Abstract
The flooding of 1861-1862 in California and Oregon is the most severe flood event documented in the far western USA and stands as a benchmark for a worst-case atmospheric-river flooding event. In western Washington, historical data are sparser, and 19th-century flood events have consequently not been well documented. We found that rainfall observations from five locations spanning western Washington had no detectable bias when compared to nearby 20th and 21st-century comparator stations. Time series of the four-day precipitation sum revealed an event in December 1867 that was greater than any of the last century at three locations, and in the top two events at the other two locations. Summing over all locations, the regional three-day or four-day peak precipitation in 1867 exceeded the 150-yr recurrence magnitude by nearly 150 mm, indicative of non-stationarity of precipitation extremes. Newspapers and historical accounts document flood damage to settlements, farms, and bridges from the Columbia River to central Puget Sound. Reported high water levels at two locations indicate floodplains under more than a meter of water. Reanalysis data (20CRv3) is poorly spatially constrained in 1867, and underestimates the magnitude of this event, but it clearly shows the atmospheric-river cause of the event and supports snowmelt as a significant contributor to flooding. Compared to the most recent extensive flooding in 1996, the 1867 floods were likely of a similar extent but centered further north, and with notably more precipitation and enhanced by snowmelt. The 1867 rainfall amounts were also greater than those produced by the 2006 atmospheric river, though flooding in 2006 was not enhanced by snowmelt and record stream discharges were limited to mountain catchments. The combined rainfall and flood evidence from 1867 shows the potential for events more extreme than have occurred in recent history in the major urban corridors of western Washington.
Publisher
Public Library of Science (PLoS)