Water Resources of Washington
State

Magnitude and Frequency of Floods in Washington

By S.S. Sumioka, D.L. Kresch, and K.D. Kasnick

Water-Resources Investigations Report 97-4277

ABSTRACT

Annual maximum instantaneous
discharge data for 527 gaging stations with at least 10 years of
peak-discharge record on unregulated streams were used to compute the
magnitude and frequency of floods on Washington streams. Flood magnitudes
having exceedance probabilities of 0.5, 0.1, 0.04, 0.02, and 0.01 were related
to physical and climatic characteristics for 504 of the 527 gaging stations
using generalized least-squares regression analyses. The regression relations
can be used to estimate flood discharges having these exceedance probabilities
for ungaged stream basins.

Table of Contents

INTRODUCTION

Purpose
and Scope
Previous Flood
Reports for Washington
Description of
the Study Area

Climate

Characteristics
of Flood Discharge

FLOOD-FREQUENCY
ANALYSES
REGRESSION
ANALYSES

Application and
Limitations of Regression Equations
SUMMARY

REFERENCES
CITED
ALL FIGURES,
1-10
ALL TABLES,
1-5 (download only)

INTRODUCTION
Estimates of the magnitude and frequency of floods are
used by engineers in the design of bridges, culverts, dams, and embankments, and
by land-use managers to assess the hazards related to the development of flood
plains. The U.S. Geological Survey (USGS) published estimates of flood
frequencies in Washington in 1985 for stream-gaging stations with 10 or more
years of annual flood records (Williams and others, 1985a,b; and Williams and
Pearson, 1985a,b). However, these estimates were based on data collected only
through the 1979 water year (the 12-month period from October 1, 1978, through
September 30, 1979).

In 1993, the U.S. Geological Survey, in cooperation with the Washington State
Department of Transportation, began a study to update these flood frequency
estimates, incorporating data collected through the 1992 water year, and to
develop regional regression equations by which flood discharges could be
estimated at ungaged sites. However, because some very large floods occurred
after 1992, the USGS, in cooperation with the Washington State Department of
Transportation and the Washington State Department of Ecology, expanded that
study to include all data collected through the 1996 water year. Flood peaks
that occurred during a prolonged and intense storm in February 1996 exceeded
previous historic peak discharges recorded at several gaging stations in
Washington.

The preferred method of expression of flood frequency is the annual
exceedance probability. The annual exceedance probability is the probability
that a flood of a certain magnitude will occur or be exceeded in any 1-year
period. Thus, a flood discharge with an exceedance probability of 0.02, or 2
percent, has a one-in-fifty chance of being equalled or exceeded each year.
Another method of expressing flood frequency is as a recurrence interval---the
average time interval in years between consecutive occurrences of an annual peak
discharge equal to or greater than a certain magnitude. The recurrence interval
corresponding to the annual peak discharge for a certain exceedance probability
flood is given by the reciprocal of the exceedance probability. Thus, a flood
with an exceedance probability of 0.02 has a recurrence interval of 50 years.

Purpose and Scope
This report presents the results of
flood-frequency analyses for 527 gaging-station records on unregulated streams
(defined in this report as being not significantly affected by reservoir
operations, diversions, or urbanization) that have at least 10 years of annual
maximum instantaneous discharge data. Also presented are equations and
techniques by which the magnitude and frequency of floods can be estimated for
any ungaged site on naturally flowing (unregulated) streams. The data and
results are presented on figures 1 to
10 and in tables 1 to 5 at the end of the report.

The flood-frequency analyses used annual maximum instantaneous
discharge data collected through the 1996 water year, which are stored in the
USGS National Water Information System (NWIS) database. The results of the
flood-frequency analyses were used in conjunction with data stored in the NWIS
basin-characteristics file to develop regression equations for estimating flood
frequencies at ungaged sites. The State was divided into nine regions for the
regression analyses on the basis of hydrologic unit code boundaries.

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Previous Flood Reports for Washington
Descriptions of
floods and the computation of flood magnitude and frequency for specific river
basins in Washington can be found in reports by Anderson (1948) for the Puyallup
and Chehalis River Basins, Bailey (1960) for the Nooksack River Basin,
Richardson (1965) for the upper Green River Basin, Walters (1974) for the
Okanogan River Basin, Walters and Nassar (1974) for the Methow River Basin,
Nassar and Walters (1975) for the Palouse River Basin, and Drost and Lombard
(1978) for the Skagit River Basin. Results of regional and State-wide
flood-frequency analyses can be found in reports by Rantz and Riggs (1949),
Bodhaine and Thomas (1960 and 1964), Williams and Pearson (1985a and 1985b), and
Williams and others (1985a and 1985b). Techniques for estimating flood magnitude
and frequency at ungaged stream basins were developed by Bodhaine and Robinson
(1952) for western Washington, Thomas and others (1963) for the Snake River
Basin, Bodhaine and Thomas (1964) for Pacific Slope Basins, Cummans and others
(1975) for the entire State, and Haushild (1979) for small, ephemeral streams in
eastern Washington.

Description of the Study Area
The State of Washington
encompasses several physiographic provinces (Fenneman, 1931): the Puget Border
Province, consisting of the Puget Sound Basin, the Olympic Mountains, and the
lowlands west of the Cascade Range extending southward to the Columbia River;
the Sierra-Cascade Province, consisting of the Cascade Range; the Columbia
Plateau Province, consisting of the area east of the Cascade Range and south of
the Columbia River in Washington; and the Northern Rocky Mountain Province
consisting of the Pend Oreille, Okanogan, and Sanpoil River Basins in
Washington. The topography of the State varies from lowlands at or near sea
level to the mountainous areas of the Olympic Mountains and the Cascade Range.
Land use and land cover in the State vary greatly, ranging from forested and
agricultural areas to densely populated urban and suburban areas.

Climate
The Cascade Range separates Washington into two
climatically different regions. Western Washington, influenced by the Pacific
Ocean, has a predominantly marine climate, characterized by cool, dry summers
and mild, wet winters. Eastern Washington has a continental climate,
characterized by warm, dry summers and cold, clear winters.

Local and regional variations in precipitation are influenced primarily by
the Olympic Mountains and the Cascade Range. In western Washington, mean annual
precipitation ranges from less than 20 inches in the rain shadow of the Olympic
Mountains to more than 220 inches along the crest of the Olympic Mountains (U.S.
Department of Commerce, 1965). Precipitation along the crest of the Cascade
Range exceeds 140 inches in some places. In eastern Washington, mean annual
precipitation ranges from less than 10 inches in parts of the Columbia River
Basin to about 40 inches near the southeastern and northeastern corners of the
State.

Characteristics of Flood Discharge
Several types of
floods occur in Washington. In most parts of western Washington, floods
generally occur in late fall and winter as a result of prolonged rainstorms.
These floods may be augmented by water from snowmelt if rain falls on snow. The
rain-on-snow floods are usually of short duration. In basins at higher
elevations, floods may occur in the spring as a result of rapid snowmelt. These
floods are usually of longer duration than the winter floods.

In eastern Washington, floods generally occur in the foothills of the
Cascade Range and in the highlands of northeastern Washington during spring
snowmelt. In some areas of eastern Washington, flooding may occur during the
winter when rain or unseasonably warm weather melts accumulations of snow.
Flooding may also occur in small basins in response to summer thunderstorms.

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FLOOD-FREQUENCY ANALYSES
The data used in the
flood-frequency analyses were the annual maximum instantaneous discharges for
each of the 527 gaging stations on unregulated streams in Washington (fig. 1) with
10 or more years of record (table 1). In this report, these data will be
referred to as peak flows, and the annual series of peak flows during the period
of record for a particular gaging station will be referred to as the systematic
record for that station. In some instances, the years of record for a station
listed in table 1 may not agree with the number of peaks used in the
flood-frequency analysis listed in table 2 because of the exclusion of peaks
designated as being affected by regulation and of peaks that are outside of the
range of discharges defined by high and low outlier thresholds. In some cases,
peak flows have been determined for floods outside of the period when regular,
systematic records of discharge have been kept. These peak flows, referred to as
historic peaks, can be used to extend systematic records to longer historical
periods (U.S. Water Resources Council, 1981).

The stations given in table 1 and all other tables containing
gaging-station data are listed in downstream order by USGS gaging station
numbers. The first two digits (12, 13, or 14) designate major basin subdivisions
of the State. Stations with numbers beginning with 12 are located on streams
tributary to the Pacific Ocean or the Columbia River upstream from the mouth of
the Snake River; stations with numbers beginning with 13 are located on streams
tributary to the Snake River; and stations with numbers beginning with 14 are on
streams tributary to the Columbia River below the mouth of the Snake River.

Estimates of flood discharge and frequency were computed for all 527 gaging
stations using an interactive version of USGS computer program J407 (Kirby,
1981), which implements guidelines established by the U.S. Water Resources
Council (1981). The U.S. Water Resources Council suggests that separate
flood-frequency curves be computed for each type of flood from a mixed
population of floods (U.S. Water Resources Council, 1981), but the detailed
study required to segregate peak flows by cause was beyond the scope of this
project; therefore, no attempt was made to analyze any of the annual flood
series separately for mixed populations. Statistical procedures were used to
identify high and low outliers in systematic station records. High outliers
known or believed to be the highest during an extended period of time were
treated as historic flood peaks. Flood peaks identified as low outliers were
deleted and then a conditional probability adjustment was applied to all the
remaining peaks to determine the flood frequency estimates. In a few cases,
visual inspection of the frequency curves identified small peaks that departed
from the fitted relation (a sharp downward break in the curve), but were not
identified as low outliers by the statistical procedures. A user-defined
low-discharge threshold was used to omit such peaks from the flood-frequency
analysis.

Not all of the available peak-flow data could be used in the flood-frequency
analyses. Stations for which more than 25 percent of the peak-flow record
consisted of zero flows were not included in the analyses (U.S. Water Resources
Council, 1981, p. 5-1). Also, if part of the systematic record for a station
included periods of regulated flow, those periods were not included in the
analysis. In some cases, part of the systematic record for a station was
excluded from the frequency analysis because only gage heights (and not
discharge) were recorded for some years. Flood-frequency results were not
included for stations at which the peak discharge data appeared to be
representative of and significantly influenced by mixed populations of floods
and for which it appeared that no single flood-frequency curve would adequately
fit the peak discharge data.

In this study, a log-Pearson Type III distribution was fit to the data
for each station using the method of moments as described by the U.S. Water
Resources Council (1981). The base 10 logarithms of the mean, standard
deviation, and skew coefficient were used to compute the logarithm of the
discharge, , at a selected exceedance probability using the following
equation:

, (1)
where

= mean of the logarithms of peak flows,

= factor that is a function of the logarithm of the skew
coefficient and the selected exceedance probability, and

= standard deviation of the logarithms of peak flows.

Values of  were obtained from the table in appendix 3 of the U.S. Water
Resources Council guidelines (1981). For this study, the skew coefficient used
to obtain  in the above equation was estimated by weighting the station
skew coefficient and a generalized skew coefficient in inverse proportion to
their individual mean-square errors. The generalized skews and their mean-square
errors used for this study were those determined by the U.S. Water Resources
Council (1981). A detailed study of the regional (generalized) skews for the
State of Washington was not done for this study.

Flood-frequency estimates were determined graphically for a small
number of gaging-station records because the log-Pearson Type III distribution
did not accurately fit the peak discharge data.

Flood discharges computed for exceedance probabilities of 0.5, 0.1,
0.04, 0.02, and 0.01 are given in table 2 (first line of data for each station),
along with the 95-percent confidence interval for each computed flood magnitude
(second line of data). This interval is the range that, with a probability of 95
percent, contains the true flood magnitude for a particular exceedance
probability. Confidence intervals were not determined for flood-frequency
estimates determined by graphical techniques.

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REGRESSION ANALYSES
Flood magnitudes determined for 504
of the 527 gaging stations included in the flood-frequency analyses were used in
conjunction with selected physical and climatic basin characteristics to develop
generalized least-squares regression equations for estimating flood magnitudes
and frequencies for ungaged, unregulated (hereafter referred to as ungaged)
stream basins. Stations not used were omitted for a variety of different
reasons, including uncertainties regarding the correct location of drainage
boundaries and the amount of a basin that contributes to runoff. Stations for
which a significant portion of the upstream drainage basin was located outside
of the State boundaries were not used because they may not be representative of
hydrologic conditions within the State. Also, stations for which flood frequency
estimates were determined graphically were not included because the log-Pearson
Type III station skew---one of the variables used in the regression
analyses---determined for them would not be representative of the graphically
determined relations.

A review of previous flood-frequency studies for Washington indicated that
various combinations of nine identified physical and climatic basin
characteristics should be sufficient to produce equations by which flood
magnitudes and frequencies can be accurately estimated. Brief descriptions of
the nine identified basin characteristics are as follows:

Contributing drainage area, , in square miles, upstream from the gaging station;
determined by planimetering the contributing drainage area on a topographic
map;

Main channel slope, , in feet per mile; determined by extending the main stream
channel upstream to
the basin divide and then locating a point that is 10
percent of the distance from the gage to the divide and another point on the
stream that is 85 percent of the distance from the gage to the divide; the
slope is equal to the difference in elevation between the two points divided
by the distance between the points;

Stream length, , in miles, from the gage to the basin divide; measured
along the channel from the gage to the basin divide;

Basin elevation, ---the mean basin elevation, in feet above sea level;
determined by finding the average elevation of at least 20 equally spaced
points in the drainage basin as indicated by a transparent grid overlay;

Storage, ---the percentage of the contributing drainage area made up
of lakes, ponds, and swamps; determined by placing a transparent grid over the
drainage-basin map and counting the number of squares occupied by water or
swamp surfaces; to prevent computing the logarithm of zero, storage values of
zero percent were changed to 0.01 percent before running regression analyses;

Forest cover, ---the percentage of the contributing drainage area covered
by forests; estimates of forest cover were obtained from the Washington State
Department of Natural Resources; forest cover may also be determined by the
grid-sampling method on a suitable large-scale map; to prevent computing the
logarithm of zero, forest cover values of zero percent were changed to 0.01
percent before running regression analyses;

Annual precipitation, ---mean annual precipitation, in inches, determined by
locating the basin on the precipitation map (plate 1 and U.S. Weather Bureau,
1965), placing a transparent grid over the map, and averaging the values
determined at each grid intersection;

Precipitation intensity, ---the 24-hour rainfall, in inches, that can be expected on
the average of once every 2 years within the basin, obtained by using
the grid-sampling method from precipitation-frequency maps for the State of
Washington (Miller and others, 1973); and

Minimum January temperature, ---the mean minimum January air temperature, in degrees
Fahrenheit; determined by using the grid-sampling method on an
equal-temperature map from the U.S. Department of Commerce (1965).

All
nine of these basin characteristics were used in ordinary least-squares
regression analyses to obtain an indication of which explanatory variables
(basin characteristics) were most significant for each of nine State regions.
Generalized least-squares regression analyses were used to develop equations for
estimating flood magnitudes and frequencies for ungaged stream basins in each
region. The goal was to develop sets of equations using only a few easily
obtainable basin characteristics that could be used to estimate flood discharges
at selected exceedance probabilities for sites on ungaged streams.

Flood-frequency determinations for all of the gaging stations and their
associated basin characteristics were placed in a single data set for an initial
regression analysis to attempt to determine the geographic regions within the
State for which separate regression equations should be developed. The residuals
(the differences between the flood magnitudes obtained from the flood-frequency
analyses and the flood magnitudes obtained from the regression equations) were
plotted on a statewide map. Using this same procedure, Cummans and others (1975)
were able to subdivide the State into 12 regions, each with its own set of
regression coefficients and constants. However, no meaningful grouping of the
residuals computed during this study could be made. Adjacent basins and, in some
cases, parts of the same basin were located in different regions when residuals
were used as the grouping criteria. Therefore, hydrologic-unit code boundaries
were used instead to group the stations into separate geographic regions
believed to have different hydrologic characteristics. The State of Washington
is divided into eight hydrologic units, each composed of several cataloging
units (U.S. Geological Survey, 1976). Each cataloging unit, in turn, contains
one to several major stream basins. For this study, an additional region was
created by dividing the Columbia River Basin hydrologic unit into two subunits:
one encompassing the Columbia Plateau (Region 7) and the other encompassing
mostly those areas east of the Cascade Range and north or west of the Columbia
River (Region 4). The nine geographic regions are shown on figure 1, and
the distribution of gaging stations used for the regression analysis in each
region is shown on figures 2-10.

Regression analyses for each region included the use of the ordinary
least-squares technique to obtain a preliminary list of which basin
characteristics were most influential in affecting peak flows at the 5 percent
level of significance. All nine of the basin characteristics identified in
previous reports as being the most important in determining flood discharges
were considered for inclusion in each regional regression equation. In addition,
a subjective decision was made in cases where two or more basin characteristics
were about of equal significance in estimating flood magnitude. In these cases,
the relative ease by which values for the basin characteristics could be
obtained was considered in the selection of which characteristics to use in the
development of the regression equations for estimating flood magnitudes and
frequencies for ungaged streams. The objective in making such decisions was to
greatly simplify the application of an equation with only a minor decrease in
its accuracy. Consequently, a regression equation may contain a basin
characteristic that is slightly less significant than another characteristic
that was considered but not included in the equation or it may contain fewer
basin characteristics than the "best" regression equation defined by the
regression analysis.

The physical and climatic basin characteristics determined most significant
in predicting flood discharge were contributing drainage area and mean annual
precipitation for regions 1, 2, 3, 4, 6, and 9; and contributing drainage area
only for regions 5, 7, and 8. All basin characteristic values were computed in
accordance with both the NWIS basin-characteristics file guidelines and the
National Handbook of Recommended Methods for Water-Data Acquisition (U. S.
Department of the Interior, 1977). The basin characteristic values for the
stations used in the regression analyses are given in table 3.

Main channel slope, one of the basin characteristics found to be
statistically significant in the development of the regression equation for
region 5, was removed from the final regression equation because it had a
negative regression coefficient, which doesn't seem physically realistic. It is
believed that its statistical significance in the regression analysis may be the
result of either a spurious relationship or the fact that it may be a surrogate
for one or more other basin characteristics.

Once it was determined which basin characteristics should be used for
each regression region, then the flood-frequency data for each region were
analyzed using the generalized least-squares technique (Tasker and Stedinger,
1989) to determine equations for estimating flood magnitude and frequency for
ungaged sites within the regions. This method differs from the ordinary
least-squares method by weighting each station used in an analysis on the basis
of the number of years of peak flows in the station record and by the distance
between stations. Ordinary least squares were not used to develop the regression
equations because two assumptions that are made in the use of that method---that
the residuals of the data have equal variances and that each residual is
independent of all others---are usually violated to some degree in hydrologic
regression analysis. In the case of peak-flow data, both of these assumptions
are often violated. Peak-flow records are usually of differing lengths, and
nearby basins may be affected by the same weather patterns, leading to unwanted
correlations in the peak flows.

The mathematical model used to define the relation between flood discharge
and basin characteristics for each region was

, (2)
where

= flood discharge, in cubic feet per second,

= basin characteristics,

= regression constant, and

= regression coefficients.
This model was
converted to the following linear form by transforming the variables (, ) to base 10 logarithms:

. (3)
Logarithmic transformations were performed
so that least-squares linear regression techniques could be used during the
analyses.

The regression equations developed for the nine flood-frequency
regression regions are presented in table 4. The standard error of prediction
(given in table 4) is a measure of how well the regression equation
predicts  from the data used in the analysis; a higher degree of
uncertainty is associated with a higher standard error. The highest percent
standard error of prediction is found in Region 8 (133 percent for an exceedance
probability of 0.5). Although the exact source of this high value is not known,
it is probably a combination of the time sampling error and model error. The
time sampling error is the error due to using a peak flow record that might not
represent the entire range of peak flows possible at a site. The model error is
the error due to not having the most influential variable in the regression
equation. Although the time sampling error may be reduced by accumulating more
years of peak-flow data, a reduction in model error is limited by the
availability of basin-characteristic data and the cost and effort involved in
collecting other types of basin data. Also, including more explanatory variables
in the regression equation may reduce the model error at the cost of ease of use
and data availability for the user. The equivalent years of record (given in
table 4) is also a measure of the predictive ability of the regression equation,
expressed as the number of years of actual peak-flow data required to achieve
results equal to those obtained from the regression equation.

Weighted estimates of flood magnitude for the 504 gaging stations used
in the regression analyses (table 2) were obtained using the weighting
procedures presented in appendix 8 of the guidelines of the U.S. Water Resources
Council (1981), in which two different estimates of flood magnitude (from the
frequency analysis and from the regression equation) are weighted inversely
proportional to their variance. The weighted estimates generally provide better
estimates of the true flood discharges than those determined from either the
flood-frequency analysis or the regression analysis alone.

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Application and Limitations of Regression Equations
The
flood magnitude for a desired exceedance probability for an ungaged site in an
ungaged stream basin can be computed as follows:

1. From the figure showing the hydrologic regions (fig. 1),
select the region in which the basin is located.

2. From table 4, find the desired equation for the selected region and
exceedance probability.

3. Determine which basin characteristics are required for the selected
regression equation and obtain or determine the required data.

4. Substitute the data values obtained in step 3 into the regression
equation and compute the flood magnitude.
For example, to determine
the magnitude for a flood with an exceedance probability of 0.01 at an ungaged
site in Region 1, the appropriate equation from table 4 would be

. (4)
Assuming the drainage area of the basin is
100 mi2 and the mean annual precipitation is 40 inches,

However, the flood discharge at an ungaged site
on a gaged stream can be computed by the following equation (Thomas and others,
1994), if the drainage area of the ungaged site is between 50 and 150 percent of
the drainage area of the gaged site:

, (5)
where

= flood discharge, in cubic feet per second, at the
ungaged site,

= weighted flood discharge, in cubic feet per second, at
the gaged site (from table 2),

= contributing drainage area, in square miles, at the
ungaged site,

= contributing drainage area, in square miles, at the
gaged site, and

= exponent for each region as follows.

The exponent  for each region was determined by regressing the discharges
for each exceedance probability on the drainage areas for the stations in the
region. The drainage area exponents determined from the regression analyses for
each of the exceedance probabilities were averaged to produce the single
exponent for each region.

Equation 5 should only be used if the basins of the gaged and ungaged
sites have similar basin characteristics. Therefore, if a large tributary enters
the stream between the gaged and ungaged sites, and the tributary basin has much
different topography, vegetation, or other basin characteristics that could
affect flood discharges, then the appropriate regression equation from this
report should be used to estimate flood-frequency discharges for the ungaged
site.

The following is an example of the determination of the flood discharge
with an exceedance probability of 0.01 at an ungaged site on the Naselle River
(Region 1). The contributing drainage area at the ungaged site () is 47 square miles. The contributing drainage area at the
gage () on the Naselle River (station number 12010000 in table 2)
is 54.8 square miles.

The drainage area of the ungaged site , expressed as a percentage of the drainage area of the gaged
site , is given by:

,
which satisfies the requirement for the use of
equation 5.

The weighted discharge,  (obtained from table 2, third line for station 12010000) is
12,100 cubic feet per second. The exponent, , for Region 1 is 0.92. The computed discharge, , is

There are some limitations to the use of the
regional flood-frequency equations presented in this report. One is that the
equations should not be used for streams in which the flow is significantly
regulated or diverted because the regression coefficients were computed from
gaging-station data for naturally flowing (unregulated) streams. Another
limitation is that the equations should not be used to compute discharges for
basins in which the values for one or more of the basin characteristics are
significantly outside the range of values used in the development of the
equations. Maximum and minimum basin characteristic values used in the
development of the regression equations are listed in table 5.

A further limitation on the use of the regression equations is that
they should not be used for basins in which urbanization has taken place.
Because the regression equations were developed using data from unurbanized
basins, application of the equations to urbanized basins could produce
misleading results. Techniques for estimating flood discharges on urban streams
are presented by Sauer and others (1983).

SUMMARY
The procedures recommended by the U.S. Water
Resources Council (1981) were applied to the peak-flow records of 527 gaging
stations on unregulated streams in the State of Washington to obtain discharges
with exceedance probabilities of 0.5, 0.1, 0.04, 0.02, and 0.01. The frequency
analyses included peak-flow data collected through the 1996 water year.

The results of the frequency analysis were combined with physical and
climatic basin characteristics values to develop equations that can be used to
compute peak discharges for ungaged streams. The State was divided into nine
regions, based on hydrologic unit boundaries, and a separate set of equations
was developed for each region. Basin characteristics shown to be important in
estimating flood discharge include contributing drainage area and mean annual
precipitation.

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REFERENCES CITED
Anderson, I.E., 1948, Floods of the
Puyallup and Chehalis River Basins, Washington: U.S. Geological Survey
Water-Supply Paper 968-B, p. 61-124.

Bailey, E.G., 1960, Floods in the Nooksack River Basin, in
Water resources of the Nooksack River Basin and certain adjacent streams:
State of Washington, Department of Conservation, Division of Water Resources
Water-Supply Bulletin No. 12, p. 95-98.

Bodhaine, G.L. and Robinson, W.H., 1952, Floods in Western Washington,
Frequency and magnitude in relation to drainage basin characteristics:
Geological Survey Circular 191, 124 p.

Bodhaine, G.L. and Thomas, D.M., 1960, Floods in Washington, magnitude
and frequency: U.S. Geological Survey Open-File Report, 25 p.

---------1964, Magnitude and frequency of floods in the United States,
part 12, Pacific Slope Basins in Washington and upper Columbia River Basin:
Geological Survey Water-Supply Paper 1687, 335 p.

Cummans, J.E., Collings, M.R., and Nassar, E.G., 1975, Magnitude and
frequency of floods in Washington: U.S. Geological Survey Open-File Report
74-336, 46 p.

Drost, B.W. and Lombard, R.E., 1978, Water in the Skagit River Basin,
Washington: State of Washington, Department of Ecology Water-Supply Bulletin 47,
247 p.

Fenneman, N.M., 1931, Physiography of western United States: New York,
McGraw-Hill Book Company, Inc., 534 p.

Haushild, W.L., 1979, Estimation of floods of various frequencies for
the small ephemeral streams in eastern Washington: U.S. Geological Survey Water-
Resources Investigations Open-File Report 79-81, 22 p.

Kirby, William, 1981, Annual flood frequency analysis using U.S. Water
Resources Council guidelines (Program J407), chapter I, section C of WATSTORE
user's guide: U.S. Geological Survey Open-File Report 76-435, v. 4, p. C-1 to
C-57.

Miller, J.F., Frederick, A.H., and Tracey, R.J., 1973, Precipitation
atlas of the western United States, vol. IX, Washington: National Oceanic and
Atmospheric Administration NOAA Atlas 2, 43 p.

Nassar, E.G. and Walters, K.L., 1975, Water in the Palouse River Basin,
Washington: State of Washington, Department of Ecology Water-Supply Bulletin 39,
246 p.

Rantz, S.E. and Riggs, H.C., 1949, Magnitude and frequency of floods in
the Columbia River Basin, in U.S. Geological Survey, 1949, Floods of
May-June 1948 in Columbia River basin: Geological Survey Water-Supply Paper
1080, p. 317-469.

Richardson, Donald, 1965, Effect of logging on runoff in upper Green
River basin, Washington, a progress report: U.S. Geological Survey Open-File
Report, 45 p.

Sauer, V.B., Thomas, W.O., Jr., Stricker, V.A., and Wilson, K.V., 1983,
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Survey Water-Supply Paper 2207, 63 p.

Tasker, G.D., and Stedinger, J.R., 1989, An operational GLS model for
hydrologic regression: Journal of Hydrology, v. III, nos. 1-4, p. 361-275.

Thomas, B.E., Hjalmarson, H.W., and Waltemeyer, S.D., 1994, Methods for
estimating magnitude and frequency of floods in the southwestern United States:
U.S. Geological Survey Open-File Report 93-419, 211 p.

Thomas, C.A., Broom, H.C., and Cummans, J.E., 1963, Magnitude and
frequency of floods in the United States, part 13, Snake River basin: Geological
Survey Water-Supply Paper 1688, 250 p.

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methods for water-data acquisition: U.S. Geological Survey Office of Water Data
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Washington: Reston, Va, U.S. Geological Survey, 1 sheet, scale 1:500,000.

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flow frequency: U.S. Water Resources Council Bulletin 17B, 28 p., 14 appendixes.

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---------1985b, Streamflow statistics and drainage-basin
characteristics for the southwestern and eastern regions, Washington, Volume II,
Eastern Washington: U.S. Geological Survey Open-File Report 84-145-B, 662 p.

Williams, J.R., Pearson, H.E., and Wilson, J.D., 1985a, Streamflow
statistics and drainage-basin characteristics for the Puget Sound region,
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Open-File Report 84-144-A, 330 p.

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Report 84-144-B, 420 p.

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For more information,
contact the District Chief, U.S. Geological Survey, 1201 Pacific Avenue, Suite
600, Tacoma, WA 98402.

For additional technical information from the
principal author, send email to Steve Sumioka ([email-masked]) or call him at (253)
428-3600 x2645.

This on-line report is adapted from Magnitude and
Frequency of Floods in Washington, 1998, by S.S. Sumioka, D.L. Kresch, and K.D.
Kasnick, which is published as U.S. Geological Survey Water-Resources
Investigations Report 97-4277. Copies of the report may be purchased from the
USGS Information Services, Box 25286, Denver, Colorado 80225, telephone
888-ASK-USGS (888-275-8747).

USGS--Water Resources of Washington
State
USGS--Water Resources of the
United States
U.S. Geological
Survey

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Last modified: Tue Jun 16 15:25:12 1998
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