Climate Change Impacts on River Flooding

This study looks into rainfall and river flow data throughout King County to determine whether large storm events and large flooding events are happening more often than they used to. This study also looks into results of computer modeling efforts at the University of Washington to understand whether such changes might happen in the future. This study was done to support planning efforts by King County’s Rivers and Floodplain Management Section, to help ensure that flood protection efforts consider an appropriate range of flow levels based on best available information.

Overall, there is some evidence that large storms and large floods in King County are occurring more frequently than they did in the 1960s. This evidence, while still preliminary, is in line with the computer model results from the University of Washington looking at how climate change might affect Pacific Northwest rivers. Rising air temperatures and changing storm patterns are likely to lead to more rain in November than used to occur, and also a shift over time from snow to rain in the mountains in the winter. Both of these changes would result in bigger and more frequent floods, although it is unknown how many years before these changes become significant and how large these changes may be. In general, given the preliminary evidence of changes to date, and the anticipated future changes based on computer modeling, it seems reasonable to pursue management strategies that facilitate adaptation to potential future increases in the frequency and magnitude of large floods.

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Download the Climate Change Impacts on River Flooding Report (1,424 KB)

Download the Climate Change Impacts on River Flooding Appendices (all) 7,052 KB