This study evaluated annual peak flow data recorded at stations representing regulated and unregulated stream and river systems draining relatively undeveloped areas of King County. The objective of the study was to characterize the severity of high flows resulting from the large storm that affected all of western Washington in January 2009 and to update the 1 percent AEP (100-yr return period) discharge for comparison to estimates used in the most recent flood studies.

Record flows were observed during the 2009 flood event at three locations, all within the Snoqualmie River Basin. Near record peak flows (ranked 2nd and 3rd highest) were observed at nine other gaging stations located across the county representing almost every major river basin except the Sammamish. Issaquah Creek in the Sammamish River basin experienced the 4th largest peak flows on record, although Lake Sammamish moderated the effects of these flows. The peak flow observed in the Sammamish River during the January 2009 storm was ranked 20th. The comparison of the 1 percent AEP calculated in this study to the estimates in the most recent flood studies indicated that most differences were relatively small and within the 95 percent confidence intervals of the 1 percent AEP.