When the ground stops drinking
Why Do Rivers Flood?
A flood is not extra rain — it is rain the ground could not swallow fast enough.
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Step 1 of 5
Soil is mostly gaps.
About a quarter of ordinary soil is empty space between grains. Rain fills those gaps, and while there is room the water simply disappears downward.
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Step 2 of 5
Once the gaps are full, water travels sideways.
Saturated soil cannot take another drop. Everything that falls now slides across the surface toward the lowest point, gathering as it goes.
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Step 3 of 5
Hard surfaces skip the soaking entirely.
Roads and roofs have no gaps at all. A town sends almost all of its rain to the drains within minutes, instead of over the following days.
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Step 4 of 5
A river can only carry so much.
The channel has a fixed capacity. When arriving water exceeds it, the extra has nowhere to go but sideways, over the banks and across the floodplain.
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Step 5 of 5
Timing matters as much as amount.
Rain spread over a week vanishes into the soil. The same rain in three hours arrives all at once and the river peaks far above its banks.
A flood peak often arrives hours after the rain has stopped, because the water is still making its way downhill.
The short version
Rivers flood when rain arrives faster than soil can absorb it, so the excess runs straight into a channel that is already full.
Try it yourself
Pour water on a dry sponge, then keep pouring. The moment it stops absorbing and starts running off is exactly what a floodplain does.
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