Flash floods are sudden and unexpected, taking place when very intense rains fall over a very brief period . A flash flood may do its damage miles from where the rain actually falls if the water travels far down a dry streambed.
People try to protect areas that might flood with dams, and dams are usually very effective. But high water levels sometimes cause a dam to break and then flooding can be catastrophic. People may also line a river bank with levees, high walls that keep the stream within its banks during floods. A levee in one location may just force the high water up or downstream and cause flooding there.
Floods are also responsible for moving large amounts of sediments about within streams. These sediments provide habitats for animals, and the periodic movement of sediment is crucial to the lives of several types of organisms.
When the amount of water in a drainage exceeds the capacity of the drainage, there is a flood.
Floods are made worse when vegetation is cleared, when the land is already soaked, or when hillsides have been logged.
People build dams and levees to protect from flooding.
Floods are a source of nutrients on a floodplain.