A reliable supply of water is vital for our communities, businesses, industries, environment, and quality of life, but too much precipitation at one time means reservoirs become overburdened with no way to capture water that is needed at other times of the year. The extreme weather events early in 2023 that hit California showed how climate change is increasing the intensity of weather events, leaving municipalities unable to capture – or cope with – unusual amounts of precipitation.
Flooding impacts both water quantity and water quality. It can contaminate private and municipal water systems, and carve new river channels that threaten infrastructure. Large deposits of silt and debris can pollute streams and rivers, making it impossible for salmon to live. Property damage and crop losses threaten the food supply, and human lives are endangered and families displaced. Even in areas that are not low-lying or adjacent to a waterway can be affected by flooding; water-saturated soil combined with other factors contributed to the deadly landslide in Oso, Washington in 2014.
Climate change is increasing the frequency and severity of flood events in the Puget Sound region. Extreme precipitation events like atmospheric rivers are happening more often, resulting in substantial winter rainfall and major flooding west of the Cascades. Heavy rainfall events are expected to become more severe, leading to higher peak streamflows and when combined with warmer mountain temperatures melting snow, the flood risk becomes great. Models project that the heaviest 24-hour rain events in western Washington and Oregon will intensify by +22% on average by the 2080s.
While mountain snowfields normally act as natural reservoirs to store precipitation from the cool season, as temperatures warm earlier in the season the snow melts and runs into rain-swollen swollen rivers. There is over an 80% chance that 10 or more flood events will happen in any given year in the state, and the frequency of events is increasing with climate changes.
Tidal Flooding and Sea Level Rise
Sea level rise and higher sediment loading will also affect flood risk. Sea level is projected to increase by +28 inches on average in the Puget Sound region by 2100 (Table 2). Rising sea levels will increase the extent, depth, and duration of river flooding by making it more difficult for flood waters to drain into Puget Sound. For example, the area flooded in the Skagit River floodplain during a 100-year event is projected to increase by +74% on average by the 2080s when considering both sea level rise and larger flood flows. A similar study found that the 10-year flood event would flood +19% to +69% more area in the Snohomish River floodplain by the 2080s.