U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2015–5173

Prepared in cooperation with the Federal Emergency Management Agency and

Snohomish County Department of Public Works

Preliminary Assessment of Aggradation Potential in the 

North Fork Stillaguamish River Downstream of the 

State Route 530 Landslide near Oso, Washington
Cover: Photograph showing downstream view of the North Fork Stillaguamish River 

where it incised back into the SR 530 Landslide deposit near Oso, Washington. 

(Photograph by Christopher S. Magirl, U.S. Geological Survey, May 15, 2014)
Preliminary Assessment of Aggradation 

Potential in the North Fork Stillaguamish 

River Downstream of the State Route 530 

Landslide near Oso, Washington

By Christopher S. Magirl, Mackenzie K. Keith, Scott W. Anderson, 

Jim E. O’Connor, Robert Aldrich, and Mark C. Mastin

Prepared in cooperation with the Federal Emergency Management Agency and 

Snohomish County Department of Public Works

Scientific Investigations Report 2015–5173

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Magirl, C.S., Keith, M.K., Anderson, S.W., O’Connor, J.E., Aldrich, Robert, and Mastin, M.C., 2015, Preliminary 

assessment of aggradation potential in the North Fork Stillaguamish River downstream of the State Route 530 

landslide near Oso, Washington: U.S. Geological Survey Scientific Investigations Report 2015–5173, 20 p., 

http://dx.doi.org/10.3133/sir20155173.

ISSN 2328-0328 (online)
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................4

Description of Study Area ...................................................................................................................4

Methods to Determine Aggradation Potential..........................................................................................5

Calculating Erosion Volumes ..............................................................................................................5

Calculating Unit Stream Power..........................................................................................................7

Calculating Bed-Material Transport Capacity ................................................................................7

Assessing Aggradation Potential in the 

Context of Sediment Waves ..................................................................................................9

Sediment Erosion Volumes...........................................................................................................................9

Downstream Aggradation Potential .........................................................................................................12

Energy Slope and Unit Stream Power.............................................................................................12

Bed-Material Transport Capacity.....................................................................................................12

Zones of Potential Aggradation........................................................................................................12

Additional Flood Risk Issues ......................................................................................................................17

Conclusions...................................................................................................................................................17

Acknowledgments.......................................................................................................................................18

References Cited..........................................................................................................................................18
iv

Tables

1. Discharge or stage-only streamgages installed by the U.S. Geological Survey

along the North Fork Stillaguamish River, Washington .........................................................5

2. Lidar and aerial imagery used in the assessment of aggradation potential in the

North Fork Stillaguamish River downstream of the State Route 530 Landslide near

Oso, Washington ..........................................................................................................................6

3. Summary of grain-size distributions used to simulate bed-material transport along

the North Fork Stillaguamish River, Washington.....................................................................8

Figures

1. Map showing Stillaguamish River Basin, the North Fork Stillaguamish River, 

U.S. Geological Survey streamgages, and the location of the State Route 530 

Landslide near Oso, Washington, March 22, 2014 ..................................................................2

2. Map showing lidar-derived topography showing extent of the State Route 530

Landslide, the maximum extent of impoundment lake, and U.S. Geological Survey

rapid-deployment streamgages in operation near the landslide .........................................3

3. Graphs showing discharge values plotted at 15-minute intervals for the North Fork

Stillaguamish River near Arlington and impoundment lake elevation, March 1 to

July 1, 2014, near Oso, Washington.........................................................................................10

4. Graphs showing landslide-deposition and water-surface elevation and sediment

volume eroded from landslide deposition relative to distance downstream of

C-Post Bridge, near Oso, Washington.....................................................................................11

5. Graphs showing the pre-slide active channel width, post-slide energy grade slope

and median grain size, pre- and post-slide unit stream power, and bed-material

transport capacity for the 0.5 annual exceedance probability peak-flow event for

the North Fork Stillaguamish River, Washington...................................................................13

6. Map showing North Fork Stillaguamish River showing unit stream power by

500-meter reach intervals downstream of the State Route 530 Landslide and graph

showing bed-material transport capacity from RKM 16 to RKM 33 ..................................14

7. Diagram showing conceptual model of typical sediment-wave dynamics in

mountainous rivers.....................................................................................................................15

8. Graphs showing shifts in stage-discharge rating at the U.S. Geological Survey

streamgage North Fork Stillaguamish River near Oso, Washington, 

March 28–July 1, 2014................................................................................................................16
v

Conversion Factors 

SI to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

meter (m) 1.094 yard (yd) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2

) 0.0002471 acre 

square meter (m2

) 10.76 square foot (ft2) 

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

cubic meter (m3

) 35.31 cubic foot (ft3)

cubic meter (m3

) 1.308 cubic yard (yd3) 

cubic meter (m3

) 0.0008107 acre-foot (acre-ft) 

Flow rate

cubic meter per second (m3

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

Mass

kilogram (kg) 2.205 pound avoirdupois (lb)

Density

kilogram per cubic meter (kg/m3

) 0.06242 pound per cubic foot (lb/ft3) 

Power

watts per square meter (W/m3

) 0.005290 BTU per square foot per minute 

[(BTU/ft2

)/min] 

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

Volume

cubic foot (ft3

) 0.02832 cubic meter (m3) 

Flow rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

Datums

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

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.

Preliminary Assessment of Aggradation Potential in the 

North Fork Stillaguamish River Downstream of the 

State Route 530 Landslide near Oso, Washington

By Christopher S. Magirl1, Mackenzie K. Keith1, Scott W. Anderson1, Jim E. O’Connor1, Robert Aldrich2, and 

Mark C. Mastin1

Abstract

On March 22, 2014, the State Route 530 Landslide 

near Oso, Washington, traveled almost 2 kilometers (km), 

destroyed more than 40 structures, and impounded the North 

Fork Stillaguamish River to a depth of 8 meters (m) and 

volume of 3.3×106 cubic meters (m3). The landslide killed 

43 people. After overtopping and establishing a new channel 

through the landslide, the river incised into the landslide 

deposit over the course of 10 weeks draining the impoundment 

lake and mobilizing an estimated 280,000±56,000 m3 of 

predominantly sand-sized and finer sediment. During the 

first 4 weeks after the landslide, this eroded sediment caused 

downstream riverbed aggradation of 1–2 m within 1 km of 

the landslide and 0.4 m aggradation at Whitman Road Bridge, 

3.5 km downstream. Winter high flows in 2014–15 were 

anticipated to mobilize an additional 220,000±44,000 m3

of sediment, potentially causing additional aggradation and 

exacerbating flood risk downstream of the landslide. Analysis 

of unit stream power and bed-material transport capacity along 

35 km of the river corridor indicated that most fine-grained 

sediment will transport out of the North Fork Stillaguamish 

River, although some localized additional aggradation was 

possible. This new aggradation was not likely to exceed 0.1 m 

except in reaches within a few kilometers downstream of 

the landslide, where additional aggradation of up to 0.5 m 

is possible. Alternative river response scenarios, including 

continued mass wasting from the landslide scarp, major 

channel migration or avulsion, or the formation of large 

downstream wood jams, although unlikely, could result in 

reaches of significant local aggradation or channel change. 

Introduction

On March 22, 2014, an 8×106 cubic meter (m3) landslide 

(Iverson and others, 2015) originating from a 190 m bluff 

of unconsolidated glacial till moved almost 2 km across the 

flood plain of the North Fork Stillaguamish River 6 km east 

of Oso, Washington (fig. 1). The destructive landslide traveled 

from north to south across the river and through a rural 

neighborhood, destroyed more than 40 structures, traversed 

a 1.4-km section of Washington State Route 530 (SR 530), 

and killed 43 people. The landslide covered about 1 square 

kilometer (km2) of the valley bottom with deposits as thick as 

20 m. The SR 530 Landslide (hereinafter landslide) dammed 

the North Fork Stillaguamish River by 8 m, forming an 

impoundment lake about 3 km long, 1.5 km2 in area, 

and 3.3×106 m3 in volume (fig. 2). After 25 hours, the 

impoundment lake overtopped the landslide deposit in a 

topographically low trough establishing a new river-channel 

location. Because of the relatively shallow slope of the 

new river channel through the landslide deposit, the rate of 

erosion was moderate without the rapid dam breaching and 

outburst flood sometimes associated with landslide-dammed 

impoundments (Costa and Schuster, 1988; Korup, 2002; 

O’Connor and Beebee, 2009). 

From March 23 through May 2014, the new river channel 

slowly incised into the landslide deposit. To aid the searchand-recovery effort, a pilot channel through the eastern section 

of the landslide was excavated in early April by Snohomish 

County, its consultants, and cooperating agencies. This 

channel accelerated river incision and expedited the drainage 

of the impoundment lake. By June 1, 2014, there was no 

longer a pool within the impoundment for river flows less than 

about 57 m3

/s (2,000 ft3/s). Some degree of impoundment, 

however, was expected to reappear during high winter 

flows as the relatively narrow channel through the landslide 

deposit exerted hydraulic control on the upstream river reach. 

1

U.S. Geological Survey.

2

Snohomish County Public Works.
2 Preliminary Assessment of Aggradation Potential, North Fork Stillaguamish River, State Route 530 Landslide, Oso, Washington watac15-1025_fig01 Lake Cavanaugh Lake Stevens Big Lake

S ak gi Rt vi er

Deer Creek

Sauk River

Arrow Creek

Sauk River

Suiattle River

i P hcl evi R ku r

Squ ri e r C kee

i P cl huck Cr

ts mr A Cgnor r

Stillaguamish River

N

Fk Stillaguamish River

S Fk Stillaguamish River

Boulder River

S Fk Sultan River

Oso

Cicero Darrington

GraniteFalls

Arlington

5

Stanwood

5

530

530

9

9

0

50

40

30 20 10

Skagit

Bay

Port

Susan

Bay

PUGET

SOUND

12166240 12166400

12166495

12166300 12166185

12167000 12166150

WASHINGTON

Maparea

EXPLANATION

Total landslide extentStillaguamish River River kilometer

Basin boundary

USGS streamgage 12166495

20

121°30' 121°40' 121°50' 122° 122°10' 122°20'

48°20'48°10'48° 0 2 4 6 8

0 2 4 6 MILES

10 12 KILOMETERS

Base map modified from U.S. Geological Survey digital data (10-meter resolution) and other digital data, various scales. Coordinate reference system: UTM Zone 10N, North American Datum of 1983

Figure 1. Stillaguamish River Basin, the North Fork Stillaguamish River, U.S. Geological Survey streamgages, and the location of the State Route 530 Landslide near Oso, Washington, March 22, 2014.
Introduction 3

watac15-1025_fig 02

530 WHITMAN ROAD

530

C-POST ROAD

12166300

12166240

12166185

12166150

12166190

12166200

12166220

EXPLANATION

Approximate impoundment lake extent

Slide extent

Overlap of extents

12166190 USGS streamgage 

0 0.5 1 MILES

0 0.5 1 1.5 2 KILOMETERS

Base map modified Puget Sound Lidar Consortium digital data (1-meter resolution), U.S. 

Geological Survey digital data (10-meter resolution), and other digital data, various scales. 

Coordinate reference system: UTM Zone 10N, North American Datum of 1983

121°53' 121°52' 121°51' 121°50' 121°49'

45°

17'

45°

16'

Figure 2. Lidar-derived topography showing extent of the State Route 530 Landslide, the maximum extent of impoundment lake, and 

U.S. Geological Survey (USGS) rapid-deployment streamgages in operation near the landslide.

In summer of 2014, we anticipated that larger flows during 

the 2014–15 winter flood season would progressively widen 

and incise the river channel through the landslide deposit 

toward a future equilibrium condition with a quasi-stable 

hydraulic geometry. 

The evolution of localized accumulations of alluvium 

deposited from geomorphic disturbances, also known as 

sediment waves, is well studied in the geomorphology 

literature (Gilbert, 1917; Nicholas and others, 1995; Lisle 

and others, 2001; James, 2006; Lisle, 2008). Fine-grained 

sediment eroded by the river from the landslide traveled 

as far as Port Susan Bay in Puget Sound in the days and 

weeks following the event, but some portion of the bedload, 

composed of sands and gravels, accumulated in the river 

reach downstream of the landslide. Localized aggradation, 

followed by incision, was expected to continue in the 

2014–15 flood season. Flume studies, mechanistic theory, 

and landslide case studies (Lisle, 2008) indicate that sediment 

waves composed of coarser grained material (that is, sands, 

gravels, and cobbles) in mountainous rivers with Froude 

numbers greater than 0.4 evolve primarily though dispersion; 

whereby sediment from the newly added deposit transports 

downstream and causes aggradation, and background bedload 

in the river accumulates just upstream of the new sediment 

deposit because of localized decrease in transport capacity. 

If the landslide sediment wave is dispersion dominant, river 

reaches within a few kilometers downstream and upstream of 

the landslide will be most strongly affected by aggradation 

and effects farther downstream will be negligibly small. In 

contrast, where the sediment deposit is dominated by sands 

and finer particles and Froude number is less than 0.4, the 

sediment wave evolves with both dispersion and translation. 

If sediment-wave dispersion and translation occurs, reaches 

far from the landslide may accumulate detectable quantities of 

new sediment.

To assess the potential for anticipated aggradation in the 

North Fork Stillaguamish River during the 2014–15 flood 

season, the Federal Emergency Management Agency and 

Snohomish County Department of Public Works requested the 

U.S. Geological Survey (USGS) to investigate the potential 

of aggradation downstream of the landslide, thus identifying 

potential increases in flood risk as the sediment wave evolves. 

The assessment in this report was completed largely using data 

and analysis completed in summer of 2014.
4 Preliminary Assessment of Aggradation Potential, North Fork Stillaguamish River, State Route 530 Landslide, Oso, Washington

Purpose and Scope

This report represents a preliminary assessment of 

aggradation potential in the river reach downstream of the 

landslide using datasets and models available during summer of 

2014, before the 2014–15 flood season. This report is neither a 

comprehensive analysis of sediment transport in the North Fork 

Stillaguamish River nor is it intended to precisely estimate the 

depth of aggradation. Instead, this report identifies river reaches 

prone to aggradation and offers preliminary analysis to estimate 

a range of expected aggradation depths. With more data and 

analysis, the models and approaches discussed herein may be 

updated, improved, and finalized in future reports.

Description of Study Area

The North Fork Stillaguamish River drains 736 km2

of the Cascade Range of western Washington and joins the 

South Fork Stillaguamish River near Arlington becoming the 

Stillaguamish River, which flows into Port Susan Bay in Puget 

Sound near Stanwood (Collings and Hill, 1973; Embrey, 1987; 

Beechie and others, 2001) (fig. 1). Compared to other rivers in 

the greater Puget Sound watershed (Konrad, 2015), the North 

Fork Stillaguamish River drains a lower elevation catchment 

with maximum basin elevation of 2,078 m (6,820 ft) and a 

mean basin elevation of 625 m (2,050 ft) (U.S. Geological 

Survey, 2014). 

The North Fork Stillaguamish River flows down an 

alluvial-controlled corridor comprised of late Pleistocene and 

Holocene sediment with few bedrock outcrops (Dragovich 

and others, 2003). Active geologic processes, including 

landslides from adjacent bluffs (Haugerud, 2014), lahars, 

and eruptive deposits from Glacier Peak (Beechie and others, 

2001; Dragovich and others, 2003) and major drainage 

realignments (Beechie and others, 2001), presumably increase 

the lateral instability of the river channel through geologic 

time frames. This lateral instability may have been reduced 

from anthropogenic revetments and development; the river 

channel is now predominantly single threaded. Ubiquitous 

point, lateral, and mid-channel bars act as sources and sinks of 

likely active bedload of gravels and cobbles transported during 

peak-flow events. Boulders deposited in many bars throughout 

the river corridor indicate that, for large floods, the river has 

the competence to transport sizeable clasts. Sand is prevalent 

throughout the bar substrate indicating a relatively ample 

supply of background sediment. At modest flows, non-cohesive 

sand-sized sediment is likely the dominant sediment class in 

bedload transport.

Flooding on the river is caused by heavy winter rainfall 

associated with atmospheric rivers (Neiman and others, 

2011), and Henn and others (2015) determined that the 

21–45-day period preceding the landslide had anomalously 

high precipitation with return periods that ranged from 2 to 

88 years, depending on the analytical approach. Streamflow 

gaging on the North Fork Stillaguamish River near Arlington 

(USGS streamgage 12167000) began in 1928. Sumioka 

and others (1998) reported 0.5 (2-year), 0.04 (25-year), and 

0.01 (100-year) exceedance probabilities of annual peak 

flows (annual exceedance probability; AEP) during water 

years 1929–96 to be 646 m3/s (22,800 ft3/s), 1,020 m3/s 

(35,900 ft3/s), and 1,140 m3/s (40,300 ft3/s), respectively. 

However, the five largest peak-flow events during the period 

of record occurred after 2002. Using all 85 systematic annual 

peak flows from 1929 to 2013, a Log Pearson Type III analysis 

(U.S. Interagency Advisory Committee on Water Data, 1982) 

that used a multiple Grubbs-Beck low outlier test, a weighted 

station and regional skew value, and a final estimate weighted 

with the regional regression estimate (Sumioka and others, 

1998) yields updated estimates of the 0.5 , 0.04, and 0.01 AEP 

events to be 678 m3/s (24,000 ft3/s), 1,160 m3/s (41,100 ft3/s), 

and 1,350 m3/s (47,700 ft3/s), respectively.

A river centerline convention was established based 

on the 2013 channel location: river kilometer (RKM) 0.0 

was positioned at the confluence of the North and South 

Forks Stillaguamish River near Arlington. The landslide 

affected a section of the river from RKMs 34.1 to 32.7. 

Just after the landslide, the USGS installed streamgages at 

Whitman Road Bridge at RKM 29.2 (12166300, North Fork 

Stillaguamish River near Oso) and upstream of the landslide 

at RKM 36.2 (12166150, North Fork Stillaguamish River 

near Swede Heaven) (table 1; fig. 2). A stage-only gage was 

installed at RKM 34.8 on C-Post Bridge (12166185, North 

Fork Stillaguamish River at C-Post Bridge near Oso). From 

March 28 to May 31, three floating buoys provided stage‐only 

data from the impoundment lake (12166190, North Fork 

Stillaguamish River Northeast Pooled Slide Area near Oso; 

12166200, North Fork Stillaguamish River East Pooled Slide 

Area near Oso; 12166220, North Fork Stillaguamish River 

Southwest Pooled Slide Area near Oso). Another stage-only 

streamgage was later installed at RKM 32.3 just downstream 

of the landslide (12166240, North Fork Stillaguamish River 

at Rowan). The main stem streamgage (12167000, North 

Fork Stillaguamish River near Arlington) is located at 

RKM 11.1. Deer Creek, the largest tributary to the North Fork 

Stillaguamish River, enters at Oso, at RKM 23.9.

As of summer 2014, the river flowed through a 1-km 

long section of landslide deposit composed of 10–20-m 

tall hummocky terrain sourced from the bluff bounding the 

northern extent of the flood plain and consisting of re-deposited 

Pleistocene glacial till, glacial outwash, and lacustrine units 

(Dragovich and others, 2003). Of seven auger samples of the 

landslide deposit, four samples were finer than 2 mm and three 

samples were predominantly fine grained with all samples 

containing at least 85 percent finer than 2 mm (Keaton and 

others, 2014). We presumed this sand-sized sediment of the 

landslide deposit was the dominant size class in bedload 

transport during high flows as the incision channel deepened 

and widened. The river also flows past a larger landslide 

deposit 2 km downstream of the SR 530 Landslide with similar 

hummocky terrain informally known as the Rowan landslide 

(Dragovich and others, 2003; Iverson and others, 2015). The 

age of the Rowan landslide is unknown, but it likely predates 

European settlement of the valley. 
Methods to Determine Aggradation Potential 5

Table 1. Discharge or stage-only streamgages installed by the U.S. Geological Survey along the 

North Fork Stillaguamish River, Washington. 

[Abbreviation: km2, square kilometer]

Streamgage 

No. Streamgage name River kilometer

Drainage 

area

(km2

)

Provisional

data used

12166150 North Fork Stillaguamish River near 

Swede Heaven

36.2 344 Stage, discharge

12166185 North Fork Stillaguamish River at 

C-Post Bridge near Oso

34.8 373 Stage only

12166190 North Fork Stillaguamish River 

Northeast Pooled Slide Area 

near Oso

34.4 394 Stage only

12166200 North Fork Stillaguamish River East 

Pooled Slide Area near Oso

34.4 394 Stage only

12166220 North Fork Stillaguamish River 

Southwest Pooled Slide Area 

near Oso

34.4 394 Stage only

12166240 North Fork Stillaguamish River 

at Rowan

32.3 398 Stage only

12166300 North Fork Stillaguamish River 

near Oso

29.2 416 Stage, discharge

12167000 North Fork Stillaguamish River 

near Arlington

11.1 679 Stage, discharge

Methods to Determine Aggradation 

Potential

The general methodological framework of this study was 

built around four primary tasks: (1) calculating the volume 

of sediment eroded from the landslide through the summer 

of 2014 and anticipated erosion volumes during the 2014–15 

flood season, (2) analyzing longitudinal diversion in unit 

stream power downstream of the landslide to indicate reaches 

with a propensity to aggrade or incise, (3) analyzing bedmaterial-transport capacity of the river during high flows to 

identify downstream reaches prone to aggradation, and 

(4) assessing downstream aggradation potential in the context 

of sediment waves.

High-resolution digital elevation models (DEMs), aerial 

imagery, and one-dimensional hydraulic modeling (Hydrologic 

Engineering Center River Analysis System; HEC-RAS) 

were used with stream-power and transport-capacity theory 

to identify reaches of the North Fork Stillaguamish River 

downstream of the SR 530 Landslide prone to aggradation in 

the 2014–15 flood season. Increased discharge with winter 

rains was expected to widen and incise the river channel 

into the landslide deposit mobilizing sediment. Some of this 

sediment was likely retained in the downstream channel, 

causing some degree of aggradation.

Hydrologic conditions were analyzed using provisional 

discharge and stage data from the USGS streamgages. 

The impoundment water volume was calculated from the 

deficit discharge measured at the downstream streamgage 

and confirmed with hypsometric analysis of DEMs. Aerial 

imagery and DEMs also were used to determine extent of the 

impoundment. The evolution of the water-surface elevation 

of the impoundment lake was determined after March 28 

using the four USGS stage-only streamgages installed in 

the impoundment lake and stage data from the streamgages 

to de-trend fluctuations of the hydrograph. Manual survey 

measurements of the impoundment-lake elevation were made 

by Snohomish County, March 24–27 (D. Lucas, Snohomish 

County, written commun., 2014). 

Calculating Erosion Volumes

The volume of sediment eroded from the landslide by the 

river was calculated using DEM data, bathymetric data, and 

aerial imagery for two periods: March 24–April 6 and 

April 6–July 1, 2014 (table 2). The Puget Sound Lidar 

Consortium acquired high-resolution light and detection 

ranging (lidar) data of the North Fork Stillaguamish River 

valley from April to July 2013. After the landslide, the 

Washington State Department of Transportation (WSDOT) 

commissioned and processed two lidar overflights of the 
6 Preliminary Assessment of Aggradation Potential, North Fork Stillaguamish River, State Route 530 Landslide, Oso, Washington

landslide area on March 24 and April 6, 2014. The lidar 

data from March 24 did not penetrate the surface of the 

impoundment lake but provided elevation data of the deposit 

above the level of the impoundment and the river watersurface elevation downstream of the landslide. Bathymetric 

data were collected in the impoundment lake in early April 

by Snohomish County and its consultant, David Evans and 

Associates, Inc. WSDOT and David Evans and Associates, 

Inc. (J. Dasler, David Evans and Associates, Inc., written 

commun., 2014), combined the April 6 lidar data with the 

surveyed bathymetry and available 2013 lidar data to construct 

a comprehensive DEM of the landslide deposit as of April 

6. WSDOT also acquired high-resolution aerial imagery 

on July 1, 2014, showing the progress of river incision and 

widening. Aerial imagery from April 1, April 14, and May 1, 

2014, also was used for geomorphic interpretation of the river 

corridor near the landslide (table 2). 

Sediment volume eroded from the landslide deposit 

between March 24 and April 6 was determined by comparing 

DEM-generated cross sections for those dates spaced at 

50-m segments along the July 1 river centerline. For the 

March 24 DEM, the westernmost cross sections were 

unaffected by the impoundment, allowing direct comparison 

with the April 6 DEM to calculate volume change. The 

easternmost March 24 cross sections were in an area free of 

river incision before April 6. For cross sections in the middle 

of the deposit, where incision occurred before April 6 and 

where the presence of the impoundment lake precluded precise 

determination of the submerged topography, the terrestrial 

surface was estimated using exposed hummocks adjacent 

to the future river channel and intermediate aerial imagery 

collected by WSDOT on April 1. 

To calculate sediment volume eroded from April 6 to 

July 1, the same cross sections were analyzed, assuming the 

river incised a prismatic trapezoidal shape from the landslide 

deposit with linear side slopes and wetted bottom width and 

exposed top widths determined with the high-resolution July 1 

imagery. The water-surface elevation of the bottom of the 

trapezoid was determined using survey data of river elevation 

collected by Snohomish County on August 6 (K. Hanson, 

Snohomish County, written commun., 2014). Turbidity data 

collected downstream indicated negligible incision occurred 

between July 1 and August 6. Based on a reasonable range of 

potential future side slopes, an anticipated degree of channel 

migration and adjustment, and geomorphic judgment, we 

estimated the uncertainty of the sediment volume change 

calculations reported here to be about 20 percent.

To assess the potential downstream aggradation in the 

2014–15 flood season, we estimated the sediment volume 

likely to be eroded from the landslide under a future 

equilibrium condition. This estimate entailed assuming (1) the 

river channel will remain in its summer 2014 alignment, 

(2) the width of the river will approach the mean width of the 

river where it flows adjacent to the Rowan landslide 2-km 

downstream, (3) the side slopes will retreat to 34 degrees, an 

angle of repose typical of loose, unconsolidated colluvium 

(Allen, 2001), and (4) the water-surface elevation of the river 

will approach the pre-slide river profile. This postulated future 

condition was compared to the river morphology on July 1 to 

estimate the volume of sediment expected to mobilize in the 

2014–15 flood season.

Table 2. Lidar and aerial imagery used in the assessment of aggradation potential in the North 

Fork Stillaguamish River downstream of the State Route 530 Landslide near Oso, Washington. 

[Abbreviations: Lidar, light detection and ranging; PSLC, Puget Sound Lidar Consortium; WSDOT, Washington 

State Department of Transportation]

Date Data format Source Horizontal resolution

(meters)

04-17-13 to 07-31-13 Lidar PSLC 1 × 1

03-24-14 Lidar WSDOT 1 × 1

03-24-14 Aerial imagery WSDOT 0.3 × 0.3

04-01-14 Aerial imagery WSDOT 0.08 × 0.08 

04-06-14 Lidar WSDOT 1 × 1

04-14-14 Aerial imagery WSDOT 0.08 × 0.08

05-01-14 Aerial imagery WSDOT 0.08 × 0.08

07-01-14 Aerial imagery WSDOT 0.08 × 0.08
Methods to Determine Aggradation Potential 7

Calculating Unit Stream Power

Unit stream power in watts per square meter, ω, was 

calculated along the North Fork Stillaguamish River using the 

equation (Bagnold, 1966): 

ω ρ = gQS

w , (1)

where

ρ is the density of water, assumed to be 

1,000 kg/m3;

g is the gravity constant, taken to be 9.81 m/s2 ;

Q is the discharge, taken here to be the discharge 

of the 0.5 AEP peak-flow event; 

S is the energy slope; and

w is the active, unvegetated channel width.

The 0.5 AEP peak flow was calculated at the USGS 

streamgage near Arlington (12167000). This 0.5 AEP peak 

flow was hydrologically scaled to the full river corridor 

(accounting for changes in flow resulting from tributaries 

entering along the 55-km long study reach) using the relative 

variability of mean annual discharge from the National 

Hydrologic Dataset (NHD) linked to the NHDPlus dataset, 

a hydrologically conditioned DEM with 30-m resolution 

(U.S. Environmental Protection Agency, 2008). Energy 

slope of the river long profile for the 0.5 AEP discharge 

was determined using Hydrologic Engineering Center’s 

River Analysis System (HEC-RAS) Version 4.1 (U.S. Army 

Corps of Engineers, 2010a, 2010b, 2010c) one-dimensional, 

steady-state, step-backwater model constructed for 56 km of 

the North Fork Stillaguamish River by the Federal Emergency 

Management Agency and the U.S. Army Corps of Engineers 

(T. Perkins, Federal Emergency Management Agency, and 

T. Ball, U.S. Army Corps of Engineers, written commun., 

2014). The HEC-RAS model constructed for the analysis 

assumed an idealized trapezoidal shape and standard hydraulic 

depth (updated bathymetric data were unavailable), contained 

cursory bridge geometry data, was neither calibrated nor 

validated, and was never intended to provide precise estimates 

of water-surface elevation. Instead, the model was used as an 

analytical tool to estimate relative values of energy grade and 

hydraulic influence in the river corridor in order to estimate 

stream power and transport capacity at a coarse resolution. 

Using the energy elevation predicted at HEC-RAS cross 

sections, energy slope was calculated using a weighted mean 

at 500-m river segments. The average active-channel width 

for each 500-m segment was determined by digitizing the 

unvegetated portion of the river channel (Wallick and others, 

2011) using imagery from the 2011 National Agriculture 

Imagery Program (U.S. Department of Agriculture, 2011). 

Calculating Bed-Material Transport Capacity 

Bed-material transport capacity was calculated with 

the aid of the U.S. Forest Service Bedload Assessment of 

Gravelbed Rivers (BAGS) macro-enabled spreadsheet (Pitlick 

and others, 2009; Wilcock and others, 2009) in a manner 

similar to that of Wallick and others (2010, 2011). BAGS 

required several inputs for this preliminary analysis, including 

bed-material size distribution, cross-section geometry, 

discharge, and energy slope (from hydraulic modeling). 

Field measurements of bed-material size and cross-section 

geometry, discharge, and energy slope from the HEC-RAS 

model were applied to the bed-material transport equations of 

Parker (1990a, 1990b) and Wilcock and Crowe (2003) using 

BAGS software. These two sediment-transport equations 

are good for gravel-bedded rivers with ample sand supply as 

observed in the North Fork Stillaguamish River and have been 

successfully used in gravel-bedded rivers in Oregon (Wallick 

and others, 2010, 2011). For this study, transport capacity 

was assumed to be a suitable proxy for actual bed-material 

transport because the assumption of unlimited sediment supply 

(Pitlick and others, 2009) was likely valid in the 2014–15 

flood season.

For the BAGS calculations, we primarily used 

grain‐size information collected at five locations along the 

North Fork Stillaguamish River in 2014 (table 3) using a 

modified Wolman (1954) pebble count method. Additional 

grain-size data collected between 2005 and 2009 (J. Brown, 

Stillaguamish Tribe of Indians, written commun., 2014) were 

used, where needed, to supplement data collected in July 2014. 

Another set of pebble-count data were collected from the 

landslide deposit in the hummocky terrain adjacent the river 

to determine the size of sediment prone to mobilization in 

the 2014–15 flood season. Although we observed occasional 

cobble- and boulder-sized particles spaced throughout the 

landslide deposit, the material adjacent the river channel 

subject to future erosion was predominantly sand sized and 

smaller. Three Wolman pebble counts performed on the 

hummocky terrain just south of the river channel on May 15, 

2014, indicated that 67 percent of the surficial deposits were 

2 mm or finer (table 3).

Bed-material transport was calculated for three flow 

scenarios—the 0.5 AEP peak flow, one-half the 0.5 AEP 

peak flow, and one-quarter 0.5 AEP peak flow. As noted in 

section, “Calculating Unit Stream Power,” peak-flow values 

were scaled along the length of the river corridor using 

mean-annual flow from the NHDPlus dataset. For each of the 

flow scenarios, we calculated the corresponding hydraulic 

conditions with the HEC-RAS model, assuming steadystate and subcritical flow. Hydraulic results for each of the 

three discharge scenarios became the input for the BAGS 

calculations, which were taken from just downstream of the 

landslide at RKMs 32.7–16.2. 
8 Preliminary Assessment of Aggradation Potential, North Fork Stillaguamish River, State Route 530 Landslide, Oso, Washington

Table 3. Summary of grain-size distributions used to simulate bed-material transport along the North Fork Stillaguamish River, 

Washington.

[Percent finer than: Categories are in millimeters]

Grain-size 

distribution 

identifier

River

kilometer Descripiton Date Source

Percent finer than

2.0 4.0 8.0 16.0 32 64 128 256 512

USGS1 35.0–34.7 Composite of three

pebble counts

collected near 

C-Post Road Bridge

2014 U.S. Geological

Survey

39 40 40 46 66 89 100 100 100

USGS_LSD 33.2 Composite of three

pebble counts from

top of hummocky

terrain of landslide

deposit just south of

river channel

05-15-14 U.S. Geological

Survey

67 3 2 3 3 0 0 0 0

RP9 32.7 Composite of two

pebble counts

2005, 2006 Stillaguamish

Tribe of Indians

0 14 20 36 56 74 87 97.5 100

RP8 32.4 Composite of two

pebble counts

2005, 2006 Stillaguamish

Tribe of Indians

0 36 44 56 69 81 92 95 100

RP7 31.9 Composite of two

pebble counts

2005, 2009 Stillaguamish

Tribe of Indians

0 23 26 36 46 59 78 93 100

RP4/5 30.4 Composite of three

pebble counts

2005, 2005, 

2008

Stillaguamish

Tribe of Indians

0 15 19 24 38 54 76 87 100

RP2 29.7 One pebble count 2005 Stillaguamish

Tribe of Indians

0 39 43 50 63 80 92 96 100

USGS2 29.3–29.1 Composite of three

pebble counts

collected near

Whitman Road Bridge

2014 U.S. Geological

Survey

15 22 27 37 51 77 94 99 99

USGS3 26.5 One pebble count

collected near 

the upstream 

SR530 Bridge

2014 U.S. Geological

Survey

25 25 26 32 49 69 98 100 100

USGS4 24.3 One pebble count

collected near 

221st Avenue bridge

2014 U.S. Geological

Survey

1 1 2 17 59 87 100 100 100

USGS5 16.3–16.0 Composite of three

pebble counts

collected near 

the downstream

SR530 Bridge

near Cicero

2014 U.S. Geological

Survey

16 21 28 38 63 91 100 100 100
Sediment Erosion Volumes 9

Assessing Aggradation Potential in the 

Context of Sediment Waves

River reaches where unit stream power or transport 

capacity transitions from large to small values represent 

areas with greater relative potential for aggradation. Full 

sediment-transport analyses are required to fully estimate the 

depth of anticipated aggradation and are beyond the scope 

of this study; however, using insight from sediment-wave 

dynamics on mountainous rivers elsewhere (Lisle, 2008) 

and by projecting the downstream distribution of anticipated 

sediment erosion during the coming flood season, estimates 

of the magnitude of aggradation depth in different reaches 

is possible. 

Relative change in stage at Whitman Road Bridge 

from March to July 2014 was determined using the shift in 

the stage-discharge rating as measured by USGS (Rantz, 

1982). Although not a direct measurement of aggradation, 

changes in stage at a streamgage have been used as a proxy 

for aggradation (Smelser and Schmidt, 1998; Juracek and 

Fitzpatrick, 2009; Czuba and others, 2010). 

To gain insight in sediment-wave behavior, the 

dimensionless Froude number, Fr, for an irregular, 

low‐gradient channel with steady, one-dimensional flow was 

calculated using discharge measurements made from March to 

June at the Whitman Road Bridge streamgage and the equation 

(Henderson, 1966):

Fr Q B

gA =

2

3 , (2)

where

B is the top width of the wetted channel; and

A is the cross-sectional area of the wetted 

channel.

Sediment Erosion Volumes

From March 22 to July 1, 2014, discharge values in the 

North Fork Stillaguamish River were typical for late spring 

and early summer. As measured near Arlington, discharge 

ranged from 28.1 m3/s (994 ft3/s) to 264 m3/s (9,340 ft3/s) 

through the end of May with three peaks greater than 170 m3/s 

(6,000 ft3/s) (fig. 3). 

Before the landslide, the low-flow water-surface 

elevation at C-Post Bridge (12166185; fig. 1), based on 2013 

lidar, was about 83.4 m (273.6 ft). Just after the landslide, the 

water-surface elevation of the impoundment increased steeply 

to 89.3 m (293 ft) on March 23 (fig. 3). Aerial imagery and 

DEM data indicated that the impoundment lake extended 

about 4 km upstream of the landslide. The volume of water 

in the impoundment was estimated to be about 3.3×106 m3. 

By April 1, the impoundment elevation declined to 86.9 m 

(285 ft). Through the middle of April, the impoundment 

declined by only about 0.5 m. The completion of the pilot 

channel in middle April, combined with a series of moderate 

flow events, promoted erosion of the landslide deposit and 

lowering of the impoundment lake to about 83.9 m (275.3 ft) 

by May 1. Continued, but less rapid, incision reduced the lake 

level to 83.4 m (273.6 ft) by late May, effectively eliminating 

the impoundment at low discharge (fig. 3).

Measurements of the water-surface elevation of the 

river through the landslide showed the upstream progression 

of the erosion knickpoint with time (fig. 4). At the apex of 

the landslide deposit, the maximum impoundment water 

depth (that is, the increase in water-surface elevation relative 

to pre-slide river conditions) was about 8 m (fig. 4). By 

March 24, the river had topped the landslide deposit and the 

erosion knickpoint was effectively in the center of the deposit 

about 1,000 m downstream of C-Post Bridge. The landslide 

deposit extended to 2,000 m downstream of C-Post Bridge, 

but landslide sediment eroded by the new river on March 23 

and 24 quickly filled the downstream river channel 1–2 m 

at least as far as 2,500 m downstream of C-Post Bridge. By 

April 6, the knickpoint moved upstream another 200 m, and 

the river naturally incised into the deposit as much as 5 m. 

At the downstream end of the landslide deposit, April 6 net 

aggradation was still 1–2 m. By August 6, the knickpoint was 

about 700 m from C-Post Bridge and the river had further 

incised with an elevation only about 1.5 m higher than the 

pre-slide water-surface elevation. At the downstream extent of 

the landslide deposit between 1,500 and 2,000 m downstream 

of C-Post Bridge, the river was nearly back to its pre-slide 

elevation indicating the continued evolution of the sediment 

wave (fig. 4). 

From March 24 to April 6 and from April 6 to July 1, 

120,000±24,000 m3

 and 160,000±32,000 m3, respectively, 

eroded from the landslide deposit. The total sediment eroded 

from March 24 to July 1 was 280,000±56,000 m3. Sediment 

erosion as a consequence of future channel incision and 

widening to an equilibrated geometry is projected to total 

220,000±44,000 m3. Without additional mass wasting from 

the landslide scarp or major channel migration or avulsions, 

the total volume of sediment eroded by the river is expected to 

be about 500,000±100,000 m3, or 6 percent of the total slide 

volume of 8×106 m3. 
10 Preliminary Assessment of Aggradation Potential, North Fork Stillaguamish River, State Route 530 Landslide, Oso, Washington

watac15-1025_fig 03

Fill period of impoundment lake

Discharge, in cubic meters per second 

Discharge, in cubic feet per second 

Impoundment lake elevation, in meters above 

North American Vertical Datum of 1988

Impoundment lake elevation, in feet above 

North American Vertical Datum of 1988.

5,000

10,000

15,000

20,000

142

283

425

566

83

84

85

86

87

88

89

90

272.3

275.6

278.9

282.2

285.4

288.7

292.0

295.3

March April May June

B

A

2014

Figure 3. Discharge values plotted at 15-minute intervals for the North Fork Stillaguamish River 

near Arlington (12167000) (A) and impoundment lake elevation (B), March 1 to July 1, 2014, near Oso, 

Washington. Manual survey measurements of the impoundment lake elevation from March 24 to 27 

are shown in B as data points; the data gap represents the transition from manual measurements to 

continuous gaging.
Sediment Erosion Volumes 11

watac15-1025_fig 04

Landslide deposition

Elevation, in meters above North American Vertical Datum of 1988

Downstream distance from C-Post Bridge, in meters

Impoundment lake

Downstream distance from C-Post Bridge, in meters

Eroded volume, in thousands of cubic meters

Eroded volume, in thousands of cubic yards

2,500 2,000 1,500 1,000 500 0

74

76

78

80

82

84

86

88

90

92

245

300

290

295

250

255

260

265

270

275

280

285

Elevation, in feet above North American Vertical Datum of 1988

0

10

20

30

40

50 65

60

50

55

45

40

35

30

20

25

15

10

5

0

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

Landslide deposition

B

A

120,000 m3

160,000 m3

220,000 m3 (projected)

EXPLANATION

Water-surface elevation

March 24, 2014

April 6, 2014

July 1, 2014 

July 2013 (pre-slide)

River-bed elevation

August 6, 2014 

April 6, 2014

EXPLANATION

Volume eroded since 

March 24, cumulative

Estimated future 

equilibrium condition

April 6, 2014

July 1, 2014

Figure 4. Landslide-deposition and water-surface elevation (A) and sediment volume eroded from landslide 

deposition relative to distance downstream (B) of C-Post Bridge, near Oso, Washington. Graph B also shows the 

projected sediment erosion from the deposit under a future equilibrium condition assuming the river channel 

remains in its July 1 location. Uncertainty of the volume projections in graph B is 20 percent.
12 Preliminary Assessment of Aggradation Potential, North Fork Stillaguamish River, State Route 530 Landslide, Oso, Washington

Downstream Aggradation Potential

The sediment eroded from the landslide—likely totaling 

about 500,000±100,000 m3 in the near term—was expected to 

contribute to downstream channel aggradation. If all of that 

mobilized sediment remained in lower 40 km of the North 

Fork Stillaguamish River, the channel would aggrade by an 

average of 0.25 m, assuming a 50-m wide active channel, but 

this is an unrealistic overestimate because most sediment will 

be transported out of the study reach toward Port Susan Bay in 

Puget Sound and patterns of aggradation will vary temporally 

and spatially as the sediment moves downstream encountering 

complex reach-specific hydraulic conditions. Unit stream 

power and bed-material transport capacity analyses provide 

spatially explicit estimates of potential aggradation.

Energy Slope and Unit Stream Power

In the lower 40 km of the North Fork Stillaguamish 

River, pre-slide, active-channel widths varied from 43 to 

133 m, with the narrowest river reach adjacent to the Rowan 

landslide near RKM 31.6 (fig. 5). Sparse pebble-count data 

indicated that typical median particle size near the SR 530 

Landslide ranges from 10 to 50 mm. Post-slide energy slope 

in the river was relatively constant with values of about 0.002 

from RKMs 5 to 20 (fig. 5). Locally, energy slope increased to 

near 0.004 at the confluence with the South Fork Stillaguamish 

River and 0.0052 near the SR 530 Bridge east of Oso. At 

the slide, the energy slope was a maximum value of 0.009, 

and slope approached 0.0 in the impoundment lake. Before 

the SR 530 Landslide, unit stream power ranged from 50 to 

280 W/m2, with localized peaks near the Rowan landslide and 

near the SR 530 Bridge east of Oso due to narrowing of the 

active channel. Unit stream power also increased downstream 

of Deer Creek with increasing discharge. After the landslide, 

unit stream power was affected from RKMs 30 to 39. The 

peak post-slide unit stream power was 380 W/m2 at the 

downstream section of the landslide, and unit stream power in 

the impoundment lake was near zero (fig. 5).

Bed-Material Transport Capacity

Results of the BAGS-calculated bed-material transport 

capacity from the Parker (1990a, 1990b) and Wilcock and 

Crowe (2003) equations showed decreasing capacity in 

the river reach 2 km downstream of the landslide, with a 

localized minimum near RKM 30.5 (fig. 6). Transport capacity 

generally increased from RKM 30.5 downstream to Whitman 

Road Bridge then decreases toward the SR 530 Bridge east 

of Oso with localized minimum values near RKMs 28.8 and 

26.8. At the SR 530 Bridge east of Oso (RKM 26.4), trends 

in transport capacity are complex with a localized maximum 

at the bridge. The SR 530 Bridge east of Oso is co-located 

with an early 20th-century railroad bridge (now named 

the Whitehorse Trail) constructed with relatively narrow 

bridge abutments that constrict the river and likely cause the 

transport-capacity localized maximum. The transport-capacity 

data indicate the bridge will not be a location with pronounced 

aggradation. Between the SR 530 Bridge and Deer Creek, 

there are localized transport-capacity minima. The Parker 

(1990a, 1990b) equation indicates a minimum near RKM 26 

and the Wilcock and Crowe (2003) equation indicates a 

minimum near RKM 24.3. Transport capacity increased at 

the Deer Creek confluence because of increased discharge. 

The analysis indicated a general decreasing transport-capacity 

trend downstream of Deer Creek to about RKM 16 (fig. 6). 

The localized transport-capacity minima likely reflected 

natural decreases in slope or increased active-channel width 

in particular reaches; however, anthropogenic influences, 

particularly the early 20th-century railroad bridge with its 

relatively narrow abutments, also affect transport-capacity 

values locally.

Notably, simulated transport rates reflected the river 

capacity to move bed material at a coarse resolution; actual 

transport rates will vary with available sediment supply 

and geomorphic response. Estimates of bed-materialtransport rates of the analytical approach used in this study 

are notoriously approximate (Pitlick and others, 2009; 

Wallick and others, 2010, 2011) due to complexities in the 

sediment-transport process and river mechanics. Modeled 

transport capacities also vary with transport equation used, 

input flow, grain-size, and energy slope data. The hydraulic 

model was constructed as a cursory tool that provided a 

first-order estimate of sediment-transport potential, and 

datasets used to build the model were incomplete. The 

simulated transport rates, therefore, represented first-order 

approximations of how the system might respond and were 

not intended to represent precise prediction of river-system 

behavior. Transport-rate predictions would improve with more 

grain-size data, improved energy slope estimates, and a refined 

hydraulic model. 

Zones of Potential Aggradation

Longitudinal differences in unit stream power and 

transport capacity along the river corridor can indicate 

areas prone to aggradation. With continued river erosion 

of the landslide deposit, increased bedload would 

presumably be in transport with the potential for localized 

aggradation in reaches with decreasing unit stream power or 

transport capacity. 
Downstream Aggradation Potential 13

watac15-1025_fig 05

SR 530

Landslide

extent

0

10

20

30

40

50

60

 100

 200

 300

 400

Stream power, in watts per square meters

0

Median grain size, in millimeters

Active channel width, in meters Energy grade slope, in meters per meter

Streamgage

12167000

Deer

Creek

Whitman

Road Bridge

SR 530

Bridge

SR 530 Bridge,

near Cicero

C-Post 

Road

 105

 104

 103

 102

 101

0

20

40

60

80

100

120

140

0

0.002

0.004

0.006

0.008

0.01

B

A

C

D

Bed-material transport capacity moving 

average, in kilograms per minute

Pre-slide active-channel 

width

EXPLANATION

Pre-slide stream power

Post-slide stream power

EXPLANATION

Wilcock and Crowe (2003)

Parker (1990a, 1990b)

EXPLANATION

Post-slide energy slope

and median grain size

EXPLANATION

Pre-slide

Post-slide 

0 5 10 15 20 25 30 35 40

Distance upstream of confluence with South Fork Stillaguamish River, in kilometers

Figure 5. (A) Pre-slide active channel width, (B) post-slide energy grade slope and median grain size, (C) pre- and 

post-slide unit stream power, and (D) bed-material transport capacity for the 0.5 annual exceedance probability peakflow event for the North Fork Stillaguamish River, Washington.
14 Preliminary Assessment of Aggradation Potential, North Fork Stillaguamish River, State Route 530 Landslide, Oso, Washington

watac15-1025_fig 06 SR 530 bridge, near Cicero

Deer Creek

SR 530 Bridge

SR 530

Landslide

 extent

C-Post Road

35

30

25

20

15

10

5

SR 530 Landslide

A

B

SR 530

Bridge Deer

Creek

SR 530 Bridge,

near Cicero

Whitman Rd.

Bridge

12167000

C-Post 

Road

Whitman Road Bridge

25 percent of annual exceedance probability

50 percent of annual exceedance probability

0.5 annual exceedance probability 

Parker

(1990a,1990b)

Wilcock and

Crowe (2003)

EXPLANATION

Modeled discharge

Transport capacities, moving average 

EXPLANATION

Unit stream power, in watts per square meter

5.38–19.5

19.5–62.4

62.4–104

104–119

119–135

135–147

147–161

161–183

183–227

227–379

Landslide extent

USGS streamgage

River kilometers 20

0 1 2 MILES

0 1 2 3 4 KILOMETERS

12167000

122°04' 122°02' 122° 121°58' 121°56' 121°54' 121°52' 121°50'

48°

16'

48°

14'

Transport capacity, in kilograms per minute

River kilometer

15 20 25 30 35 0.001

0.01

0.1

1

10

100

1,000

10,000

100,000

1,000,000

Base map modified Puget Sound Lidar Consortium digital data (1-meter resolution), U.S. 

Geological Survey digital data (10-meter resolution) and other digital data, various scales. 

Coordinate reference system: UTM Zone 10N, North American Datum of 1983

Figure 6. North Fork Stillaguamish River showing unit stream power by 500-meter reach intervals downstream of the State 

Route 530 Landslide (A) and bed-material transport capacity from RKM 16 to RKM 33 (B).
Downstream Aggradation Potential 15

Just downstream of the landslide, in the reach from 

RKM 32.5 downstream to 27.3, unit stream power decreased 

from 380 to 140 W/m2, thus indicating the potential for 

aggradation (fig. 5). Analysis of transport capacity showed 

decreasing transport capacity downstream to RKM 30.2 then 

a slight increase until local minima present near RKM 28.2 

and 26.2 (fig. 6). Considering analysis of both unit stream 

power and transport capacity, river reaches from the landslide 

downstream to RKM 30.2 and near RKM 28.8 and 26.8 

were expected to be most likely to experience measurable 

aggradation. Farther downstream, the river reach between 

the SR 530 Bridge east of Oso (RKM 26.4) and Deer Creek 

(RKM 23.9) could have experienced measurable aggradation 

as this reach showed both decreasing unit stream power and 

transport capacity. Similarly, river reaches near RKM 20 and 

RKMs 6–16 could see localized aggradation, although the 

amount of aggradation in these downstream reaches would 

likely be small compared to reaches immediately downstream 

of the landslide.

In sediment-wave studies, pulses of sediment commonly 

translate, disperse, or exhibit both characteristics (fig. 7). 

Lisle (2008) suggested that sediment waves both disperse 

and translate when Fr is less than 0.4 and the sediment wave 

material is dominated by sand and is finer than the existing 

bed material. For the landslide, the sediment mobilized 

from the deposit is predominantly sand. Moreover, Fr at the 

Whitman Road Bridge streamgage, measured on 11 occasions 

from March 25 to June 24, averaged 0.30 and ranged from 

0.18 to 0.44. Thus, the sediment wave would be expected to 

disperse downstream with some degree of translation. In this 

instance, we expected the river reach closest to the landslide to 

experience the most aggradation. 

Dispersing wave

Translating wave

Translating and dispersing wave

Direction of flow

Figure 7. Conceptual model of typical sediment-wave dynamics in mountainous rivers, 

from Lisle (2008).
16 Preliminary Assessment of Aggradation Potential, North Fork Stillaguamish River, State Route 530 Landslide, Oso, Washington

Post-slide lidar and aerial photographs help confirm 

that the sediment wave was primarily dispersive. The lidar 

DEMs in March and April showed aggradation of 1–2 m at 

2,000–2,500 m downstream of C-Post Bridge (fig. 4), this was 

confirmed with aerial-imagery analysis indicating the burial 

of a 1–2 m glacial erratic boulder in the river near RKM 32.4. 

By July 1, the river in this reach had incised back down into 

the aggraded river bed almost to the pre-slide river elevation. 

At the Whitman Road Bridge streamgage, analysis of the data 

indicated that from late March to late April, the stage shift 

in water-surface elevation for comparable flows increased 

about +0.4 m (fig. 8). Then, from May to July, the stage 

shift decreased to about +0.25 m relative to pre-slide river 

conditions. This river response at the streamgage is consistent 

with downstream translation and dispersion of a sediment 

wave, similar to that described by Lisle (2008). 

Before July 1, 280,000±56,000 m3 of sediment was 

eroded from the landslide and entered the channel, with 

the most pronounced downstream river response occurring 

in March and April. As the river continues to widen and 

incise through the deposit with future high flows, another 

220,000±44,000 m3 of sediment is expected to mobilize. 

Deposition of this additional sediment as bed material is 

expected to more strongly affect the water-surface elevation 

of smaller and intermediate flows (0.1 or 0.2 AEP peak‐flow 

events) as opposed to larger floods (0.01 or 0.02 AEP 

peak‐flow events). Larger events convey flood waters through 

the flood plain and also would readily mobilize the finer 

0.0

0.5

1.0

1.5

March Landslide on March 22

April May June

0.0

0.15

0.31

0.46

2014

Shift in stage-discharge rating at 28.3 

cubic meters per second, in feet

Shift in stage-discharge rating at 28.3 

cubic meters per second, in meters

Figure 8. Shifts in stage-discharge rating at the U.S. Geological 

Survey streamgage North Fork Stillaguamish River near Oso (12166300), 

Washington, March 28–July 1, 2014.

grained material sourced from the landslide into suspension 

and off the bed, thus mitigating influence of aggradation. In 

addition, for reaches nearest the landslide, a series of smaller 

peak-flow events greater than the largest flows seen since the 

landslide (283 m3/s or 10,000 ft3/s) but less than the 0.5 AEP 

peak-flow event (about 678 m3/s or 24,000 ft3/s), would lead 

to greater aggradation than if a 0.1 or 0.2 AEP event were to 

occur early in the flood season, which would likely transport 

mobilized sediment far downstream.

Although it is difficult to predict the depth of downstream 

aggradation, a worst case scenario was considered whereby 

all 220,000 m3 of sediment from the landslide mobilizes in 

a single event, then immediately deposits in the relatively 

low transport-capacity reach between RKMs 30.2 and 31.9. 

Over the 1.7-km long reach, active-channel widths average 

50 m. Assuming 20 percent of the mobilized sediment moves 

as bedload and covers the bed uniformly, the total depth of 

aggradation would be about 0.5 m. However, such focused 

aggradation is unlikely; spreading the same volume of eroded 

sediment over the several kilometers of depositional reaches 

downstream of the landslide would produce maximum 

aggradation of less than 0.2 m. Notably, this depth of potential 

aggradation is less than that documented just downstream of 

the landslide using the March 24 and April 6 lidar data (fig. 4), 

indicating that the worst aggradation from the landslide in the 

first 1.7 km had likely already occurred in the weeks following 

the event.
Conclusions 17

Additional Flood Risk Issues

The previous analysis assumes the river channel through 

the landslide does not migrate substantively or avulse to a 

new location, reflecting a likely anticipated river response 

in the 2014–15 flood season. However, other river-behavior 

scenarios were possible. New mass movements from the 

landslide scarp caused by heavy winter rain saturating the 

unvegetated hillslope could affect river response. Such 

mass movements include small-scale slumping of clay-rich 

lacustrine deposits adjacent to the river, debris flows entering 

the incised channel causing new impoundments, or large-scale 

remobilization of the landslide mass, which could increase 

the volume of sediment transported downstream relative to 

anticipated 220,000±44,000 m3. 

If large mass movements again dam the river, a new 

channel could erode through a different section of the 

landslide deposit mobilizing more sediment and increasing 

the amount of downstream aggradation; however, rapid 

dam breaching from a new channel avulsion is unlikely 

due to the low gradient of the deposit in the flood plain. In 

contrast, if a debris flow entered the narrow incision channel, 

an impoundment as deep as 8 m could conceivably reform 

upstream of the debris-flow deposit. The gradient of the river 

overtopping a localized debris-flow deposit could be relatively 

steep, causing rapid erosion and impoundment outburst flood. 

This sequence of events that could lead to rapid breeching 

is unlikely, but the hazards associated with this scenario and 

potential effects to downstream residents are significant. 

In addition to sediment, large woody debris transported 

from the landslide deposit could accumulate on downstream 

bridges, gravel bars, or boulder-dominated riffles causing 

localized increases in water-surface elevation during high 

flows or redirecting the momentum of the flowing river 

into revetments or low areas of the flood plain. Most of the 

landslide-derived large woody debris prone to downstream 

movement was likely transported out of the reach during 

spring flows, but large peak events in the winter will likely 

mobilize additional wood.

Our analysis assumed much of the sediment-wave 

behavior would be dispersive, affecting river reaches close 

to the landslide more than downstream reaches. There is 

a possibility, however that sand load will travel several 

kilometers and accumulate in sandy reaches. In the North Fork 

Stillaguamish River, the reaches near RKMs 2–3 and RKMs 

6–9 may accumulate sand in detectable quantities. Although 

beyond the scope of this study, the main stem Stillaguamish 

River near Stanwood contains sand-dominant reaches that 

also could be prone to new sand accumulation sourced from 

the landslide.

Conclusions

On March 22, 2014, the State Route 530 Landslide 

near Oso, Washington, caused 43 fatalities and released a 

major pulse of sand-dominated sediment into the North Fork 

Stillaguamish River. This sediment was expected to affect 

channel geomorphology and potentially increase flood risk 

in the 2014–15 flood season. Using a combination of aerial 

imagery, high-resolution digital elevation models, hydraulic 

models; and principles of sediment-wave behavior, stream 

power, and bed-material transport theory, we completed a 

preliminary assessment of aggradation potential in the river 

channel downstream of the landslide in summer of 2014.

From March 24 to April 6 and from April 6 to July 1, 

an estimated 120,000±24,000 cubic meters (m3) and 

160,000±32,000 m3 of sediment, respectively, eroded from 

the landslide deposit. The cumulative sediment eroded from 

March 24 to July 1 was 280,000±56,000 m3

. Assuming 

the river remains in its current location, incises down to 

an elevation similar to the pre-slide elevation, and widens 

to about 43 meters (m) (the width of the active channel 

downstream adjacent the larger Rowan landslide deposit), 

another 220,000±44,000 m3 of sediment was expected to 

mobilize during the 2014–15 flood season. If the river remains 

in its current location through the landslide deposit, the total 

volume of sediment eroded by the river is expected to be about 

500,000±100,000 m3, or 6 percent of the total slide volume of 

8×106 m3.

In the weeks following the landslide, 1–2 m of new 

sedimentation was documented just downstream of the 

landslide and 0.4 m of aggradation was documented at the 

Whitman Road Bridge, 3.5 kilometers (km) downstream of the 

landslide. By July 1, incision in the reach just downstream of 

the landslide brought the river water surface back to nearly its 

pre-slide elevation.

Analysis of unit stream power and bed-material transport 

capacity indicated that in the 2014–15 flood season, the river 

reach 1.7 km downstream of the landslide would see the 

most additional aggradation, with other reaches near river 

kilometers (RKM) 28.8, 26.8, and RKM 26.4–23.9 also being 

prone to aggradation. The depth of accumulation was expected 

to be less than 0.1 m, although the reach just downstream of 

the landslide could have as much as 0.5 m of aggradation, 

a depth less than the aggradation documented in the weeks 

immediately following the event.

In this study, we focused on the most likely river 

scenarios in the 2014–15 flood season, but geomorphic

and hydraulic conditions could create alternative, albeit 

less likely, scenarios of river response. Sand mobilized by the 

landslide could translate far downstream as a sediment wave 
18 Preliminary Assessment of Aggradation Potential, North Fork Stillaguamish River, State Route 530 Landslide, Oso, Washington

accumulating in deposits that could affect flood-conveyance 

capacity. Reaches of the North Fork Stillaguamish River near 

RKMs 2–3 and 6–9 could see detectable sand accumulation, 

and reaches in the main stem Stillaguamish River near 

Stanwood also could experience localized sand accumulation. 

New mass wasting from the landslide scarp could increase 

the volume of sediment anticipated to mobilize. Less likely, 

but potentially more hazardous, a new mass movement at 

the landslide could partially or completely block the new 

river channel through the landslide deposit, reforming the 

impoundment lake with the possibility of rapid breaching and 

release of an outburst flood downstream. 

Acknowledgments

Jody Brown and Jason Griffith of the Stillaguamish 

Tribe of Indians provided particle-size data. Ted Perkins of 

the Federal Emergency Management Agency and Travis Ball 

of the U.S. Army Corps of Engineers provided the HEC-RAS 

model used in the analysis. Kirt Hanson and Dave Lucas of 

Snohomish County Department of Public Works provided 

impoundment and river survey data. JoJo Mangano provided 

assistance with the BAGS analysis. Jon Dasler and his team 

at David Evans and Associates, Inc., collected and processed 

bathymetric data from the impoundment lake. Casey Kramer 

of the Washington State Department of Transportation and 

Paul DeVries of R2 Resource Consultants, Inc. provided 

analytical insight into the analytical approaches used in the 

study. Funding for the study was provided by the Federal 

Emergency Management Agency.

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Magirl and others—Preliminary Assessment of Aggradation Potential, North Fork Stillaguamish River, State Route 530 Landslide, Oso, Washington—Scientific Investigations Report 2015–5173

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20155173