Assessing tidal marsh vulnerability to sea-level rise in the Skagit Delta

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