Presentation Abstract
Historical aerial photos, from 1937 to the present, show Skagit Delta tidal marshes prograding into Skagit Bay throughout most of the record, but the progradation rates have been steadily declining and become negative in recent decades, i.e., the marshes have begun to erode. If past trends continue, the marshes will continue eroding despite the large suspended sediment load provided by the river. Within the bay-fringe region between the North and South Fork Skagit River distributaries, an area isolated from direct riverine sediment supply by anthropogenic blockage of historical distributaries, 0.5-m tall marsh cliffs along with concave marsh profiles indicate wave erosion is contributing to marsh retreat. This is further supported by a “natural experiment” provided by rocky outcrops that shelter high marsh in their lee, while being bounded by 0.5-m lower eroded marsh to windward and on either side. GIS analysis of the bay-fringe marsh indicates that relative to modern LIDAR data the seaward edge of the 1972 marsh (its maximal extent) was 15 cm lower in elevation than the 2012 marsh edge; this comparison assumes no change in marsh slope during the last 40 years. Tide gauge data show sea level rose 7.9 ± 4.6 cm during this time, suggesting sea level rise accounts for a substantial portion of the elevation difference, while erosion may have contributed to the rest. Coastal wetlands with high sediment supply are considered resilient to sea level rise, but the Skagit Delta shows this is not necessarily true. A combination of sea level rise and wave-generated erosion may overwhelm sediment supply. Additionally, constructed levees intensify river jet momentum thereby delivering most suspended sediment far offshore where it is unavailable for marsh accretion. Adaptive response to the threat of sea level rise should consider the efficacy of restoring historical distributaries and managed retreat of constrictive river levees to maximize sediment delivery to delta marshes.
Session Title
Session S-07H: Assessing, Planning and Adapting to Climate Change Impacts in Skagit River Watershed
Conference Track
Shorelines
Conference Name
Salish Sea Ecosystem Conference (2014 : Seattle, Wash.)
Document Type
Event
Start Date
1-5-2014 3:30 PM
End Date
1-5-2014 5:00 PM
Location
Room 607
Genre/Form
conference proceedings; presentations (communicative events)
Contributing Repository
Digital content made available by University Archives, Heritage Resources, Western Libraries, Western Washington University.
Subjects – Topical (LCSH)
Salt marshes--Climatic factors--Washington (State)--Skagit River Delta; Sea level--Environmental aspects--Washington (State)--Skagit River Delta; Climatic changes--Washington (State)--Skagit River Delta
Geographic Coverage
Salish Sea (B.C. and Wash.); Skagit River Delta (Wash.)
Rights
This resource is displayed for educational purposes only and may be subject to U.S. and international copyright laws. For more information about rights or obtaining copies of this resource, please contact University Archives, Heritage Resources, Western Libraries, Western Washington University, Bellingham, WA 98225-9103, USA (360-650-7534; heritage.resources@wwu.edu) and refer to the collection name and identifier. Any materials cited must be attributed to the Salish Sea Ecosystem Conference Records, University Archives, Heritage Resources, Western Libraries, Western Washington University.
Type
Text
Language
English
Format
application/pdf
Included in
Assessing tidal marsh vulnerability to sea-level rise in the Skagit Delta
Room 607
Historical aerial photos, from 1937 to the present, show Skagit Delta tidal marshes prograding into Skagit Bay throughout most of the record, but the progradation rates have been steadily declining and become negative in recent decades, i.e., the marshes have begun to erode. If past trends continue, the marshes will continue eroding despite the large suspended sediment load provided by the river. Within the bay-fringe region between the North and South Fork Skagit River distributaries, an area isolated from direct riverine sediment supply by anthropogenic blockage of historical distributaries, 0.5-m tall marsh cliffs along with concave marsh profiles indicate wave erosion is contributing to marsh retreat. This is further supported by a “natural experiment” provided by rocky outcrops that shelter high marsh in their lee, while being bounded by 0.5-m lower eroded marsh to windward and on either side. GIS analysis of the bay-fringe marsh indicates that relative to modern LIDAR data the seaward edge of the 1972 marsh (its maximal extent) was 15 cm lower in elevation than the 2012 marsh edge; this comparison assumes no change in marsh slope during the last 40 years. Tide gauge data show sea level rose 7.9 ± 4.6 cm during this time, suggesting sea level rise accounts for a substantial portion of the elevation difference, while erosion may have contributed to the rest. Coastal wetlands with high sediment supply are considered resilient to sea level rise, but the Skagit Delta shows this is not necessarily true. A combination of sea level rise and wave-generated erosion may overwhelm sediment supply. Additionally, constructed levees intensify river jet momentum thereby delivering most suspended sediment far offshore where it is unavailable for marsh accretion. Adaptive response to the threat of sea level rise should consider the efficacy of restoring historical distributaries and managed retreat of constrictive river levees to maximize sediment delivery to delta marshes.
