Wetlands were drained for centuries because standing water was seen as wasted land. Engineers now recognise what was lost: a marsh absorbs a river's flood water and releases it slowly, so a drained catchment sends the same rain downstream in a rush. Building walls higher along the river only moves the problem to the next town. Restoring wetlands upstream is usually cheaper than concrete defences and it does not fail catastrophically when a flood exceeds the design limit, though it requires land that farmers currently use.