If heavy rainfall occurs repeatedly over a wide area, then river or mainstem flooding becomes more likely, in which the main rivers of a region swell and inundate large areas, sometimes well after rainfall has ended. The 1993 Midwest floods were caused by 77 events over several months where rainfall of greater than one inch occurred over areas 100 to 200 miles wide and 400 to 600 miles long. Both flash flooding and river flooding threaten life and property, although the former causes more deaths and the latter more property damage. The average flooding toll has increased in recent decades while deaths from tornadoes and hurricanes have dropped. Almost half of all flash-flood deaths are connected to stream crossings or highway travel.
Victims often underestimate the power of water when driving into flooded areas.
It takes only 18 inches of water to float a typical vehicle. Few locations have rainfall records of more than a century, so 100-year flood values are estimates rather than certainties. Changes in watershed management, land use, and the like can affect streamflow characteristics and alter the likelihood of a given flood. Moreover, climatic patterns themselves can change. Because flood risk is the result of both environmental and societal factors, it is difficult to single out the impact of climate variations.