Keeping Tidal Forces at Bay

In 2020, Ducks Unlimited Canada (DUC) and the Canadian Wildlife Service (CWS) restored a wetland located at the Wallace Bay National Wildlife Area (NWA) that was experiencing a breached and eroding dyke from sea level rise. Wallace Bay in northern Nova Scotia is perched on the shores of the Northumberland Strait and the NWA consists of more than 1,433 acres (580 hectares) of marshes, fields, and forests. In the 1970s, DUC and CWS created managed wetlands that became home to species like the ring-necked duck, blue-winged teal, and American black duck. Impounded freshwater wetlands were developed over a large section of lands that were once drained and dyked for agriculture. However, with recent sea level rise, tides were topping the dyke, speeding up its erosion, and making it harder to maintain. The mix of saltwater and freshwater in the ecosystem wasn’t as welcoming to plants and wildlife that originally called the area home. In 2016, management goals and objectives were established through the Wallace Bay National Wildlife Area Management Plan. To address the impacts of climate change, the plan’s management approach included identifying and removing barriers to promote inland migration of salt marshes in response to rising sea levels, and identifying and mitigating areas of high erosion on impoundment dikes. The restoration project in 2020 involved removing a dyke and water control systems to reintroduce the tidal influence to a wetland impoundment, allowing reconnection of the tidal regime. The free flow of tidal water provided conditions for salt marsh restoration to occur. The project resulted in approximately 23 hectares of saltmarsh habitat restored. DUC has partnered with the Confederacy of Mainland Mi’kmaq (CMM) to monitor both the Wallace Bay site and a control salt marsh, located just a kilometre away.


Identifying Actions

Management approaches were established to ensure wetland habitats will continue to provide habitat for migratory birds including waterfowl, waterbirds, and marsh birds under a changing climate. These approaches include monitoring habitat changes and coastal erosion within the NWA through annual aerial photography, identifying and removing barriers to inland migration of salt marshes in response to rising sea levels, and identifying and mitigating areas of high erosion on impoundment dikes.

In this case, continued dyke maintenance at Wallace Bay #3 was no longer feasible due to continued scour and erosion from tidal hydrodynamics. Furthermore, restoration of the marsh platform and tidal regime was required to make room for tidal marsh migration inland. Flood risk scenarios, LiDAR DEM, topographic maps, current and historic aerial imagery, and solicitation of local knowledge, were used together to advise the restoration plan. Adjacent landowners were involved and supportive of the project goals. Following consultation, DUC and CWS agreed to reintroduce tidal influence on the impoundment by removing the hard infrastructure. The free flow of tidal water would provide conditions for salt marsh restoration to occur and allow free fish passage by virtue of channel reconnections. Three attributes were critical to restoration success: reconnection of tidal channels, adequate sediment supply, and suitable marsh elevation. Presence of a halophytic vegetation seedbank was also considered before proceeding.

Implementation

In September 2020, CWS and DUC began work to restore the site to the original salt marsh through managed dyke breaches. The densely vegetated access road was mulched to allow equipment to transit. Impoundment water levels were slowly reduced to reveal the marsh platform. Following dewatering, excavation equipment leveled the dyke material, depositing the excess soil back into the impoundment borrow pit. Later, two associated water control structures were also removed, and the main and secondary channel were reconnected to the tide. By reintroducing the tidal influence on this impoundment, conditions were established for salt marsh restoration to occur. Reconnection of tidal channels was also important to re-establish dynamic equilibrium and increase soil solidification rates on the marsh.

As the tide comes into the salt marsh, soil particles flocculate out of the water and deposit on the surface of the marsh, building up the marsh over time. With the tide flowing, salt water is pouring back into the marsh, revealing mud flats and letting the seeds of salt grasses buried deep in the soil pop back to life. In patches of marsh that are shallower, freshwater plants and cattails might hold on for years before giving way to reedy cordgrasses. As the salt marsh develops, the vegetation will grow and change along with it. There’s the first colonizer cordgrass species that takes root in the initial plunge of new water. As the marsh rises and elevates with more sediments, the successional plants come in. And all the while, these changes are crucial for combatting climate change.

Outcomes and Monitoring Progress

The reintegration of floodplain has many benefits that are well documented in the neighboring Bay of Fundy. However, due to similar ecology, this project is a pilot to inform other potential dyke realignment projects in the Northumberland Strait. DUC has partnered with the Confederacy of Mainland Mi’kmaq (CMM) to monitor both the Wallace Bay site and a control salt marsh, located just a kilometre away. Before the work started, teams from CMM took drone video of the two sites in July 2020 to get baseline data and a snapshot of the conditions of the site prior to restoration. Monitoring will continue on an annual basis for at least three years. It has also been established as field research site by local universities.

Next Steps

Managers are hopeful that no other intervention action will be necessary at the site. Ecological monitoring efforts aim to reveal how the restoration is progressing and will identify any adaptive management required. Managers hope to see a return of several fish species, including striped bass, killifish, trout, and gaspereau. In addition, CMM is hoping to see a resurgence of sweetgrass, a common salt marsh plant, and one that’s particularly important to the Mi’kmaq and is used ceremonially and for smudging.

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