Sediment and vegetation monitoring during a levee removal project on the Stillaguamish River Delta at Port Susan Bay, WA

Sea levels around the world are on the rise in due to the effects of climate change. Coastal wetlands and estuaries are at risk of being submerged as water levels continue to increase, unless they can move inland or gain surface elevation. These wetland systems provide vital ecosystem services that would be difficult or impossible to provide by other means. In the Puget Sound, Washington, 80% of the original estuarine and coastal wetland habitat has been replaced by human infrastructure, making the monitoring, preservation, and restoration of the remaining stock important both ecologically and economically. The objective of this project was to monitor the restoration of an estuarine system on the Stillaguamish River delta. The project involves the removal of levees and reintroduction of tidal flow into a subsided farmland that was formerly part of the estuary, and to determine the sustainability of the Stillaguamish River delta and similar Puget Sound estuaries with rising sea-levels. The scope of this monitoring project includes the installation and yearly sampling of surface elevation tables (SETs), vegetation surveys and quantification of the net primary productivity (NPP) within the leveed area, immediately outside the levees, in an adjacent area outside the farmland, and within an un-leveed reference site across the main river channel. SET sampling, before the levee removal, revealed a positive trend in elevation gain at 8 of the 11 SETs of over 1 cm/year, well above current rate of RSLR at 0.19cm/year. Sediment markers revealed that most of that gain can be attributed to sediment accretion, indicating adequate sources of sediment and therefore sustainability of the estuary under current rates of sea level rise. Primary productivity sampling in the late summer of 2012 yielded an average of 420 DW(g)/m²/year in the high marsh and 327 DW(g)/m²/year in the low marsh sites. Vegetation consisted predominantly of Schoenoplectus americanus, Schoenoplectus acutus, and Schoenoplectus maritimus, with elevation delineating the greatest shifts in community structure and abundance. The exception to this was within the portion of leveed farmland, where surface elevations were below the surrounding estuary and vegetation consisted primarily of a Schoenoplectus americanus monoculture.