Floodplain function is assessed using connectivity, land cover, and land use for the 17 major rivers of Puget Sound. Areas that have natural land cover and unrestricted river flow are expected to be the most functional and provide the most ecosystem services. Floodplain areas with non-natural land cover and restricted river flow due to constraints (e.g. roads, railroads, levees) will have impaired or loss of floodplain function.
2011 baseline floodplain condition assessment for the 17 major rivers of Puget Sound. Total area is shown in acres for each category of floodplain condition.
Key Indicator Results
Floodplain function has been quantified for 2011, and a comparison with one or more additional years will determine if we are making progress or not for no net loss of floodplain function.
36% (156,361 acres) of Puget Sound’s major river floodplains are highly functional (categorized as connected natural land cover); 64% (282,571 acres) of Puget Sound’s major river floodplains have reduced function.
Over half of the region’s floodplain areas are found in three of the 17 major rivers: Nooksack, Skagit, and Snohomish Rivers.
Since 2011, at least 39 activities managed by RCO’s PRISM have actively conserved 731 high-quality floodplain acres and 188 activities have improved or restored 8,162 floodplain acres (see Restoration of Floodplains indicator for more details).
By 2020, have no net loss of floodplain function in any watershed relative to a 2011 baseline.
Floodplain Extent: Delineated by NOAA Status and Trends Habitat Program, Northwest Science Center, and where NOAA data were lacking, FEMA 500 year (0.2%) and USGS Low Floodplain (Konrad 2015) were used.
Connectivity: USGS refined methods from Konrad (2015) using roads, railroads, and regional and local levee data
Land Use: Washington State Department of Agriculture (2011) and local agriculture in pilot watersheds, if available
Land Cover: NOAA CCAP Land Cover (2011), WDFW High Resolution Land Cover (2011)