This report presents a method of determining the magnitude and frequency of expected floods applicable for any area in western Washington (fig. 1). A composite frequency curve was developed that expresses the relation to mean annual floods of floods having recurrence intervals from 1.03 to 50 yr. This composite frequency curve was based on the combined frequency curves for all gaging stations in the region with 5 or more years of record. Records for 131 stations, with drainage areas ranging from l to 2,700 sq mL, and recorded flood peaks ranging from 30 to 210,000 cfs were combined to obtain the composite curve. An equation was derived expressing the relationship between mean annual floods and basin characteristics. Those basin characteristics that may easily be determined from topographic maps and that significantly affect the peak discharge were compiled for all streams in the region on which 5 or more years of stream-flow records are available. These basin characteristics were correlated with mean annual flood of the streams, and as a result of this correlation a formula for the computation of the mean annual flood for any basin was derived. The limitations of the formula and its probable accuracy are discussed in the report.
Included in the report are curves of magnitude and frequency of floods at all gaging stations in the region with 15 or more years of record. Also included is an enveloping curve of the maximum recorded discharges in the region.