Streams and Floodplains

Floodplains provide invaluable ecosystem services including critical habitat for the health, growth, and survival of Pacific salmon and steelhead, flood damage mitigation, improved water quality, vital habitat for a suite of flora and fauna, recreational opportunities, and economically valuable farmlands.

Population growth and related development needs continue to modify floodplains and excaberate seasonal anomalies in stream flows. These changes have potentially adverse consequences for people, property, habitats, and the species that depend on streams and floodplains.

This Vital Sign tells us about the extent and condition of streams, rivers, and associated floodplain and riparian habitats that are important to salmon recovery and overall Puget Sound health.


Key Vital Sign Messages

  • Intact, healthy floodplains protect an estimated $18 billion dollars of residential, commercial, and industrial development and infrastructure through the storage of flood waters (Floodplains by Design 2016).
  • The 17 major rivers of Puget Sound have lost or experienced a reduction in over 60% of their floodplain function in the last 100 years. The loss is predominantly in response to increased population growth and development leading to constrained river flow and non-natural land cover.
  • Summer low flows in the Puget Sound basin respond to a variety of drivers including rainfall, snowfall, temperature, evapotranspiration, land-use conversion, forest practices, and human water use. The summer low flows indicator aims to describe the change pattern in summer low flow based on the net effects of all factors combined; it does not evaluate the potential impact of individual factors on a trend.
  • Climate change impacts on Puget Sound streams and floodplains is expected to further stress these ecosystems and impact the multiple benefits they provide (UW Climate Impacts Group 2016).
    • Some floodplain communities are experiencing more frequent and severe winter floods and the pattern is expected to continue.
    • At the same time, summer stream flows are expected to be lower and warmer, further reducing floodplain connectivity with the river.
    • Both of these climate impacts will impede salmon use of floodplains and the viability of floodplains to continue to provide critical flood storage capacity to reduce the downstream effects of flooding.
    • Well-functioning floodplains with low levels of development are likely to be more resilient to climate change impacts.
    • Compromised floodplains in urban watersheds may need to implement adaptive measures such as relocating structures in the floodplain to reduce losses due to increased likelihood of damaging floods.

Background Documents

Implementation Strategy

The Partnership and its affiliated network of researchers works with the three Strategic Initiative Lead Teams on Implementation Strategy development and operationalization. Please read more about these teams and our shared work at

Indicator Targets

Other Resources

Contributing Partners