Flooding is an overflow of water on land that is normally dry. Generally, this happens when rain is heavy and widespread, or when a dam or levee breaks. Other factors can also cause flooding, including rapid ice melt, wind over water and tsunamis.
Floods often carry material that clogs waterways, like debris, trees and roofing materials. This can make them slow or dangerous. In addition, the buildup of debris can obstruct the natural flow and path of waters, which can push flood waters farther onto riverbanks or coastline. The accumulation of sand or gravel can also change how high the water rises, especially in a delta where sediment builds up.
When the floodwaters recede, they leave behind rich deposits of nutrients that help fertilize the land. This is one reason ancient civilizations grew along rivers that seasonally flooded their deltas.
In areas that are prone to flooding, people can do several things to reduce the risk of damage and illness. They can build homes that are designed to withstand a temporary inundation, and they can install porous paving and drainage channels that allow stormwater to seep into the ground rather than flowing off into waterways. Communities can also adopt building codes that require new construction to be built on high ground or with a raised foundation and piers.
Floods also provide clean freshwater for ecosystems in arid and semi-arid regions, where precipitation is sparse throughout the year. This water gets absorbed into the soil and percolates through layers of rock, reaching underground aquifers that supply springs, wells and lakes with freshwater.