No targets are currently set for this indicator.
This indicator measures the amount (acres and percent) of floodplain area in functional condition in Puget Sound’s 17 major rivers. Floodplain function is assessed at a regional scale using river connectivity and land use and cover. Areas that have natural land cover and unrestricted river flow are expected to be the most functional and provide the most ecosystem services. Floodplain function is impaired in areas with non-natural land cover or restricted river flow due to constraints or barriers (for example, roads, railroads, and levees).
Floodplain condition assessment across Puget Sound's 17 major rivers. Total area in acres for the four categories of floodplain condition at two time periods: 2011 baseline and circa 2021 update.
Floodplains are an important part of the Puget Sound landscape, particularly in the lowlands along large rivers. They are used by people for cities, farms, and transportation. Floodplains provide critical habitat for fish and wildlife, which in turn play a role in human wellbeing, meeting economic, cultural, and recreational needs.
During floods, a wide, connected floodplain allows the river to move and reduces damage to nearby areas by absorbing stormwater and routing water through river corridors while also holding some water to recharge groundwater reserves.
The Puget Sound Partnership and its partners are responsible for developing, maintaining, and updating these data in coordination with Lead Entities.
Floodplains Conditions Assessment (Environmental Science Associates and Puget Sound Partnership 2023)
This indicator describes floodplain function in Puget Sound’s 17 major river watersheds based on river connectivity and categories of land use and land cover. We assessed floodplain conditions at two time periods: a 2011 baseline and a circa 2021 update (update based on data from 2016 through 2023, see Table 1 below). The update allows for a more recent assessment of floodplain status and supports the analysis of trends in Puget Sound floodplain function.
Local data for the floodplain extent, barriers, land use, and land cover were provided for three pilot watersheds (Dungeness, Green-Duwamish, and Stillaguamish) to refine and improve the baseline assessment data in those watersheds with their technical guidance.
Floodplain Extent
The floodplain extent represents the historic or geomorphic floodplain boundary. This covers the relatively level surface extending laterally from the river channel edge (Beechie et al. 2017). Within each river, the floodplain includes the upland freshwater as well as brackish estuarine or delta areas.
Condition Assessment
The purpose of the condition assessment is to delineate Puget Sound floodplains based on connectivity to their associated rivers and their land use and cover as a proxy for floodplain function. We combined the prepared data sources (Table 1) and attributed areas of the floodplain under four high-level categories of function:
Connectivity
Connectivity is defined as "the unrestricted movement of water, biota, sediment, wood, and other materials between rivers and floodplains” (Konrad 2015). We use the presence of levees, roads, and railroads to identify disconnected areas within the floodplain.
With support from the Puget Sound Ecosystem Monitoring Program (PSEMP) Spatial Data Work Group, Environmental Science Associates (ESA) coordinated with local, regional, state, and federal entities to develop a standardized levee dataset for the Puget Sound region. For more information on the regional levee standardization project, see the Technical Memo available online at https://pspwa.box.com/s/qoolcw8dai3v1lg9kd3mh7670d9pp3p4.
Land Use and Cover
We categorize land cover as natural or non-natural based on classifications under NOAA’s Coastal Change Analysis Program (C-CAP) Regional Land Cover and Washington Department of Fish and Wildlife’s (WDFW) High Resolution Land Cover.
Agricultural lands are identified from Washington State Department of Agriculture (WSDA) Agriculture Land Use data.
Floodplain Extent | Puget Sound Delta Boundaries 2016 (NOAA SHSTMP) |
Puget Sound Floodplains 2016 (NOAA SHSTMP) | |
USGS Low Floodplain (Konrad 2015) | |
FEMA 500-year Flood, 0.2% (County data compiled, various years) | |
Condition Assessment 2011 Baseline |
Floodplain Connectivity Assessment (USGS 2015) |
C-CAP Regional Land Cover (NOAA 2011) | |
High Resolution Land Cover (WDFW 2011) | |
Agricultural Land Use (WSDA 2011) | |
Hydrology (WA DNR 2014) | |
Condition Assessment Circa 2021 Update |
Regional Standardized Levees (ESA 2023) |
Roads and Railroads (US Census 2021) | |
C-CAP Regional Land Cover (NOAA 2016) | |
High Resolution Land Cover (WDFW 2017) | |
Agricultural Land Use (WSDA 2021) | |
Hydrology (WA DNR 2023) |
References:
Beechie, T. J., O. Stefankiv, B. Timpane-Padgham, J. E. Hall, G. R. Pess, M. Rowse, M. Liermann, K. Fresh, and M. J. Ford. 2017. Monitoring Salmon Habitat Status and Trends in Puget Sound: Development of Sample Designs, Monitoring Metrics, and Sampling Protocols for Large River, Floodplain, Delta, and Nearshore Environments. U.S. Department of Commerce, NOAA Technical Memorandum NMFS-NWFSC-137. https://doi.org/10.7289/V5/TM-NWFSC-137
Konrad, C.P., 2015, Geospatial assessment of ecological functions and flood-related risks on floodplains along major rivers in the Puget Sound Basin, Washington: U.S. Geological Survey Scientific Investigations Report 2015–5033. http://dx.doi.org/10.3133/sir20155033
These data are intended for watershed and regional scales and are therefore useful in evaluating regional trends and targets. The data should not be used at site-specific scales or for the purpose of prioritizing and planning projects.
Puget Sound has 443,000 acres of historic geomorphic floodplain on the 17 major rivers. All of Puget Sound’s major rivers have some level of impaired function. The extent and condition of floodplain area in Puget Sound varies by geographic region. The rivers of the Strait of Juan de Fuca and Hood Canal originate in the northern and eastern Olympic Mountains and enter Puget Sound as relatively steep, narrow floodplains. Subsequently, the surface area of these river floodplains is much less than that of the broad, low gradient areas draining the Cascade Range. The Skagit and Snohomish river floodplains are uniquely large. Together, these deltas represent 43 percent of Puget Sound’s mapped geomorphic floodplain area.
Protection and restoration efforts have made important improvements to the region’s floodplains and activities are continuing. Regional strategies for floodplain restoration are documented in a Floodplain and Estuaries Habitat Implementation Strategy. The NEP Funded Habitat Strategic Implementation Lead (SIL) is leading these effort, for more information consult the Habitat SIL website.
Snohomish County’s Benefits of a Healthy Floodplain and Hazards of Flooding
The Nature Conservancy’s Coastal Resilience and Floodplain by Design Tool
Federal Emergency Management Agency (FEMA) Benefits of Natural Floodplains
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Condition Assessment |
Baseline (2011), Update (2021)
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