magnitude of flow 

only 12 times (supplemental data file DF1). An exceedance 

probability analysis of annual maximum daily mean 

streamflow since 1981 (fig. 9) shows that 150 m3/s streamflow 

has an annual exceedance probability of about 0.27 (a roughly 

4-year recurrence-interval flow). Thus, enhanced runoff was 

a major factor influencing the geomorphic response of this 

basin in the immediate aftermath of the eruption.

Water year

200

400

600

800

Streamflow, in cubic meters per second

Feb. 1982 flood

Feb. 1996 flood

Nov. 2006 flood

Dec. 2015 flood

1980 1985 1990 1995 2000 2005 2010 2015

Dec. 1982 flood

Feb. 1986 flood

EXPLANATION

Daily mean streamflow at North Fork 

Toutle River below SRS (FTP)

Measured

Estimated

Annual peak streamflow at FTP

Annual peak streamflow at KID

Figure 7. Time series of daily mean streamflow and annual peak streamflow of North Fork Toutle River below 

the sediment retention structure (SRS) near Kid Valley (USGS streamgage 14240525, FTP, see fig. 2). For periods in 

which measurements were not available, daily mean streamflow estimated using a regression on the difference 

between streamflow measured at North Fork Toutle River at Kid Valley (USGS streamgage 14241100, KID, see fig. 

2) and at Green River above Beaver Creek near Kid Valley (USGS streamgage 14240800, GRE, see fig. 2) or using 

regression relations among Toutle River at Tower Road (USGS streamgage 14242500, TOW, see fig. 2), FTP, and 

the differences between KID and GRE (see supplemental data file DF1). Major floods are annotated. Annual peak 

streamflow for water year 1982 (963.4 m3

/s), caused by the March 19, 1982, lahar, is not shown. Dashed horizontal 

line highlights daily mean streamflow of 150 m3

/s, a flow having an annual exceedance probability of 0.27 (a 

recurrence interval of about 4 years) (see fig. 9). Shaded region shows period when streamflow was regulated by 

passage through culverts in SRS (see fig. 8).
12 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

Table 1. Key hydrogeomorphic events in post-eruption channel development of upper North Fork Toutle River.

[Estimated volume is water (landslide) volume. Q is discharge, where subscript denotes type of measurement or estimate; E, estimated value; m3/s, cubic meters 

per second; SRS, sediment retention structure]

Date Event

Estimated 

volume

(103 m3)

Qpeak

(m3/s)1

Qdaily mean

(m3/s)2

Qpeak

(m3/s)3

Qdaily mean

(m3/s)4 Source

Dec. 26, 1980 Flood 174.8 E No data 238.4 Gage record

Feb. 19, 1981 Flood 181.1 E No data 297.5 Gage record

Jan. 24, 1982 Flood 194.0 E No data 379.7 Gage record

Jan. 25, 1982 Flood—Breach of ponds on 

Coldwater Lake levee

379 95.3 E No data 176.8 Simon (1999)

Feb. 14, 1982 Flood—Breach of ponds along 

north side of valley above Coldwater reach

710 120.1 E No data 212 Simon (1999)

Feb. 16, 1982 Flood 154.3 E No data 270.6 Gage record

Feb. 20, 1982 Flood—Breach of Jackson Lake 

adjacent to Elk Rock 

2470 261.8 E 5960.6 427.9 Simon (1999); gage 

record

Mar. 19, 1982 Lahar 10,000–

13,000

57.5 E 963.4 79.3 Waitt and others (1983); 

Pierson(1999); gag

record

Nov. 5, 1982 Spirit Lake pumping begins at 

5.1 m3/s

24.2 E 41.4 Simon (1999); gage 

record

Dec. 3, 1982 Flood 174.3 E 725.4 308.9 Gage record

Mar. 10, 1983 High flow 65.9 E No data 117.6 Gage record

Jul. 13, 1983 High flow—3 days 40.3 E No data 67.4 Gage record

Jan. 24, 1984 Flood 131.9 E 416.5 265.8 Gage record

Apr. 5, 1985 Spirit Lake pumping ends; tunnel 

opens

Simon (1999)

Feb. 23, 1986 Flood 144.2 E 617.7 325.9 Gage record

Nov. 24, 1986 Flood 118.6 E 410.9 279.4 Gage record

Jan. 9, 19906 Flood 141.4 138.0 328.7 253.0 Gage record

Feb. 8, 1996 Flood 371.2 311.7 Gage record
Hydrologic Setting of Upper North Fork Toutle River Basin 13

Table 1. —Continued

Date Event

Estimated 

volume

(103

 m3)

Qpeak

(m3/s)1

Qdaily mean

(m3/s)2

Qpeak

(m3/s)3

Qdaily mean

(m3/s)4 Source

Jan. 1, 1997 Flood 232.4 173.1 Gage record

Jan. 31, 2003 Flood No data 152.9 E Reconstructed

Jan. 2006 Landslide near cross section NF110 600 Major and others (2018)

Nov. 7, 2006 Flood and debris flow 280.8 198.1 Gage record

Dec. 9, 2015 Flood 512.9 283.4 Gage record

1,2Streamflows estimated for or measured at U.S. Geological Survey gage North Fork Toutle River below SRS near Kid Valley (streamgage 14240525). 

Station record begins October 1, 1989. See supplemental data file DF1 for estimated streamflow values.

3,4Streamflows measured at U.S. Geological Survey gage North Fork Toutle River at Kid Valley (streamgage 14241100). Period of record is June 10, 1980, to 

September 30, 1994.

5

 Unpublished secondary streamflow peak (K.R. Spicer, U.S. Geological Survey, written commun., 2019).

6

Streamflow on North Fork Toutle River below SRS was strongly affected by regulation of flow through culverts in SRS. In other basins within Toutle River 

watershed (South Fork Toutle River and Green River), this flood was a substantial hydrologic event in the streamflow record.

Culvert outlets

Spillway

A B Mount St. Helens

Figure 8. Views of sediment retention structure on North Fork Toutle River looking upstream. A, Oblique aerial view. B, Water 

passing through rows of culverts embedded in face of structure (see A for location). U.S. Army Corps of Engineers photographs from 

approximately 1990.
14 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

men19-7369_fig09

Annual maximum daily mean streamflow, in cubic meters per second

0.01

0.1

1

Exceedance probability

North Fork Toutle River (at FTP)

1981–2017, full record

1981–2017, excluding enhanced runoff period (1981–85)

1981–2017, excluding enhanced runoff period (1981–85) 

and SRS regulation period (1988–97)

1996

2007

2016

Annual peak streamflow, in cubic meters per second

0.01

0.05

0.1

0.2

0.5

1

Exceedance probability 

North Fork Toutle River (at FTP), 1990–2017

Toutle River (at TOW), 1981–2017

1996

2007

2016

2-yr RI

3.7-yr RI

0 100 200 300 400 500

0 400 800 1,200 1,600 2,000

EXPLANATION

EXPLANATION

Figure 9. Exceedance probability plots for streamflow at North Fork Toutle River below the sediment retention structure (SRS) 

(at FTP, USGS streamgage 14240525) and at Toutle River at Tower Road (at TOW, USGS streamgage 14242580). Top, Exceedance 

probabilities of annual maximum daily mean streamflow. Bottom, Exceedance probabilities of annual peak streamflow.

Fluvial Geomorphic Evolution of Upper Methodology

North Fork Toutle River Basin

The fundamental framework of the drainage system of 

upper NFTR basin developed rapidly following the May 1980 

eruption. By the mid-1980s, the drainage system was established and defined (see D.F. Meyer and others, 1986; Meyer 

and Dodge, 1988; Simon, 1999), and since then it has undergone an evolutionary process of enlargement and refinement 

(for example, Major and others, 2019). Here, we examine the 

evolutionary trajectory on two different time-scales—first 

through decadal-scale snapshots of change within the whole 

basin, followed by a more temporally refined analysis. We also 

examine evolutionary development on two spatial scales—a 

landscape scale that encompasses a broad view of the area 

around the Spirit Lake blockage (and farther downstream for 

the decadal-scale snapshots of change), and a more discrete, 

local scale viewed through the lens of repeat surveys of channel cross sections. We first discuss our methods of analysis 

before presenting results.

We examine broad-scale geomorphic changes by creating, and then examining differences between, digital terrain 

models (DTMs)—digital representations of surface elevation. 

DTMs were generated from a variety of sources (table 2). 

Those sources included topographic maps, traditional vertical and low-elevation-oblique aerial photographs, and lidar 

surveys. These sources comprise a range of scales. Pre-1980 

15-minute-quadrangle topographic maps have a scale of 

1:62,500; vertical aerial photographs from 1980–85 were 

acquired at 1:9,600, other vertical aerial photographs have 

scales of 1:12,000 to 1:14,000, and oblique aerial photos have 

variable scales; and lidar surveys have horizontal and vertical 

resolutions ranging from centimeters to meters.

DTMs constructed from aerial photographs were 

generated using structure-from-motion (SfM) and traditional 

photogrammetric techniques (for example, Turner and others, 

2012; Javernick and others, 2014; Gomez and others, 2015; 

Bakker and Lane, 2017). For 1980–1996 SfM-derived DTMs, 

negatives or contact prints were scanned, pre-processed as 
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 15

needed in Adobe Photoshop software to improve contrast 

and sharpen detail, oriented to remove as much distortion as 

possible, and processed in Agisoft Photoscan Pro software 

using invariant objects (such as logs, large rocks, and other 

stable landscape features) identified in a georeferenced 2015 

DTM as ground control. A calibrated digital camera system 

was used for the September 2015 aerial photography (table 

2), eliminating the need to scan and pre-process imagery 

prior to DTM construction. For the September 2018 aerial 

photography, we used a non-calibrated digital camera and 

acquisition methods described by Mosbrucker and others 

(2017). DTMs based on lidar surveys are bare-earth surface 

models—vegetation has been removed using a filtering 

algorithm. Over much of the area of interest on the Spirit 

Lake blockage, there is little vegetation other than thin ground 

cover, although dense riparian vegetation is present locally. 

Therefore, the DTMs generated from aerial photographs are 

largely bare-earth models. However, vegetation obscures the 

ground surface in parts of those photogrammetry-derived 

models, particularly on and adjacent to hillsides and in areas 

where springs emerge or where depressions contain ponds, 

especially since the middle to late 1990s.

Once DTMs were generated, we produced digital terrain 

models of topographic difference (DoDs) to examine changes 

in landscape topography between DTMs. The DoDs were 

created by subtracting surface elevations of grid cells in one 

Table 2. Available digital terrain models (DTMs), source of topography, and method of creation.

[SfM, structure from motion; lidar, airborne laser scanning]

DTM from those at identical grid cells in another DTM. The 

convention adopted here is to subtract elevations in an earlier 

DTM (DTM1) from those in a subsequent DTM (DTM2) 

(DTM2 – DTM1). Thus, if the difference in surface elevation 

at a grid-cell after subtracting DTM1 from DTM2 is positive, then the ground-surface elevation at that cell increased 

through deposition. Conversely, if the difference in surface 

elevation at a grid-cell after subtracting DTM1 from DTM2 is 

negative, then the ground-surface elevation lowered through 

erosion. Apparent loss of elevation at a grid cell along a river 

channel can happen if the river incises and lowers the channel 

bed, or if a channel bank erodes laterally. Removal of bank 

material can result in very large negative changes in elevation 

at a grid-cell if banks are tall. Apparent gain of elevation at a 

grid cell along a river channel can happen if a river deposits 

sediment on its channel bed or floodplain, if a river debouches 

from a canyon and deposits a fan of sediment, or if sediment 

is externally introduced to a channel, such as by a landslide. 

Thus, DoDs show where erosion and deposition have taken 

place within the landscape and can identify geomorphic “hot 

spots” of activity. Here, we focus principally on changes that 

occur on and around the Spirit Lake blockage resulting largely 

from fluvial processes that initiated and subsequently sculpted 

river channels.

A threshold value of change was used to distinguish 

geomorphic signal from noise and error within each DoD. 

Year of DTM Date of topography Source of topography Method of creation1

Pre-1980 Photography 1952; maps 1953–1958 15-minute topographic maps (Mount St. 

Helens, Spirit Lake, Elk Rock, Cougar)

Digitized topographic maps

1980 Sept. 5, 1980 Vertical aerial photography; 1:9,600 SfM (1)

1981 July 27, 1981 Vertical aerial photography; 1:9,600 SfM (1)

1982 Sept. 22, 1982 Vertical aerial photography; 1:9,600 SfM (1) 

1984 July 7, 1984 Vertical aerial photography; 1:9,600 SfM (1) 

1985 July 25, 1985 Vertical aerial photography; 1:9,600 SfM (1) 

1987 June 6–11, 1987 Vertical aerial photography; 1:12,000 Traditional photogrammetry (2)

1996 Aug. 29, 1996 Vertical aerial photography; 1:9,600 SfM (1) 

1999 Sept. 3, 1999 Vertical aerial photography; 1:14,000 Traditional photogrammetry (2)

2003 Sept. 19–Oct. 2, 2003 Airborne laser scanning Lidar (3)

2007 Oct. 22–29, 2007 Airborne laser scanning Lidar (4) 

2009 Sept. 16–20, 2009 Airborne laser scanning Lidar (5)

2015 Sept. 27, 2015 Vertical aerial photography, 1:12,000 SfM (1)

2017 Sept. 2017 Airborne laser scanning Lidar (6)

2018 Sept. 26, 2018 Oblique aerial photography SfM (1)

1

Data sources: (1) Mosbrucker and Sweeney (2019); (2) Mosbrucker (2019); (3) University of Washington libraries (2019); (4) Mosbrucker (2015); 

(5) Mosbrucker (2014); (6) Mosbrucker (2020).
16 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

Owing to photo distortion and challenges identifying and 

testing adequate ground control, the change-detection 

threshold was based on empirical trial-and-error that 

minimized apparent change on areas across each DTM known 

to be stable. This empirical determination of a threshold value, 

rather than a more rigorous error-propagation protocol (as 

described in Brasington and others, 2003; Major and others, 

2018; Anderson, 2019, for example), is admittedly coarse and 

probably masks real geomorphic changes. But because we do 

not attempt to quantify volumetric changes along channels, 

and locally use cross-section surveys to quantify amounts of 

incision and widening, this coarse approach is sufficient to 

illustrate timing and gross magnitudes of channel change. For 

most DTMs, the change-detection threshold is set at 2 m (for 

a few with tighter constraints, it is set at 1 m). However, the 

DoDs for 1980–81 and 1984–85 required a 4-m threshold. 

In those DoDs, signals are generally large enough to see 

distinctive erosion and deposition, but such changes cannot 

be confidently quantified because the DTMs were distorted, 

tilted, and could not be co-registered precisely. We include 

those DoDs for completeness and for what they can tell us 

heuristically about geomorphic evolution of upper NFTR 

basin, but more precise quantification of changes between 

those time periods can be extracted only locally from changes 

observed in resurveyed cross sections. In several DoDs, 

apparent erosion and deposition on Harry’s Ridge, Windy 

Ridge, and the ridge bounding the east side of the blockage 

(for locations, see fig. 4) are largely artifacts owing to 

distortion, poor georeferenced control along edges of imagery, 

and, in later years, growth of vegetation. In some instances, 

apparent erosion and deposition are the result of differencing 

a photogrammetry-derived DTM (which locally includes 

vegetation) against a bare-earth, lidar-derived DTM. Such 

artifacts are especially apparent around emergent springs (for 

example, the informally named “Carbonate springs”; fig. 4) 

that support relatively dense riparian vegetation.

Discrete, local changes along major channels in the 

drainage network are monitored through repeat surveys of 

channel cross-sections. An initial network of sections was 

established in the first months to years after the 1980 eruption 

(D.F. Meyer and others, 1986), and several sections have been 

resurveyed episodically over the nearly four decades since. 

Various methods have been used to survey cross-sections. 

Early surveys used an electronic distance meter (EDM) 

coupled with a theodolite to measure distances and angles to a 

rod-mounted prism. In the 1990s, total stations were adopted 

to measure distances and angles to rod-mounted prisms, and 

since 2012 reflectorless total-station technology has been 

adopted. Since 2009, section surveys also have benefitted 

from integrated global positioning system (GPS) technologies 

(Mosbrucker and others, 2015).

Cross-section surveys recurred irregularly. In the early 

1980s, resurveys were frequent and commonly documented 

morphological changes after each major storm or other 

significant hydrological event. Survey frequency declined after 

1984, and since the 1990s surveys typically have recurred less 

than once per year (Mosbrucker and others, 2015).

Each cross-section is georeferenced to monumented 

endpoints which, since the late 1990s, have been located using 

GPS. Episodically, monuments lost to erosion were reset. 

Elevations for cross-section surveys are shown in meters 

above the North American Vertical Datum of 1988 (NAVD 88) 

(Mosbrucker and others, 2015), and horizontal distance along 

a cross section is relative to a left bank monument. In general, 

elevations from on-the-ground measurements are accurate 

to within tens of centimeters, and horizontal locations are 

accurate to within hundreds of centimeters, especially along 

very long lines that extend many hundreds of meters to more 

than 1 km (Mosbrucker and others, 2015).

Broad Evolution of Upper North Fork Toutle 

Basin and Relation to Hydrologic Events

In the first few years after the 1980 eruption, the drainage 

network in upper NFTR basin evolved swiftly. However, 

key processes operated largely in spatially disparate reaches. 

For example, a substantial lake breakout in February 1982 

(Jackson Lake; Simon, 1999, fig. 29) produced one of the 

largest floods and greatest daily mean streamflows in upper 

NFTR valley (fig. 7; table 2), but it originated far downstream 

from the Spirit Lake blockage. In contrast, rainfall runoff, 

debris flows, and water pumped from Spirit Lake from 1982 

to 1985 caused significant erosion along Loowit Creek and 

Truman channels, which bound the west and north sides of the 

Spirit Lake blockage, respectively (fig. 4).

In the immediate aftermath of the eruption, debrisavalanche and mantling pyroclastic deposits were easily 

eroded. Measurements of suspended-sediment delivery 

downstream, when linked to streamflow magnitudes, show 

that small- to moderate-magnitude streamflow, not just largemagnitude streamflow, transported vast amounts of sediment 

(Major, 2004). Repeat measurement of cross sections along 

Loowit Creek channel and upper NFTR channel illustrate 

rapid evolution—both channel incision and widening—that 

occurred within the first 5 years after the eruption when the 

fluvial system was greatly out of equilibrium. Along these 

channels, vertical incision of 10 meters or more and lateral 

erosion of 100 meters or more was common (fig. 10).
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 17

men19-7369_fig10a

1982 March

1983 February

1984 June

1985 August

1986 June

A. Cross section LO030

Elevation, in meters above NAVD 88

1986 June

1987 October

1992 June

1997 September

Distance from left monument, in meters

1997 September

2004 August

2007 June

2009 August

EXPLANATION

EXPLANATION

EXPLANATION

0 200 400 600 800 1,000 1,200

1,120

1,140

1,160

1,180

1,120

1,140

1,160

1,180

1,120

1,140

1,160

1,180

Figure 10 (pages 17–22). Plots of repeat cross-section surveys along Loowit Creek and mainstem North Fork 

Toutle River channels. See figure 4 for cross section locations. A, Cross section LO030; B, cross section LO033; 

C, cross section LO040; D, cross section NF100; E, cross section NF105; F, cross section NF110; G, cross section 

NF120. View is looking downstream.
18 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

men19-7369_fig10b

1,020

1,030

1,040

1,050

1,060

1981 April

1982 September

1983 October

1984 June

1987

1,020

1,030

1,040

1,050

1,060

1987

1992 July

1999

2003 September

1,020

1,030

1,040

1,050

1,060

2003 September

2005 September

2007 May

2009 September

2014 August

B. Cross section LO033

Elevation, in meters above NAVD 88

Distance from left monument, in meters

−100 −50 0 50 100 150 200

EXPLANATION

EXPLANATION

EXPLANATION

Figure 10 (pages 17–22). Continued
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 19

men19-7369_fig10c

1,010

1,020

1,030

1,040

1,050

1982 March

1983 October

1984 June

1985 August

1986 June

1,010

1,020

1,030

1,040

1,050

1986 June

1987 June

1992 June

1996 September

1,010

1,020

1,030

1,040

1,050

1996 September

2005 September

2007 June

2014 August

2016 February

C. Cross section LO040

Elevation, in meters above NAVD 88

Distance from left monument, in meters

0 50 100 150 200 250 300

EXPLANATION

EXPLANATION

EXPLANATION

Figure 10 (pages 17–22). Continued
20 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

men19-7369_fig10d

920

940

960

980

1,000

1982 November

1983 January

1984 November

1985 August

1986 June

920

940

960

980

1,000

1986 June

1987 September

1989 June

1990 July

1998 July

920

940

960

980

1,000

1998 July

2005 August

2007 June

2011 July

2017 August

Elevation, in meters above NAVD 88

Distance from left monument, in meters

0 100 200 300 400 500 600

EXPLANATION

D. Cross section NF100

EXPLANATION

EXPLANATION

Figure 10 (pages 17–22). Continued
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 21

men19-7369_fig10e

900

920

940

960

980

1983 February

1984 January

1987

2003 September

2009 September

2016 July

E. Cross section NF105

Elevation, in meters above NAVD 88

Distance from left monument, in meters

200 400 600 800 1,000 1,200 1,400 1,600

EXPLANATION

860

880

900

920

1983 September

1984 July

1987

1998 July

860

880

900

920

1998 July

2003 September

2006 May

2007 June

2016 September

F. Cross section NF110

Elevation, in meters above NAVD 88

Distance from left monument, in meters

0 100 200 300 400 500

EXPLANATION

EXPLANATION

Figure 10 (pages 17–22). Continued
22 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

men19-7369_fig10g

760

770

780

790

800

1983 February–April

1984 June

1985 July

1987 October

760

770

780

790

800

1987 October

1990 July

1992 May

1997 August

760

770

780

790

800

1997 August

2007 July

2011 June

2012 August

2016 September

Elevation, in meters above NAVD 88

Distance from left monument, in meters

0 100 200 300 400

EXPLANATION

G. Cross section NF120

EXPLANATION

EXPLANATION

Figure 10 (pages 17–22). Continued
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 23

Mobility of this volcanic sediment when initially subject 

to even modest amounts of water is well illustrated along Truman channel (fig. 11). Along the distal end of Truman channel, 

just upstream from its confluence with Loowit Creek channel, 

repeat surveys show considerable incision and widening in late 

1982, within a month of water being pumped from Spirit Lake. 

The channel at cross-section TR065 (fig. 11B) incised about 

15 m and widened nearly 100 m between late August and late 

December 1982 (full pumping began on November 5, 1982; 

Paine, 1984; Glicken and others, 1989). Slightly farther downstream, the channel at cross-section TR070 (fig. 11C) incised 

nearly 15 m between mid-November 1982 and mid-January 

1983, and by nearly 20 m by August 1983. Closer to the 

source of outflow from the pumping operations, at TR060B 

(fig. 11A), the channel incised 11 m between September and 

December 1982, 7 m of which occurred within 2 weeks of the 

pumps becoming fully operational (Meyer and Dodge, 1988; 

Mosbrucker and others, 2015).

As channel geometries evolved and channel beds 

coarsened (Simon and Thorne, 1996), rates and magnitudes of 

incision declined (Simon and Thorne, 1996; Zheng and others, 

2014). Along Truman channel, geomorphic change effectively 

ceased once pumping operations halted in spring 1985 and 

water drained from Spirit Lake through a newly bored bedrock 

tunnel. Presently, the channel hosts limited local runoff and 

flow from groundwater seepage and harbors substantial 

riparian vegetation. Along several locations of upper NFTR 

and the uppermost reaches of Loowit channel, bed-elevation 

by the mid- to late 1980s fluctuated around a quasi-static 

level, though inexorable modest incision persisted locally. 

In contrast, pervasive lateral erosion persisted. Topographic 

differences between decade-spanning DTMs show large 

(multi-meter) magnitudes of vertical elevation change along 

channel corridors—owing to both vertical and lateral erosion 

from 1980–87 (fig. 12A), and largely to lateral erosion after 

1987 (figs. 12B, C; Major and others, 2018). After 1999, 

1,030

1,050

1,070

1,090

1982 September

1982 November

1983 August

1984 June

1985 April

1,030

1,050

1,070

1,090

1985 April

1986 June

1990 July

1998 August

A. Cross section TR060B

Elevation, in meters above NAVD 88

Distance from left monument, in meters

−200 −100 0 100 200 300

EXPLANATION

EXPLANATION

Figure 11 (pages 23–24). Plots of repeat cross-section surveys along Truman channel. See figure 4 for cross section 

locations. A, Cross section TR060B; B, cross section TR065; C, cross section TR070. View is looking downstream.
24 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

men19-7369_fig11b

980

1,000

1,020

1,040

1982 August

1982 November

1982 December

1983 July

1984 August

980

1,000

1,020

1,040

1984 August

1985 April

1986 June

1988 August

1989 June

1990 July

B. Cross section TR065

Elevation, in meters above NAVD 88

Distance from left monument, in meters

100 200 300 400 500

EXPLANATION

EXPLANATION

960

980

1,000

1,020

1982 November

1983 January

1983 August

1984 June

1986 June

960

980

1,000

1,020

1986 June

1991 August

1992 May

1999 July

C. Cross section TR070

Elevation, in meters above NAVD 88

Distance from left monument, in meters

100 200 300 400 500 600

EXPLANATION

EXPLANATION

Figure 11 (pages 23–24). —Continued
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 25

men19-7369_fig12

Elevation change, in meters

>15

10 to 15

5 to 10

3 to 5

1 to 3

Below detection level

>15

10 to 15

5 to 10

3 to 5

1 to 3

Deposition Erosion

EXPLANATION

0 1 2 3 4

1 2 3 4

5 KILOMETERS

0 5 MILES N

Elk

Rock

Castle

Lake

Spirit Lake

Coldwater Lake

Tunnel

A. 1980 to 1987

Elk

Rock

Castle

Lake

Spirit Lake

Coldwater Lake

MOUNT ST. HELENS

MOUNT ST. HELENS

MOUNT ST. HELENS

B. 1987 to 1999

Sediment retention structure

N1

Tunnel

Elk

Rock

Castle

Lake

Spirit Lake

Coldwater Lake

Sediment retention structure

N1

Tunnel

C. 1999 to 2009

Figure 12. Digital terrain models of topographic difference (DoDs) based on digital terrain 

models (DTMs) derived from aerial photography or airborne lidar. DoDs have been draped over a 

shaded-relief topographic model derived from a 2009 airborne lidar survey (table 2). These DoDs 

provide decadal-scale snapshots of geomorphic changes in upper North Fork Toutle River basin. N1, location of former small retention 

structure constructed in early 1980s. A, DoD from 1980 to 1987. Apparent broad-scale deposition along the western edge of the model 

is an artifact of poor topographic control and warping along the edge of the 1980 DTM. B, DoD from 1987 to 1999. The deep erosion at 

the sediment retention structure spillway is an artifact of construction activity. C, DoD from 1999 to 2009. Parts B and C are modified 

from Major and others (2018).
26 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

holistic, decadal-scale snapshots of topographic change show 

that the footprint of substantial erosion had greatly diminished, 

but that erosional hotspots persisted locally, particularly 

along upper NFTR and Loowit Creek channels (Major and 

others, 2018; fig. 12C). Reaches of persistent, focused erosion 

are along the volcano’s lower north flank (upstream from 

cross-section LO030), across the Pumice Plain (between 

cross-sections LO030 and NF100), and near Coldwater Lake 

between cross-section NF120 and the Castle Creek/Coldwater 

Creek confluence (Major and others, 2018) (figs. 4, 12C). On 

the volcano’s lower north flank, a few to a few tens of meters 

of lateral bank erosion can produce many tens of meters of 

elevation change owing to erosion of very tall banks, especially 

along upper Loowit Creek and Step creek (informally named). 

Oblique aerial photographs from late November 2006 indicate 

some, and perhaps much, of the documented erosion along 

upper Loowit Creek channel between 1999 and 2009 happened 

during a large storm in early November 2006 (Major and others, 

2018) that delivered 338 mm of rainfall in 5 days (November 

5–9) to upper NFTR (Natural Resources Conservation Service, 

2019). Those photographs also show a 4.5 million m3

 mass 

failure on the crater floor, which became a debris flow that 

traveled down Loowit Creek channel (Mosbrucker and others, 

2019).

Following an initial phase of rapid geomorphic change 

and channel development in the early 1980s under even lowto moderate-magnitude flows, channel evolution became 

predominantly event driven. Comparing changes in channel 

geometry documented by measurements of channel cross 

sections to the hydrological record shows that the most 

distinctive channel changes from 1990 to 2017 are associated 

mainly with some of the largest magnitude flows that have 

occurred since the early 1980s. The three principal hydrological 

events from 1996 to 2016—a flood in February 1996, a flood 

and debris flow in November 2006, and a flood in December 

2015, as well as a landslide from debris-avalanche sediment 

perched along the south face of Johnston Ridge in January 

2006—left clear imprints on channel morphology, at least as 

observed through the lens of spatially discrete snapshots of 

channel geometry. [We note, however, that the elapsed time 

between surveys makes any impact of the December 2015 

flood event less distinctive.] For example, along Loowit Creek 

channel, high streamflow and a debris flow in November 

2006 triggered ~10 m of incision and tens of meters of lateral

erosion (cross-sections LO033, LO040; figs. 10B, C). Farther 

downstream along the reach of upper NFTR channel that hugs 

Johnston Ridge, channel incision from this event ranged 3–10 

m (cross-sections NF100, NF105, and NF110), with local 

lateral erosion to tens of meters (figs. 10D, E, F). But floods 

and debris flows are not the only processes to affect channel 

evolution. In January 1996, a large mass failure of debrisavalanche material deposited about 600,000 m3

 of sediment in 

NFTR channel near cross-section NF110 (Major and others, 

2018; figs. 4, 10F, 12C). That deposit forced the river to erode 

laterally by tens of meters.

In addition to secondary failure of debris-avalanche 

sediment affecting channel geometry, many debris flows from 

the crater (of order 100,000 m3) have deposited considerable 

sediment on a fan at the base of Loowit and Step canyons 

(Waitt and others, 1993; Cameron and Pringle, 1990; Pierson 

and Waitt, 1999; Pringle and Cameron, 1999; Mosbrucker 

and others, 2019). Such deposition has, over the years, caused 

Loowit Creek to change course—at times draining directly 

toward Spirit Lake and at other times draining toward NFTR 

(fig. 13). Aerial photographs show the Loowit Creek and 

Step creek channels began to form around November 1980. 

Since then, Loowit Creek has avulsed multiple times. From 

1980 to 1984, the creek flowed toward NFTR. But in 1984, 

streamflow began to bifurcate with most flowing toward NFTR 

and some flowing toward Spirit Lake. From 1984 through 

1985, bifurcated streamflow discharged both ways in roughly 

equal proportions. In 1986, bifurcation ceased and streamflow 

drained toward NFTR, but from 1987 to 1989 streamflow 

again switched and flowed toward Spirit Lake. Between 1989 

and 1994, flow direction switched yet again toward NFTR. 

Following significant debris-flow deposition in September 1997 

(Major and others, 2005), streamflow switched course, and from 

1997 to 2002, it drained toward Spirit Lake. The course briefly 

shifted toward the NFTR in 2002, and again toward Spirit Lake 

in 2003. The present drainage of Loowit Creek toward NFTR is 

the result of yet another avulsion in 2004.

The Step-Loowit fan is susceptible to avulsion primarily 

because its channels are poorly incised at the fan head. Near 

the fan apex, the nexus of most avulsions (fig. 13), the Loowit 

Creek channel is about 1 m deep. As a result, it takes little 

sediment accumulation to cause channel avulsion. Debris flows 

from the crater commonly deposit sediment fill of 0.5–1 m thick 

or more. In contrast, channels are more deeply incised farther 

down the fan and across the Pumice Plain. Thus, avulsions 

downstream on the fan are less likely. Such physiography 

has important geomorphic implications (de Haas and others, 

2018a,b; Densmore and others, 2019). If avulsion occurs 

near the fan apex, there is a high probability that streamflow 

direction will switch completely from one side of the fan to the 

other, thereby crossing the debris-blockage drainage divide. 

In contrast, if avulsions happen farther down fan, the change 

in streamflow direction will be more local and more likely to 

remain on the same side of the blockage drainage divide.

From 1999 to 2009, reaches of focused erosion along upper 

NFTR channel were punctuated by reaches of deposition (Major 

and others, 2018). At the base of the volcano’s north flank, but 

above cross-section LO030 (fig. 4), as much as 4 m of sediment 

accumulated on the valley floor (fig. 12C). Downstream from 

the fan of landslide debris that filled the channel near crosssection NF110 (fig. 4), fluvial and debris-flow deposits from the 

large storm in November 2006 left a 6-m-thick fill on the valley 

floor (fig. 12C). Sediment also accumulated on the valley floor 

farther downstream; in November 2006 deposition displaced 

the confluence of NFTR and Coldwater Creek several hundred 

meters downstream (Major and others, 2018).
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 27

men19-7369_fig13

A

B

Drainage divide

1997–2002

1984–1985

1987–1989

2002–2004

2004–2006

1990–1997

2002

Loowit Creek

Step creek

1980–1984

1985–1986

2006–2018

Spirit Lake

N 0 1,000 FEET

0 500 METERS

Figure 13. Step-Loowit fan. A, Schematic diagram showing history of channel avulsion of Loowit 

Creek near head of fan. Arrows show predominant flow direction of Loowit Creek for time period 

indicated. Dashed line shows approximate drainage divide on Spirit Lake blockage. Colors show 

relative elevation; warmer colors indicate higher elevations. Base map derived from a 2009 

airborne lidar survey (table 2). B, Oblique aerial view of fan. USGS photograph by John Pallister, 

October 6, 2004.
28 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

The erosion, channel avulsion, and localized channel 

deposition documented following emplacement of the Spirit 

Lake blockage illustrate the very dynamic character of upper 

NFTR basin. Though many channel reaches now appear to 

be approaching a state of equilibrium with respect to vertical 

incision and modification of the river’s longitudinal profile 

(Zheng and others, 2014), the sediment composing the fluvial 

landscape in this valley remains highly susceptible to erosion. 

Such erosion is especially intense when abundant, concentrated 

runoff is first introduced to these deposits. It is also clear that 

even though chronic incision of the drainage system on and 

around the blockage has largely diminished, large storm events 

can drive acute incision locally on the order of several meters. 

Such acute localized incision can set up geomorphic instabilities 

that may migrate headward as knickpoints or knickzones 

(abrupt steps in the channel profile).

Fine-Time-Scale Landscape Evolution from 

1980 to 2018 and Linkages to Hydrology and 

Hydrologic Events

Although decadal-scale DoDs provide useful holistic 

snapshots of the geomorphic evolution of upper NFTR valley, 

finer-time-scale DoDs permit a better grasp of linkages 

among topographic change, hydrology, topography, and 

surface geology. We thus generated DoDs showing annual 

to multi-year topographic changes but focused on changes 

on and around the Spirit Lake blockage. These DoDs are 

generated from DTMs using various data sources (table 2), 

and they represent varying time intervals. The fine-time-scale 

DoDs show initial development of the NFTR network as it 

integrated the upper basin, along with subsequent adjustments 

and sculpting. They show that early evolution was dominated 

largely by vertical channel incision and network extension, 

whereas later changes shifted more toward lateral erosion and 

greater definition of the emergent drainage network (see also 

Major and others, 2019).

Pre-1980 to 1980

A DoD of pre-eruption (approximately 1952) to posteruption (September 5, 1980) elevation change shows the 

massive accumulation of debris-avalanche and pyroclastic 

deposits that filled upper NFTR valley, as well as scour on 

the lower north flank of the volcano caused by passage of the 

debris avalanche, the blast PDC, and subsequent pyroclastic 

flows (fig. 14). This DoD represents the initial condition from 

which the drainage network develops.

Spirit Lake

Johnston Ridge

Pumice Plain

MOUNT ST. HELENS

Elevation change, in meters, 

from pre-eruption to 1980

>15

12 to 15

Below detection level

12 to 15

>15

Deposition

Erosion

EXPLANATION

122°14’ 122°12’ 122°10’

46°16’

46°14’

0 1 KILOMETER

0 1 MILE

Figure 14. Digital terrain model 

of topographic difference (DoD) of 

uppermost North Fork Toutle River 

basin from pre-1980 topography 

(approximately 1952) to September 

5, 1980. The pre-1980 topography 

is based on topographic maps; the 

1980 topography is based on vertical 

aerial photographs. The contour 

depicting the shoreline of Spirit Lake 

represents the shoreline from a 2009 

airborne lidar survey. See table 2 for 

sources of topography and methods 

used to create digital terrain models. 

Base map is a shaded-relief model of 

pre-1980 topography. The area of DoD 

coverage is limited by coverage of the 

1980 aerial photographs.
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 29

1980–81

The DoD representing change from September 5, 1980, 

to July 27, 1981, illustrates initial development of the nascent 

drainage network (fig. 15). Despite known distortions, this 

DoD shows initial erosion of the lower north flank of the 

volcano in the areas that eventually became the headwaters 

of Step creek and Loowit Creek, the birth of the debris fan at 

the mouth of Loowit and Step canyons, nascent development 

of Loowit Creek channel across the Pumice Plain and the 

beginning of a drainage channel along the eastern margin and 

toe of Floating Island lava flow. This DoD also shows closed, 

sediment-filled depressions along the foot of Johnston Ridge. 

These depressions are loosely connected, or are beginning 

to be connected, by small segments of incised channels. 

When this DoD is compared to a pre-1980 topographic 

map (fig. 3), it is evident that the nascent drainage network 

strongly resembles the location of the pre-eruption drainage 

network. This resemblance indicates fundamental factors 

such as general topography and gross geology (discussed in 

“Geomorphic Processes and Relations with Topography and 

Surface Geology” section) strongly influence the location of 

drainage channels at Mount St. Helens.

1981–82

Between July 27, 1981, and September 22, 1982, the 

upper NFTR drainage network expanded and sharpened 

its definition substantially. This geomorphically active 

period was characterized by (1) widening along Loowit 

Creek channel across the Pumice Plain, which enlarged 

and sharpened the shape of that channel; (2) deposition 

at the foot of the volcano, which expanded and elevated 

the Step-Loowit fan; (3) erosion by both surface-water 

runoff and a lahar in March 1982, which defined a nascent 

channel at the base of Johnston Ridge along the present-day 

location of lower Truman channel (see fig. 4); (4) substantial 

expansion of deposition within a depression at the base of 

Johnston Ridge known colloquially as the pumice pond, 

likely as a result of deposition by the March 1982 lahar; 

(5) breaching of depressions farther downstream, which 

enlarged and sharpened the definition of small channel 

segments connecting them, perhaps owing to erosion by 

the March 1982 lahar; and (6) initiation and enlargement 

of new channel segments caused by groundwater seepage 

in the vicinity of present-day Carbonate springs (fig. 16), 

possibly owing to a breaching of a small lake that formed 

Loowit Creek

Spirit Lake

Johnston Ridge

Pumice Plain

MOUNT ST. HELENS

Floating Island 

lava flow

Step-Loowit fan

Elevation change, in meters, 

from 1980 to 1981

Greatest

Least

Below detection level

Greatest

Least

Deposition Erosion

EXPLANATION

122°14’ 122°12’ 122°10’

46°16’

46°14’

0 1 KILOMETER

0 1 MILE

Figure 15. Digital terrain model 

of topographic difference (DoD) of 

uppermost North Fork Toutle River basin 

from September 5, 1980, to July 27, 1981, 

based on digital terrain models (DTMs) 

derived from aerial photography. The 

contour depicting the shoreline of Spirit 

Lake represents the shoreline from a 

2009 airborne lidar survey. See table 2 

for sources of topography and methods 

used to create DTMs. Base map is a 

shaded-relief model of September 1980 

topography. A threshold of 4 meters 

was used to detect elevation change. 

The area of DoD coverage is limited by 

coverage of the 1980 aerial photographs.
30 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

men19-7369_fig16

Loowit Creek

Carbonate

springs

Spirit Lake

Johnston Ridge

Pumice

pond

Loowit canyon Step canyon

Step-Loowit fan

Pumice Plain

MOUNT ST. HELENS

Elevation change, in meters, 

from 1981 to 1982

>15

10 to 15

5 to 10

2 to 5

Below detection level

>15

10 to 15

5 to 10

2 to 5

Deposition Erosion

EXPLANATION

122°14’ 122°12’ 122°10’

46°16’

46°14’

0 1 KILOMETER

0 1 MILE

Figure 16. Digital terrain model 

of topographic difference (DoD) of 

uppermost North Fork Toutle River basin 

from July 27, 1981, to September 22, 

1982, based on digital terrain models 

(DTMs) derived from aerial photography. 

The shoreline of Spirit Lake represents 

the shoreline from a 2009 airborne 

lidar survey. See table 2 for sources of 

topography and methods used to create 

DTMs. Base map is a shaded-relief 

model of July 1981 topography. The area 

of DoD coverage is limited by coverage 

of the 1982 aerial photographs.

there (Simon, 1999). Although the March 1982 lahar was a 

significant hydrologic event (fig. 17), and it left an imprint 

on the topography as noted above, it generally had little 

detectable impact, within our threshold limits, other than 

noted above on the landscape evolution of upper NFTR 

basin. In general, its deposit is less than 2 m thick (Waitt 

and others, 1983; Pierson, 1999), and thus the depositional 

imprint of that event, except perhaps in the pumice pond, is 

below the level of detectable change achieved in this DoD.

Geomorphic changes represented by the 1980–81 and 

1981–82 DoDs occurred during a period when the basin 

experienced significant and varied hydrologic adjustments. 

As noted previously, a daily mean streamflow of 150 m3

/s (as 

estimated on NFTR below the SRS), a streamflow having an 

approximately 0.27 annual exceedance probability (a 4-year 

recurrence-interval flow; fig. 9), was exceeded 7 times from 

1981 to 1983. Thus, this initial phase of network development 

occurred under conditions of distinctly enhanced runoff (as 

well as being affected by volcanically induced hydrologic 

events). Although this period encompasses one of the largest 

floods recorded (960 m3

/s on February 20, 1982; table 1) on 

NFTR (NFTR at Kid Valley, streamgage 14241100; KID in 

fig. 2), that flood resulted mainly from breaching of Jackson 

Lake—a lake impounded at the debris-avalanche-deposit 

margin downstream from Castle Lake (Simon, 1999). Hence, 

that event had no direct impact on fluvial-network evolution in 

upper NFTR basin above Castle and Coldwater Lakes.

1982–84

Some of the most extensive evolution of the channel 

network in upper NFTR basin occurred between late 1982 and 

summer 1984, especially along Truman channel (fig. 18). This 

period encompasses the pumping operations enacted to lower 

and stabilize Spirit Lake while a suitable outlet was designed 

and constructed. Water from Spirit Lake was pumped through 

a conduit shallowly buried within the debris-avalanche deposit 

to a stilling basin west of the crest of the Spirit Lake blockage, 

from which it then flowed across the blockage surface. Full 

pumping operations began November 5, 1982, and ended in 

late April 1985 when the present-day outlet tunnel became 

fully operational. Extensive erosion by continuous flow of 

5.1 m3/s—including more than 10 m of incision and many tens 

of meters widening—is evident in the DoD (see also fig. 11). 

Much of the erosion along Truman channel occurred within 

weeks to a few months of the start of full pumping operations. 
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 31

By summer 1984, the geometry of Truman channel had largely 

stabilized and equilibrated to the steady water discharge 

delivered (fig. 11), and since then extensive riparian vegetation 

has established.

The 1982–84 DoD also shows significant erosion, deposition, and sculpting of channels across the blockage landscape. 

During this 2-year period, Loowit Creek channel became 

more prominently defined, both incising and eroding laterally (figs. 10B, C). The mainstem NFTR channel downstream 

from the Loowit Creek-Truman channel confluence eroded 

and enlarged, but also aggraded and accumulated sediment 

below the erosional reach. The channel formed by streamflow emanating from the Carbonate springs area accumulated 

sediment, and sediment continued to aggrade the Step-Loowit 

fan at the foot of the volcano. Hydrologically, daily mean 

streamflow greater than 150 m3/s occurred in water year1 1983 

and approached that magnitude of streamflow in water year 

1984 (fig. 7), but there were no exceptional hydrologic events. 

Nevertheless, basin hydrology remained in a state of disequilibrium compared to pre-eruption hydrology (Major and Mark, 

2006). In addition, the volcano remained in a state of active 

eruption and triggered melt-induced floods and debris flows 

that affected upper NFTR basin. Relatively small eruptions 

in February 1983 and May 1984 triggered slushflows, debris 

flows, and small-magnitude floods that emanated from the 

crater and swept across Step-Loowit fan and through channels 

across the Pumice Plain (Pierson and Janda, 1994; Pierson 

and Waitt, 1999; Pringle and Cameron, 1999; Major and others, 2005). Narrow fingers of flow from these events reached 

Spirit Lake. These flows contributed to further aggradation of 

the Step-Loowit fan and likely helped sculpt Loowit Creek 

channel through lateral erosion (see cross sections LO033 and 

LO040, fig. 10B, C).

1

A water year is the 12-month period from October 1 through September 30 

and is designated by the calendar year in which it ends.

Figure 17. Lahar generated by melting of crater snow during a small explosion on March 19, 1982. Spirit Lake is in the lower 

left corner of the image. U.S. Geological Survey photograph by T. Casadevall.
32 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

men19-7369_fig18

Loowit Creek

North Fork

Toutle River

Carbonate

springs

Spirit Lake

Johnston Ridge

Truman channel

Loowit canyon Step canyon

Step-Loowit fan

Pumice Plain

MOUNT ST. HELENS

LO030

LO033

LO040

NF110 NF105 NF100 TR070

TR065

TR060B

Cross section

Elevation change, in meters, 

from 1982 to 1984

>15

10 to 15

5 to 10

2 to 5

Below detection level

>15

10 to 15

5 to 10

2 to 5

Deposition Erosion

EXPLANATION

122°14’ 122°12’ 122°10’

46°16’

46°14’

0 1 KILOMETER

0 1 MILE

Figure 18. Digital terrain model 

of topographic difference (DoD) of 

uppermost North Fork Toutle River 

basin from September 22, 1982, to 

July 7, 1984, based on digital terrain 

models (DTMs) derived from aerial 

photography. The shoreline of Spirit 

Lake represents the shoreline from 

a 2009 airborne lidar survey. See 

table 2 for sources of topography 

and methods used to create DTMs. 

Base map is a shaded-relief model of 

September 1982 topography. The area 

of DoD coverage is limited by coverage 

of the 1984 aerial photographs.

1984–85

Little geomorphic change is detected in the DoD 

from July 7, 1984 to July 25, 1985 (fig. 19). This lack of 

detectable change results in part because water years 1984 

and 1985 were hydrologically muted, with few daily mean 

streamflows approaching 150 m3/s (fig. 7), and because the 

DoD for this period has a large (4 m) threshold of detection. 

Aside from a minor eruption-triggered debris flow in May 

1984 (Pringle and Cameron, 1999), only modest amounts 

of streamflow (largely less than 1.5 m3/s) issued from the 

crater. Cross sections along Loowit Creek channel (LO033, 

LO040) show localized sediment deposition (figs. 10B, 

C), whereas farther downstream, at cross sections NF100, 

NF105, and NF120, channel evolution largely involved 

incision and widening (figs. 10D, E, G). Cross section 

NF110, however, displays aggradation and widening 

(fig.10F). By the mid-1980s, geomorphic evolution aroun

the blockage and through upper NFTR basin became more 

event driven, requiring moderate- to large-magnitude flows 

(greater than 150 m3/s as measured on NFTR below the 

SRS) to do much geomorphic work beyond nibbling channel 

margins. Relatively small flows along the major channels 

eroded channel banks, but otherwise left little geomorphic 

signal in the DoD, in part because any cumulative erosion 

or deposition along the channel floor was below the 4-m 

detection threshold; channel widening, though detectable, 

was neither extensive nor ubiquitous. The greatest 

geomorphic changes were perhaps most relevant on StepLoowit fan, but that area is largely outside the footprint of 

this DoD. Although we cannot confidently detect substantive 

change to the fan, sediment accumulation there was likely 

the agent responsible for driving the avulsions of Loowit 

Creek visible in aerial imagery (see fig. 13).

1985–87

From July 1985 to July 1987, geomorphic modifications 

to the channels in upper NFTR basin were relatively minor—

limited largely to modest channel enlargement promoted by 

channel aggradation and lateral erosion (figs. 10, 20). These 

minor modifications owe largely to gradually increasing 

landscape stability under relatively benign hydrological 

conditions and because flow from the crater was directed largely 

toward Spirit Lake rather than toward NFTR (fig.13). Pe

daily mean streamflow in water years 1985–87 was less than 

150 m3/s at NFTR below the SRS (fig. 6), and dome-building 

eruptions triggered only one small debris flow from the crater 

in May 1986, which attenuated swiftly upon reaching the flat 

topography of Step-Loowit fan (Cameron and Pringle, 1990).
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 33

men19-7369_fig19

Loowit Creek

North Fork

Toutle River

Carbonate

springs

Spirit Lake

Johnston Ridge

Truman channel

Step-Loowit fan

Pumice Plain

MOUNT ST. HELENS

LO030

LO033

LO040

NF110 NF105 NF100 TR070

TR065

TR060B

Cross section

Elevation change, in meters, 

from 1984 to 1985

Greatest

Least

Below detection level

Greatest

Least

Deposition Erosion

EXPLANATION

122°14’ 122°12’ 122°10’

46°16’

46°14’

0 1 KILOMETER

0 1 MILE

Figure 19. Digital terrain model of 

topographic difference (DoD) of uppermost 

North Fork Toutle River basin from July 

7, 1984, to July 25, 1985, based on digital 

terrain models (DTMs) derived from aerial 

photography. The shoreline of Spirit Lake 

represents the shoreline from a 2009 

airborne lidar survey. See table 2 for 

sources of topography and methods used 

to create DTMs. Base map is a shadedrelief model of July 1984 topography. A 

threshold of 4 meters was used to detect 

elevation change. The area of DoD 

coverage is limited by coverage of the 

1984 aerial photographs.

Loowit Creek

North Fork

Toutle River

Carbonate

springs

Spirit Lake

Johnston Ridge

Truman channel

Step canyon

Pumice Plain

MOUNT ST. HELENS

LO030

LO033

LO040

NF110 NF105 NF100 TR070

TR065

TR060B

Cross section

Elevation change, in meters, 

from 1985 to 1987

>15

10 to 15

5 to 10

4 to 5

Below detection level

>15

10 to 15

5 to 10

4 to 5

Deposition Erosion

EXPLANATION

122°14’ 122°12’ 122°10’

46°16’

46°14’

0 1 KILOMETER

0 1 MILE

Figure 20. Digital terrain model 

of topographic difference (DoD) of 

uppermost North Fork Toutle River basin 

from July 25, 1985, to mid-June 1987 

based on digital terrain models (DTMs) 

derived from aerial photography. The 

shoreline of Spirit Lake represents 

the shoreline from a 2009 airborne 

lidar survey. See table 2 for sources of 

topography and methods used to create 

DTMs. Base map is a shaded-relief 

model of July 1985 topography. The area 

of DoD coverage is limited by coverage 

of the 1987 aerial photographs.
34 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

1987–96

The frequency of remotely sensed topographic data 

declined sharply after 1987, and as a result our ability to 

holistically analyze fine-time-scale geomorphic change 

in upper NFTR basin diminished. By this time, however, 

enhanced runoff had largely diminished (Major and Mark, 

2006), the long-profile of NFTR channel had begun to 

stabilize (Zheng and others, 2014), and the landscape had 

begun equilibrating to the imposed hydrologic regime. 

Landscape dynamics had also shifted toward an eventdriven dynamic to achieve substantive geomorphic change, 

especially regarding further channel incision. The 1987–96 

DoD (fig.21)—documenting changes from June 1987 t

August 1996—shows lateral adjustments along margins of 

Loowit Creek channel and renewed channel development near 

Carbonate springs. Cross-section surveys indicate channel 

refinement consisted of channel incision and widening above 

NF110 and aggradation and widening downstream (fig. 10).

From 1987 through 1995, basin hydrology was pedestrian 

and perhaps even a bit on the dry side. Daily mean streamflow 

below the SRS was commonly less than 100 m3

/s (fig. 6), a 

streamflow typically exceeded every 18 months on average 

(fig. 9), and peak streamflow was less than 150 m3/s. In 

the early 1990s, peak daily mean streamflow approached 

150 m3/s. We note, however, that streamflow from 1988 

to 1998 was regulated by culverts in the face of the SRS 

(fig.8). In January 1990, substantial peak streamflo

issued from Green River and South Fork Toutle River 

basins, but the measured peak streamflows on NFTR at KID 

(USGS streamgage 14241100) and FTP (USGS streamgage 

14240525) (fig. 2) were muted (table 1; fig. 7). In February 

1996, a strong and sustained atmospheric river delivered 

substantial warm rainfall onto an extensive and low-elevation 

snowpack (Marks and others, 1998). The combination of 

abundant warm rainfall and induced snowmelt generated the 

peak streamflow of record along lower Toutle River at Tower 

Road (1,750 m3/s; USGS streamgage 14242580; labeled 

TOW in fig. 2) and the second largest peak streamflow, but 

largest daily mean streamflow, measured at NFTR below the 

SRS (USGS streamgage 14240525) since the gage became 

operational in 1989 (fig. 7; table 1).

Loowit Creek

North Fork

Toutle River

Carbonate

springs

Spirit Lake

Johnston Ridge

Truman channel

Step canyon

Pumice Plain

MOUNT ST. HELENS

LO030

LO033

LO040

NF110 NF105 NF100 TR070

TR065

TR060B

Cross section

Elevation change, in meters, 

from 1987 to 1996

>15

10 to 15

5 to 10

2 to 5

Below detection level

>15

10 to 15

5 to 10

2 to 5

Deposition Erosion

EXPLANATION

122°14’ 122°12’ 122°10’

46°16’

46°14’

0 1 KILOMETER

0 1 MILE

Figure 21. Digital terrain model 

of topographic difference (DoD) of 

uppermost North Fork Toutle River basin 

from mid-June 1987 to August 29, 1996, 

based on digital terrain models (DTMs) 

derived from aerial photography. The 

shoreline of Spirit Lake represents 

the shoreline from a 2009 airborne 

lidar survey. See table 2 for sources of 

topography and methods used to create 

DTMs. Base map is a shaded-relief model 

of June 1987 topography. The area of DoD 

coverage is limited by coverage of the 

1987 aerial photographs.
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 35

The rare 1996 flood event is likely responsible for 

substantial lateral and vertical channel adjustments detected 

in both the DoD (fig. 21) and cross-section measurements 

(fig. 10), especially upstream from cross-section NF105. 

Renewed channel development near Carbonate springs likely 

owes to eastward shifting of groundwater seepage (compare 

the 1987–96 DoD [fig. 21] with that of 1981–82 [fig. 16]), 

but it is unclear whether the 1996 flood event influenced 

spring location or discharge magnitude. Water year 1996 

Table 3. Annual mean streamflow and total runoff of North Fork Toutle River below sediment retention structure near Kid Valley (U.S. 

Geological Survey streamgage 14240525).

[m3/s, cubic meters per second; m3, cubic meters; E, estimated value]

Water 

year

Annual mean streamflow

(m3

/s)1

Annual total runoff

(million m3

)

1981 18.2 E 573.8 E

1982 22.7 E 716.7 E

1983 23.7 E 746.3 E

1984 26.1 E 824.0 E

1985 20.5 E 646.2 E

1986 20.3 E 640.7 E

1987 17.9 E 565.8 E

1988 16.7 E 527.6 E

1989 17.1 E 540.5 E

1990 22.8 719.0

1991 22.9 721.4

1992 15.9 502.5

1993 16.8 530.8

1994 15.3 483.5

1995 22.7 715.1

1996 30.4 961.7

1997 28.9 911.9

1998 22.2 699.7

1999 26.8 E 846.1 E

Water 

year

Annual mean streamflow

(m3

/s)1

Annual total runoff

(million m3

)

2000 21.0 E 663.2 E

2001 13.2 415.8

2002 23.6 744.6

2003 17.6 E 554.9 E

2004 18.3 E 578.2 E

2005 17.2 E 541.2 E

2006 20.9 E 660.4 E

2007 21.4 673.9

2008 21.9 691.6

2009 18.2 573.1

2010 24.8 783.6

2011 30.0 944.6

2012 27.4 867.7

2013 22.9 723.5

2014 22.6 711.3

2015 18.8 594.1

2016 25.8 816.0

2017 32.1 1,013.1

2018 19.9 629.1

1

Values for water years 1981–89 are computed using regression relations for differences between streamflow at North Fork Toutle River at Kid Valley (USGS 

streamgage 14241100) and Green River above Beaver Creek, near Kid Valley (USGS streamgage 14240800). Values for water years 1999, 2000, and 2003–06 

are estimated using regression relations among streamgages 14240525, 14240800, 14241100, and streamflow measured on Toutle River at Tower Road (USGS 

streamgage 14242580). See supplemental data file DF1 for streamflow estimates. All other water years are based on streamflow measured at North Fork Toutle 

River below sediment retention structure near Kid Valley (USGS streamgage 14240525).

recorded the greatest volume of annual runoff from upper 

NFTR basin since the eruption, and it was substantially 

larger than any other annual runoff during 1987–96 (table 3). 

Therefore, it is possible that abundant precipitation input 

to the local groundwater system invigorated and increased 

shallow groundwater flow, and that the increased groundwater 

discharge caused the channel change detected. If so, this is 

yet another example of the sensitivity of this landscape to 

changes in discharge regime.
36 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

1996–99

Except along the deeply incised canyons of Step creek 

and Loowit Creek on the lower north flank of the volcano, 

relatively minor changes are detected in the DoD that 

represents change from August 1996 to September 1999 

(fig. 22). It is unclear if the substantial deposition detected 

in Step and upper Loowit canyons is real or an artifact of 

uncontrolled imagery near the boundaries of the terrain 

models, because nearly identical footprints of equivalent 

erosion appear in the 1999−2003 DoD (fig. 23). Major 

channels (Loowit Creek, mainstem NFTR) show modest 

lateral adjustments as channel banks continued to erode, and 

deposition at the head of Step-Loowit fan and along upper 

Loowit channel is evident. Deposition along the eastern 

margin of Step-Loowit fan owes mainly to small debris flows 

emanating from channel headwaters in September 1997 

(Major and others, 2005)—a geomorphic process common 

in early fall when the first substantial rains strike the region. 

Notably, flow from Loowit Creek during this period was 

directed toward Spirit Lake rather than toward NFTR as it 

had been previously (fig.13). Deposition detected alo

upper Loowit channel appears to coincide with an area of 

erosion just upslope, and this may reflect a small mass failure 

from hummocky areas on the debris avalanche, another 

geomorphic process commonly observed on this landscape. 

This period is characterized by hydrologic conditions typical 

of the long-term average along lower NFTR (fig.7)—neith

extreme events nor notably dry conditions (though water 

year 1998 appears to be below average).

Loowit Creek

North Fork

Toutle River

Carbonate

springs

Spirit Lake

Johnston Ridge

Truman channel

Loowit canyon Step canyon

Step-Loowit fan

Pumice Plain

MOUNT ST. HELENS

LO030

LO033

LO040

NF110 NF105 NF100 TR070

TR065

TR060B

Cross section

Elevation change, in meters, 

from 1996 to 1999

>15

10 to 15

5 to 10

2 to 5

Below detection level

>15

10 to 15

5 to 10

2 to 5

Deposition Erosion

EXPLANATION

122°14’ 122°12’ 122°10’

46°16’

46°14’

0 1 KILOMETER

0 1 MILE

Figure 22. Digital terrain model of topographic difference (DoD) of uppermost North Fork Toutle River 

basin from August 29, 1996, to September 3, 1999, based on digital terrain models (DTMs) derived from aerial 

photography. The shoreline of Spirit Lake represents the shoreline from a 2009 airborne lidar survey. See table 

2 for sources of topography and methods used to create DTMs. Base map is a shaded-relief model of August 

1996 topography. The area of DoD coverage is limited by coverage of the 1996 aerial photographs.
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 37

1999–2003

Relatively benign hydrologic conditions characterize 

the period from September 1999 to September 2003 (fig.7)

Consequently, little except local geomorphic change is 

detected (fig. 23). Peak daily mean streamflow of NFTR 

below the SRS approached or exceeded 150 m3/s on few 

occasions, and during a few water years was largely less 

than 50 m3/s. Average annual mean streamflow (and average 

total annual runoff) from NFTR basin during this period 

is 25 percent lower than that from 1996–99 (table 3). The

greatest geomorphic changes occurred in headwater areas 

of Step and Loowit canyons, where apparent erosion of 

tall banks is evident, although as noted previously, this 

footprint of erosion may be an artifact of the terrain-model

processing. Nevertheless, sediment was deposited locally 

along Step creek channel, the eastern margin of Step-Loowit 

fan, and thinly along channels draining toward Spirit Lake 

(fig. 23). Through much of this period, flow from the crater 

was directed toward Spirit Lake and not along Loowit Creek 

(fig. 13). Apparent hillside erosion around the margins of the 

DoD is an artifact of differencing a photogrammetry-derived 

DTM (that includes vegetation) (1999) and a bare-earth, 

lidar-derived DTM (2003). But this does not fully explain the 

substantial changes apparent within the steep confines of Step 

and Loowit canyons.

Loowit Creek

North Fork

Toutle River

Carbonate

springs

Spirit Lake

Johnston Ridge

Truman channel

Loowit canyon Step canyon

Step-Loowit fan

Pumice Plain

MOUNT ST. HELENS

LO030

LO033

LO040

NF110 NF105 NF100 TR070

TR065

TR060B

Cross section

Elevation change, in meters, 

from 1999 to 2003

>15

10 to 15

5 to 10

2 to 5

Below detection level

>15

10 to 15

5 to 10

2 to 5

Deposition Erosion

EXPLANATION

122°14’ 122°12’ 122°10’

46°16’

46°14’

0 1 KILOMETER

0 1 MILE

Figure 23. Digital terrain model of topographic difference (DoD) of uppermost North Fork Toutle River basin from 

September 3, 1999, to late September 2003 based on digital terrain models (DTMs) derived from aerial photography 

and airborne lidar. The shoreline of Spirit Lake represents the shoreline from a 2009 airborne lidar survey. See table 

2 for sources of topography and methods used to create DTMs. Base map is a shaded-relief model of September 

1999 topography. The area of DoD coverage is limited by coverage of the 1999 aerial photographs.
38 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

2003–07

Extensive geomorphic adjustments associated with 

substantial channel incision, widening, and mass-movement 

sculpted the fluvial network from September 2003 to October 2007 (figs. 10, 24). Locally, 10 m of incision and tens of 

meters of channel widening occurred, mainly along Loowit 

Creek channel but extending downstream from the Truman 

channel-Loowit Creek channel confluence. Mass failure of 

debris-avalanche sediment perched on the south face of Johnston Ridge in January 2006 (Major and others, 2018) deposited 

several meters of sediment in the NFTR channel and forced 

the river to migrate and laterally erode its bank (fig. 10F). 

Although this period is characterized by relatively normal 

hydrologic conditions (fig. 7), and annual mean streamflow is 

6 percent less than that from 1999–2003 (table 3), a substantial storm in November 2006 produced the third greatest peak 

streamflow and third greatest daily mean streamflow measured 

on NFTR below the SRS since the gage became operational 

in 1989 (figs.7, 9). This flood and an associated debris flo

(Mosbrucker and others, 2019) are largely responsible for the 

geomorphic response detected during this period. This event, 

and its consequent geomorphic response, illustrate not only 

the important role that substantial hydrologic events have on 

evolution of this landscape, but also reinforce the notion that 

this landscape remains sensitive to infrequent, but not extreme, 

hydrologic events (see, for example, cross sections LO033, 

LO040, and NF100, fig. 10B, C, D).

Loowit Creek

North Fork

Toutle River

Carbonate

springs

Spirit Lake

Johnston Ridge

Truman 

channel

Loowit canyon Step canyon

Step-Loowit fan

Pumice Plain

MOUNT ST. HELENS

LO030

LO033

LO040

NF110 NF105 NF100 TR070

TR065

TR060B

Cross section

Elevation change, in meters, 

from 2003 to 2007

>15

10 to 15

5 to 10

2 to 5

Below detection level

>15

10 to 15

5 to 10

2 to 5

Deposition Erosion

EXPLANATION

122°14’ 122°12’ 122°10’

46°16’

46°14’

0 1 KILOMETER

0 1 MILE

Figure 24. Digital terrain model of topographic difference (DoD) of uppermost North Fork Toutle River basin from late 

September 2003 to late October 2007 based on digital terrain models (DTMs) derived from airborne lidar. The shoreline 

of Spirit Lake represents the shoreline from a 2009 airborne lidar survey. See table 2 for sources of topography and 

methods used to create DTMs. Base map is a shaded-relief model of September 2003 topography. The area of DoD 

coverage is limited by coverage of the 2007 lidar survey.
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 39

2007–09

The 2007–09 DoD, reflecting change from October 2007 

to September 2009, reveals persistent, lateral sculpting of 

the drainage system (fig. 25), especially along Loowit Creek, 

consistent with local observations from surveys of channel 

cross sections (fig. 10). Deposition along the valley floor is 

also evident and widely dispersed along the channel network. 

Substantial deposition (~5m) is evident, however, along a flat 

reach of lower Step creek at its confluence with Loowit Creek 

between cross sections LO030 and LO033. That sediment 

deposition appears to have filled an area of erosion caused 

by the November 2006 event (see fig. 24), and its magnitude 

appears to exceed that of bank erosion immediately upstream. 

We suggest this sediment deposition may be due to a debris 

flow from the Step canyon headwaters, although we have no 

solid evidence of such an event at this time. Although water 

years 2008 and 2009 appear to have been hydrologically 

modest (see fig. 7), with only brief instances of daily mean 

streamflow approaching 150 m3/s at the NFTR streamgage 

below the SRS, there was a substantial storm in January 2009. 

Unfortunately, a gage malfunction prevented measurement of 

peak streamflow below the SRS. But data from the Natural 

Resources Conservation Service Spirit Lake snow telemetry 

(SNOTEL) station (site number 777) document 185 mm of 

rainfall at temperatures well above freezing (1–6 degrees 

C) during January 6–8, 2009. This rainfall, 60 percent of 

which fell on January 7, could have triggered a small debris 

flow. Data from acoustic flow monitors on the Pumice Plain 

downstream from cross-section LO033 are inconclusive (K.R. 

Spicer, U.S. Geological Survey, written commun., 2019). 

Water years 2008 and 2009 were otherwise rather benign 

hydrologically, with average annual mean streamflow less than 

the long-term average from 1981 to 2018 (table 3).

Loowit Creek

North Fork

Toutle River

Carbonate

springs

Spirit Lake

Johnston Ridge

Truman channel

Step-Loowit fan

Pumice Plain

MOUNT ST. HELENS

LO030

LO033

LO040

NF110 NF105 NF100 TR070

TR065

TR060B

Cross section

Elevation change, in meters, 

from 2007 to 2009

>15

10 to 15

5 to 10

2 to 5

Below detection level

>15

10 to 15

5 to 10

2 to 5

Deposition Erosion

EXPLANATION

122°14’ 122°12’ 122°10’

46°16’

46°14’

0 1 KILOMETER

0 1 MILE

Figure 25. Digital terrain model of topographic difference (DoD) of uppermost North Fork Toutle River basin from 

late October 2007 to late September 2009 based on digital terrain models (DTMs) derived from airborne lidar. The 

shoreline of Spirit Lake represents the shoreline from the 2009 lidar survey. See table 2 for sources of topography 

and methods used to create DTMs. Base map is a shaded-relief model of October 2007 topography. The area of DoD 

coverage is limited by coverage of the 2007 lidar survey.
40 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

2009–15

From September 2009 to September 2015, channel 

margins continued to enlarge, particularly at channel bends 

(figs. 10, 26). The greatest sculpting occurred just downstream 

from the Step creek-Loowit Creek confluence upstream from 

cross section LO033. That flat reach continued to accumulate 

sediment supplied from upstream, and this aggradation likely 

drove the documented channel widening. Modest sculpting 

within Loowit and Step canyons is evident, as is moderate 

deposition near the head of Step-Loowit fan. Bank erosion 

is evident along small channels leading into Spirit Lake. The 

most obvious change over this period is advancement of 

Crater Glacier in the headwater reaches of Step and Loowit 

canyons. Apparent deposition in the Carbonate springs area is 

an artifact of vegetation. The 2009 DTM is a bare-earth, lidarderived model, whereas the 2015 DTM is a photogrammetryderived model that includes vegetation. These “deposition 

artifacts” owing to vegetation appear on several hillsides 

in this DoD. The modest, but inexorable, channel changes 

detected over this 6-year period attest to the modest hydrologic 

conditions characteristic of the period (fig. 7). There were no 

substantial floods or debris flows and daily mean streamflow 

remained below 150 m3

/s. However, average annual mean 

streamflow was 8 percent greater than the long-term average, 

and annual mean streamflow (and total annual runoff) in water 

year 2011 was the third greatest recorded or estimated for the 

NFTR below the SRS (table 3). Thus, geomorphic sculpting 

was done largely under conditions characterized by lowmagnitude, but long-duration, high-volume streamflow.

2015–17

The largest peak streamflow measured on NFTR below 

the SRS (FTP in fig. 2), since it became operational in 1989, 

occurred in early December 2015 (fig. 7). This flood also 

generated the second greatest daily mean streamflow measured 

at or estimated for that gage (figs. 7, 9). Based on the hypothesis that substantive geomorphic work in upper NFTR basin 

is now event-driven, we anticipated that the DoD reflecting 

Loowit Creek

North Fork

Toutle River

Carbonate

springs

Spirit Lake

Johnston Ridge

Truman channel

Loowit canyon Step canyon

Step-Loowit fan

Pumice Plain

MOUNT ST. HELENS

LO030

LO033

LO040

NF110 NF105 NF100 TR070

TR065

TR060B

Cross section

Elevation change, in meters, 

from 2009 to 2015

>15

10 to 15

5 to 10

2 to 5

Below detection level

>15

10 to 15

5 to 10

2 to 5

Deposition Erosion

EXPLANATION

122°14’ 122°12’ 122°10’

46°16’

46°14’

0 1 KILOMETER

0 1 MILE

Figure 26. Digital terrain model of topographic difference (DoD) of uppermost North Fork Toutle River basin from 

late September 2009 to September 27, 2015, based on digital terrain models (DTMs) derived from airborne lidar and 

aerial photography. The shoreline of Spirit Lake represents the shoreline from a 2009 airborne lidar survey. See table 

2 for sources of topography and methods used to create DTMs. Base map is a shaded-relief model of September 2009 

topography. The area of DoD coverage is limited by coverage of the 2015 aerial photographs. Apparent deposition in 

the Carbonate springs area results from differencing a DTM derived from aerial photography (which is influenced by 

dense vegetation) (2015) and a bare-earth, lidar-derived DTM (2009).
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 41

change from September 2015 to September 2017 would show 

another period of significant erosion. Although the DoD and 

surveyed cross sections (figs. 10, 27) show channel widening 

(for example, at cross sections NF100 to NF120), particularly 

at channel bends, the modest extent of widening and general 

lack of incision belie the anticipated response based on prior 

geomorphic responses to large flood events. (The apparent 

erosion in the Carbonate springs area is again an artifact of 

vegetation.) Reasons for this relatively muted response are 

unclear. It may be that although the storm was significant and 

generated substantial runoff, the duration of that runoff was 

relatively short-lived in the upper basin. Runoff from upper 

NFTR basin during the 5-day period that encompasses the 

bulk of that storm (December 7–11) accounted for only about 

7 percent of the annual runoff for water year 2016. Although 

the February 1996 and November 2006 storms also delivered 

7 to 8 percent of their respective annual runoff over five days 

each, the December 2015 storm delivered the least amount of 

precipitation to upper NFTR basin as measured at Spirit Lake 

(Natural Resources Conservation Service SNOTEL site 777, 

Spirit Lake). Total precipitation during December 7–11, 2015, 

was 268 mm, compared to 328 mm during February 6–10, 

1996, and 338 mm during November 5–9, 2006. Furthermore, the 1996 and 2006 storms remained warm and delivered rainfall throughout, whereas the December 2015 storm 

swiftly transitioned to snowfall shortly after peak streamflow 

occurred. It is likely that the trajectories of storm tracks and 

spatial distributions of precipitation among those three storms 

differed substantially. Even though the 2015 storm generated 

substantial peak and daily mean streamflow on the NFTR 

below the SRS, that storm apparently did not affect upper 

NFTR basin as strongly as did the other two storms. As a 

result, the December 2015 storm had a lesser geomorphic 

impact on the upper NFTR basin even though 40 km downstream from the volcano it generated one of the most substantial floods since the 1980 eruption.

Loowit Creek

North Fork

Toutle River

Carbonate

springs

Spirit Lake

Johnston Ridge

Truman channel

Loowit canyon Step canyon

Step-Loowit fan

Pumice Plain

MOUNT ST. HELENS

LO030

LO033

LO040

NF110 NF105 NF100 TR070

TR065

TR060B

Cross section

Elevation change, in meters, 

from 2015 to 2017

>15

10 to 15

5 to 10

2 to 5

Below detection level

>15

10 to 15

5 to 10

2 to 5

Deposition Erosion

EXPLANATION

122°14’ 122°12’ 122°10’

46°16’

46°14’

0 1 KILOMETER

0 1 MILE

Figure 27. Digital terrain model of topographic difference (DoD) of uppermost North Fork Toutle River basin from 

September 27, 2015, to late September 2017 based on digital terrain models (DTMs) derived from aerial photography 

and airborne lidar. The shoreline of Spirit Lake represents the shoreline from a 2009 airborne lidar survey. See table 2 

for sources of topography and methods used to create DTMs. Base map is a shaded-relief model of September 2015 

topography. Apparent erosion in the Carbonate springs area results from differencing a bare-earth, lidar-derived DTM 

(2017) and a DTM derived from aerial photography (which is influenced by dense vegetation) (2015).
42 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

2017–18

To determine whether single-year change is detectable 

during a typical water year nearly 40 years after the 1980 

eruption, we generated a DoD representing changes from 

September 2017 to September 2018 (fig. 28). In October 2017, 

small debris flows triggered by one of the first heavy autumn 

rainstorms issued from the crater and swept along StepLoowit fan. That storm also generated the peak streamflow for 

water year 2018 measured at NFTR below the SRS (table 1). 

Other than this early rainstorm causing a flush of sediment, 

the remainder of water year 2018 was benign, with no large 

storms or flood events. Peak streamflow at NFTR below the 

SRS was less than 150 m3/s, and peak daily mean streamflow 

was less than 100 m3

/s (fig. 7). As a result, we find little 

change on the landscape other than minor erosion along channel margins and some detectable deposition along Step-Loowit 

fan. These results emphasize and reinforce our hypothesis that 

the channel system of upper NFTR basin has largely equilibrated to the long-term average hydrologic condition that now 

characterizes the basin, and that large storms, and consequent large floods and debris flows (perhaps coincident with 

≥10-year return interval peak streamflow), are now required 

to generate significant geomorphic change along the evolving drainage network. Nevertheless, inexorable bank erosion 

continues and helps maintain abnormally elevated sediment 

delivery from this basin relative to pre-eruption conditions 

(see Major and others, 2019).

Loowit Creek

North Fork

Toutle River

Carbonate

springs

Spirit Lake

Johnston Ridge

Truman channel

Loowit canyon Step canyon

Step-Loowit fan

Pumice Plain

MOUNT ST. HELENS

LO030

LO033

LO040

NF110 NF105 NF100 TR070

TR065

TR060B

Cross section

Elevation change, in meters, 

from 2017 to 2018

>15

10 to 15

5 to 10

2 to 5

Below detection level

>15

10 to 15

5 to 10

2 to 5

Deposition Erosion

EXPLANATION

122°14’ 122°12’ 122°10’

46°16’

46°14’

0 1 KILOMETER

0 1 MILE

Figure 28. Digital terrain model of topographic difference (DoD) of uppermost North Fork Toutle River basin from 

late September 2017 to September 26, 2018, based on digital terrain models (DTMs) derived from airborne lidar and 

aerial photography. The shoreline of Spirit Lake represents the shoreline from a 2009 airborne lidar survey. See table 

2 for sources of topography and methods used to create DTMs. Base map is a shaded-relief model of September 

2017 topography. The area of DoD coverage is limited by coverage of the 2018 oblique aerial photographs. Apparent 

deposition on hillsides, near the shore of Spirit Lake, and near Truman channel results from differencing a bare-earth, 

lidar-derived DTM (2017) and a DTM derived from aerial photography (which is influenced by dense vegetation) (2018).
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 43

Geomorphic Processes and Relations with 

Topography and Surface Geology

To examine the extent to which surface geology of 

the blockage affected drainage development between 1980 

and 1985, we examined spatial relations between DoDs and 

surface geology mapped in 1980 (Lipman and Mullineaux, 

1981). Mapped surface geology, however, does not capture the 

three-dimensional depositional sequences (for example, Brand 

and others 2014, 2016) that certainly influence geomorphic 

development, particularly after initial channel development 

ensued. For example, pyroclastic-flow deposits overlie 

the debris-avalanche deposit, have variable stratigraphic 

texture, and contain many internal structures having variable 

erosional resistance. In places where it appears that surficial 

pyroclastic deposits may have influenced channel evolution, it 

is possible that the debris-avalanche deposit also, or perhaps 

predominantly, influenced channel evolution. With that 

limitation in mind, we examine example areas where we might 

anticipate that surface geology may influence the location and 

extent of geomorphic change (fig. 29).

Surface geology of the blockage is composed of 

exposures of debris-avalanche, blast, and pyroclastic-flow 

deposits (figs. 4, 5, 29). The irregular, hummocky topography 

of the debris-avalanche deposit appears to have steered initial 

network development through fill-and-spill among depressions 

and by headward incision around hummocks (Rosenfeld 

and Beach, 1983; Janda and others, 1984; Parsons, 1985; 

fig. 30). These observations indicate surface topography 

may be a dominant influence on channel position rather than 

deposit geology. In the blockage, hummocks are composed 

largely of stratigraphically intact, but shattered, pieces of the 

mountain, whereas inter-hummock areas are composed of 

both intact and blended pieces of the mountain rarely larger 

than coarse gravel. Although it may be tempting to also 

interpret this steering as a result of an erodibility contrast 

between hummock and inter-hummock areas, the co-location 

of topographic gradients and geologic variation precludes 

fully disentangling the effect of substrate composition from 

the effect of deposit topography. Once a channel established, 

bank erosion and channel widening carved into hummocks, 

suggesting that these features are certainly erodible (fig. 31). 

Indeed, as of 2019, many hummocks have been sculpted 

or removed by fluvial erosion that has triggered slumping 

of hummock sediment. The surfaces of the blast and 

pyroclastic-flow deposits are very susceptible to erosion and 

were dominated by channel incision during initial channel 

development. Although channel locations are close to contacts 

between blast and pyroclastic-flow deposits, indicating a 

possible erodibility contrast, in some cases they crosscut 

contacts, indicating that channel initiation was more strongly 

influenced by topographic slope (fig. 32). Despite a lack of 

clear control on geomorphic evolution of the blockage by 

the geologic composition of the 1980 deposits, pre-existing 

geologic features, such as Floating Island lava flow, did 

influence location of drainage development (fig. 15).

Area of 

figure 31

Area of figure 32

Spirit Lake Area of

figure 30

Johnston Ridge

N

0 1 KILOMETER

0 1 MILE

Debris avalanche

Blast

Pyroclastic flow

Lahar

Eruption deposits

EXPLANATION Figure 29. Simplified surface geology 

of the Spirit Lake blockage (adapted from 

Lipman and Mullineaux, 1981). The geology 

has been draped over a shaded-relief 

topographic model derived from 1982 aerial 

photography (see table 2). Boxes delineate 

detailed areas shown in figures 30–32.
44 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

men19-7369_fig30

Debris avalanche

Pyroclastic flow

Eruption deposits

EXPLANATION

A B

Hummock

Depression

N

0 100 METERS

0 500 FEET

Figure 30. Example of topographic influence on location of channel development by the rugged surface texture 

of the debris-avalanche deposit. Two hummocks of rugged debris-avalanche topography in 1980 (A) dictate 

location of channel incision in 1981 (B). Simplified surface geology adapted from Lipman and Mullineaux (1981). 

See figure 29 for location. Base images are digital terrain models derived from aerial photographs (see table 2).

A. 1980 B. 1981 C. 1982 D. 1984

Hummock

N

0 1,000 FEET

0 500 METERS

Figure 31. Examples of initial topographic influence on location of channel development by a hummock and depression, 

followed by lateral erosion in the debris-avalanche deposit. This image sequence, from 1980 (A) to 1984 (D), shows 

channel development was more strongly influenced by topography than by deposit geology. As the channel became more 

fully formed and a stronger topographic gradient emerged, the river easily eroded hummock material. Simplified surface 

geology adapted from Lipman and Mullineaux (1981). See figure 29 for location. Base images are digital terrain models 

derived from aerial photographs (see table 2).

Location of the drainage network is largely settled, and 

the overall rate and magnitude of landscape change since the 

mid-1980s has decreased (see also Major and others, 2018, 

2019). Hence, it is unclear whether the geology of the Spirit 

Lake blockage has played a role in more recent geomorphic 

evolution. Specifically, it is not entirely obvious whether 

exhumed contacts (for example, that between the pyroclasticflow and debris-avalanche deposits) provide a local base level 

that regulates channel incision, or whether the overall topographic regime has set the most influential base-level control. 

Observations east of the blockage crest (C. Crisafulli, U.S. 

Forest Service, written commun., 2019) indicate that small 

channels have cut through pyroclastic deposits but not deeply 

into the debris-avalanche deposit, suggesting that both geology and local topographic base level (in this case Spirit Lake) 

may be influencing geomorphic evolution of those channels. 

West of the blockage crest, however, the base-level control 

(the SRS) is well below the elevation of geologic contacts, and 

both Loowit Creek and NFTR have incised below geologic 

contacts and into debris-avalanche deposit. This indicates that 
Fluvial Geomorphic Evolution of Upper North Fork Toutle River Basin 45

men19-7369_fig32

Debris avalanche

Blast

Pyroclastic flow

Eruption deposits

EXPLANATION

Nascent Loowit Creek channel

0 1,000 FEET

0 500 METERS

Step creek channel

N

Figure 32. Example of how channel development may be influenced by location of geologic contacts between the 

blast pyroclastic density current deposit and pyroclastic-flow deposits. Nascent channels that ultimately became 

main drainages for Loowit Creek and Step creek largely followed topographic gradients within pyroclastic-flow 

deposits. Headwater extensions of other channels draining the lower north flank of the volcano, however, have 

steered around outcroppings of blast deposit and preferentially eroded pyroclastic-flow deposits. See figure 29 

for location. Simplified surface geology adapted from Lipman and Mullineaux (1981). The geology has been draped 

over a shaded-relief topographic model derived from 1982 aerial photography (see table 2).

topographic control, rather than geologic control, exerts a 

stronger influence on channel incision and evolution. Future 

work refining the influence of blockage geology on proximal 

channel evolution may benefit from more detailed consideration of correlations among individual units mapped within 

the debris-avalanche deposit (Glicken, 1996) and channel 

evolution.

The greater spatial extent of geomorphic change in the 

1980–81 DoD compared to later 1980s DoDs demonstrates 

how erosion differs between the early and later stages of 

drainage development. As the drainage network established, 

the locations of channels appear to be dictated by the overall 

topographic gradient (for example, subparallel channels on the 

Pumice Plain, which slopes to the north-northwest; see figs. 

3, 6) except where surface roughness elements are significant 

enough to steer channel incision (figs. 31, 32). But once the 

drainage network became established, most landscape change 

occurred through channel modification and sculpting rather 

than additional channel initiation or integration. The nature of 

subsequent enlargement and evolution of the channel network, 

rather than initiation of additional channels, indicates that 

geologic properties and erodibility contrasts along with individual hydrological events, which greatly influence sediment 

supply and channel equilibrium, subsequently played a more 

substantial role in later-stage channel evolution (although 

topographic influence, such as avulsion on Step-Loowit fan, 

persists).

Our analyses indicate that overall topographic gradient 

(fig. 6) and pre-existing geologic controls, such as the Floating Island lava flow, largely set the location of the drainage 

network. But we suggest that once channels were established, 

the pattern of channel evolution was influenced by an interplay between channel depth, substrate geology, and the spatial 

organization of geomorphic processes. For example, at the 

head of Step-Loowit fan where channel depths are about 1 

m and debris flows of similar depth frequently pass, multiple 

avulsions have occurred (fig. 13). In contrast, channels farther 

down the network are more deeply incised (fig. 10) and have a 

lower frequency of hydrologic events of sufficient scale to fill 

them and trigger avulsion. Nevertheless, event-driven erosion, such as that near cross section NF110 caused by the 2006 

landslide from Johnston Ridge (fig. 10F) or that along Loowit 

Creek channel driven by the November 2006 flood event, 

can significantly influence the evolution of existing channel geometry. Importantly, these forms of channel evolution 

are influenced strongly by composition of the channel banks 

within the context of overall base-level control and transport 

capacity relative to sediment supply.
46 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

Implications for Future Geomorphic 

Development in Response to 

Management Options

The approximately 40-year history of fluvial response of 

upper NFTR to volcanic events, storms, lake breakouts, and 

pumping of Spirit Lake provides a strong foundation for forecasting future geomorphic behavior of this landscape. Here 

we consider how landscape evolution of upper NFTR basin 

provides insight useful for forecasting likely trajectories of 

future change in response to various potential outlets for Spirit 

Lake. We consider two potential management scenarios: (1) 

operation of some form of closed conduit outlet (the existing 

or an upgraded outlet tunnel, or a conduit buried across the 

debris blockage); and (2) an open channel across the blockage. 

Other management options are possible. For example, a conduit draining water to the Muddy River basin would remove 

any future influence of Spirit Lake outflow on geomorphic 

evolution and sediment transport in NFTR basin, but such 

loss of water could affect fish recovery in the Toutle watershed and the increase in flow to the Muddy River basin would 

temporarily increase its sediment delivery. Our goal here is 

not to be exhaustive, but rather to consider the hydrogeomorphic implications of either an open-channel or closed-conduit 

outlet through the lens of what we know about the historical 

hydrogeomorphic behavior of the landscape. Furthermore, the 

following discussion is not tied to any specific outlet designs. 

It is possible that proper design could mitigate many of the 

issues we highlight. Our purpose here is to emphasize the 

hydrogeomorphic functioning of the Spirit Lake blockage and 

surrounding terrain in its present state to provide geologic, 

hydrologic, and geomorphic context for design and long-term 

management of an outlet for Spirit Lake. A more extensive 

assessment of risks and vulnerabilities associated with various 

possible outlet options is provided by Grant and others (2017).

Closed-Conduit Outlet

If the outlet to Spirit Lake is managed using some form 

of closed conduit, we expect that the trajectories of landscape 

change that have occurred in upper NFTR basin since 1980, as 

documented here, will continue along current trajectories. Specifically, we anticipate that the dominant erosional processes 

will continue to be persistent lateral adjustment and localized 

widening of the existing channel network, with limited vertical 

downcutting except perhaps during exceptional hydrological events. The gradual shift from dominance of vertical to 

lateral erosion that we and others have documented (Simon 

and Klimetz, 2012; Zheng and others, 2014; Major and others, 

2018; 2019) results from both the overall adjustment of channel grade (Zheng and others, 2014) to the prevailing streamflow regime and sediment supply in upper NFTR, and to the 

coarsening of the bed over time (Simon and Thorne, 1996). 

Provided streamflow and sediment supply remain relatively 

consistent with what they are today, we do not expect dramatic, widespread vertical incision, although continued local 

incision is likely (see Major and others, 2019). As documented 

in November 2006, however, episodic storm events that generate substantial floods and debris flows still retain the capacity 

to promote vertical incision of many meters (figs. 10B, C, 

24). This forecasted behavior assumes no other direct human 

modifications or infrastructure that would change the fundamental hydrogeomorphic functioning of the landscape; it also 

assumes that volcanic activity continues to be modest and in 

keeping with the types of events that have occurred since the 

end of major explosive activity in 1980—namely relatively 

benign dome growth and infrequent eruption-triggered lahars.

Future erosion that produces substantial geomorphic 

response will likely be event-based rather than resulting 

from a regime of continuous erosion over a broad range of 

streamflows like the hydrogeomorphic behavior that characterized the first decade of response following the 1980 eruption 

(Major, 2004). As discussed, streamflow and debris flows 

associated with floods having daily mean magnitude exceeding 150 m3

/s (as measured on NFTR below the SRS) are now 

typically required to perform notable geomorphic work during 

a single event. Daily mean streamflow of this magnitude has 

a roughly a 4-year recurrence interval (fig. 9); more frequent 

events of lower magnitude generally do not result in appreciable change to channel geometry. Larger hydrologic events, 

such as those that occurred in February 1996 and November 

2006, can cause large changes in channel geometry, and we 

expect such behavior to continue. But not all large hydrologic 

events result in substantive changes in all parts of the basin. 

The contrasting responses between the November 2006 and 

December 2015 events indicate that the nature of the hydrologic event—including storm trajectory, temperature history, 

precipitation distribution, and generation or absence of debris 

flows, as well as local geological factors, can be important. It 

is also possible that the dimensions of the channel system may 

have begun adjusting and stabilizing to large discharge events. 

This is not to say that slow, inexorable change does not have 

an effect. Even in the absence of large floods and debris flows, 

channel banks continue to erode, and over multi-year to multidecadal time frames the cumulative slow, persistent erosion 

has substantial geomorphic effect.

Two major uncertainties affect this forecast: future volcanic activity and climate change. Any volcanic activity that 

loads the channel system with sediment, including pyroclastic 

flows, lahars, landslides, or abundant tephra fall, would likely 

result in channel aggradation, although steep channel headwaters closest to the volcano could erode. Such events have 

occurred episodically since the 1980 eruptions (Waitt and others, 1983; Cameron and Pringle, 1990; Pringle and Cameron, 

1999; Pierson, 1999; Major and others, 2005) and may occur 

again when the volcano reawakens. The effects of relatively 

small channel-loading events, such as small debris flows from 

the crater, tend to be concentrated in the uppermost reaches 

of NFTR basin, particularly along the Step-Loowit fan and 

immediately downstream. Larger and more mobile flows (such 
Implications for Future Geomorphic Development in Response to Management Options 47

as eruption-triggered lahars or large glacier-outburst floods) 

can travel much farther, to tens of kilometers. In addition, Crater Glacier now poses a source of abundant water that could be 

released during a future eruption, especially during an explosive eruption. Should an eruption melt glacier ice and liberate 

a large volume of water, such a flood would likely erode sediment across the blockage and potentially initiate geomorphic 

instabilities along perennial and ephemeral channels, which 

could have significant impact on channel stabilities.

Climate change may alter streamflow and sediment supply in NFTR basin, and thus its hydrogeomorphic functioning. Landscape-evolution modeling that considered climate 

variations showed that substantive valley erosion of the basin 

is likely to persist for many more decades (Meadows, 2014). 

Furthermore, a key factor of the November 2006 event was 

heavy precipitation falling as rainfall at high elevations on barren volcanic slopes lacking seasonal snowpack. That lack of 

snowpack allowed heavy precipitation to fall directly on loose 

volcaniclastic sediment, permitting rapid runoff and sediment 

mobilization. Most climate predictions do not call for significantly increased precipitation in the Pacific Northwest, but 

they do call for warmer temperatures, more intense rainfalls, 

and a shift from snow to rain at high elevations (International 

Panel on Climate Change [IPCC], 2014). Therefore, conditions that characterized the November 2006 event may become 

more frequent. If so, notable dynamic adjustments to the channel system may also become more frequent.

As time passes, vegetation will continue to expand across 

the valley. But growth and expansion of vegetation coverage 

is likely to have mixed results on short-term channel evolution and stability. Many valley banks along NFTR drainage 

network are several meters to a few tens of meters tall. Consequently, valley bank heights exceed rooting depths, and thus 

vegetation established atop those high surfaces will have little 

effect on channel erosion and bank stability. Until channel 

and valley widths exceed the active channel migration zone, 

vegetation will be unlikely to anchor the bases of valley banks 

and impede erosion. Thus, until the channel network achieves 

a sufficient degree of geomorphic stability, ecological recovery along channel margins is unlikely to develop sufficiently 

to impede lateral channel erosion (see Gran and others, 2015; 

Major and others, 2019).

We anticipate future switching of the channel course 

across the Step-Loowit fan, like what has occurred multiple 

times in the past. Although the channels draining the crater 

currently flow northwesterly into NFTR, the low relief of 

the fan and high sediment loads supplied from the crater will 

likely cause future avulsions and channel changes and result in 

drainage flowing northeast toward Spirit Lake as it has in the 

past. Overall, such channel avulsions should not pose substantial risks to a closed-conduit outlet, though it could expose an 

outlet intake to a higher degree of risk when flow is directed 

toward the lake. Under these forecast conditions it is unlikely 

that the lake would be “captured” by channel switching across 

the fan and cause a breach of the blockage.

Open-Channel Outlet

An open-channel outlet across the blockage introduces 

greater uncertainty with respect to hydrogeomorphic behavior 

of the drainage system than does a closed-conduit outlet. First, 

there is currently no developed design, engineering scheme, or 

location for an open channel identified, so specific issues that 

might accompany any such plan are unknown. Second, aside 

from the Truman channel, which was active in the mid-1980s 

when pumping was used to maintain a safe lake level and 

which partly crosses the blockage, no channel has crossed the 

blockage, so the geomorphic consequences must be inferred. 

Finally, future volcanic or geomorphic events have the potential to more directly impact an open channel than a closed 

conduit, yet the type, location, and magnitude of such events 

can only be broadly generalized and consequent risks hypothesized. Such uncertainties are intrinsic to introducing a new 

outlet technology into a dynamic landscape. Grant and others 

(2017) summarize specific risks to an open-channel outlet.

Despite the greater uncertainty associated with an openchannel outlet compared to that associated with a closed 

conduit, the geomorphic history of upper NFTR basin offers 

insight into potential issues surrounding an open channel. 

Accordingly, we assume that all geomorphic issues and trajectories associated with a closed conduit, as discussed above, are 

also associated with an open channel. The channel would presumably be designed solely to provide an outlet for Spirit Lake 

and maintain safe lake levels, and no other source of water 

would feed into it other than the lake. Further, we assume that 

the channel would deliver water to upper NFTR much as the 

Truman channel did. Indeed, we anticipate that a reasonable 

alignment for an open channel would follow that of Truman 

channel for several reasons. First, such an alignment broadly 

follows the topographically lowest ground across the blockage and is the location where a natural channel would likely 

have established had it been allowed to do so. Second, that 

alignment is located as far as possible from Mount St. Helens, 

providing the least possible exposure to future volcanic events. 

Third, excavation of an open channel would require removal 

of large amounts of sediment from the blockage. If an open 

channel followed the incised Truman channel, the amount of 

sediment removal may be minimized, possibly reducing construction costs and effects. Further geophysical analyses are 

required to determine if this is the most feasible alignment for 

an open-channel outlet.

A key aspect of an open channel across the blockage 

is that it would inject a large source of flowing water into a 

highly erodible landscape. The geomorphic history of the 

blockage shows that whenever large, or even modest, volumes 

of moving water have access to the material comprising the 

blockage, very rapid erosion can occur. This was evident 

throughout the 1980s during the initial phase of channel 

adjustments, when even streamflow less than or equal to 

a 2-year recurrence-interval streamflow caused extensive 

channel erosion and sediment transport (Major, 2004). Rapid 
48 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

erosion of the Truman channel from 1982 to 1984 (fig. 11) 

exemplifies the processes and timescales involved. Moreover, 

the entire stratigraphic sequence of the blockage, and not 

just the upper pyroclastic-flow and ashcloud deposits, is 

susceptible to rapid fluvial erosion. Although the debrisavalanche deposit appears to offer greater resistance to 

erosion locally than do the overlying pyroclastic deposits, 

it is nevertheless a highly erodible deposit as is evident by 

extensive incision and widening along NFTR downstream 

from cross-section NF100 and along Truman channel (see 

figs.10–12; Major and others, 2019

Implications of our analysis are that an open channel will 

likely have to be heavily armored, and that any water flowing 

in an open channel must be fully isolated from the surrounding 

debris-blockage deposits, otherwise rapid and uncontrolled 

erosion may occur. Isolation includes hardened bed and banks, 

sufficient freeboard to prevent overtopping of channel walls, 

and measures to prevent water from discharging around 

the channel entrance rather than through it. An unarmored, 

naturalized open channel is not a viable option considering the 

steep gradient across the blockage, the mobility of blockage 

sediment, and the potential consequences of an uncontrolled 

release of water from the lake (see Scott, 1988; Grant and 

others, 2017).

Full or partial blockage of an open channel, either by volcanic events such as lahars or pyroclastic flows, or by weatherinduced geomorphic events such as debris flows or landslides, 

poses specific risks (Grant and others, 2017). A channel-filling 

blockage and consequent ponding of water could possibly lead 

to overtopping of the channel walls or an “end run” around the 

channel entrance, thereby diverting water across the blockage. 

Resultant erosion and probable geomorphic instabilities, such 

as knickpoint development and migration, could potentially 

destabilize the channel through undercutting, or bypass it completely, resulting in uncontrolled fluvial erosion.

An extreme-case scenario is that an uncontrolled release 

of water and subsequent erosion could result in knickpoints 

Table 4. Summary of scientific, engineering, and societal tradeoffs among outlet alternatives.

retrogressively migrating toward Spirit Lake and releasing 

lake water in a runaway break-out event. Although such a scenario has a low (but unquantified) probability of occurrence, 

the consequences of such an event would be catastrophic 

because of the very large volumes of water involved and 

because the timescales for intervention (assuming one could 

be mounted) would be measured in hours to days. In contrast, failure of a closed conduit, such as a tunnel or a buried 

conduit, would have a much longer timescale for intervention 

(weeks to months), because failure of the blockage following 

failure of a conduit outlet requires lake level to rise to the contact between the debris avalanche and overlying pyroclastic 

deposits (Grant and others, 2017).

Tradeoffs Among Outlet Alternatives

Scientific, engineering, and social complexities and 

values must be evaluated when considering a long-term 

management solution for the security of Spirit Lake. These 

complexities and values revolve around engineering performance, scientific understanding of hazards, timescales for 

potential intervention in the event of outlet failure, social 

values particularly as regard fish, wildlife, environmental 

and cultural significance, and public safety. Tradeoffs among 

these complexities are summarized in table 4. The ultimate 

long-term management solution is the one that best minimizes 

risk of outlet failure and breaching of the blockage within the 

context of values that society prizes most.

Our intent here is not to argue for or against any singular 

management decision, but rather to emphasize insights that 

analysis of channel evolution on and around the blockage 

reveals in terms of the intrinsic erodibility of this landscape. 

Any decision regarding long-term management of outflow 

from Spirit Lake should be made with the historical context 

of channel evolution in mind, as it is an important guide to 

potential future geomorphic behavior and functioning of this 

dynamic landscape.

Outlet performance Conduit-style outlet Open-channel outlet

Known engineering design and performance Yes No

Potential for mechanical failure Yes No

Outflow scales with inflow No Yes

Vulnerability to principal regional hazards1

 Hydrologic High Moderate

 Volcanic Low2 High

 Seismic Low2 High

 Geomorphic Very low Moderate to high

Timescales of lake-level recession post-hydrologic event1 Weeks to months Days

Timescale for intervention in event of failure1 Weeks to months Hours to days

Passes fish No Yes

1

See Grant and others (2017).

2

Vulnerability of a conduit buried across the blockage is greater than that of a tunnel bored through bedrock.
Summary and Conclusions 49

Summary and Conclusions

The cataclysmic 1980 eruption of Mount St. Helens reset 

the topography of upper North Fork Toutle River (NFTR) 

basin. Sediment from a colossal debris avalanche, a laterally 

directed (“blast”) pyroclastic density current, and later pyroclastic flows thickly filled basin headwaters, raised the bed 

and surface of Spirit Lake and transformed its basin, blocked 

its outlet, and severed hydrologic connection between the 

upper and lower parts of the basin. Reconnection required 

establishment of a new drainage network. To prevent catastrophic breaching of Spirit Lake, a tunnel outlet was bored 

through bedrock, and water from Spirit Lake now bypasses 

basin headwaters and enters NFTR downstream from Coldwater Lake. In this study, we documented initiation and 

nearly 40 years of geomorphic evolution of the fluvial channel network on and around the Spirit Lake blockage without 

overland flow from Spirit Lake. Our analyses show the posteruption drainage network bears a strong resemblance to the 

pre-eruption drainage network, and thus we infer that gross 

topography exerts a first-order control on channel location 

within the network. In addition, blockage geology—to first 

approximation the mapped surface distributions of debrisavalanche, blast, and pyroclastic-flow deposits—has also 

influenced channel location. In particular, the location of 

Loowit Creek channel, which drains the Mount St. Helens 

crater and traverses the Pumice Plain west of the blockage 

crest, may have been influenced by contrasts in erosional 

resistance between the blast and pyroclastic-flow deposits or 

contrasts within the pyroclastic-flow deposits themselves. The 

mounded topography of the debris-avalanche deposit (hummocks) and possible erosional heterogeneity of its composition also guided channel location locally.

The basic architecture of the developing fluvial network 

established within a few years after the cataclysmic eruption. Initial channel development occurred during a time 

of enhanced runoff owing to transient hydrologic changes 

to the landscape. During this time, streamflow peaks (both 

primary and secondary) were a few percent to many tens 

percent greater for a given precipitation input than they 

were before the eruption, and a range of streamflows, from 

small-magnitude (less than 2-year return-interval) to largemagnitude (greater than 10-year return-interval) discharges, 

played fundamental roles eroding and transporting sediment and establishing the fluvial network. Some of the most 

dramatic channel evolution was caused by modest streamflow 

(5.1 m3/s) pumped from Spirit Lake while the present tunnel 

outlet was designed and constructed.

Within several years of the 1980 eruption, the established fluvial network began equilibrating to average hydrologic conditions and the consequent upstream sediment 

delivery. As a result, geomorphic evolution of the drainage 

network switched gradually from a regime of modification 

by a broad range of streamflow to one that is more eventdriven. Since the mid-1980s, moderate- to large-magnitude 

streamflows, particularly those generating daily mean streamflows greater than about 150 m3

/s—as measured on NFTR 

below the sediment retention structure (USGS streamgage 

14240525)—are needed to do much geomorphic work beyond 

modest erosion of channel margins. Though relatively smallmagnitude streamflows (those having daily mean discharges 

less than 150 m3/s) were important during the initial phase 

of channel development, by the mid-1980s they had largely 

become agents of channel refinement rather than agents of 

major channel change. Nevertheless, inexorable erosion of 

channel banks, and not just major geomorphic changes driven 

by large-magnitude floods and debris flows, continues to 

maintain sediment delivery from this basin at levels that are 

elevated compared to pre-eruption conditions.

Documentation of acute erosion, channel avulsion, and 

persistent channel refinement on and around the Spirit Lake 

blockage illustrates the sensitive and dynamic nature of upper 

NFTR fluvial system. It is clear from analyses presented 

here that the blockage sediments are highly susceptible to 

erosion, especially when water is introduced to parts of the 

landscape lacking well-established channels or when channels 

are subject to streamflow to which they are not well-adjusted. 

Considerable sediment mobility, now driven largely by floods 

and debris flows generated by large storms, may establish 

geomorphic instabilities such as knickpoints or knickzones 

that can migrate headward rapidly and potentially trigger 

additional instabilities in smaller channels tributary to main 

trunk channels.

The erosional susceptibility documented here has significant implications—and cautionary ramifications—for an 

open-channel outlet for Spirit Lake. The most logical placement of an open-channel outlet is along or near the present 

alignment of Truman channel. But that channel is adjusted 

to a mean discharge of 5.1 m3/s. Streamflow of substantially 

greater magnitude, almost certain through an open channel 

because discharge would be unregulated and vary with lake 

level, is very likely to induce additional channel incision and 

widening unless the channel is heavily fortified, especially 

given the overall 3-percent gradient (125-m elevation drop 

over 4 km) from Spirit Lake to the Truman channel-NFTR 

confluence. Furthermore, if water somehow escaped an open 

channel and breached the blockage drainage divide, it would 

flow largely over landscape lacking channels adjusted to such 

flow. Because the un-channelized landscape is very susceptible to erosion, an intense overland flow could generate 

geomorphic instabilities with possibly catastrophic consequences. These risks could be reduced by careful design and 

engineering that accounts for the hydrogeomorphic behavior 

of this landscape.
50 A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington

Acknowledgments

This work was conducted in cooperation with the U.S 

Forest Service Gifford Pinchot National Forest. We thank Liz 

Safran, Matt Collins, Fred Swanson, Charlie Crisafulli, Rene 

Renteria, Jonathan Berry, Claire Landowski, and Regan Austin 

for comments and discussions that sharpened our thinking 

and presentation. A U.S Geological Survey Mendenhall 

Postdoctoral fellowship provided partial funding for Kristin 

Sweeney.

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Menlo Park Publishing Service Center, California 

Manuscript approved for publication March 13, 2020

Edited by Claire Landowski and Regan Austin

Layout and design by Kimber Petersen 

Illustration support by JoJo Mangano
Major and others—A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington—Scientific Investigations Report 2020–5027, 

April 2020

ISSN 2328-0328 (online)