Wetlands were drained for centuries as unproductive ground. The functions they perform are now measurable, and they address flooding and heat through the same physical process.
Storage flattens the flood peak
A wetland holds water in saturated soil, vegetation and surface storage, and releases it downstream over days rather than hours.
That delay lowers the peak flow that arrives downstream, because the same total volume passes through spread across a longer period.
Drained land does the opposite. Water runs off quickly through ditches and hard surfaces, so more of it arrives at once and the peak is higher.
Evaporation converts heat into water vapour
Evaporating water absorbs a large amount of energy, and that energy comes from the surrounding air and surface rather than raising their temperature.
A wetland with abundant water and vegetation therefore runs cooler than dry ground receiving identical sunlight, and it cools the air passing over it.
The effect extends beyond the wetland itself, which is why wet areas within or near urban zones measurably reduce local peak temperatures during hot weather.
The two functions depend on the same saturation
Both storage and evaporative cooling require the wetland to be wet, and a wetland that dries out provides neither.
Partial drainage often produces exactly that condition: enough water removed to lose the cooling and storage capacity, but not enough gained for reliable agriculture.
Degraded peat wetlands go further, releasing stored carbon as the exposed organic material oxidises, which reverses their contribution rather than merely removing it.
Coastal wetlands add wave and surge attenuation
Mangroves, salt marsh and reed beds slow water movement through friction with stems and roots, which reduces wave energy reaching the shore behind them.
Wide belts of vegetation lower storm surge heights measurably, and the protection scales with width rather than with the presence of vegetation alone.
These systems also trap sediment and build elevation over time, which is the mechanism by which they can keep pace with rising sea levels where sediment is available.
Restoration is constrained by hydrology, not planting
Recreating a wetland requires restoring the water regime, including how water arrives, how long it stays and how it leaves, before vegetation can establish.
Where drainage networks and upstream abstraction remain unchanged, planted wetlands fail because the underlying water balance is still that of drained land.
This is why successful projects usually begin with blocking ditches, removing embankments or reconnecting floodplains rather than with any planting at all.