River floods

Dateline article| David Murphy|

by David Murphy

River Floods, as the name implies, are floods that occur along large rivers. These floods have different causes and behave differently than other types of floods.

River floods can be among the most costly and destructive floods, because they often cover very wide areas. A river may overflow its banks for dozens or even hundreds of miles at a time, affecting property along both banks. The number of homes and businesses affected can easily climb into the thousands. However, river floods are not necessarily the most deadly flood type, because they often develop slowly and are foreshadowed by the flooding of a river's tributaries, which sends an advance warning to riverside residents.

River floods have several causes. In the winter or spring, a sudden, prolonged thaw can melt a great volume of snow which enters the river. Ice jams (large chunks of river ice that break up and flow downstream) can also pile up around bridges and shallow stretches, causing upstream water to spill over banks. The remnants of tropical storms and hurricanes can put down enough rain water to choke creeks and streams and eventually rivers. Several strong rainstorms in quick succession can also raise creek and stream levels until downstream rivers flood. Often, these storms are the result of a stalled frontal boundary sitting over upstream locations and channeling storm after storm through the same area.

In each of these cases hydrologists, who monitor flood gauges and stream flow along rivers and their tributaries, can usually compute flooding potential in advance and give plenty of warning.

However, there is one cause of river floods that's far more sudden and less predictable. That's when a dam or levee along the river's edge breaks. In this case, great volumes of water can rush into a large area, submerging it almost without warning. While failures like this often happen during times of heavy rain, signaling potential danger to those behind the dam or levee, it's not always possible to get word to those at risk, or to convince them to seek safer ground prior to an actual break.

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