Concept · Ch 17
Coastal squeeze (nowhere to retreat)
The coastal wetlands are pinned between a rising sea in front and a hard human edge behind — levees, roads, canal estates and cane. As the sea rises the whole tidal system tries to migrate landward, but where the land behind is built or walled, it has nowhere to go, and the marsh is squeezed out of existence.
First, meet: Estuarine zonation (the tide sorts the bands)
The coastal wetlands of this coast are caught in a slow vice. In front of them the sea is rising; behind them, more and more, stands something that will not move — a levee bank thrown up to keep cane paddocks dry, a road on its embankment, a canal estate, a wall of houses. The marsh is pinned between the two.
To see why that is fatal, remember that the tidal bands — saltmarsh, mangrove, seagrass — are not fixed. Each sits at the one height where it can just bear the salt and the soaking, so when the sea rises, every band tries to shuffle landward to keep its footing: the saltmarsh climbing onto what was dry ground, the mangrove following behind it. On a natural shore, with an open slope at its back, an estuary can do exactly this — rise with the sea by walking inland, and lose nothing. That is the reassuring half of the story, and it is true as far as it goes.
The trouble is the other half. Migration only works where there is somewhere to migrate to, and a levee or a road or a canal estate is precisely the removal of that somewhere. Pressed forward by the water and stopped dead by the wall, the bands are crushed against the barrier and then drowned as the sea keeps coming. This is coastal squeeze, and it is why “the wetland will just move landward” is a dangerous thing to half-believe on a developed coast. The most useful thing you can do for a squeezed marsh is often to give it its escape room back — set the levee back, or drop it — so the country behind can once again take the water, and the bands have somewhere to climb.
In depth
Estuarine plants live in bands set by the tide — seagrass, mangrove and saltmarsh, each at the one height it can just tolerate the salt and the soaking (estuarine-zonation). Those bands are not fixed lines painted on the mud; they are the current position of a system that moves. When sea level rises, every band tries to migrate landward, tracking the height it needs — the saltmarsh climbing onto ground that used to be dry, the mangrove following the saltmarsh, and so on up the slope. Given room, an estuary can rise with the sea by simply walking inland.
Coastal squeeze is what happens when that room is gone. On a natural shore the land behind the marsh is a gentle, open slope the bands can climb. On a developed shore it is a hard edge — a levee bank, a road embankment, a canal estate, a cane paddock defended by a bund — and that edge does not move. So the wetland is caught between two things it cannot argue with: the sea advancing from the front and the wall holding firm at the back. The bands are compressed against the barrier, then drowned as the water keeps rising, and the marsh is squeezed out of existence with nowhere to retreat.
The trap in reading this is the reassuring half-truth that "wetlands just migrate landward, so a rising sea only moves them." They do migrate — where there is room. The whole point of the squeeze is that the room is exactly what a hard human edge removes, and much of this coast's low country now sits behind precisely such edges (see estuary, paperbark). It is why simply protecting a marsh in place is not enough under sea-level rise, and why the most effective defence is often to give the land behind it back — setting levees back, or lowering them, so the bands have somewhere to climb as the water comes.
Primary sources & further reading — the doorway
- McPhee, D. (2017). Environmental History and Ecology of Moreton Bay. CSIRO Publishing. — Moreton Bay estuarine ecology and the pressures on tidal wetlands, including sea-level rise against hardened shores.