Re-evaluation of the 1921 Peak Discharge at 

Skagit River near Concrete, Washington

Scientific Investigations Report 2007–5159

U.S. Department of the Interior

U.S. Geological Survey
Cover: Photograph of Darrin Miller, U.S. Geological Survey, surveying a high water 

mark left from the 2003 flood on the right bank of the Skagit River on the mouth of the 

Dallas Gorge near Concrete, Washington. (Photograph taken by Jeffrey Paulat, U.S. 

Geological Survey, August 2004).
Re-evaluation of the 1921 Peak Discharge 

at Skagit River near Concrete, Washington

By M.C. Mastin

Scientific Investigations Report 2007–5159

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

Mastin, M.C., 2007, Re-evaluation of the 1921 peak discharge at Skagit River near Concrete, Washington: U.S. 

Geological Survey Scientific Investigations Report 2007–5159, 12 p.
iii

Contents

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

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

Purpose and Scope .......................................................................................................................................2

Field Survey of the 2003 Flood .....................................................................................................................2

Field Survey of the 2006 Flood......................................................................................................................4

Calculated Water-Surface Slope for the 2006 Flood and the n Value for the 2003 Flood ..................5

 Water-Surface Slope for the 2006 Flood..........................................................................................5

n-Value for the 2003 Flood ...................................................................................................................5

Recalculating the 1921 Peak Discharge ....................................................................................................6

Historic Peak Discharges Based on the Stage-Discharge Rating ......................................................10

Re-evaluating the Historic Peak Discharges ..........................................................................................11

Summary........................................................................................................................................................12

References Cited..........................................................................................................................................12

Figures

Figure 1. Topographic map of the slope-area measurement reach on the Skagit River

near Concrete showing the three cross sections (XS1, XS2, and XS3), the

streamflow-gaging station, and HWMs from the 2003 flood and the 2006 flood

surveyed by the U.S. Geological Survey ................................................ 3

Figure 2. Profile showing high-water marks (HWMs) from peak flow of November 6,

2006, that was surveyed November 21, 2006, by the U.S. Geological Survey on

the right bank between cross sections 2 (XS2) and 3 (XS3) on the Skagit River

about 3,000–5,000 feet downstream of the streamflow-gaging station, Skagit

River near Concrete, Washington (station No. 12194000) .............................. 4

Figure 3. Cross section XS2 as surveyed in 1923 and 2004, n values for the particular

subareas (n), peak water surface for the 1921 and 2003 floods, and the points

at which the cross sections were subdivided into two subareas ..................... 6

Figure 4. Historical photographs showing only small, sparse brush on the island/bar

near cross section 2, Skagit River, Washington.......................................... 7

Figure 5. Profile showing high-water marks (HWMs), cross sections (XS), and

water-surface profile from the peak flow of November 27, 1949 on the Skagit

River about 3,000–5,000 feet downstream of the streamflow-gaging station,

Skagit River near Concrete, Washington (station No. 12194000) ..................... 8

Figure 6. Cross sections B, C, and D and the n values (n) used in the n-verification

study based on the 1949 peak discharge at Skagit River near Concrete,

Washington ................................................................................. 9

Figure 7. Stage-discharge rating based on revised 1921 peak discharge recalculation

extended to a gage height of 69.3 feet and the current stage-discharge rating

for Skagit River near Concrete, Washington, showing selected discharge

measurements, published historical peak discharges, and the proposed

revised historic peak flows ............................................................... 10
iv

Conversion Factors

Multiply By To obtain

cubic foot per second (ft3

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

foot (ft) 0.3048 meter (m)

foot per second (ft/s) 0.3048 meter per second (m/s)

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

Table

Table 1. Revised peak discharges for Skagit River near Concrete, Washington ............ 11
Re-evaluation of the 1921 Peak Discharge at Skagit River 

near Concrete, Washington

By M.C. Mastin

Abstract

The peak discharge record at the U.S. Geological 

Survey (USGS) gaging station at Skagit River near Concrete, 

Washington, is a key record that has come under intense 

scrutiny by the scientific and lay person communities in the 

last 4 years. A peak discharge of 240,000 cubic feet per second 

for the flood on December 13, 1921, was determined in 1923 

by USGS hydrologist James Stewart by means of a slope-area 

measurement. USGS then determined the peak discharges 

of three other large floods on the Skagit River (1897, 1909, 

and 1917) by extending the stage-discharge rating through 

the 1921 flood measurement. The 1921 estimate of peak 

discharge was recalculated by Flynn and Benson of the USGS 

after a channel roughness verification was completed based 

on the 1949 flood on the Skagit River. The 1949 recalculation 

indicated that the peak discharge probably was 6.2 percent 

lower than Stewart’s original estimate but the USGS did not 

officially change the peak discharge from Stewart’s estimate 

because it was not more than a 10-percent change (which is 

the USGS guideline for revising peak flows) and the estimate 

already had error bands of 15 percent. All these flood peaks 

are now being used by the U.S. Army Corps of Engineers to 

determine the 100-year flood discharge for the Skagit River 

Flood Study so any method to confirm or improve the 1921 

peak discharge estimate is warranted. 

 During the last 4 years, two floods have occurred on 

the Skagit River (2003, 2006) that has enabled the USGS 

to collect additional data, do further analysis, and yet again 

re-evaluate the 1921 peak discharge estimate. Since 1949, an 

island/bar in the study reach has reforested itself. This has 

complicated the flow hydraulics and made the most recent 

recalculation of the 1921 flood based on channel roughness 

verification that used 2003 and 2006 flood data less reliable. 

However, this recent recalculation did indicate that the original 

peak-discharge calculation by Stewart may be high, and it 

added to a body of evidence that indicates a revision in the 

1921 peak discharge estimate is appropriate. 

The USGS has determined that a lower peak-discharge 

estimate (5.0 percent lower) similar to the 1949 estimates is 

most appropriate based on (1) a recalculation of the 1921 flood 

using a channel roughness verification from the 1949 flood 

data, (2) a recalculation of the 1921 flood using a channel 

roughness verification from 2003 and 2006 flood data, and 

(3) straight-line extension of the stage-discharge relation at the 

gage based on current-meter discharge measurements. Given 

the significance of the 1921 flood peak, revising the estimate 

is appropriate even though it is less than the 10-percent 

guideline established by the USGS for revision. Revising the 

peak is warranted because all work subsequent to 1921 point 

to the 1921 peak being lower than originally published. 

Introduction 

The peak discharge record at the U.S. Geological 

Survey (USGS) gaging station Skagit River near Concrete, 

Washington, is a key record being used by the U.S. Army 

Corps of Engineers to determine the 100-year flood discharge 

for the Skagit River Flood Study. The four largest annual 

peak discharges of record that are used to determine the 

100-year flood discharge for this station occurred in 1897, 

1909, 1917, and 1921. A peak discharge of 240,000 ft3/s for 

the flood on December 13, 1921, was determined in 1923 by 

USGS hydrologist James Stewart by averaging the results of 

a contracted-opening measurement and slope area. The peak 

discharges of the other three largest floods were determined 

from a stage-discharge rating after extending it on the basis of 

the slope-area peak discharge of the 1921 flood. The accuracy 

of the peak discharge of the 1921 flood has been questioned, 

thus putting into question the accuracies of the other three 

largest peak discharges. The peak discharge of the 1921 flood 

was first checked in the 1950s with a channel roughness 

coefficient (n value) verification study based on the 1949 peak 

discharge of 153,000 ft3

/s that included a survey of high-water 

marks (HWMs) and four cross sections in the same reach used 

by James Stewart for his original slope-area measurement. 
H.C. Riggs and W.H. Robinson (U.S. Geological Survey, 

written commun., 1950) used this data to calculate an n value 

for the reach. In 1952, F.J. Flynn and M.A. Benson (U.S. 

Geological Survey, written commun., 1951–52) revised the 

n-verification study and used the revised n value to recalculate 

the 1921 peak discharge with channel geometry data compiled 

by James Stewart. They calculated a peak discharge of 

225,000 ft3/s for the 1921 peak discharge. G.L. Bodhaine 

(Area engineer, U.S. Geological Survey, written commun., 

1954) agreed with the analysis of F.J. Flynn and M.A. Benson, 

and reasoned that the other historical peak discharges also 

should be lower than the published values. Based on extension 

of the stage-discharge rating for the gage that was constrained 

to pass through the 1921 revised discharge point, he lowered 

peak-discharge values for all other historical peak discharges 

except for the 1815 flood, which remained the same. However, 

he noted that because none of the revisions were more than the 

10 percent guideline used to decide if revisions are needed, the 

published values should not be changed.

 The validity of the original slope-area measurement of 

the 1921 flood was recently re-evaluated by recalculating the 

peak discharge using an n value based on an n-verification 

analysis done following the flood on October 21, 2003. The 

peak discharge associated with the 2003 flood was determined 

to be 166,000 ft3/s from the current stage-discharge rating 

(Mastin and Kresch, 2005). However, a considerable amount 

of variation in the elevation of surveyed HWMs for the flood 

on October 21, 2003, precluded the determination of a single 

definitive water-surface slope for use in the n-verification 

analysis. The variability in surveyed HWMs for the flood on 

October 21, 2003, was attributed to the length of time between 

the event and the actual field survey, which was not completed 

until 9 months later in July–August 2004. Consequently, a 

range of water-surface slopes produced a range of possible n

values (Mastin and Kresch, 2005). 

A peak discharge of 145,000 ft3/s on November 6-7, 

2006, determined from the current stage-discharge rating 

provided an opportunity to survey a peak water-surface slope 

soon after the flood using high-water marks. Access to the left 

bank of the study reach had to be made by boat, which was 

unavailable at the time, so only the HWMs on the right bank 

of the slope-area reach were surveyed, but the survey provided 

an accurate water-surface slope between cross sections 2 

and 3 (James Stewart and Mastin and Kresch [2005] both 

used the same cross-section locations and identification; cross 

section 2 is upstream of the most downstream cross section, 

cross section 3). The field survey of the 2003 flood included 

HWMs on both sides of the river and detailed cross-sectional 

data; therefore, the water slope surveyed in 2006 was used in 

combination with the channel-geometry information for the 

2003 flood to compute an n value for the 2003 flood. Using 

this recalculated n value, the discharge for the 1921 peak 

discharge was recalculated and re-evaluated. It was thought 

at the time of the 2006 survey that this data could be used to 

improve on the previous n-verification studies and provide a 

better verification of the 1921 peak discharge calculation. 

Purpose and Scope

This report describes a re-evaluation of the 1921 

peak discharge based on three lines of evidence: (1) the 

n-verification studies using the 1949 flood data, (2) the nverification study using the 2003 and 2006 flood data, and 

(3) the location of the 1921 peak discharge data point on the 

stage-discharge rating—a relation defined by current-meter 

measurements made at the site. 

The previous n-verification studies that used data from 

the 1949 flood are discussed in detail by Mastin and Kresch 

(2005). A re-examination of those studies is discussed in 

this report and results of that study are the main thread of 

evidence for the re-evaluation of the 1921 historic flood. The 

n-verification study using the 2003 and 2006 flood data is 

discussed first in this report because the results of the field 

survey of the flood HWMs on November 6-7, 2006, have not 

been documented in any previous report and the resulting 

well-defined water-surface slope was substituted into the 2003 

flood channel roughness verification study to re-verify the 

roughness coefficient. This is followed by a discussion of the 

stage-discharge rating curve and how this curve is important 

as a third piece of evidence supporting the re-evaluation of the 

historical discharges at the gaging station on the Skagit River 

near Concrete, Washington. 

Field Survey of the 2003 Flood 

The HWMs and channel cross sections of the 2003 flood 

were surveyed in July–August 2004 at the same location used 

in the slope-area measurement made in 1923 (J.E. Stewart, 

U.S. Geological Survey, written commun., 1923). The site is 

a fairly straight reach of the Skagit River located downstream 

of the Dalles Gorge bedrock constriction, which is just 

downstream of the gaging station Skagit River near Concrete, 

Washington (station No. 121940000, fig. 1). A total station 

survey instrument was used to survey the cross sections and 

HWMs. During the cross-section survey, 2×2-inch wooden 

hubs were established near the site as survey control points 

based on Washington Department of Transportation survey 

monuments on the nearby highway. Elevation for the field 

survey was adjusted to the gage datum. 

   Re-evaluation of the 1921 Peak Discharge at Skagit River near Concrete, Washington
Figure 1. Topographic map of the slope-area measurement reach on the Skagit River near Concrete showing the three cross 

sections (XS1, XS2, and XS3), the streamflow-gaging station, and HWMs from the 2003 flood and the 2006 flood surveyed by 

the U.S. Geological Survey.

wa19-0151_Figure 01

EXPLANATION

Cableway

Cross section

Surveyed high-water mark (HWM)

 2003 flood

2006 flood

USGS streamflow-gaging station and No.

100 200

500 1000 1500 2000 FEET

0 300 400 500 600 METERS

0

Flow

XS1

XS2

XS3

SKAGIT

RIVER

Sand

Sand

Mile

54

Mile

53

121°46’

48°32’

T.

35

N.

R. 8 E.

12194000

12194000

Field Survey of the 2003 Flood    
 HWMs were particularly difficult to find near cross 

section 1 (labeled XS1 in fig. 1) and marks that were found 

were rated poor. Most of the marks were scour lines, flooddeposited sand, mud lines or small debris piles. Cross 

section 1 is near a rapidly expanding section just downstream 

of the Dalles Gorge, where it is difficult to assess the amount 

of energy loss due to expansion. Slope-area calculations are 

best applied to reaches where bed friction losses dominate and 

are less accurate when applied to reaches with expansions. 

Because only limited, poor quality HWMs could be located 

and the difficulties in estimating the energy loss due to 

expansion, XS1 was not used and only XS2 and XS3 were 

used in a two-section slope-area calculation to estimate the n

value. Peak-flow discharge was determined from the stagedischarge rating for the gaging station supported by a currentmeter measurement of 138,000 ft3

/s on the day of the 2003 

peak discharge. The water-surface elevation was well defined 

by HWMs at XS2, but there was about 2 ft of scatter of the 

good and excellent HWMs at XS3. Because of the uncertainty 

of the water-surface elevation at XS3, only a range of plausible 

n values (0.024 to 0.032) could be calculated for the 2003 

flood. 

Comparisons of channel cross sections and historical 

photographs (Mastin and Kresch, 2005) show that the channel 

has changed little in this particular reach of the Skagit River 

since 1923 except for the channel island (or channel bar at 

lower stages) crossed by XS2. This island/bar was likely 

barren and void of vegetation during the 1920s due to the 

series of large floods from 1897 to 1921. A series of historical 

photographs beginning in 1937 (historical photographs on 

file at the U.S. Geological Survey, Washington Water Science 

Center) shows a barren island/bar that has gradually increased 

in the amount of established vegetation. This island/bar is now 

densely forested.

Field Survey of the 2006 Flood

On November 21, 2006, 2 weeks after the flood, a twoman crew from the USGS surveyed HWMs on the right bank 

from XS2 to XS3 using the same total station as used in the 

previous survey in 2004. On the day of the flood after the 

river had crested, Malcolm Leytham (Northwest Hydraulics 

Consultants) marked the high-water line with 1×2-inch hubs 

labeled with red poker chips (written comm., November 

20, 2006). Five of these HWM hubs were found by USGS 

personnel, who also located 25 additional HWMs, each 

subjectively rated Excellent, Good, Fair to Poor (E, G, F, and 

P) based on the condition, type, and accuracy of the mark. A 

straight stationing reference line was drawn on a map along 

the line of HWMs to provide stationing for each HWM in a 

longitudinal profile (fig. 2). 

Figure 2. High-water marks (HWMs) from peak flow of November 6, 2006, that was surveyed November 

21, 2006, by the U.S. Geological Survey on the right bank between cross sections 2 (XS2) and 3 (XS3) on 

the Skagit River about 3,000–5,000 feet downstream of the streamflow-gaging station, Skagit River near 

Concrete, Washington (station No. 12194000). 

31

32

33

34

35

36

37

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

STATIONING, IN FEET

ELEVATION, IN FEET (GAGE DATUM)

Estimated slope = 0.00114

HWMs located November 21, 2006 by USGS

HWMs located by Northwest Hydraulics Consultants

NOTE: All HWMs were rated as Good except where

 indicated by an F for fair or an E for excellent

XS2

XS3

F F

E

E

E

 Re-evaluation of the 1921 Peak Discharge at Skagit River near Concrete, Washington
 Several 2×2-inch survey control hubs from the previous 

survey in 2004 were found and used to establish the survey 

control for the 2006 survey. The survey began near XS2 with 

two 2004 hubs that agreed within 0.025 ft in the horizontal 

and 0.038 ft in the vertical with the 2004 survey. The survey 

ended near XS3 on a 2004 hub (on a sand bank and found 

at an angle), checking the old coordinates within 0.082 ft in 

the vertical and 3.003 ft in the horizontal. The error in the 

horizontal is quite large, but this error may be partially due 

to movement of the hub in the sand bank. Positional checks 

were made between every total-station setup in the survey and 

they were all within 0.07 ft in the horizontal and 0.02 ft in the 

vertical. Ten setups were used along a baseline of about 2,100 

ft to complete the survey. A 3-foot error in the horizontal 

length of the river (2,108 ft) used to determine the water slope 

in this reach equals an error in the slope of ±0.15 percent. 

Calculated Water-Surface Slope for the 

2006 Flood and the n Value for the 2003 

Flood

Information from the 2003 and 2006 floods was 

combined to re-evaluate the n value for the reach. The 2003 

flood survey provided detailed channel geometry and HWMs 

on both sides of the river, but had a poorly defined watersurface slope due to the wide scatter of HWMs at XS3. The 

2006 flood survey accurately measured the water-surface 

slope, but it did not include a survey of the cross sections or 

left-bank HWMs. If the slope estimated for the 2006 flood 

is substituted into the two-section equation for a slope-area 

measurement using the geometry surveyed for the 2003 flood, 

and utilizing the peak discharge for the 2003 flood, the n value 

for this reach for the 2003 flood can be calculated. 

 Water-Surface Slope for the 2006 Flood

Figure 2 shows a profile of the HWMs from the peak of 

November 6, 2006, with the location of the cross sections and 

a straight line approximating the water surface at the peak. 

As a peak recedes, it may leave additional HWMs that are 

identified in the field. As a result, it is common to identify 

HWMs lower than nearby marks. A peak water surface 

generally is drawn with a bias to be nearest the highest water 

marks. Additional weight is given to water marks with the 

highest quality ratings. Despite having some scatter in the 

2006 HWMs, the scatter of the HWMs is considerably less 

than the scatter of HWMs from the 2003 flood at XS3. Instead 

of providing a possible range in water-surface slopes as was 

done with the 2003 flood data, a single peak water-surface 

profile can be drawn for the 2006 flood that closely aligns 

with several HWMs at XS3 and several excellent HWMs just 

upstream of XS3. The water surface approximated in figure 2

has a slope of 0.00114. This slope is in the range of values 

estimated from the survey of the HWMs from the 2003 flood, 

0.00064 to 0.00124, and it is very close to the slope of 0.00120 

calculated by Stewart in 1923 for the 1921 flood (James 

Stewart, U.S. Geological Survey, written commun., November 

1923).

Two recent medium-to-high discharge measurements plot 

close to the current rating without applying shifts. A discharge 

measurement made from a cableway on November 9, 2006, 

measured 53,000 ft3

/s and plotted +7.1 percent off the current 

rating without shifts. A discharge measurement of 138,000 

ft3/s on October 21, 2003, plotted +1.5 percent off the same 

rating without shifts. These discharge measurements indicate 

that the hydraulics of the reach have changed little between 

the 2003 and 2006 floods. We, therefore, expect that the slope 

for the 2006 flood should be similar to the slope of the 2003 

flood.

n-Value for the 2003 Flood

Initially, the surveyed cross section points for XS2 

and XS3 for the 2003 flood were entered into the computer 

program Slope Area Computation, v97-01 (SAC, Fulford, 

1994). The elevation of the water surface for XS2 was held 

to an elevation of 37.50 ft, a relatively accurate water-surface 

elevation determined from the survey of HWMs from the 2003 

flood. The elevation of XS3 was set to 34.82 ft, the calculated 

elevation from applying a slope of 0.00114 to the reach length. 

As was done by Stewart in his 1923 calculation of the 1921 

flood, no subdivision of the cross sections was made and n 

values were held the same for both cross sections. By trial 

and error with the SAC program, the estimated n value for the 

2003 flood using the water-surface slope from the 2006 flood 

that resulted in a discharge of 166,000 ft3/s (the peak discharge 

for the 2003 flood) was calculated to be 0.0306. Calculated 

velocities were 11.1 and 12.1 ft/s and Froude numbers 0.43 

and 0.46 for XS2 and XS3, respectively. This n value is at the 

high end of the range of n values calculated earlier by Mastin 

and Kresch (2005)—0.024 for the flattest estimated watersurface slope and 0.032 for the steepest water-surface slope. 

James Stewart used the Chezy and Kutter equations (Corbett, 

1962, p. 81-83) along with an n value of 0.033. Stewart, 

however, used the water-surface slope in his calculations 

rather than the energy slope that is used today, which results in 

slightly different peak-discharge values for the same n values.

A close examination of the shape of XS2 indicated 

that a subdivision of the cross section should be done when 

performing one-dimensional hydraulic computations. The 

decision to subdivide follows guidelines outlined by Davidian 

(1984, p. 21) for “bench panhandle shapes” of cross sections. 

Davidian’s guidelines state that cross sections should be 

subdivided when the ratio of the length of the panhandle to its 

depth is 5 or greater. In the case of XS2, the ratio is roughly 

15. A second n-verification computation was made with XS2 

subdivided into two subareas, a main channel on the left 

Calculated Water-Surface Slope for the 2006 Flood and the n Value for the 2003 Flood 
half of the cross section and a high-water bench on the right 

half of the cross section containing the forested island/bar. 

The plot of XS2 from the 2004 survey indicates that three 

subareas may be warranted (left main channel, high-water 

bench, and right main channel). However, because the plot 

of XS2 from the 1923 survey shows only two subareas (on 

main channel and one panhandle area) only two subareas for 

the 2004-surveyed XS2 were used for consistency between 

the two calculations. No distinct panhandle shape is found in 

cross section XS3 so no subdivision of XS3 was used in the 

calculations. XS2 was subdivided at station 346 (fig. 3) for the 

cross section data surveyed in 2004. Using a weighted-wetted 

perimeter technique (Arcement and Schneider, 1989) for the 

panhandle portion of XS2, the n value of the right subarea was 

calculated to be 0.066 based on an estimated n value of 0.120 

for the forested island/bar portion (station 346 to 486, wetted 

perimeter = 140 ft) and 0.033 for the remainder of the subarea 

(wetted perimeter = 231 ft). Therefore; 0.066 = ((0.120 * 140) 

+ (0.033 * 231)) / 371. The n value for the forested island 

(0.120) was estimated from photographs of verified n values 

for floodplain reaches (Arcement and Schneider, 1989). The n

value of the remainder of the right subarea of XS2 was set to 

Stewart’s original n value for the reach, 0.033. 

 Several trial-and-error runs of the SAC program were 

performed to determine the n value for the left subarea of XS2 

and all of XS3, which are considered to be the main channel 

in this reach of the Skagit River. Running the SAC program 

to produce a discharge as close to the 2003 flood peak as 

possible (166,000 ft3/s), an n value of 0.033 was calculated for 

the main channel areas (a discharge of 168,000 ft3/s resulted in 

an n value of 0.033, and a discharge of 163,000 ft3/s resulted 

in an n value of 0.034). 

Recalculating the 1921 Peak Discharge

The final n value of 0.033 for the main channel areas that 

was calculated for the 2003 flood and the slope for the 2006 

flood was used to recalculate the 1921 peak discharge using 

cross-section geometry, reach length, and fall from Stewart’s 

1923 survey (James Stewart, U.S. Geological survey, written 

commun., November 1923). For this recalculation, XS2 from 

the 1923 survey was subdivided into two areas at station 256 

(see fig. 3) based on the guidelines discussed previously. 

Figure 3. Cross section XS2 as surveyed in 1923 and 2004, n values for the particular subareas (n), peak water surface 

for the 1921 and 2003 floods, and the points at which the cross sections were subdivided into two subareas. 

[Note: Stationing for the 1923 surveyed cross section was adjusted to fit the stationing surveyed in 2004.]

-100 0 100 200 300 400 500 600 700 800

STATIONING, IN FEET FROM LEFT BANK

Surveyed March 7, 1923

Surveyed July 28, 2004

1921 flood, peak water surface

2003 flood, peak water surface

Station 346

Station 256

-10

-20

0

10

20

30

40

50

60

ELEVATION, IN FEET (CURRENT GAGE DATUM)

n=0.033 n=0.066

n=0.033 n=0.033

   Re-evaluation of the 1921 Peak Discharge at Skagit River near Concrete, Washington
Even though XS2 was subdivided, an n value of 0.033 was 

used for both subareas because the island/bar was assumed 

to be bare of vegetation at the time of the 1921 peak with 

similar roughness elements as the main channel. The resulting 

recalculated peak discharge is 219,000 ft3/s. 

A re-examination of the n-verification done by Flynn and 

Benson (U.S. Geological Survey, written commun., August 

1952) using the 1949 flood data also computed an n value 

that could be used to recalculate the 1921 peak discharge. 

In both recalculations, Stewart’s surveyed data for the reach 

between XS2 and XS3 were used with the different n values to 

calculate different peak discharges for the 1921 flood. 

Recalculation of the 1921 peak using an n value 

calculated from 2003 and 2006 data assumes that the channel 

roughness conditions and hydraulics of the reach have not 

changed since 1921. However, this assumption may not 

be valid because the island/bar that probably was bare of 

vegetation in 1921 has built up slightly from deposition (fig. 3) 

and is now densely forested (Stewart’s survey notes for his 

indirect measurements in 1923 do not mention anything about 

the vegetative character of the island). This alteration of the 

island/bar probably has complicated the flow hydraulics, 

which makes the recalculation of the 1921 flood using 2003 

and 2006 flood data less reliable.

A more reliable recalculation of the 1921 flood peak may 

be obtained using the analysis of the 1949 flood because the 

conditions in the reach in 1949 may have been more similar to 

the conditions in 1921. In fact, a 1932 and a 1948 photograph 

show the island/bar mostly bare of vegetation (fig. 4). The 

1948 photograph indicates that the influence of vegetation on 

the island/bar was minimal prior to the November 1949 flood 

that was used for the n-verification study by Flynn and Benson 

(U.S. Geological Survey, written commun., August 1952). 

B. Skagit River near Concrete, Washington. Looking upstream. 

 Photograph taken from helicopter by F. Veatch., U.S. 

 Geological Survey, May 8, 1948. 

A. Skagit River near Concrete, Washington. Looking 

 downstream from top of gage house. Photograph 

 taken by J. Bonner, U.S. Geological Survey, June 

 or July 1932.

Figure 4. Historical photographs showing only small, sparse brush on the island/bar near cross section 2, Skagit River, 

Washington. Historical photographs on file at the U.S. Geological Survey, Washington Water Science Center.

Recalculating the 1921 Peak Discharge 
wa19-0151_Figure 05

21

22

23

24

25

26

27

28

29

0 500 1,000 1,500 2,000 2,500 3,000 3,500

STATIONING, IN FEET

ELEVATION, IN FEET (GAGE DATUM)

XS-A

XS-B

XS-C

XS-D

Left Bank HWMs

Right Bank HWMs

Water-surface slope used in

n-verification analysis

Figure 5. High-water marks (HWMs), cross sections (XS), and water-surface profile from the peak flow of 

November 27, 1949 on the Skagit River about 3,000–5,000 feet downstream of the streamflow-gaging station, Skagit 

River near Concrete, Washington (station No. 12194000). 

Flynn and Benson (F.J. Flynn and M.A. Benson, U.S. 

Geological Survey, written commun., August 1952) computed 

an n value of 0.0305 for the 1949 flood data, and using that n

value, Flynn and Benson recalculated the 1921 peak discharge 

to be 225,000 ft3/s. They surveyed four cross sections labeled 

A through D in downstream order beginning with A at the 

upstream end about 300 ft downstream of Stewart’s cross 

section 1. Cross sections B and C fell on either side of 

Stewart’s cross section 2 and cross section D was about 700 ft 

upstream of Stewart’s cross section 3. Reach A-B was not 

used because of the large expansion in the reach. The surveyed 

HWMs provided a reasonably accurate water-surface profile 

for the 1949 flood (fig. 5); however, the analysis did not 

subdivide any of the cross sections. 

Looking at the shape of cross sections B and C (fig. 6), 

and using the guidelines that are now available for subdividing 

(Davidian, 1984), the cross sections clearly needed to be 

subdivided. Based on these guidelines, cross sections B and 

C were subdivided and the 1949 analysis was recomputed in 

order to come up with a revised n value for the 1949 study. 

Through trial and error with the SAC program using different 

n values, an n value of 0.0315 provided a calculated discharge 

of 154,000 ft3

/s for the reach from B to D, which is very near 

the 1949 flood discharge of 153,000 ft3

/s. Using this n value in 

the recalculation of the 1921 flood using Stewart’s data with 

a subdivided cross section 2 as described above, resulted in a 

discharge of 228,000 ft3/s. 

In summary, two recalculations of the 1921 peak 

discharge based on two n-verification analyses, one using 

1949 peak-flow data and one using the 2003 and 2006 peakflow data, both resulted in discharges less than Stewart’s 

original calculated discharge. The recalculated value of 

228,000 ft3

/s for the 1921 peak discharge that used 1949 peakflow data and subdividing techniques is 5.0 percent less than 

Stewart’s original value of 240,000 ft3/s and the recalculated 

value of 219,000 ft3/s that used 2003 and 2006 peak-flow data 

and subdividing techniques is 8.8 percent less than Stewart’s 

original value. The slope-area recalculation of the 1921 peak 

flow using the n value from 1949 peak n-verification analysis 

and subdivision is considered a fair measurement within 15 

percent of the actual value (Benson and Dalrymple, 1967). 

Note that Stewart’s calculation falls well within the error 

bounds of this recalculated peak discharge. The roughness due 

to vegetation on the island/bar in 1949 is likely to have been 

minimal as it was in 1921, but much different than the 2003 

or 2006 peak flows. Due to the complication of the hydraulics 

of flow through and around the forested island/bar, more 

confidence can be given to the recalculation of the 1921 peak 

based on the 1949 peak when the reach conditions were more 

similar to 1921 than in 2003 or 2006. The recalculation of the 

1921 peak discharge using the 2003 and 2006 peak discharge 

information is only 2.7 percent less than the recalculation 

based on the 1949 peak discharge information. Despite some 

doubt in the accuracy due to the complications of accounting 

for the hydraulics around and through the forested island/

bar, the recalculation of the 1921 peak using 2003 and 2006 

information supports the assertion that Stewart’s original value 

for the 1921 peak is too high. 

 Re-evaluation of the 1921 Peak Discharge at Skagit River near Concrete, Washington
wa19-0151_Figure 06

STATIONING, IN FEET

n=0.0315

Cross section D, station 3,120 feet

-15

-10

-5

0

5

10

15

20

25

30

-15

-10

-5

0

5

10

15

20

25

30

-15

-10

-5

0

5

10

15

20

25

30

-100

ELEVATION, IN FEET (GAGE DATUM)

n=0.0315 n=0.0315

Cross section C, station 1,953 feet

0 100 200 300 400 500 600 700 800 900 1,000

n=0.0315 n=0.0315

Cross section B, station 726 feet

Subdivisions

Figure 6. Cross sections B, C, and D and the n values (n) used in the n-verification study based on the 

1949 peak discharge at Skagit River near Concrete, Washington. 

Recalculating the 1921 Peak Discharge   
Historic Peak Discharges Based on the 

Stage-Discharge Rating

Another indicator that the published historical discharges 

may be too high is the location of the peak-discharge data 

points that plot to the right of the current stage-discharge 

rating curve at the site. This rating was extended based on 

current high-flow current-meter measurements; the highest is 

measurement number 475 that was made on October 21, 2003, 

at a discharge of 138,000 ft3/s (fig. 7). The primary channel 

feature that controls the stage-discharge rating at this site is a 

bedrock gorge that is relatively immune to typical scour and 

fill activities common to many rating controls. The lack of 

scatter in the long history of current-meter measurements on 

the plot of the stage-discharge rating supports this conclusion 

of a stable rating that has not changed substantially over the 

last 80 years. The channel geometry does not substantially 

change at the highest stages, which indicates that a straightline extension of the rating beyond the highest current-meter 

measurement is a reasonable method to estimate high flows. 

Water year 1925 discharge measurements are highlighted in 

figure 7 because they represent the stage-discharge relation 

near the time of the historic floods. 

In order to revise the historical peak discharges based 

on a revised peak discharge for the 1921 peak discharge 

(228,000 ft3/s), a new rating was drawn. Using the water year 

1925 discharge measurements, the highest current-meter 

measurements, and constraining the rating curve to pass 

directly through the recalculated 1921 peak discharge data 

point, a rating curve was redrawn and extended with a straight 

line to a gage height of 69.3 feet, the estimated gage height for 

the 1815 peak discharge (fig. 7).

Figure 7. Stage-discharge rating based on revised 1921 peak discharge recalculation extended to a gage height of 

69.3 feet and the current stage-discharge rating for Skagit River near Concrete, Washington, showing selected discharge 

measurements, published historical peak discharges, and the proposed revised historic peak flows. 

SKAGIT RIVER NEAR CONCRETE, WASHINGTON 12194000

14

15

16

17

18

19

20

30

40

50

60

70

80

90

5 10 20 50 100 200 500 700

GAGE HEIGHT, IN FEET

DISCHARGE, IN THOUSAND CUBIC FEET PER SECOND

385

11

7

44

41

426 405

365 425

433

438

361

503

431

397

467

371

442

39

1A

Water year 1925 discharge measurements

Published historical peak discharges

Revised historic peak flow

Current rating

Revised rating

Selected discharge measurements 

 and all measurements greater than

 25,000 cubic feet per second

10 Re-evaluation of the 1921 Peak Discharge at Skagit River near Concrete, Washington
Re-evaluating the Historic Peak 

Discharges

A SAC run using the 2003 and 2006 flood data to 

calculate the discharge for various roughness coefficients 

calculated a velocity of 12.1 ft/s for XS3, but the velocities 

calculated in the subareas for XS2 were 14.3 ft/s for the left 

main channel and 4.9 ft/s in the right high-water bench. With 

the obstruction of the forested island/bar and such a large 

difference in the velocities in the two subareas, flows during 

peaks are not expected to be parallel to the banks of the reach 

because they would be going around the forested island/bar. 

Non-parallel flows would introduce some head losses that are 

not accounted for in the calculation of the energy equation, 

and for these reasons introduce some error in the calculation 

that are impossible to quantify. A detailed two-dimensional 

flow model could possibly account for these losses, but one 

is not currently available for this analysis. The n-verification 

study done for the 1949 flood did not have this problem 

because the island/bar was practically barren and flows were 

more likely to be parallel to the bank. Due to these hydraulic 

complications in the 2003 and 2006 peak flow n-verification 

calculations and the fact that the study reach in 1949 was more 

similar to the reach in 1921 than it was in 2003 or 2006, there 

is more confidence in the n verification and subsequent 1921 

peak-flow recalculation based on the 1949 peak flow than the 

calculations using only the 2003 and 2006 peak-flow data.

Novak (1985) discusses the USGS criteria for revisions of 

peak flows as follows: 

“Extremes and peaks greater than base discharge 

should be revised if the difference in discharge 

between old and new data is more than about 10 

percent. Revisions may be made for errors less than 

10 percent if they are needed to maintain the correct 

relationship between the annual maximum discharge 

or to keep annual maximums in the proper order of 

magnitude.” 

The revised discharge values reported by Flynn and 

Benson in their unpublished report from1952 is less than 

10 percent different than the published value, and because the 

magnitudes of the three other historic peaks that occurred in 

1897, 1909, and 1917 are dependent on the magnitude of the 

1921 peak discharge, Bodhaine, the USGS area engineer at the 

time, felt that a revision would not be needed (G.L. Bodhaine, 

U.S. Geological Survey, written commun., May 1954). Since 

that time, two new independent n-verification studies as 

described in this report came up with a similar conclusion that 

the currently published peak flows are too high. Together with 

the straight-line extension of the stage-discharge relation, there 

are now three independent estimations that all are lower than 

the published values. 

 The accuracy of the peak discharge of the four historic 

floods at this site is essential to the current flood-reduction 

study by the U.S. Army Corps of Engineers and the Skagit 

County Flood Insurance Study Update by FEMA. In order to 

provide the best data available for the current flood studies, 

the USGS revised historic floods at Skagit River near Concrete 

despite the fact that their estimates are all less than 10 percent 

from the currently published discharges (table 1). The 1921 

peak discharge is determined from the recalculation using 

Stewart’s data, subdivision of cross section 2, and the n value 

calculated from the n verification using 1949 peak-flow data 

as described in this report. The other historical peak discharges 

are revised based on the rating extension described in this 

report and rounded to the nearest 5,000 ft3

/s (table 1). 

Table 1. Revised peak discharges for Skagit River near 

Concrete, Washington.

[ft3/s, cubic foot per second]

Year

Currently 

published peak 

discharges

(ft3

/s)

Gage height 

(ft)

(current 

datum)

Revised peak 

discharges

(ft3

/s)

Percent 

difference 

in revised 

discharge

1815 500,000 69.3 510,000 2.0

1856 350,000 57.3 340,000 -2.9

1897 275,000 51.1 265,000 -3.6

1909 260,000 49.1 245,000 -5.8

1917 220,000 45.7 210,000 -4.5

1921 240,000 47.6 228,000 -5.0

Re-evaluating the Historic Peak Discharges 11
Summary

In 1952 and 1954, the U.S. Geological Survey 

investigated a revision of the historic floods at Skagit River 

near Concrete based on a revision of the 1921 peak-flow 

discharge that used a revised channel roughness coefficient 

calculated from the 1949 peak discharge. At the time, the 

recomputations were not used to change the published values 

because the proposed revisions were all less than 10 percent. 

Recently, a new channel roughness coefficient verification 

was made using the water-surface slope of the 2006 flood at 

Skagit River near Concrete along with channel geometry and 

high-water marks from the 2003 flood. The recalculation of 

the 1921 peak discharge using this new roughness coefficient 

and a subdivision of the cross sections also indicated that the 

original computation of the 1921 peak discharge by James 

Stewart in 1923 was too high although well within the margin 

of error. However, the island/bar in the slope-area reach 

thought to have been barren during the 1921 and 1949 peak 

discharges has re-vegetated into a dense forest, and it is likely 

altering the flow patterns during high flows. The slope-area 

computation does not solve for these complex flow patterns 

in two dimensions, and therefore, the recalculation based on 

a verified channel roughness coefficient using the 1949 flood 

data and channel characteristics and subdividing techniques 

is thought to more accurately reflect 1921 conditions. The 

recalculated peak discharge for the 1921 flood using the 

channel roughness coefficient verified using 1949 flood data 

is 5.0 percent less than the published value computed in 

1923. The current stage-discharge relations at the station also 

indicate that the lower peak discharge value is more reasonable 

than the published value. With these several lines of evidence 

that all indicate that the published value of the 1921 peak is 

too high, the USGS will revise the historical peak discharges. 

The 1921 peak discharge will be revised to 228,000 ft3

/s or 

5.0 percent less than the published value, and correspondingly, 

the other historical peak discharges will be revised from 

straight-line extension of a revised stage-discharge rating that 

passes through the revised 1921 peak-discharge data point.

References Cited

Arcement, G.J., Jr., and Schneider, V.R., 1989, Guide for 

selecting Manning’s roughness coefficients for natural 

channels and flood plains: U.S. Geological Survey WaterSupply Paper 2339, 38 p.

Benson, M.A., and Dalyrymple, T., 1967, General field and 

office procedures for indirect discharge measurements: 

U.S. Geological Survey Techniques of Water-Resources 

Investigations, Book 3, Chap. A1, 30 p.

Corbett, D.M., 1962, Stream-gaging procedure—A manual 

describing methods and practices of the Geological Survey: 

U.S. Geological Survey Water-Supply Paper 888, 245 p.

Davidian, J., 1984, Computation of water-surface profiles 

in open channels: U.S. Geological Survey Techniques of 

Water-Resources Investigations, Book 3, Chap. A15, 48 p.

Fulford, J.M., 1994, User’s guide to SAC, a computer program 

for computing discharge by the slope-area method: U.S. 

Geological Survey Open-File Report 94-360, 31 p.

Mastin, M.C., and Kresch, D.L., 2005, Verification of 1921 

peak discharge at Skagit River near Concrete, Washington, 

using 2003 peak-discharge data: U.S. Geological Survey 

Scientific Investigations Report 2005-5029, v. 2.0, 18 p.

Novak, C.E., 1985, WRD data reports preparation guide: U.S. 

Geological Survey Water Resources Division, 199 p. 

12 Re-evaluation of the 1921 Peak Discharge at Skagit River near Concrete, Washington
Manuscript approved for publication, August 8, 2007

Prepared by the USGS Publishing Network, 

Bill Gibbs 

Bob Crist

Bobbie Jo Richey

Linda Rogers

For more information concerning the research in this report, contact the 

Washington Water Science Center Director, 

U.S. Geological Survey, 934 Broadway — Suite 300

Tacoma, Washington 98402

http://wa.water.usgs.gov
SIR 2007–5159 Mastin Re-evaluation of the 1921 Peak Discharge at Skagit River near Concrete, Washington