Salt marshes have persisted through changing sea levels for thousands of years by moving. What threatens them now is not water depth so much as the absence of anywhere to move to.
A marsh builds its own elevation
Marsh plants trap sediment from tidal water around their stems, and their roots and decaying matter accumulate organic material below the surface.
Together these processes raise the marsh surface over time, which is how a marsh can keep pace with slowly rising water rather than being submerged by it.
The rate of accumulation is finite. Where water rises faster than sediment and root growth can build, the plants spend more hours flooded and eventually fail.
Sediment supply is the limiting input
Vertical growth depends on suspended sediment arriving with the tide, and most of that sediment originates upstream in rivers.
Dams trap it, channelization carries it past the marsh into deep water, and dredging removes it from the system entirely. The delivery has been reduced in many estuaries.
A sediment-starved marsh may look healthy for years while quietly losing the ability to keep up, and the failure appears suddenly as interior ponding.
Landward migration is the other survival route
Historically a marsh responded to rising water by expanding inland, with salt-tolerant plants colonizing low ground that had previously been too dry for them.
The marsh as a system persisted while its actual location shifted, losing area at the seaward edge and gaining it at the landward one.
This requires land at a suitable elevation immediately inland, and it requires that land to be available rather than built on.
Coastal squeeze is a built problem
Where a road, seawall, railway or subdivision sits at the upland edge, the marsh cannot move. The seaward edge continues to erode and nothing replaces it.
The marsh narrows year by year until the remaining band is too small to function, at which point the shoreline it was protecting is exposed directly.
This is the mechanism behind marsh loss in developed estuaries, and it is a land use outcome as much as an oceanographic one.
Migration corridors are the practical response
Coastal planners map the low-lying ground a marsh would need decades from now and secure it through easements, acquisition or rules limiting hard shoreline armoring.
Living shoreline approaches use graded slopes and planted vegetation instead of vertical walls, preserving the gradient a marsh needs to advance across.
Protecting land that is currently dry and unremarkable is a hard case to argue locally, which is why the corridor is usually the piece missing from a coastal plan.