There are more than 80,000 dams in the United States, according to the 2007 update to the National Inventory of Dams. Approximately one third of these pose a "high" or "significant" hazard to life and property if failure occurs.
Dam failure or levee breeches can occur with little warning. Intense storms may produce a flood in a few hours or even minutes for upstream locations. Flash floods occur within six hours of the beginning of heavy rainfall and dam failure may occur within hours of the first signs of breaching.
Other failures and breaches can take much longer to occur, from days to weeks, as a result of debris jams or the accumulation of melting snow.
For preparedness tips on what to do before, during and after a flood, visit Ready.gov.