Oh the Places You'll Go...Marsh Migration Model - WMAP Blog

Oh the Places You’ll Go…Marsh Migration Model

Written on: March 19th, 2025 in Wetland Research, Wetland Restoration

By Kenny Smith, DNREC’s Wetland Monitoring and Assessment

In 2024 the Delaware Coastal Program and the Wetland Monitoring and Assessment Program worked together to update the Delaware Marsh Migration Model originally created in 2017. New and updated layers were added, as well as some refinement of scoring to better understand where our tidal wetlands may migrate to as sea levels rise. This effort should be used as a guidance tool for landowners and professionals to aid in land management and resiliency planning.

Tidal wetlands are found in coastal areas where the land meets water and are flooded and drained twice a day by tides. A natural, healthy tidal wetland will receive nutrients and suspended sediments from tidal waters that help plant growth and replenish wetland sediment which allows tidal wetlands to maintain proper elevations. Ideally, tidal wetlands could build their surface elevation over time to match rising water levels. However, tidal wetlands can fall behind in sediment building and become permanently flooded and eventually convert to mudflats or open water.

*Tidal marsh migrating into a forest (left) and dying trees showing evidence of migration happening already (right).*

Another technique tidal wetlands use is to move slowly towards higher ground or migrate inland, which is called marsh migration. Tidal wetlands, when able, will slowly start a landward based movement as water level conditions begin to threaten vegetation survival. This can be visually seen by an increase in water, a slow die off of upland vegetation, and a transition into wetland plants. The model that was updated is predicting where tidal wetlands could potentially migrate. However, migration can only occur if adjacent upland habitat is natural and undeveloped and if conditions such as slope make it possible.

The updated Delaware Marsh Migration Model predicts where tidal wetlands have the potential to migrate under a 4ft. sea level rise scenario. The model uses various mapping layers to predict where migration is likely to occur. Model inputs include the slope of the land, land use/land cover (LULC), soil type, and distance to the nearest tidal wetland. It is important to note, this model does not provide any timeline for migration and should only be a guidance tool for consideration of future land management decisions.

*Example maps from the model of land use and land cover (LULC) and soil type.*

Each of the mapping layers were used to assign a potential migration score ranging from 0 to 12. Areas with 0 score are not suitable for migration, while areas scoring 1-3 are least suitable, 4-6 less suitable, 7-9 suitable, and 10-12 are highly suitable. While this model shows where marsh migration is suitable, there are many other components that go into the potential for future habitat transition. For example, land use and ownership are the most important things to consider when using the marsh migration model. There may be an area that has high suitability for marsh migration but if it is privately owned and the land is being used for profit then it is doubtful that area will actually have marsh migration. The inverse also could be true; there could be an area that is only suitable but is publicly owned and can be maintained to foster marsh migration, not combat it.

For our analysis we focused on conditions with 4ft. of sea level rise as it is a likely scenario to occur in the next 75 years. Across Delaware the model predicts 34,211 acres total of migration potential, with 21,449 acres of that being scored as highly suitable.

*Example model outputs showing migration into a coastal community (left) and agriculture land use (right).*

According to the Delaware Sea Level Rise Vulnerability Assessment, 99% of tidal wetlands will be lost to 4ft. of sea level rise by 2100, where the 21,449 acres of highly suitable marsh migration pathways would only replace 21% of these lost tidal wetlands. Of the highly suitable areas, 72% are found on private property, with the most being in an agriculture land use (6,906 acres; 45%) and existing non-tidal vegetated wetlands (4,757 acres; 31%). The remaining highly suitable migration areas are found on public property (6,101 acres; 28%).

*Highly Suitable Areas + Land Use + Private Ownership*

While these results only provide a prediction of where tidal marshes may migrate in the future, they can be used to inform current and future management decision at both the public and private levels. Some examples of how this tool might be used are provided below:

Public Lands:
- Identify suitable areas for marsh migration and create or set aside corridors for future tidal marsh habitat. This might look like removing hardened structures or re-grading steep slopes
Private Lands:
- Conservation partners could identify suitable areas for marsh migration that are adjacent to existing protected lands and purchase these areas to increase protection of potential migration pathways.
- Agricultural landowners might identify locations on their land that have the potential to lose yield via saltwater intrusion. These lands might be restored back to wetlands and set aside as potential migration pathways.
- Private landowners might use the tool to identify whether suitable lands fall within their property lines. Where present, landowners might remove physical barriers (i.e. sea walls) and reduce preventative activities (i.e., mowing) in order to facilitate migration.

Keep a lookout for our report and informational handout coming soon!