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Hydrology Report
Nisqually River, WA
January 2019
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1. INTRODUCTION ............................................................................................................................................. 3
1.1. PURPOSE OF STUDY ........................................................................................................................................... 3
1.2. SCOPE OF WORK ............................................................................................................................................... 3
1.3. WATERSHED CHARACTERISTICS AND FLOODING HISTORY .......................................................................................... 4
1.4. PREVIOUS STUDIES ............................................................................................................................................ 5
2. GAGE ANALYSIS AND ADJUSTMENTS ............................................................................................................. 5
2.1. GAGE DATA ..................................................................................................................................................... 5
2.2. BULLETIN 17C STATISTICAL ANALYSIS ................................................................................................................... 6
3. DISCHARGES AT UNGAGED SITES................................................................................................................... 8
3.1. STREAMSTATS .................................................................................................................................................. 8
3.2. REGIONAL REGRESSION EQUATIONS ..................................................................................................................... 8
3.3. DISCHARGE VS. DRAINAGE AREA .......................................................................................................................... 9
3.4. MAJOR LAKES AND RESERVOIRS ......................................................................................................................... 10
3.5. FINAL DISCHARGE GRIDS .................................................................................................................................. 10
4. REFERENCES ................................................................................................................................................ 11
APPENDIX A ................................................................................................................................................ 13
APPENDIX B ................................................................................................................................................. 34
LIST OF FIGURES
Figure 1: Location and Study Area Map for the Nisqually River ................................................................................... 3
Figure 2: Peak discharges vs. drainage area-frequency curves based on Nisqually River gages ................................... 9
LIST OF TABLES
Table 1: USGS Peak Streamflow Gages .......................................................................................................................... 6
Table 2: Gage Discharges ............................................................................................................................................... 7
Table 3: Effective, historic and estimated peak flows at McKenna ............................................................................... 8
Table 4: Peak discharge vs. Drainage area coefficients ................................................................................................. 9
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1. Introduction
1.1. Purpose of Study
The purpose of this study is to perform hydrologic and hydraulic analyses of the Nisqually River to
revise the Flood Insurance Studies (FIS) for Thurston County, WA, and Pierce County, WA. The
Strategic Alliance for Risk Reduction (STARR II) was contracted to complete this study for the Federal
Emergency Management Agency (FEMA), under contract number HSFE60-15-D-0005, task order
number HSFE10-17-J-0005.
1.2. Scope of Work
The Nisqually River, which has been identified as of high priority for detailed restudy, is located within
the Nisqually Watershed (HUC 17110015), encompassing areas of Pierce, Thurston and Lewis
Counties, Washington. All effective Nisqually River floodplain is currently classified as Zone A (FEMA
2017, 2018a). Figure 1 depicts the scope of work, which consists of 34.4 stream miles to be studied
with detailed methods, and 26.6 stream miles to be studied with approximate methods.
Figure 1: Location and Study Area Map for the Nisqually River
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This report will discuss the analyses performed to estimate new 10-, 4-, 2-, 1-, 0.2-, and 1-percent plus
annual-chance peak discharges for the study area, which followed methods for frequency analysis of
streamflow gage data described in Bulletin 17C (England and others, 2018). To address community
preference, a methodology to minimize the impacts of flow regulation was devised and applied to
gage data prior to performing frequency analysis.
1.3. Watershed Characteristics and Flooding History
The Nisqually River is one of the major rivers in Western Washington, forming Pierce County’s
southern boundary. From Puget Sound to just upstream of Alder Lake near Elbe, the river separates
Pierce and Thurston Counties; moving upstream from Elbe, the river separates Pierce and Lewis
Counties.
Topography changes from flat, or moderately hilly, in the northwestern portion of the watershed near
Puget Sound, to rolling upland toward the central part and foothills, and mountainous areas in the
southeast. Elevations increase in the southeasterly direction, ranging from near sea level around
Puget Sound, to a maximum of 14,410 feet at the top of Mount Rainier.
The Nisqually River is fed continuously by rain and snowmelt from the Nisqually Glacier at Mount
Rainer. Floods generally occur during the winter months due to intense rainfall, often augmented by
snowmelt (FEMA, 2017, 2018a). Besides surface water flooding, the watershed has sustained other
significant flood hazards, such as channel migration and groundwater flooding (Pierce County Public
Works and Utilities, 2008).
With an estimated 50,000 cfs observed at the City of McKenna (FEMA, 2017), the flood of February
1996 remains the largest event on record, exceeding the effective 0.2-percent annual chance flow.
Besides 1996, the most severe floods recorded near or at McKenna happened in January 1965
(25,700 cfs), January 1974 (23,200 cfs) and December 1975 (30,700 cfs). A flood in December 1933,
before gage records near McKenna were available, had an estimated peak of 42,000 cfs (FEMA, 2017,
2018a).
The Nisqually River has been affected by flow regulation since the early 1900’s. The most significant
source of regulation is the Nisqually River Hydroelectric Project, owned and operated by Tacoma
Power. The project is located at La Grande Canyon, the physical divide between the Upper and Lower
Nisqually basins, and is comprised of Alder Dam and La Grande Dam (Tacoma Power, 2018). Originally
built as a diversion dam in 1912, La Grande Dam was replaced in 1945 and created a small reservoir
with 45-acre surface area at its maximum elevation. Completed in 1945, Alder Dam was built two
miles upstream, impounding a larger volume of water to create the 3,000-acre Alder Lake Reservoir.
Besides power generation, the highest priority for reservoir water level operation is to maintain
downstream river flows above the required minimum year-round. During summer months, operation
is also required to maintain Alder Lake water levels high to sustain recreational activities. Despite
there is no flood control requirement, Tacoma Power lowers lake levels whenever possible during Fall
and Winter (Tacoma Power, 2018), so that peak flow attenuation may be provided.
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1.4. Previous Studies
Countywide Flood Insurance Studies (FIS) are available for both Pierce and Thurston Counties: Pierce
County FIS (FEMA, 2017) became effective on March 7, 2017, while Thurston County FIS (FEMA,
2018a) became effective on May 15, 2018. A Thurston County Preliminary FIS, issued on June 29,
2018, includes a Physical Map Revision (PMR) to the Lower Chehalis watershed, not affecting the
Nisqually River or its tributaries. The Lewis County Countywide FIS is on-hold due to revised Levee
Analysis and Mapping Procedures (LAMP) for non-accredited levees. The FIS for Lewis County and
Unincorporated Areas became effective on July 17, 2006 (FEMA, 2006), but does not include studies
related to the Nisqually River.
The Nisqually River has experienced significant channel migration since the last time it was studied in
detail (FEMA, 2017, 2018a). Channel migration made the effective floodplain inaccurate and not
suitable for a conventional redelineation process. As a result, all Zone AE (detailed) areas have been
converted to Zone A (approximate) and effective discharges along the river have not been published
on the countywide FIS reports (FEMA, 2017, 2018a).
Both countywide FIS reports do not describe the last detailed study, which can only be found in
historic FIS reports for Unincorporated Pierce (FEMA, 1987) and Unincorporated Thurston (FEMA,
1999) Counties. Completed by the U.S. Geological Survey (USGS) in November 1980, the hydrologic
analysis consisted in developing and applying regression equations to estimate 10-, 2-, 1-, and
0.2-percent annual chance peak discharges based on drainage area and mean annual precipitation.
The analysis used records from more than 60 streamflow gages, most within Pierce and Thurston
Counties, and included at least one regulated gage on the Nisqually River. Peak discharge estimates
for the quantiles of interest were obtained by fitting a log-Pearson Type III to the annual peak series
at each gaged location; these peak estimates were then used to develop regression equations for each
of the above-mentioned quantiles.
Local communities have expressed concerns that effective flows resulting from the previous detailed
study were low and underestimated risk; the flood of 1996 reinforced such concerns. One- and
0.2-percent annual chance effective flows near the City of McKenna were respectively 32,000 cfs and
44,000 cfs, while the peak in 1996 was estimated in 50,000 cfs. Most communities within the Nisqually
River watershed regulate to higher standards than those required by FEMA, using 1996 peak
discharges as standard. Communities are particularly concerned with flow regulation and would
prefer to see effective flows estimated for natural conditions, ignoring or minimizing the impacts of
regulation.
2. Gage Analysis and Adjustments
2.1. Gage Data
Table 1 lists all USGS peak streamflow gages located along the Nisqually River, with respective periods
of record and regulation status. Gages 12082500 (near National), 12086500 (at La Grande) and
12089500 (at McKenna), shown in Figure 1, are active long-recording gages that were selected for
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analysis. Records from three inactive gages – 12084000, 12088400 and 12088500 – were used to fill
gaps or provide historical information to support analyses.
Table 1: USGS Peak Streamflow Gages
*Drainage area values as reported on USGS National Water Information System (NWIS) website.
Community preference that hydrologic analysis reflect natural conditions, ignoring or minimizing the
impacts of flow regulation, led to the development and application of a methodology that consists in
(1) identifying, within the regulated record, flood events that were not significantly impacted by
regulation, or minimally regulated; (2) extending the minimally regulated series of annual events using
peak flow data from a long-term unregulated gage; and (3) performing frequency analysis to the
resulting extended series. This methodology, including how minimally regulated events are defined
and identified, and its application to Nisqually River streamflow gage data, are described in more
detail in Appendix A.
2.2. Bulletin 17C Statistical Analysis
USGS PeakFQ 7.2 (Flynn and others, 2006; Veilleux and others, 2014) was used to perform statistical
analysis on the stream gage data described in section 2.1. PeakFQ performs flood frequency analysis
in accordance with procedures outlined in both Bulletin 17B (IACWD, 1982) and Bulletin 17C (England
and others, 2018), which recommend the use of systematic gage records of at least 10 years. Flood
frequency analyses performed for this project followed Bulletin 17C methodology, which includes
application of the Expected Moments Algorithm (EMA) and the Multiple Grubbs-Beck test (MGB).
Statistical analyses performed for this study are described in detail in Appendix A.
Bulletin 17C recommends using a regional, or generalized, skew coefficient to weight the station skew
when fitting a log Pearson Type III distribution to the annual peak flow series. However, this study is
based on the relationship between unregulated peak flows in the Upper Nisqually and minimally
regulated flows in the Lower Nisqually. Since the river has one unregulated and two regulated long-
USGS
Gage IDLocation
Drainage
area*
(sq.mi.)
Period of Record
(Water Years)
No. of
peaks
Largest
recorded
peak Status
Regulation
status
12082500NISQUALLY RIVER NEAR
NATIONAL, WA133 1943-2017 75
21,800 cfs (11/06/2006)
Active Unregulated
12084000NISQUALLY RIVER NEAR
ALDER, WA252 1932-1944 13
25,000 cfs (12/22/1933)
Inactive Unregulated
12086500NISQUALLY RIVER AT LA
GRANDE, WA292
1907-08, 1910-11;
1920-1931; 1945-201789
39,000 cfs (02/08/1996)
Active Regulated
12088400NISQUALLY RIVER AB POWELL
CREEK NEAR MCKENNA, WA431 1970-1979 10
30,700 cfs (12/04/1975)
Inactive Regulated
12088500NISQUALLY RIVER NEAR
MCKENNA, WA445 1942-1963 22
20,800 cfs (12/12/1955)
Inactive Regulated
12089500NISQUALLY RIVER AT
MCKENNA, WA517 1948-1968; 1978-2017 61
50,000 cfs (02/08/1996)
Active Regulated
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recording gages (70+ years) along its course, it seemed important to emphasize at-site gage data by
applying the station skew coefficient to all final gage analyses.
Nonetheless, every scenario investigated had one analysis performed using the station skew and
another using the weighted skew. Whenever the weighted skew was applied, the station skew was
weighted by the recently developed regional skew coefficient for the Pacific Northwest (Appendix A
of SIR 2016-5118). The PNW skew coefficient is applicable to the states of Idaho, Oregon, Washington
and parts of Montana, and is equal to -0.07, with a mean square error (MSE) equal to 0.18. Overall,
using the station skew instead of the weighted skew did not result in significant differences for
1% flows: when applying the station skew, estimates were always higher then when the weighted
skew was applied but they were essentially the same for both National and La Grande and within 5%
for McKenna.
One-percent plus and minus flows respectively correspond to the upper (84%) and lower (16%)
confidence limits of the 68% confidence interval for the one-percent annual chance peak discharge.
PeakFQ calculates 84% and 16% confidence limits for each quantile estimate according to Appendix 7
of Bulletin 17C.
Results from these analyses are shown in Table 2.
Table 2: Gage Discharges
Gage Location 10% 4% 2% 1% 0.2% 1% plus
Source
12082500 Nisqually River near National
12,900 16,600 19,500 22,500 30,000 29,800 PeakFQ
120865001 Nisqually River at La Grande
24,400 30,300 34,800 39,300 50,400 53,100 PeakFQ with
MOVE 2
120895001 Nisqually River at McKenna3
29,900 38,300 45,000 52,100 70,500 80,400 PeakFQ with
MOVE 2
(1) regulated gages; analysis performed on minimally regulated series
(2) see Appendix A
(3) PeakFQ results were subtracted by 800 cfs corresponding to the Centralia Power Canal diversion
A comparison against effective and historic peak flows at McKenna is presented in
Table 3. Both 1% and 0.2% peak discharges (respectively, 52,000 cfs and 70,000 cfs) are about 60%
higher than the corresponding effective discharges. However, the 1% peak is of the same magnitude
as the historic flood of 1996.
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Table 3: Effective, historic and estimated peak flows at McKenna
3. Discharges at Ungaged Sites
3.1. StreamStats
StreamStats (USGS, 2016; Ries and others, 2017) is a web-based geographic information system (GIS)
application that provides streamflow statistics for a user-selected site on a stream grid. Once a site is
selected, the application delineates its catchment and computes characteristics such as drainage area
and average precipitation. StreamStats uses these characteristics to estimate peak discharges based
on regression equations developed for unregulated rural (or un-urbanized) watersheds.
Gaged and selected ungaged sites along the Nisqually River were created as points in GIS and
submitted to StreamStats for batch processing. For Washington State, StreamStats uses a 30-meter
digital elevation model (DEM) derived from the National Hydrologic Dataset Plus (NHDPlus) to
determine catchment boundaries and other topographic characteristics (USGS, 2016; Mastin and
others, 2016). Resulting batch catchment delineations, characteristics, and point data were later used
to estimate flows for ungaged locations.
3.2. Regional Regression Equations
Discharges for ungaged sites can be estimated using regional regression equations for rural or urban
watersheds. However, regression equations are not recommended for regulated streams such as the
Nisqually River; consequently, they were not applied in this study.
For reference, regional regression equations for un-urbanized, unregulated streams in Washington
are found in USGS report SIR 2016-5118 (Mastin and others, 2016), and the Nisqually River watershed
is located in flood region 3.
Q 1% 32,000
Q 0.2% 44,000
Q 1% 52,000
Q 0.2% 70,000
February 1996 50,000December 1933 42,000
Peak discharge (cfs)
Effective
(FEMA, 1987, 1999)
Bulletin 17C +
extended minimally
regulated series
(2018)
Historic floods
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3.3. Discharge vs. Drainage Area
Discharges at ungaged sites on a gaged stream can be estimated using drainage area weighting (Ries,
2007; Mastin and others, 2016). However, applying the area weighting equation from USGS SIR 2016-
5118 between gaged locations near McKenna overestimated downstream peak flows.
As a result, peak flows at ungaged sites were estimated using discharge vs. drainage area-frequency
curves. These curves represent the relationship between flow quantiles obtained through gage
analysis (Table 2) and drainage area at the gage (Table 1). These relationships may be expressed as:
�(�) = ��(��)��
where coefficients C1 and C2 can be obtained through linear regression of the logarithms of peak flow
estimates against drainage area at the gages. Table 4 lists C1, C2 and r2 (square of linear regression
correlation coefficient r) for each annual exceedance probability (AEP) of interest, while Figure 2
shows the respective peak flow vs. drainage area curves developed.
Table 4: Peak discharge vs. Drainage area coefficients
Figure 2: Peak discharges vs. drainage area-frequency curves based on Nisqually River gages
AEP (%) C1 C2 r 2
10 571.93 0.6441 0.97
4 756.31 0.6361 0.98
2 891.90 0.6338 0.99
1 1024.3 0.6335 0.99
0.2 1318.1 0.6382 1.00
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FEMA guidance for estimating 1% plus flows on gaged streams (FEMA, 2018b) recommends the use
of synthetic statistics presented in Bulletin 17B, which are not applicable to Bulletin 17C methods. As
a result, 1% plus flows for ungaged locations were also estimated through a peak flow vs. drainage
area relationship. Similar to the method used for flow quantiles, the relationship for 1% plus flows
was developed from the values estimated at gaged locations (Table 2), resulting in an equation with
coefficients C1 and C2 equal to 799.76 and 0.7375, respectively, and r2 equal to 1.00.
3.4. Major Lakes and Reservoirs
The study area includes Alder Lake Reservoir, the 3,000-acre impoundment created by Alder Dam on
the Nisqually River, which is described in Section 1.3. Flows upstream of Alder Lake were based on
frequency analysis of the Nisqually near National gage record, while flows downstream were based
on the gage at La Grande. Frequency analyses of both gages are described in Section 2.
3.5. Final Discharge Grids
Stream grids containing the final estimated 10%, 4%, 2%, 1%, 0.2%, and 1%+ discharges were created
for use in the hydraulic analysis.
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4. References England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E.,
and Mason, R.R., Jr., 2018, Guidelines for determining flood flow frequency — Bulletin 17 C,
U.S. Geological Survey Techniques and Methods 4-B5, 148p., https://doi.org/10.3133/tm4B5.
Federal Emergency Management Agency, 1987, Flood Insurance Study, Pierce County, Washington,
and Unincorporated Areas.
Federal Emergency Management Agency, 1999, Flood Insurance Study, Thurston County, Washington
and Unincorporated Areas.
Federal Emergency Management Agency, 2006, Flood Insurance Study, Lewis County, Washington,
and Unincorporated Areas.
Federal Emergency Management Agency, 2017, Flood Insurance Study, Pierce County, Washington,
and Incorporated Areas.
Federal Emergency Management Agency, 2018a, Flood Insurance Study, Thurston County,
Washington, and Incorporated Area.
Federal Emergency Management Agency, 2018b, Guidance for Flood Risk Analysis and Mapping,
General Hydrologic Considerations.
Flynn, K.M., Kirby, W.H., and Hummel, P.R., 2006, User’s manual for program PeakFQ, Annual Flood
Frequency Analysis using 17B Guidelines, U.S. Geological Survey Techniques and Methods 4-B4, 42 p.
Interagency Advisory Committee on Water Data (IACWD), 1982, Guidelines for determining flood flow
frequency, Bulletin 17B of the Hydrology Committee, U.S. Department of the Interior Geological
Survey, Office of Water Data Coordination, Reston, VA.
Mastin, M.C., Konrad, C.P., Veilleux, A.G., and Tecca, A.E., 2016, Magnitude, frequency, and trends of
floods at gaged and ungaged sites in Washington, based on data through water year 2014 (v.1.2,
November 2017), U.S. Geological Survey Scientific Investigations Report 2016–5118, 70 p.,
http://dx.doi.org/10.3133/sir20165118.
Pierce County Public Works and Utilities, Surface Water Management, 2008, Nisqually River Basin
Plan, Volume 2, Pierce County, WA.
Ries, K.G., III, 2007, The national streamflow statistics program: A computer program for estimating
streamflow statistics for ungaged sites, U.S. Geological Survey Techniques and Methods 4-A6, 37 p.
Ries, K.G., III, Newson, J.K., Smith, M.J., Guthrie, J.D., Steeves, P.A., Haluska, T.L., Kolb, K.R., Thompson,
R.F., Santoro, R.D., and Vraga, H.W., 2017, StreamStats, version 4, U.S. Geological Survey Fact 2017–
3046, 4 p., https://doi.org/10.3133/fs20173046 .
12
Tacoma Power, 2018, Nisqually River Project, Tacoma Public Utilities, City of Tacoma, WA,
https://www.mytpu.org/tacomapower/about-tacoma-power/dams-power-sources/hydro-
power/nisqually-river-project/.
U.S. Geological Survey, 2016, The StreamStats program, online at http://streamstats.usgs.gov .
Veilleux, A.G., Cohn, T.A., Flynn, K.M., Mason, R.R., Jr., and Hummel, P.R., 2014, Estimating magnitude
and frequency of floods using the PeakFQ 7.0 program, U.S. Geological Survey Fact Sheet 2013–3108,
2 p., https://dx.doi.org/10.3133/fs20133108.
13
Appendix A
Nisqually River Hydrology
Frequency analysis of minimally regulated peak discharges
14
Appendix B
Frequency analysis of Nisqually River gages
Supplemental Information
Appendix A
Nisqually River Hydrology
Frequency analysis of minimally regulated peak discharges
14
Nisqually River Hydrology Frequency analysis of minimally regulated peak discharges
Background
The historic flood of February 1996 brought significant flooding and damage to the Nisqually River
watershed (Pierce County Public Works and Utilities, 2008). With an estimated 50,000 cfs observed at the
City of McKenna (FEMA, 2017), the 1996 flood remains the largest event on record, exceeding the
effective 500-year flow.
The magnitude of the 1996 flood reinforced local concerns that effective flows underestimate risk. Most
concerns are associated with regulation since effective flows were estimated based on frequency analysis
of regulated stream gage records (FEMA, 1999). Local communities would prefer to perform such analyses
ignoring or minimizing the impacts of regulation, replicating unregulated conditions as much as possible.
However, hydrologic analysis must contend with the fact that the Nisqually River has experienced some
degree of flow regulation since the early 1900’s.
This report describes a methodology for frequency analysis of regulated peak discharges that aims to
address both the community wish to simulate unregulated conditions and the limitations imposed by long
term regulation.
Impact of regulation on Nisqually River peak discharges
The most significant source of regulation affecting the Nisqually River is the Nisqually River Hydroelectric
Project, located at the divide between the Upper and Lower Nisqually basins. The project is comprised of
Alder Dam, completed in 1945, and La Grande Dam, built in 1912 but replaced in 1945. Besides power
generation, the highest priority for dam operation is to maintain downstream water levels year-round.
During summer months, dam operation is also required to maintain water levels in Alder Lake, the
reservoir created behind Alder Dam. Despite that there are no flood control requirements, the reservoir
may provide peak attenuation, especially for Fall and early Winter events.
The impact of regulation can be observed by comparing regulated against unregulated peak discharges
along the Nisqually River. Table 1 lists all Nisqually River USGS peak streamflow gages and their respective
periods of record. The gages near National (12082500), at La Grande (12086500) and at McKenna
(12089500) – shown in Figure 1 and highlighted on Table 1 – are long-recording gages that remain active.
Out of the three, only the near National gage, in the Upper Nisqually, is unregulated; the gages at
La Grande, just downstream of the dam, and at McKenna have their entire records affected by regulation.
15
Table 1 – Nisqually River USGS streamflow gages
*Drainage area values as reported on USGS National Water Information System (NWIS) website.
Figure 1- Nisqually River USGS gages
USGS
Gage IDLocation
Drainage
area*
(sq.mi.)
Period of Record
(Water Years)
No. of
peaks
Largest
recorded
peak Status
Regulation
status
12082500NISQUALLY RIVER NEAR
NATIONAL, WA133 1943-2017 75
21,800 cfs (11/06/2006)
Active Unregulated
12084000NISQUALLY RIVER NEAR
ALDER, WA252 1932-1944 13
25,000 cfs (12/22/1933)
Inactive Unregulated
12086500NISQUALLY RIVER AT LA
GRANDE, WA292
1907-08, 1910-11;
1920-1931; 1945-201789
39,000 cfs (02/08/1996)
Active Regulated
12088400NISQUALLY RIVER AB POWELL
CREEK NEAR MCKENNA, WA431 1970-1979 10
30,700 cfs (12/04/1975)
Inactive Regulated
12088500NISQUALLY RIVER NEAR
MCKENNA, WA445 1942-1963 22
20,800 cfs (12/12/1955)
Inactive Regulated
12089500NISQUALLY RIVER AT
MCKENNA, WA517 1948-1968; 1978-2017 61
50,000 cfs (02/08/1996)
Active Regulated
16
The February 1996 flood is the largest event ever recorded at the gages downstream of Alder Reservoir.
However, at the upstream gage near National, the 1996 peak discharge is the second largest in the record,
slightly lower than the November 2006 peak. A comparison of peak discharges at all three gages for both
events (Table 2) shows that November 2006 peak flows were significantly attenuated downstream of the
reservoir, while February 1996 flows were not.
Table 2 - Peak discharges for selected floods at Nisqually River USGS gages
Hydrographs for both events at all three Nisqually River gages (Figure 2) display the level of attenuation
downstream of La Grande Dam in more detail. While the November 2006 event was clearly attenuated
by Alder Lake reservoir storage, the February 1996 event seems to have been minimally impacted.
Peakflows
(cfs)
February
1996
November
2006
Near National
(Upper Nisqually)
At La Grande
(just d/s La Grande dam)
At McKenna
(Lower Nisqually)
21,200 21,800
39,500 7,540
50,000 12,500
17
Figure 2 – Hydrographs for February 1996 and November 2006 floods
18
A comparison of Alder Lake levels immediately before and after each event (Figure 3) underscores the
role reservoir storage play in attenuating peak flows. In 1996, reservoir level was near its maximum when
the flood happened, leaving minimal storage for flow attenuation. In 2006, however, levels were low,
providing enough storage to attenuate peak flows downstream.
Figure 3 – Alder Reservoir levels in February 1996 and November 2006
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Minimally regulated peak discharge series for the Nisqually River
Defining minimally regulated flood events
Minimally regulated events are defined as flood events that occur when reservoir storage capacity is
negligible, so that the impact of regulation on peak flows is significantly reduced. Minimally regulated
peak flows should closely resemble unregulated flows.
The February 1996 flood is an example of a minimally regulated event. It is possible that the annual peak
discharge series at Nisqually River regulated gages contain other minimally regulated events. If enough of
such events covering a wide range of flow magnitudes, not just the largest floods, could be identified, a
series of minimally regulated peak discharges could be developed and analyzed.
For gages in the same stream, it is expected for minimally regulated and unregulated peak flows to be
well correlated. In general, regulated and unregulated gages are not well correlated, as is the case for
Nisqually River gages (Figure 4).
Figure 4 – Regulated vs. unregulated annual peak discharges
Identifying minimally regulated annual flood events at La Grande
Identification of minimally regulated events was based on unregulated peak discharges near National,
regulated peaks at La Grande and Alder Reservoir levels. These events should have similar characteristics
to the 1996 flood, i.e., annual events near National should show minimal attenuation when recorded at
La Grande.
20
Identification followed the criteria below:
1. Annual peak event is the same at both near National and at La Grande gages, recorded within four
days;
2. Annual peak discharge at La Grande is significantly higher than annual peak at National, implying
minimal peak attenuation;
3. Alder Reservoir average level at the day of the peak is high, indicating reduced storage availability.
Alder Reservoir average daily elevations from January 1950 to July 2018 were provided by Tacoma Power,
owner and operator of La Grande and Alder dams. Given that the Nisqually Hydroelectric Project became
operational in 1945, only gage records from WY1946 to WY2017 were considered for identifying minimally
regulated annual events.
The resulting minimally regulated peak flow series at La Grande is listed on Table 3, containing 19 out of
the 73 annual events recorded at both National and La Grande post-1945. Events are listed by their rank
as recorded at the near National gage, ranging in magnitude from the second (February 1996) to the
67th largest event (May 1949).
Table 3 – Unregulated vs. minimally regulated annual peaks
rank date Peak (cfs) rank date Peak (cfs)
1996 2 2/8/1996 21,200 1 2/8/1996 39,500
1976 8 12/4/1975 13,200 2 12/4/1975 27,100
1981 9 12/26/1980 11,600 3 12/26/1980 21,500
1965 11 1/29/1965 11,000 9 1/30/1965 16,000
1960 13 11/23/1959 10,900 5 11/23/1959 17,900
1982 24 2/20/1982 8,280 8 2/18/1982 16,800
1956 33 12/12/1955 7,470 7 12/12/1955 16,900
2006 38 1/10/2006 7,030 15 1/11/2006 13,400
1954 39 12/9/1953 6,640 13 12/12/1953 14,500
1999 41 12/29/1998 6,350 23 12/29/1998 11,400
2014 42 3/9/2014 6,090 25 3/10/2014 11,100
1951 43 2/11/1951 6,050 10 2/10/1951 15,400
1946 50 12/28/1945 5,000 28 12/31/1945 10,600
1953 51 1/31/1953 4,760 29 2/1/1953 10,600
1961 54 2/21/1961 4,350 17 2/21/1961 13,200
1970 56 1/23/1970 4,350 40 1/27/1970 8,520
2017 57 3/15/2017 4,300 42 3/18/2017 7,960
1964 62 1/25/1964 3,560 39 1/25/1964 8,820
1949 67 5/13/1949 3,010 51 5/13/1949 6,640
Water
year
USGS 12082500, Near National USGS 12086500, at La Grande
21
Linear regression was applied to the logarithms of the 19 minimally regulated peak flows at La Grande
against the corresponding unregulated peaks near National. Peak flows are well correlated with
correlation coefficient R equal to 0.95 (R2 = 0.8928), as showed in Figure 5.
Figure 5 – Minimally regulated vs. unregulated annual events
Minimally regulated peak flow series at McKenna
Since the major source of regulation for the Lower Nisqually is the Nisqually River Project, identification
of minimally regulated peak flows at McKenna was based on the series developed for La Grande.
The record at McKenna has gaps in the mid-1940s and in the 1970s, missing a few of the events included
in the La Grande minimally regulated series, including major floods in January 1974 and December 1975.
Two inactive gages located near McKenna (USGS 12088400 and 12088500) had records for the missing
years and were used to fill the gaps. The near McKenna gages drained approximately 85% of the area
drained by the long-recording gage 12089500 at McKenna. The combined record was highly correlated
(98%) to gage 12089500, so that the resulting regression equation was applied to estimate peak flows for
the missing years. Application of a drainage area-ratio was also investigated but produced significantly
higher than expected flows for same magnitude observed peaks.
Another important source of regulation affecting flows at McKenna is the Centralia Power Canal diversion,
which diverts flows upstream of the gage and release them downstream. The Centralia Power Canal
maximum diversion is approximately 800 cfs, which seems to be consistently reached every year since
22
daily streamflow data become available in 1979. Even though the maximum diversion may be an
insignificant portion of the largest recorded events, such as 1996, it can be close to 10% of the smallest
minimally regulated peak flows identified at the gage at McKenna. Hence, 800 cfs were added to the
annual peak flows recorded at the gage for analysis purposes.
The resulting minimally regulated series at McKenna contains 18 events and is listed on Table 4 below:
Table 4 – Unregulated vs. minimally regulated annual peaks
*Annual peak flows were adjusted by 800 cfs to account for the Centralia Power Canal diversion
Events marked with an asterisk correspond to missing years at McKenna that were estimated using near
McKenna gages. The 1949 annual event at McKenna was not the same as at La Grande and near National,
therefore being excluded from the series.
A linear regression of the logarithms of peak flows for the 18 events above resulted in a correlation
coefficient R equal to 0.89 (R2 = 0.7959), showing that minimally regulated peaks at McKenna and
unregulated peaks near National are well correlated (Figure 6).
rank date Peak (cfs) rank date Peak (cfs)
1996 2 2/8/1996 21,200 1 2/8/1996 50,800
1976 8 12/4/1975 13,200 * 12/4/1975 32,800
1981 9 12/26/1980 11,600 3 12/26/1980 21,900
1965 11 1/29/1965 11,000 2 1/29/1965 26,500
1960 13 11/23/1959 10,900 4 11/23/1959 21,300
1982 24 2/20/1982 8,280 10 2/19/1982 17,000
1956 33 12/12/1955 7,470 5 12/12/1955 21,000
2006 38 1/10/2006 7,030 16 1/11/2006 16,200
1954 39 12/9/1953 6,640 9 12/10/1953 17,000
1999 41 12/29/1998 6,350 21 12/28/1998 14,200
2014 42 3/9/2014 6,090 27 3/10/2014 12,100
1951 43 2/11/1951 6,050 8 2/11/1951 17,700
1946 49 12/28/1945 5,000 * 12/31/1945 11,200
1953 50 1/31/1953 4,760 31 2/1/1953 10,790
1961 53 2/21/1961 4,350 15 2/22/1961 16,500
1970 55 1/23/1970 4,350 * 1/27/1970 11,100
2017 56 3/15/2017 4,300 32 3/18/2017 10,460
1964 61 1/25/1964 3,560 19 1/25/1964 15,100
Water
year
USGS 12082500, Near National USGS 12089500, at McKenna*
23
Figure 6 – Minimally regulated at McKenna vs. unregulated annual events
Extending minimally regulated series with Bulletin 17C MOVE
Using record extension to improve quantile estimates
Minimally regulated peak flow series at La Grande and McKenna respectively contain 19 and 18 annual
events, more than the minimum number recommended by Bulletin 17C (England and others, 2018) for
performing flood frequency analysis. Nonetheless, both series are short and may not be adequate to
estimate the more extreme, less frequent flows such as the 1% annual chance peak. For example,
frequency analysis of the observed minimally regulated events at La Grande resulted in the fitted curve
shown on Figure 7.
Bulletin 17C recommends the application of MOVE (Maintenance of Variance Extension) techniques to
extend gage records using a nearby site. Extension is particularly beneficial when the record of interest is
short, less than 20 years, and a longer, highly correlated record (R > 0.8) is available. The methodology,
described in Appendix 8 of Bulletin 17C, was applied to extend the minimally regulated series at La Grande
and McKenna.
In this study, the minimally regulated series are the short records to be extended; the 75-year long annual
peak flow series at the gage near National is the long record. As presented in the previous section, the
correlation coefficients for National vs. La Grande and National vs. McKenna are both above the critical
value of 0.8, making the extension suitable. It is recommended that short and long records have at least
10 years of concurrent data; however, it is not required that the concurrent years are consecutive.
24
Figure 7 – Frequency analysis of minimally regulated events observed at La Grande
Effective record length (ne)
One critical aspect of using MOVE is that record extension should be limited to ne number of years, where
ne is the effective length of record to be added to the short record. Therefore, different combinations of
ne years may be added to the short record, each producing extended series that may result in different
flow quantile estimates.
The effective length is a function of statistical characteristics of the original long and short records and it
was calculated following Appendix 8 of Bulletin 17C. In this application, the number of concurrent years
is the length of each minimally regulated series. Table 5 shows values for ne and MOVE extended record
length for the series at La Grande and McKenna.
Selection of which ne years are used to extend the short record can be somewhat subjective. Bulletin 17C
suggests starting with the most recent observations from the non-concurrent period and checking the
reasonableness of the resulting extended series. Inclusion of a sequence of wet years, for example, may
lead to an extension that misrepresent conditions at a given location.
25
Table 5 – MOVE extended record length
Using MOVE estimated parameters, two series with 44 annual events were developed for La Grande, and
three series with 32 events for McKenna. The first series at each location starts extension with the most
recent year that did not have a minimally regulated event as its annual event. In both cases, 2017 was the
most recent minimally regulated event, preceded by the 2014 event; hence, the first year extended was
2016, then 2015. Extension was applied to every year missing a minimally regulated event until ne years
were extended. The complete extended series displays a period of continuous record containing extended
and observed events, as shown in Figure 8. Selection of which extended series will be adopted at each
location will be discussed next.
Figure 8 – Example of MOVE extended series
La Grande McKenna
correlation coefficient
r0.95 0.89
concurrent years
n 1
19 18
effective record length
n e
25 14
extended record length
n 1 + n e
44 32
26
Extended series of minimally regulated peak flows at La Grande
Frequency analysis of annual flows at the gage near National was used to support selection of which
extended series better represented minimally regulated conditions at La Grande. Based on how minimally
regulated events at La Grande are well correlated against unregulated events near National, it is expected
that floods of the same relative magnitude have the same probability of occurrence. It is also likely that
the frequency distributions have skew coefficients that are relatively close.
The complete record at the gage near National contains 75 annual events (WY 1943-WY 2017). Flood
frequency analysis was performed with PeakFQ 7.2 (Flynn and others, 2006; Veilleux and others, 2014) in
accordance with procedures outlined in Bulletin 17C. The station skew coefficient was adopted to
emphasize at-site data characteristics that could be compared against records from other gages. The
estimated 1% annual chance flow equals 22,500 cfs, which is about the same magnitude as the 1996 and
2007 floods near National, respectively 21,200 and 21,800 cfs.
Two extended series with 44 annual events were developed for La Grande. Series I contains MOVE
generated flows based on the most recent observations, including the 2007 flood, the largest observed
near National. Series II contains MOVE generated flows from 1976 till 2006, including the 1978 flood, third
largest observed near National. The 1996 flood was the second largest near National and, being a
minimally regulated event, is present in both series.
Frequency analysis was applied to both series, the station skew coefficient was used to fit the frequency
distribution – Table 6 compares results against estimates for near National and for the observed minimally
regulated series at La Grande (19 events). Series I was selected since it presents similar characteristics to
the near National annual series: the 1% annual chance peak of 39,400 cfs is of the same magnitude as of
the 1996 flood (39,500 cfs), and the stations skew coefficients are very close (-0.034 vs. -0.023).
Table 6 – Comparison of MOVE extended series at La Grande
Observed
series
Extended
series I
Extended
series II
Continuous record n/a19 events
(1945-2017) 1988-2017 1976-2006
Station skew -0.034 0.616 -0.023 -0.219
Q1% (cfs) 22,500 46,000 39,400 36,800
1996 peak (cfs) 21,200
At La Grande
39,500
Frequency
Analysis
Near
National
27
Extended series of minimally regulated annual peak flows at McKenna
The same procedure applied at La Grande was used to select one of the three extended series developed
for the gage at McKenna. Each extended series contains 32 annual events, 14 generated using MOVE.
Similar to La Grande, series I is based on the most recent observations and includes the 2007 flood.
Series II also include the 2007 event as it contains MOVE generated flows from 1991 till 2007. Series III
covers a lengthy period without observed minimally regulated events, 1985 through 1995.
Frequency analysis was applied to all three series – Table 7 compares results against estimates for near
National and for the observed minimally regulated series at McKenna (18 events). Series III was discarded
based on the magnitude of the 1% annual chance flow of 45,700 cfs as compared to the 1996 flood
adjusted peak of 50,800 cfs. Neither series I or series II presented station skew coefficients close to the
value of -0.034 for the near National series. Ultimately, series I was selected since it was based on the
most recent observations and presented both a higher estimate for the 1% annual chance peak
(51,100 cfs) and a better fit of the upper tail of the frequency distribution. Another point to consider is
that an upward trend in western Washington peak flows has been recently reported (Mastin and others,
2016), which would be consistent with this selection.
Table 7 – Comparison of MOVE extended series at McKenna
*Annual peak flows were adjusted by 800 cfs to account for the Centralia Power Canal diversion
Observed
series
Extended
series I
Extended
series II
Extended
series III
Continuous record n/a18 events
(1945-2017) 2001-2017 1991-2007 1984-1999
Station skew -0.034 1.02 0.184 0.138 -0.148
Q1% (cfs) 22,500 61,500 51,100 50,400 45,700
1996 peak (cfs) 21,200 50,800
At McKenna *Frequency
Analysis
Near
National
28
Improving frequency analysis estimates with historical information
The flood of December 1933 remains the second largest event observed at McKenna, with an estimated
peak discharge of 42,000 cfs (FEMA, 2017). This flood was a historic basin wide event, with a recorded
peak of 25,000 cfs at the inactive gage near Alder (USGS 12084000), upstream of La Grande. Despite that
it predates the construction of Alder Reservoir, the December 1933 flood can certainly be accounted as a
minimally regulated event due to its magnitude.
Adding historic information to the series allows for a reasonableness check. Treatment of historic data is
one of the improvements of Bulletin 17C over previous guidelines. A historic flood can be added to the
systematic record as a point or as an interval; it can also be used to set a perception threshold for the
years between the event and the onset of stream gaging.
Consequently, the December 1933 was included in both minimally regulated series. The flood was added
as a point observation to the extended record at McKenna, being used to set a perception threshold of
40,000 cfs for the period between WY 1935 and WY 1944. The recorded peak at the gage near Alder was
transferred through drainage-area ratio to estimate the peak flow at La Grande, resulting in a flow of
29,000 cfs. Based on the estimated flow reported at McKenna and other extreme events recorded at both
gages (such as 1996), it is possible that the 1933 peak at La Grande was higher, perhaps up to 34,000 cfs.
For that reason, the December 1933 flood was represented at La Grande as a flow interval (29,000 to
34,000 cfs), and a perception threshold of 27,000 cfs was set for the WY1935-WY1944 period. A summary
of perceptible ranges applied in the analysis is presented on Table 8.
Table 8 – Perception thresholds for Bulletin 17C analysis of Nisqually River gages
USGS
Gage IDLocation
Start
Year
End
Year
Lower
Threshold
(cfs)
Upper
Threshold
(cfs)Comment
12082500 NEAR
NATIONAL1943 2017 0 Infinity Continuous systematic record.
1934 1944 27,000 Infinity Historical information.
1945 2017 0 Infinity
Systematic record - thresholds apply only to
years when the annual event was included in
the minimally regulated series.
1945 2017 Infinity Infinity
Systematic record - thresholds apply only to
years when the annual event was excluded
from the minimally regulated series.
1934 1944 40,000 Infinity Historical information.
1945 2017 0 Infinity
Systematic record - thresholds apply only to
years when the annual event was included in
the minimally regulated series.
1945 2017 Infinity Infinity
Systematic record - thresholds apply only to
years when the annual event was excluded
from the minimally regulated series.
12086500 AT
LA GRANDE
12089500 AT
MCKENNA
29
Frequency analysis was applied one last time to the minimally regulated series at La Grande and McKenna.
Resulting flow quantiles at all three locations along the Nisqually are presented on Table 9. Final
configuration of the minimally regulated annual series at La Grande is depicted in Figure 9, while the fitted
frequency curve in Figure 10. The minimally regulated series at McKenna and corresponding frequency
curves are depicted in Figure 11 and Figure 12. A comparison against effective and historic peak flows at
McKenna is presented on Table 10.
Table 9 – Peak flow quantiles at Nisqually River gages
*PeakFQ results for AEP and 1% plus flows at McKenna were subtracted
by 800 cfs corresponding to the Centralia Power Canal diversion
Table 10 – Effective, estimated and historic peak flows at McKenna
*Frequency analysis results were subtracted by 800 cfs corresponding to the
Centralia Power Canal diversion
Annual
Exceedance
Probability
(%)
Near
National
At
La Grande
At
McKenna *
0.2 29,950 50,410 70,470
1 22,450 39,330 52,060
2 19,460 34,760 44,950
4 16,590 30,280 38,250
10 12,940 24,430 29,910
50 6,528 13,470 15,610
1% plus 29,750 53,090 80,400
Skew -0.034 -0.049 0.165
30
Figure 9 – Minimally regulated series with 45 annual events
Figure 10 – Frequency curve for minimally regulated conditions for the Nisqually River at La Grande
31
Figure 11 – Minimally regulated series with 33 annual events
Figure 12 – Frequency curve for minimally regulated conditions for the Nisqually River at McKenna
32
Conclusion
This report presented a methodology aimed at reducing the impact of flow regulation on frequency
analysis of Nisqually River long-term gages. It introduced the concept of minimally regulated flood events,
showing how these events are well correlated with unregulated flows and how to identify them within a
regulated record.
This report also described the application of innovative approaches included in the recently released
Bulletin 17C. These approaches, meant to improve flow quantile estimation by reducing associated
uncertainty, involve record extension using MOVE techniques and treatment of historic information as to
how it may be added to the annual peak flow series.
Finally, the report compared flood frequency estimates against historical and effective FIS peak flows at
the City of McKenna. It illustrated how the estimated 1% annual chance flow of 52,000 cfs is in better
agreement with historical events than effective flows estimated using regulated data.
33
References
England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and
Mason, R.R., Jr., 2018, Guidelines for determining flood flow frequency — Bulletin 17 C, U.S. Geological
Survey Techniques and Methods 4-B5, 148p., https://doi.org/10.3133/tm4B5.
Federal Emergency Management Agency, 2017, Flood Insurance Study, Pierce County, Washington, and
Incorporated Areas.
Federal Emergency Management Agency, 1999, Flood Insurance Study, Thurston County, Washington,
and Unincorporated Areas.
Flynn, K.M., Kirby, W.H., and Hummel, P.R., 2006, User’s manual for program PeakFQ, Annual Flood
Frequency Analysis using 17B Guidelines, U.S. Geological Survey Techniques and Methods 4-B4, 42 p.
Interagency Advisory Committee on Water Data (IACWD), 1982, Guidelines for determining flood flow
frequency, Bulletin 17B of the Hydrology Committee, U.S. Department of the Interior Geological Survey,
Office of Water Data Coordination, Reston, VA.
Mastin, M.C., Konrad, C.P., Veilleux, A.G., and Tecca, A.E., 2016, Magnitude, frequency, and trends of
floods at gaged and ungaged sites in Washington, based on data through water year 2014 (v.1.2,
November 2017), U.S. Geological Survey Scientific Investigations Report 2016–5118, 70 p.,
http://dx.doi.org/10.3133/sir20165118.
Pierce County Public Works and Utilities, Surface Water Management, 2008, Nisqually River Basin Plan,
Volume 2, Pierce County, WA.
Ries, K.G., III, 2007, The national streamflow statistics program: A computer program for estimating
streamflow statistics for ungaged sites, U.S. Geological Survey Techniques and Methods 4-A6, 37 p.
Veilleux, A.G., Cohn, T.A., Flynn, K.M., Mason, R.R., Jr., and Hummel, P.R., 2014, Estimating magnitude
and frequency of floods using the PeakFQ 7.0 program, U.S. Geological Survey Fact Sheet 2013–3108,
2 p., https://dx.doi.org/10.3133/fs20133108.
Appendix B
Frequency analysis of Nisqually River gages
Supplemental Information
35
Table B 1
Water
Year
Rank at
National
Peak at
National
Peak Q at
National
(cfs)
Rank at
LaGrande
Peak at
La Grande
Peak Q at
La Grande
(cfs)
Rank at
McKenna
Peak at
McKenna
Peak Q at
McKenna
(cfs)
Elevation
at Alder
Lake
Minimally
Regulated
Event?
1946 50 12/28/1945 5,000 28 12/31/1945 10,600 - Y
1947 27 12/11/1946 8,100 22 12/14/1946 11,600 - M
1948 45 11/8/1947 5,560 41 1/2/1948 8,360 26 1/2/1948 11,500 - N
1949 67 5/13/1949 3,010 51 5/13/1949 6,640 36 12/9/1948 9,170 - Y
1950 36 11/27/1949 7,310 11 5/14/1950 15,000 24 11/28/1949 12,200 1,194.2 N
1951 43 2/11/1951 6,050 10 2/10/1951 15,400 8 2/11/1951 16,900 1,196.8 Y
1952 70 2/4/1952 2,700 34 12/28/1951 9,350 48 12/5/1951 5,280 1,184.3 N
1953 51 1/31/1953 4,760 29 2/1/1953 10,600 31 2/1/1953 9,990 1,204.0 Y
1954 39 12/9/1953 6,640 13 12/12/1953 14,500 9 12/10/1953 16,200 1,202.3 Y
1955 60 6/10/1955 3,740 50 11/21/1954 6,740 44 1/1/1955 6,020 1,197.7 N
1956 33 12/12/1955 7,470 7 12/12/1955 16,900 5 12/12/1955 20,200 1,204.7 Y
1957 61 2/26/1957 3,680 53 3/4/1957 6,160 49 3/7/1957 5,190 1,179.8 N
1958 68 4/20/1958 2,790 49 2/17/1958 6,900 52 2/26/1958 4,980 1,204.8 N
1959 46 11/12/1958 5,450 16 1/24/1959 13,300 20 1/24/1959 13,900 1,205.6 N
1960 13 11/23/1959 10,900 5 11/23/1959 17,900 4 11/23/1959 20,500 1,204.4 Y
1961 54 2/21/1961 4,350 17 2/21/1961 13,200 15 2/22/1961 15,700 1,205.8 Y
1962 55 1/7/1962 4,350 66 2/3/1962 2,900 57 12/24/1961 3,920 1,172.7 N
1963 15 11/20/1962 10,400 33 12/6/1962 9,600 40 12/6/1962 7,660 1,204.2 N
1964 62 1/25/1964 3,560 39 1/25/1964 8,820 19 1/25/1964 14,300 1,200.5 Y
1965 11 1/29/1965 11,000 9 1/30/1965 16,000 2 1/29/1965 25,700 1,201.0 Y
1966 66 5/6/1966 3,080 68 4/22/1966 2,650 56 3/9/1966 4,220 - N
1967 44 12/13/1966 5,870 20 1/28/1967 12,300 22 1/28/1967 12,600 1,201.6 N
1968 28 12/25/1967 8,070 37 2/20/1968 8,940 37 2/20/1968 9,120 1,205.5 N
1969 40 1/4/1969 6,620 46 12/9/1968 7,470 1,203.8 N
1970 56 1/23/1970 4,350 40 1/27/1970 8,520 * 1/27/1970 11,700 1,206.8 Y
1971 53 1/19/1971 4,460 52 2/15/1971 6,210 * 1/26/1971 10,100 1,206.9 N
1972 34 1/20/1972 7,460 14 2/28/1972 13,900 * 2/29/1972 20,100 1,203.5 N
1973 31 12/21/1972 7,700 48 12/27/1972 7,190 * 12/27/1972 10,200 1,201.5 N
1974 5 1/15/1974 15,000 4 1/16/1974 18,100 * 1/16/1974 27,000 1,205.1 M
1975 32 1/18/1975 7,660 35 1/19/1975 9,210 * 1/18/1975 14,700 1,203.6 M
1976 8 12/4/1975 13,200 2 12/4/1975 27,100 * 12/4/1975 36,000 1,205.9 Y
1977 73 9/4/1977 1,910 71 10/19/1976 2,380 * 3/9/1977 2,300 1,184.3 N
1978 3 12/2/1977 17,100 31 12/3/1977 9,820 17 12/2/1977 14,600 1,197.1 N
1979 69 3/7/1979 2,790 64 1/6/1979 2,920 58 2/7/1979 3,530 1,163.6 N
1980 37 12/17/1979 7,050 38 12/18/1979 8,840 34 12/18/1979 9,560 1,199.3 N
1981 9 12/26/1980 11,600 3 12/26/1980 21,500 3 12/26/1980 21,100 1,205.6 Y
1982 24 2/20/1982 8,280 8 2/18/1982 16,800 10 2/19/1982 16,200 1,202.6 Y
1983 30 12/3/1982 8,000 30 1/8/1983 10,200 28 1/8/1983 10,500 1,202.7 N
1984 29 1/25/1984 8,020 32 11/16/1983 9,640 33 11/18/1983 9,590 1,199.5 N
1985 47 6/7/1985 5,380 47 6/7/1985 7,200 42 6/8/1985 7,020 1,206.4 M
1986 25 2/23/1986 8,180 45 2/25/1986 7,530 38 2/25/1986 7,700 1,202.2 N
1987 16 11/24/1986 9,830 60 11/26/1986 4,470 41 11/24/1986 7,220 1,192.2 N
1988 18 12/9/1987 9,200 55 4/7/1988 5,620 43 4/7/1988 6,110 1,206.4 N
1989 58 10/16/1988 4,130 58 11/25/1988 4,620 53 11/24/1988 4,760 1,201.8 N
1990 6 1/9/1990 14,500 12 1/10/1990 14,800 6 1/10/1990 17,700 1,201.1 N
36
Table B 1 (Cont.)
Water
Year
Rank at
National
Peak at
National
Peak Q
at National
(cfs)
Rank at
LaGrande
Peak at
La Grande
Peak Q at
La Grande
(cfs)
Rank at
McKenna
Peak
at McKenna
Peak Q at
McKenna
(cfs)
Elevation at
Alder Lake
Minimally
Regulated
Event?
1991 12 11/24/1990 11,000 6 4/5/1991 17,400 7 4/5/1991 17,200 1,205.2 N
1992 65 1/28/1992 3,410 61 2/1/1992 3,980 51 1/31/1992 4,990 1,199.7 N
1993 64 3/23/1993 3,440 72 5/16/1993 2,220 60 3/23/1993 3,180 1,204.4 N
1994 72 3/3/1994 2,090 63 11/16/1993 3,790 59 11/16/1993 3,450 1,160.8 N
1995 35 1/31/1995 7,340 24 12/20/1994 11,200 18 12/21/1994 14,400 1,203.4 N
1996 2 2/8/1996 21,200 1 2/8/1996 39,500 1 2/8/1996 50,000 1,206.0 Y
1997 17 3/19/1997 9,820 19 12/29/1996 12,400 13 1/1/1997 15,900 1,189.3 N
1998 23 10/30/1997 8,330 59 10/31/1997 4,570 46 10/31/1997 5,950 1,196.0 N
1999 41 12/29/1998 6,350 23 12/29/1998 11,400 21 12/28/1998 13,400 1,195.6 Y
2000 20 11/25/1999 8,750 43 12/15/1999 7,720 29 12/16/1999 10,300 1,197.7 N
2001 71 10/1/2000 2,670 73 7/16/2001 1,210 61 10/1/2000 1,460 1,203.9 N
2002 21 1/8/2002 8,630 54 12/18/2001 5,750 30 12/17/2001 10,100 1,200.4 N
2003 14 1/31/2003 10,800 26 1/31/2003 10,900 11 1/31/2003 16,200 1,200.4 N
2004 52 1/29/2004 4,680 65 12/4/2003 2,900 54 1/30/2004 4,750 1,189.4 N
2005 26 1/18/2005 8,140 70 1/18/2005 2,440 50 1/18/2005 5,040 1,185.0 N
2006 38 1/10/2006 7,030 15 1/11/2006 13,400 16 1/11/2006 15,400 1,204.8 Y
2007 1 11/6/2006 21,800 44 3/24/2007 7,620 23 11/7/2006 12,500 1,202.1 N
2008 22 12/3/2007 8,470 56 5/28/2008 5,190 35 12/3/2007 9,370 1,204.2 N
2009 7 11/12/2008 13,900 36 1/7/2009 8,940 12 1/8/2009 16,100 1,186.2 N
2010 63 10/30/2009 3,470 69 6/6/2010 2,570 55 5/29/2010 4,370 1,204.0 N
2011 19 1/16/2011 9,020 27 1/15/2011 10,900 25 1/16/2011 12,200 1,195.2 N
2012 49 2/22/2012 5,260 62 4/25/2012 3,950 47 2/22/2012 5,620 1,203.7 N
2013 59 10/29/2012 3,790 67 9/30/2013 2,650 45 11/19/2012 5,960 1,198.5 N
2014 42 3/9/2014 6,090 25 3/10/2014 11,100 27 3/10/2014 11,300 1,205.9 Y
2015 10 11/25/2014 11,500 57 12/20/2014 4,720 39 1/5/2015 7,680 1,191.6 N
2016 4 12/9/2015 16,700 18 12/9/2015 12,800 14 12/9/2015 15,900 1,204.4 N
2017 57 3/15/2017 4,300 42 3/18/2017 7,960 32 3/18/2017 9,660 1,206.3 Y
Notes:1)
2)
3)
4)
5) Elevation at Alder Lake corresponds to the reservoir average daily elevation at the day of the peak at La Grande. The reservoir
maximum elevation is 1,207 ft, while the average annual maximum elevation is 1,205.61 ft.
Peak discharges for WY 1970-1977 at McKenna (marked with an asterisk) were actually recorded at inactive gage 12088400
(above Powell Creek near McKenna) and transferred based on linear correlation between logarithms of the peakflows at near
McKenna gages vs. at McKenna gage.
19 events (marked as "Yes") were selected to be included in the minimaly regulated series. Other 4 events (marked as "Maybe")
were added to the original 19 as they might be minimally regulated but the resulting correlation between La Grande and National
peaks was not as strong (r2 for the original 19 was 0.89, while for the 23 events was 0.77).
The 1949 annual event at McKenna was not the same as at La Grande and near National, therefore being excluded from the
series. So, the minimally regulated series at McKenna contains 18 events instead of 19.
Annual peak flows at McKenna listed above were either recorded at USGS gage 12089500 or transferred from USGS gage
12088400 (as explained on item 3). The resulting minimally regulated series at McKenna was adjusted by 800 cfs to account for the
Centralia Power Canal diversion, which is explained on pages 21 and 22 of Appendix A.
37
La Grande record prior to 1945
Prior to submitting for QC, an investigation on the impact of adding La Grande data prior to WY1946
was performed. Given that the original La Grande dam was a diversion dam, very likely all 16 recorded
events between WY1906 and WY1931 were minimally regulated. However, it wasn't clear how to
represent the uncertainty related to the inclusion of several years of missing information under pre-
Alder Dam regulation conditions, and how that would affect Bulletin 17C frequency analysis of
minimally regulated flows.
Also, according to the USGS, those events were recorded at a different gage, "Nisqually near La
Grande", with years prior to 1912 probably affected by La Grande dam construction. Despite
reasonable results – the 1% peak flow was 37,900 cfs, approximately 4% lower than the final estimate
of 39,300 cfs – , this analysis was dismissed for the reasons listed in the first paragraph.
On the other hand, the December 1933 was included as a historic event because it remains the second
largest observed in most of the watershed and because all events above 20,000 cfs at La Grande were
identified as minimally regulated. The December 1933 event peaked at 25,000 cfs at the gage near
Alder (85% of the area draining to La Grande), and estimated at La Grande within 29,000-34,000 cfs.
38
Perception Thresholds
There is general acknowledgement that Nisqually River effective flows are low and underestimate
risk. So, there was no reason for investigating a less conservative alternative than using INF-INF,
especially since the process of setting up perception thresholds for the minimally regulated series is
not straight forward.
Since the annual peak event will be minimally regulated only in certain years, the minimally regulated
annual series will resemble a broken record with multiple gaps. The figure below (from PeakFQ) shows
McKenna's minimally regulated series selected from the systematic gage record. Perception
thresholds are set equal to ZERO-INF for years when the annual event was included in the minimally
regulated series, and INF-INF for years when the annual event was excluded from the series.
Every year where there is a gap during the systematic record period, a flood peak was measured, just
not minimally regulated. One alternative was to use the measured value as the lower perception
threshold for that year, which led to numerous (~50) perceptible ranges and software crashing
(PeakFQ seems to be limited to 20 perceptible ranges). As suggested, another alternative was to use
a threshold value associated with the largest events observed, which seems to dismiss potential large
events (such as Dec/2006) that did not occur only because of reservoir attenuation. Also, perception
thresholds may significantly impact flow estimates and confidence intervals. For example, setting the
perception threshold to 40,000 for the gaps, lowered Q1% by more than 10%.
Given the above, the use of perception thresholds (different than INF-INF) for periods without data
was limited to the years prior to 1945 when historic information was available. For both McKenna and
La Grande, the December 1933 event was used as reference to set the thresholds for the period
between WY1934-45, which is discussed in Appendix A.
