Floodplains

Floodplain

Floodplains occur where large rivers and streams meander in their own sediments

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Floodplains form where rivers flow through a valley of alluvial sediment several times the width of the channel. During floods, the river inundates the floodplain depositing sediments. Floodplains and their rivers are fed by both Headwaters and Lowland Watersheds, and flow into River Deltas as they enter the sea. Floodplains are critical for Biodiversity and Salmon and are also vital for Agriculture. Encroachment of residential Land Use on floodplains is a driver for Flood Hazard Management. Floodplains are generally described by FEMA mapping of flood hazard areas. The Floodplains by Design effort by The Nature Conservancy of Washington aims to provide a stronger assessment of these systems. Lakes and Wetlands are all different kinds of floodplains (where shallow Groundwater is found at or near the surface for parts of the year).

Physical Context

USGS collaborates with NOAA to provide real-time data on river flow and flood stage. Mauger et al 2015 describes the anticipated change in the flow regime under Climate Change. Rivers are flows of both water and sediment and Czuba et al 2011 summarizes existing data on total sediment budget. Collins & Montgomery 2011 describes a diversity of physical systems, based on how much, and how floodplains have filled since glacial excavation.

Biological Context

Besides us, beaver are key ecosystem architects, and the reduction in beaver populations combined with flood hazard management has radically changed floodplain ecosystems.

Planning Context

Because of our dry-summer climate, moist rich floodplain soils are vital to agriculture. The Puget Sound Partnership has adopted a floodplain restoration target as part of Puget Sound Vital Signs. Floodplains by Design has promoted multi-benefit restoration strategies, with novel efforts in the Puyallup/White. Snohomish, Stillaguamish, Skagit, Nooksack, Dungeness and Skokomish basins. Floodplain habitats. The Puget Sound Watershed Characterization identified a unique set of floodplain attributes because of wetlands, including those created by beaver, re-affirming the importance of floodplains already described in salmon recovery planning, particularly for endangered Puget Sound Chinook Salmon. A constellation of authorities and agencies in Flood Hazard Management systems are focused on floodplain dynamics.

Floodplain Topic Pages

The following Topic Pages are categorized with Floodplain.

Beaver  •  Channel Migration Zone  •  Engineered Log Jams  •  Flood Hazard Management  •  Flood fencing  •  Integrated Floodplain Management  •  Riparian Buffers  •  Rivers and Harbors Act  •  Snohomish Agricultural Resilience  •  Snohomish Agriculture Resilience Plan

Floodplain Places By Region

The following Floodplain Pages are organized by region.

Fraser Lowlands

North Fork Nooksack Floodplain  •  South Fork Nooksack Floodplain

Hood Canal

Skokomish Confluence

Whidbey Basin

French Slough Floodplain and Watershed  •  Lower Skykomish Floodplain  •  Lower Stillaguamish Floodplain  •  North Fork Stillaguamish Floodplain  •  Snohomish Lord Hill Reach  •  South Fork Stillaguamish Floodplain

Floodplain Workgroups and Efforts

The following Workgroups and Efforts are associated with Floodplain.

Workgroups

Floodplains Local and Regional Exchange  •  Floodplains for the Future  •  Northwest Regional Floodplain Management Association  •  Skagit Climate Science Consortium  •  Sno-Valley Tilth  •  Snohomish Sustainable Lands Strategy  •  Western Washington Agricultural Association

Effort

Barnaby Reach Restoration  •  Black River Farm  •  Coastal Resilience to Climate Change  •  Elwha Dam Removal  •  Farmhouse Reach Floodplain Restoration  •  Floodplains by Design (FbD)  •  French Slough Drainage Management  •  Haskell Slough Protection and Restoration  •  Jim Creek Channel Enhancement  •  Larson's Reach Floodplain Restoration  •  Lower Canyon Creek Restoration  •  Lower Skykomish Floodplain Design  •  Lower Skykomish Reach Scale Plan  •  Lower Snohomish Reach Scale Plan  •  Nooksack Floodplain Integrated Management Planning  •  Nooksack Floodplain Restoration with Engineered Log Jams  •  Old Soldiers Home Levee Setback  •  Pierce County Levee Setback Planning  •  Predicting Sea Level Rise Impacts on Ag Production  •  Scatter Creek Farm and Conservancy  •  Skagit Farmland Legacy Program  •  Skykomish Braided Reach Project  •  Skykomish Habitat Mitigation Bank  •  Snoqualmie Fish, Farm and Flood Initiative  •  The Ecosystem Guild/Bio-Cultural Restoration Field Stations  •  Whatcom Drainage Maintenance Guidance  •  Working Buffer Pilot Project

Floodplain Products

The following Product pages are categorized with Floodplain.

Documents

Abbe & Montgomery 2003  •  Abbe et al 2018 design and placement of engineered log jams  •  Arcos 2012 skokomish channel stability.pdf  •  Beechie et al 2010 process based restoration of rivers  •  Beechie et al 2017 salmon habitat status monitoring.pdf  •  Benedict & Shaw 2012 shade benefits from hedgerow on ditched streams.pdf  •  Biddle et al 2022 national strategy protect restore floodplain wetland function.pdf  •  Boggs & Corey 2009 whatcom drainage maintenance guidance.pdf  •  Boggs & Corey 2009 whatcom drainage maintenance plan.pdf  •  Booth et al 2021 lower skykomish geomorphic assessment.pdf  •  Breslow 2001 farmer perception of salmon recovery.pdf  •  Brouwer et al 2015 floodplain early opportunities.pdf  •  Brown 2015 DRAFT nooksack floodplain vision and implementation.pdf  •  CIG 2016 climate change in floodplains.pdf  •  Canty et al 2012 farmland protection puget sound.pdf  •  Cardno 2020 lower skykomish channel migration zone.pdf  •  Cereghino 2015 coordinated investment snohomish framework.pdf  •  Cereghino 2016 DRAFT lower stillaguamish context.pdf  •  Cereghino 2017 DRAFT riparian framework.pdf  •  Cluer and Thorne 2014 stream evolution model  •  Collins & Montgomery 2002 restoring wood in river floodplains  •  Collins & Montgomery 2011 puget sound floodplain domains  •  Collins et al 2002 historical wood in large rivers  •  Collins et al 2012 floodplain large wood cycle  •  DeVries 2010 lower skykomish geomorphic analysis.pdf  •  DeVries 2015 lower snohomish geomorphic assessment.pdf  •  Dean 2011 farm succession strategies.pdf  •  Earth Economics 2013 pierce county flood return on investment.pdf  •  Evans & Moore 2011 snohomish sustainable lands strategy framework.pdf  •  FCWMC 2004 french creek watershed management plan.pdf  •  FEMA 2011 floodplain habitat assessment mitigation.pdf  •  FEMA 2013 environmental benefits acquisition hazard mitigation.pdf  •  FEMA 2013 model floodplain ordinance NFIP and ESA.pdf  •  GeoEngineers 2008 puyallup basin levee setback feasibility.pdf  •  Habitat Strategic Initiative 2021 estuary and floodplain implementation strategy.pdf  •  Hass 2001 snohomish ecosystem restoration opportunities.pdf  •  Herrera 2007 canyon creek restoration plan.pdf  •  KWL 2003 canyon creek alluvial fan risk assessment.pdf  •  Katz et al 2022 flood attenuation and restoration puyallup white.pdf  •  Klesick 2016 DRAFT SVPI concept.pdf  •  Knutson & Naef 1997 riparian management recommendations.pdf  •  Konrad 2015 ecological function flood risk puget sound floodplains  •  Legg & Olson 2015 identifying migrating stream channels in GIS  •  Leonetti et al 2015 north fork stillaguamish habitat change.pdf  •  Maudlin & Coe 2011 SF Nooksack ELJ effectiveness.pdf  •  Mauger & Kennard 2017 climate change and flood hazard management.pdf  •  Mauger et al 2015 snohomish downscale hydrologic projections.pdf  •  Mauger et al 2016 climate change and floodplain decision support.pdf  •  May 2003 riparian best available science puget lowland.pdf  •  McHenry et al 2007 ELJ monitoring elwha river.pdf  •  Montgomery et al 2003  •  NHC 2006 skykomish snoqualmie flood insurance report.pdf  •  Naiman et al 2010 process-based floodplain forest patterns  •  Pess et al 2011 salmon density on Elwha ELJs  •  Pollock et al 2015 beaver restoration guidebook.pdf  •  Rapp & Abbe 2003 channel migration zone.pdf  •  Roni et al 2013 DRAFT BPA columbia river restoration monitoring plan.pdf  •  Roni et al 2015 wood placement summary  •  SCD 2016 lower stilly farmland use map.pdf  •  SCSWM 2012 middle pilchuck river assessment.pdf  •  SCSWM 2012 pilchuck river geomorphic assessment.pdf  •  STNRD 2011 sediment impact on artificial chinook redds north fork stillaguamish.pdf  •  Scott 2023 valley bottom reset monitoring Deer Creek Oregon  •  Skidmore & Wheaton 2022 riverscapes as adaptation infrastructure  •  SnoCo 2014 DRAFT lower skykomish reach scale plan.pdf  •  Snohomish County 2015 hazard mitigation plan  •  Turner et al 2004 stillaguamish flood hazard management plan.pdf  •  USACE 2000 stillaguamish restoration feasibility.pdf  •  WDFW 2015 french creek drainage maintenance HPA.pdf  •  WSE 2021 snohomish hydrologic and hydraulic modelling.pdf  •  Wright 2021 integrated floodplain management puget sound.pdf  •  Yang et al 2011 model flood and tide

Maps

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Websites & Datasets

Snohomish County River Atlas

particles of clay, silt, sand, gravel, or cobble, transported by water, are called sediment.