The World's Largest Floods, Past and Present: Their Causes and Magnitudes

Original publisher: [Reston, Va.]: U.S. Geological Survey, 2004. OCLC Number: (OCoLC)56793014 Subject: Floods. Excerpt: ... Floods, Landscapes, and Hazards 9 This map shows rivers with drainage basins larger than 500,000 square kilometers. Map numbers are keyed to table 2. Figure 5. Most of the largest documented floods in the past 100,000 years resulted from the failures of natural dams, but conditions that caused some of the largest of these natural dams, such as the large ice-dam failures at the margins of continental ice sheets, are now absent. Consequently, earth history contains " flood epochs " - times when climate and topography combine to produce higher-than-typical frequencies of large floods. To produce floods with discharges 3 greater than about 1 million m / s, times of advanced continental ice sheets and rapidly changing global water balances are required. In many locations, these outsized floods from previous flood epochs were dominant forces in establishing regional drainage networks and landscape patterns that persisted into times of fewer extreme floods. Other geologically controlled cataclysmic floods are probably more evenly distributed in time, at least at time scales of hundreds to thousands of years. Conse-quently, the future likelihood and magnitude of such events can be more directly guided by analysis of past In general, larger river basins produce larger floods, Figure 6. but larger unit discharges in the moist tropics can result in floods events. Within active volcanic provinces, the likelihood of disproportionately large size. Numbers refer to basin numbers of large volcanic eruptions and the consequent flood-in figure 5 and table 2. generation processes probably does not change signifi-cantly over hundreds or even thousands of years. Simi-larly, the frequency of large valley-blocking landslides probably remains relatively constant over such time scales.