How a Flood Starts

Flooding is a natural occurrence and a yearly expectation in some parts of the world. Human activity can increase the intensity, impacts and frequency of floods.

Mechanisms

• The smallest tributaries are the first to overflow their banks. Swollen streams and creeks feed into rivers and if too many tributaries within a watershed carry too much water, river flooding occurs.

Causes

• Strong or lingering low pressure systems are the meteorological phenomena that frequently cause floods, Nor'easters and hurricanes among them. Snowmelt contributes to flooding as well and can be severe when combined with excessive precipitation.

Flash Floods

• In contrast to regular floods, flash floods occur with little to no warning and involve a rapid surge of water. They typically occur in streams and creeks and are often produced by heavy thunderstorms. Dam failures are another cause of flash floods.