River Flood Facts

A river drains an area called a watershed. When water enters a watershed faster than the river can drain it, the river overflows its banks, resulting in a flood. Several factors can cause river floods, which may develop slowly or happen within just hours or even minutes---an occurrence known as flash floods.

Runoff

• An especially heavy rainfall or snowmelt can saturate an area with water faster than the ground can absorb it. Runoff, or water not absorbed into the ground, flows into streams and rivers; excessive runoff can cause them to flood.

Construction and Wetland Loss

• Construction, such as buildings and pavement, can prevent the ground in an area from absorbing enough rainwater to prevent, or at least reduce, runoff. The loss of wetlands, which absorb rainwater, likewise contributes to river flooding. Dam and levee failures can cause flash floods.

Ice Jams

• Ice jams occur when chunks of ice accumulate at a bend, bridge or other obstacle in a river. By obstructing the flow of water, ice jams can cause a river to flood. If the jam breaks, the sudden surge of water can produce a flash flood.

Flash Flood Preparedness

• Flash floods, which can occur without warning, account for about 75 percent of flood-related deaths in the United States. The National Oceanic and Atmospheric Administration advises people to know their area's flood risk, check for flood warnings on TV or radio during storms, and evacuate immediately---going to higher ground and avoiding flood-prone areas---if advised to do so.