Project Summary

Port Susan Bay is a productive estuary found along the northwest coast of Washington State. The Nature Conservancy (TNC) manages the Port Susan Bay Preserve and is incorporating climate change into its long-term management plan. TNC staff and volunteers have removed invasive species, reintroduced large woody debris, used the Sea Level Affecting Marshes Model (SLAMM) to examine sea level rise impacts, and removed a dike to increase the area of tidal marshland. TNC hopes to use lessons learned at Port Susan Bay in future estuarine restoration projects.

Background

Port Susan Bay is located in Puget Sound at the terminus of the Stillaguamish River. Its estuaries and marshes support many migratory birds, several species of salmon, and countless other animals and habitats. TNC owns a 4,122-acre preserve in Port Susan Bay. The Bay has historically been subject to diking, levee installation, and other water diversions. Some of the levees have disconnected historical floodplains from the Stillaguamish River and subsequently altered the water dynamics of Port Susan Bay. Also, a sea dike that was built in the 1950s to create more farmland has changed the sediment loads that enter the estuary, causing tidal marsh erosion.

Sea level rise and periodic flooding are expected to further complicate habitat erosion rates in Port Susan Bay. As the sea level rises, vital habitats may become submerged and permanently altered. In addition, flooding on the Stillaguamish River could scour the embankment, transporting sediments from the estuary and depositing them into the ocean.

Implementation

When TNC acquired land in Port Susan Bay, a preliminary study revealed that the area had changed dramatically when compared to historical photos. It appeared that the estuarine ecosystem was gradually disappearing and productivity declining. The change evidenced in Port Susan Bay is likely caused by human influences such as water diversions, agriculture, community development, and more recently, climate change.

TNC is using multiple tools and techniques to enhance the health and long-term outlook of Port Susan Bay in order to restore tidal wetlands, improve connectivity between the estuary and the river, and improve flood attenuation. Staff and volunteers have removed most of the invasive plant, Spartina anglica, from the property. They launched the Port Susan Large Wood Project to reintroduce large logs to increase the estuary’s structural complexity by creating scour pools, trapping sediment, stabilizing channels, and providing habitat for animals. In 2007, twenty-six logs were helicoptered into Port Susan Bay and placed in six different areas to mimic historical logjams that would have been present within the estuary. TNC monitored channel morphology and habitat use to evaluate the effectiveness of the introduced logs in improving the health of the estuary. Researchers determined that while the large woody debris was difficult to maintain in place, it did yield some benefits in terms of stabilizing channels, promoting marsh development, and creating large pools in which estuarine fishes were observed.

In 2012, TNC completed a dike removal and setback project at the mouth of the Stillaguamish River in Port Susan Bay. The dike had been built in the late 1950s, and over time, reduced the tidal marsh area by almost half since its construction. The goal of the project was to restore 150 acres of tidal wetlands. TNC removed over 7,000 feet of dike, constructed a 5,000-foot setback dike, and built a fish bypass structure. TNC supported several years of close monitoring of the site. Results of the monitoring showed that marsh restoration occurred much slower than was originally predicted, likely due to pre-restoration soil conditions as well as the size and number of dike breaches. A 2019 study found that as sediment accumulates in the restored estuary, carbon is sequestered at a rate double that of surrounding marshes. Based on its current rate of sequestration, the site may be able to sequester 4,500–9,000 tons of carbon.

The impacts of climate change have become central to the long-term management strategy of Port Susan Bay. TNC is researching how sea level rise will affect river flow and erosion rates, as well as the effects of increased storm intensity on Port Susan Bay. Throughout the six years of estuarine monitoring, researchers found that sediment accumulation in restored marshes matches current rates of sea level rise. One tool used to assess vulnerability to climate change is SLAMM, which allows users to model changes in sediment accretion, freshwater distribution, and water height under varying sea level rise scenarios

Outcomes and Conclusions

Current accretion rates at the Port Susan Bay Reserve are expected to keep pace with rising sea levels. The greatest impact of climate change in the area will likely result from decreased river flows, while other stressors such as soil disturbance may exacerbate marsh erosion. Additional restoration activities that should be prioritized are those that increase estuarine freshwater residence time.

Citation

Feifel K, Gregg RM (2021). Effects of Sea Level Rise in Port Susan Bay, Washington [Case study on a project of The Nature Conservancy - Skagit River Office]. Version 2.0. Product of EcoAdapt’s State of Adaptation Program. (Last updated August 2021)