USGS

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

Table of Contents



 

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:

 

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, Flood characteristics of urban watersheds in the United States: U.S. Geological 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.

 U.S. Department of Commerce, 1965, Climates of the states---Climate of Washington, in Climatography of the United States, no. 60-45: U.S. Weather Bureau, 27 p.

 U.S. Department of the Interior, 1977, National handbook of recommended methods for water-data acquisition: U.S. Geological Survey Office of Water Data Coordination, variously paged.

 U.S. Geological Survey, 1976, Hydrologic unit map--1974, State of Washington: Reston, Va, U.S. Geological Survey, 1 sheet, scale 1:500,000.

 U.S. Water Resources Council, 1981, Guidelines for determining flood flow frequency: U.S. Water Resources Council Bulletin 17B, 28 p., 14 appendixes.

 U.S. Weather Bureau, 1965, State of Washington, mean annual precipitation, 1930-1957: Portland, Oreg., Soil Conservation Service, map M-4430, 1 sheet, [no scale].

 Walters, K.L., 1974, Water in the Okanogan River Basin, Washington: State of Washington, Department of Ecology Water-Supply Bulletin 34, 136 p.

 Walters, K.L. and Nassar, E.G., 1974, Water in the Methow River Basin, Washington: State of Washington, Department of Ecology Water-Supply Bulletin 38, 73 p.

 Williams, J.R., and Pearson, H.E., 1985a, Streamflow statistics and drainage-basin characteristics for the southwestern and eastern regions, Washington, Volume I, Southwestern Washington: U.S. Geological Survey Open-File Report 84-145-A, 424 p.

 ---------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, Washington, Volume I, Western and southern Puget Sound: U.S. Geological Survey Open-File Report 84-144-A, 330 p.

 ---------1985b, Streamflow statistics and drainage-basin characteristics for the Puget Sound region, Washington, Volume II, Eastern Puget Sound from Seattle to the Canadian border: U.S. Geological Survey Open-File 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 (ssumioka@usgs.gov) 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).


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