Functioning Habitat
Streams and Floodplains
Floodplain function in large and small river systems
Vital Sign Indicator
Acre (acres)

No targets are currently set for this indicator.

Mary Ramirez
Contributing Partners
Last Updated
9/11/2023 4:35:57 PM
This map displays the geomorphic footprint and condition assessment of Puget Sound’s major river floodplains. Floodplain condition is assessed at two time periods: a 2011 baseline and a circa 2021 update. Each assessment is included as a unique data layer. These data are intended for use at watershed and regional scales and are not appropriate for local or site-scale analyses or planning.

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).

Vital Sign Indicator Chart
Floodplain function in large and small river systems
By: Condition Assessment

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.

Key Vital Sign Indicator Results
  • Puget Sound’s 17 major rivers include more than 440,000 acres of floodplain. Over half (55 percent) of the region’s floodplain area is found in three of the 17 major rivers: Skagit, Snohomish, and Nooksack Rivers.
  • Across the region, 38 percent (166,294 acres) of the total floodplain area is highly functional (Category A - Connected Natural Land Cover). The remaining 62 percent has reduced floodplain function. This includes 163,374 acres (37 percent of the total floodplain area) where the floodplain is disconnected and classified as developed, cultivated, or other non-natural land cover (Category D - Disconnected Not Natural Land Cover).
  • A comparison of floodplain conditions between 2011 and circa 2021, showed the greatest change occurred in Category A (connected natural land cover), with an increase of 8,134 acres. This includes changes in connectivity, land cover, or both. The total area of Category D (disconnected non-natural land cover) declined by 4,925 acres.
  • Connected floodplain area [including natural (Category A) and non-natural land cover (Category B)] increased by 3,567 acres since the 2011 assessment. While this points to improving progress in floodplain function, the change in connectivity affects less than 1 percent of the total floodplain area and extensive portions of historical habitat remain lost or degraded across Puget Sound.
  • Changes in connectivity between the two time periods largely reflect reconnection projects, including areas associated with the Nisqually Delta Restoration, Dungeness River Reconnection, and Smith Island Estuarine Restoration in the Snohomish River.
  • The extent and composition of functional floodplain area varies by watershed. Please see Interpretation of Results section for additional summaries.
Monitoring Program

The Puget Sound Partnership and its partners are responsible for developing, maintaining, and updating these data in coordination with Lead Entities.

Data Source

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:

  1. Connected Natural Land Cover
  2. Connected Not Natural Land Cover
  3. Disconnected Natural Land Cover
  4. Disconnected Not Natural Land


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

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.

  • Natural land cover includes natural vegetation such as trees, shrubs, grasses, unconsolidated shore, and water.
  • Non-natural land cover includes developed or built classes, open space such as turf areas, cultivated, pasture/hay, grassland, and bare land.

Agricultural lands are identified from Washington State Department of Agriculture (WSDA) Agriculture Land Use data.


Table 1. Key data sources. (SHSTMP = Salmon Habitat Status and Trend Monitoring Program)
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)



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.

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.

Critical Definitions
Interpretation of Results

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.

Floodplain area (acres) for each category of floodplain function under circa 2021 conditions. Watersheds are ordered geographically.


Percentage of floodplain area for each category of floodplain function under circa 2021 conditions. Watersheds are ordered geographically.


Area (acres) of connected floodplain (categories A and B) by watershed. Data labels show the difference in connected acreage between the baseline (2011) and update (circa 2021).


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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Reporting Guidance
Reporting Instructions
Condition Assessment
Baseline (2011), Update (2021)