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The cumulative effect of a series of precipitation-laden Pacific storms over the period January 3-8, 1990, climaxing with extreme rainfall on January 9, resulted in heavy flooding on the northern Oregon coast and in western Washington from the southern border to the Seattle area. Interstate Highway 5 (1-5) was inundated with 3-5 feet of water and in the Chehalis area and was closed for several days. Four persons in the State of Washington lost their lives in the flooding. A Presidential Disaster Declaration was issued for two counties in Oregon and six counties in Washington. Total flood damage in the two states is estimated to exceed 19 million dollars. Record peak flows were recorded at 10 gaging stations. At eight of these stations, the peak flows are estimated to have recurrence intervals of about 100 years. The most extreme flooding was on the Nehalem River in Oregon and the Chehalis River in Washington. INTRODUCTION The flooding described in this report was primarily the result of a series of back-to-back storms accompanied by heavy rainfall over an 8-day period. The heaviest rainfall, which occurred on the seventh day of the storm, January 9, caused extreme flooding because it fell on soils that were saturated from the preceding rainstorms. This report describes the storm, the flood, and the flood damages, and includes a map showing the location of flood determination points and locations of selected precipitation stations (fig. 1). Flood-peak stream stages and discharges of the streams affected and those of previous floods of record are tabluated. Daily rainfall is shown for one site, and total rainfall for January 3-10 is shown for 10 sites. DETERMINATION OF FLOOD STAGES AND DISCHARGES Peak stages and discharges were obtained and compiled by the regular procedures of surface-water investigations by the U.S. Geological Survey. The usual method of determining stream discharge is the application of a stage-discharge relation to a known stage. That relation at a station is usually defined by current meter measurements through as much of the range in stage as possible. If the peak discharge is above the range of the computed stage-discharge relation, that relation may be extended by logarithmic extrapolation, by velocity-area studies, or by use of other hydraulic techniques. Peak discharges that are far beyond the range of the stage-discharge relation at the gaging stations may be determined by various types of indirect measurements. During major floods, such as the January 1990 floods in northwestern Oregon and Southwestern Washington, Flood determination point and __--__.!-_ /O__ A-l-lI \ 0 10 20 30 40 KILOMETERS Figure 1. Flood area and location of flood determination points and climatological stations in northwestern Oregon and southwestern Washington. adverse conditions often make it impossible to obtain current-meter measurements near flood peaks at some sites. Peak discharges at a few of the sites in this report were determined by indirect methods based on detailed surveys of selected channel reaches after the flood had subsided. Descriptions of the indirect methods used by the Geological Survey are given in a series of publications titled "Techniques of Water-Resources Investigations of the United States Geological Survey, Book 3." EXPLANATION OF DATA A summary table (table 1) of peak stages and discharges is given for the flood. In the summary table, the first major breakdown of columns under "Maximum floods" provides information on known floods prior to the 1990 flood. This information includes period of record, gage heights, and discharges for maximum floods during that period prior to 1990. More than one period of record are shown for some stations. The information under the second major breakdown of columns under "maximum floods" shows information on the January 1990 floods. It includes information by columns on date, maximum stage, maximum discharge, and recurrence interval for the January 1990 flood. The recurrence interval is the average inverval, in years, in which a flood of a given magnitude (the 1990 peak) will be equalled or exceeded once as an annual maximum. A flood having a recurrence interval of 50 years can be expected to occur, on the average, once in 50 years, or it is one that has a 2-percent chance of being exceeded in any one year. All recurrence intervals in the tables were determined using guidelines for determining flood frequency as shown in a report by the U.S. Water Resources Council (1981). THE STORM SYSTEM A major and complex storm system moved through northwestern Oregon and southwestern Washington during the period, January 3-10. This storm system included high winds and strong surges of precipitation, and was accompanied by high tides along the Oregon Coast. The main storm passed over the extreme northwestern corner of Oregon and traveled in a northeasterly direction, passing over the south end of Puget Sound before decreasing in intensity near the summit of the Cascade Range. Precipitation in Oregon was heaviest in the Nehalem and Wilson River basins in Clatsop and Tillamook Counties. The National Weather Service climatological station at Nehalem recorded a total of 16.6 inches of rain over the January 3-10 period (table 2). This 8-day total precipitation is more than 15 percent of the total yearly precipitation that is recorded at the Nehalem station on the average. The most rainfall that was recorded anywhere in the two states during the 8-day period was 18.4 inches at Lees Camp, Oregon (U.S. National Oceanographic and Atmospheric Administration, 1990c). The station at Lees Camp, Oregon recorded more than 5 inches of precipitation on January 9. The daily precipitation record for Grays River Hatchery, Washington (fig. 2) illustrates the surging precipitation pattern of the storm system over the 8-day period, January 3-10, 1990. The most intense precipitation in the State of Washington during the storms occurred near the headwaters of the Skookumchuck, Deschutes, and Nisqually River basins in Lewis County (U.S. Federal Emergency Mangement Agency Region X, 1990a). Eight-day precipitation totals for selected climatological stations are shown in table 2. Table 1. Flood stages and discharges. January 9-11. 1990 in northwestern Oregon and southwestern Washington 2 3 [mi = square miles, ft /s = Map Station Stream and Drainage number number place area 2 determination (mi ) 1 14301500 Wilson River 161 near Tillamook, Oregon 2 14301000 Nehalem River 667 near Foss, Oregon 3 12010000 Naselle River 54.8 near Naselle, Washington 4 12013500 Willapa River 130 near Willapa, Washington Known before Period Year of record

Floods of January 9-11, 1990 in northwestern Oregon and southwestern Washington