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Q1: Why is a big flood less common than a small flood?

[Answer]

A flood occurs when a river is carrying so much more water than normal that it completely fills up its channel then spills over its banks.

Floods are not all that common - even a small flood will only occur every three years, a large one perhaps once every hundred years or less. But when they happen they cause misery, and we are making them more common as you will see.