Floods occur when water covers or soaks land that is usually dry. The speed and duration of floods depend on the intensity and length of rainfall, topography, soil conditions and ground cover. Water generally travels from higher to lower ground. It may flow over sand and gravel rivers, or through estuaries and bays. It is more likely to flood low-lying areas when rainstorms occur repeatedly over the same area, or when a dam, levee, retention pond, or other structure fails.

Flood waters carry many materials with them, including sharp debris, pesticides, fuel, untreated sewage, and dangerous mold. This debris can be swept away, damaging buildings and roads. Water-borne contaminants increase the likelihood of infectious disease, such as hepatitis A and cholera. Receding floodwater can also create stagnant pools that are the perfect breeding grounds for mosquitoes, which transmit malaria and other diseases.

Hydrologists work with meteorologists to evaluate rainfall intensity and duration, as well as the ability of a region’s land to absorb the water. Rapid snowmelt is another major contributor to flooding, as the ice and snow melts more quickly than the surrounding landscape can absorb it. The rapid flow of runoff may overwhelm bridges and railways, disabling them. It can also deplete the soil beneath a building, causing it to shift and collapse.

Stories of great Earth-drowning floods appear in world cultures and are remarkably similar, including the biblical Genesis flood narrative, the Manvantara-sandhya legend in Hinduism, the Deluge in Greek mythology, and the legend of Utnapishtim in Mesopotamia (later incorporated into the Torah and Bible). A variation of this story is still told today in the Cheyenne, Blackfeet, and Puebloan traditions.