Restoring Tidal Flow and Enhancing Shoreline Resilience in the Nisqually River Delta
The Nisqually Delta Restoration Project is the largest tidal marsh restoration effort in the Pacific Northwest. Over four miles of dikes were removed in 2009 to return tidal flow to roughly 762 acres in the Nisqually National Wildlife Refuge in Washington State to enhance wildlife habitat and the buffering capacity of marshes to sea level rise and increased flooding. Along with other local restoration efforts, 22 miles of the historic delta system have been restored, increasing salt marsh habitat in southern Puget Sound by over 50 percent. Since 2009, scientists have closely monitored changes to the ecosystem using aerial photographs, permanent land-based panoramic photographs, sediment gauges, vegetation transects, bird and fish counts, and tidal gauges; results indicate that the historical delta ecosystem is returning and that the dike removal has increased the area’s salmon population.
The Nisqually National Wildlife Refuge, established in 1974, covers more than 3,000 acres of a variety of habitat types in the South Puget Sound region, including riparian and coniferous forest, seasonal and permanent freshwater wetlands, estuary, and open water, and the Nisqually River Delta. In 1904, the Brown Farm Dike was constructed to protect surrounding farmlands in the area from tidal surge; however, by restricting tidal flow, the dike also caused critical habitat loss for juvenile fish, birds, and marine mammals. A series of ongoing restoration efforts conducted in partnership between the Nisqually National Wildlife Refuge, Nisqually Indian Tribe, Ducks Unlimited, and several other partners have resulted in the restoration of over 900 acres of land. Removing the dike allowed salt water to spread across the delta, reestablishing old channels and sloughs to historic norms. Managers and scientists are carefully studying and documenting habitat development and biological indicators to assess the success of the restoration over time.
In fall 2009, four miles of the Brown Farm Dike was removed and a new 10,000-foot dike was installed, inundating over 760 acres of land. When combined with the 140 acres of tidal wetlands restored by the Nisqually Indian Tribe and 25 acres of riparian surge plain forest, this effort restored over 900 acres of estuary. In addition to enhancing habitat, the project also helped increase the area’s resilience to increased storms, sea level rise, and flooding.
The U.S. Geological Survey (USGS) is the lead science agency working to document and assess the effectiveness of the restoration effort with respect to changes in physical processes as well as ecosystem function. Specifically, USGS and partners are examining sediment delivery to the delta from the Nisqually River; changes in vegetation type and cover; and species response (e.g., distribution and abundance of birds and salmon).
Sediment Delivery. Scientists have also installed water level loggers in historic channels within the restoration site and in the three major surrounding waterways. With changes in water movement, there will be corresponding changes in sediment composition and transportation. To assess the effects tidal flow has on sedimentation, scientists are using a Surface Elevation Table (SET) and Feldspar Marker Horizon methods to mechanically measure changes in sediment elevation at three sites within the delta. Preliminary results indicate that dike removal has caused the mean delta elevation to increase, indicating that sediments are being deposited within the delta.
Geomorphic Change. Eleven permanent digital photograph locations have been selected throughout the Nisqually Delta. At each site, digital panoramic photos and photo-points are taken periodically to help managers qualitatively assess and describe changes that are occurring within the delta. These photos are useful for documenting vegetation colonization across the landscape as saltwater returns to the delta.
Species Response. Scientists are also monitoring biological responses to dike removal. They are tracking vegetation change using point-intercept transects and quadrats to estimate plant composition, height, percent cover of species, and the location of target plants or invasive species. Bird counts are done every month using binoculars and spotting scopes. Fish diversity is sampled using beach seining and fyke trapping, and otolith (i.e. fish ear bones) sampling is used to measure salmonid growth and residence times. Invertebrates are sampled using benthic cores along transect lines every summer.
Outcomes and Conclusions
There are early indications that the resilience of the historic ecosystem is returning to the Nisqually Delta. The removal of dikes and reconnecting of more than 35 km of tidal slough systems and floodplains has increased salt marsh habitat in the southern reach of Puget Sound by 55%. In addition, studies indicate that juvenile salmon have benefitted from the dike removal. Continued monitoring will allow managers and scientists to detect subtle changes within the delta as the system acclimates to tidal flows.
In 2011, the Nisqually Estuary Restoration Team was a recipient of the Coastal America Partnership Award in recognition of their efforts to restore and protect the estuary. In addition, the region was designated the Nisqually Reach Aquatic Reserve in 2011 by the Washington Department of Natural Resources, which expands its restoration and protection efforts into marine waters.
Feifel, K.M. and Gregg, R.M. (2021). Restoring Tidal Flow and Enhancing Shoreline Resilience in the Nisqually River Delta [Case study on a project of the Nisqually National Wildlife Refuge and partners]. Version 2.0. Product of EcoAdapt’s State of Adaptation Program. Retrieved from CAKE: http://cakex.org/case-studies/restoring-tidal-flow-and-enhancing-shoreline-resilience-nisqually-river-delta (Last updated October 2021)