The 17 major rivers of Puget Sound have experienced a more than 60% reduction of floodplain function in the last 100 years. This loss is largely due to population growth and development that increases non-natural land cover and constrains river flow. 

Intact, healthy floodplains protect billions of dollars of residential, commercial, and industrial infrastructure by storing water to reduce the downstream effects of flooding. Additionally, floodplains provide critical habitat for fish and wildlife (which in turn play a role in human wellbeing). 

The occurrence of below-normal streamflow is increasing across Puget Sound rivers. Consecutive years with below normal summer flows have increased since 1985 and especially since 2015. In 2024, summer flows were below normal most of the time at 89% of stream gages measured for the Vital Signs program.  

Summer flows in Puget Sound streams and rivers respond to a variety of drivers including rainfall, snowfall, temperature, evapotranspiration, land-use conversion, forest practices, and human water use. 

Puget Sound’s streams and floodplains are expected to be further stressed by climate change, which will impact the multiple benefits they provide.  

There are 3 additional indicators currently in development to support this Vital Sign.