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Causes of Flooding The most common cause of flooding along streams is a high rate of runoff from heavy or prolonged rain. Stream flooding in northern states and moun- tainous areas is also caused frequently by rapidly melting snow or by runoff from a combination of rainfall and mel ting snow. Some degree of flooding occurs along most streams on the average of about once every two years. Floods also occur occasionally due to failures of upstream dams. Obstruction of stream channels by ice jams) accumula tions of debris, deposi ts of sediment and thick vegeta- tion is a major factor contributing to flooding along streams. These sometimes obstruct, a channel to the point that even normal flows back up and overflow nearby areas. The debris carried by floods often adds to problems" of obstruction. While less frequent, stream flooding can also be caused by a land- slide, volcanic flow or other event which block~ a river or changes its course. Floods along streams are cate- gorized by how quickly they occur. Floods which occur within a few hours of a heavy rain or sudden release of water are called "flash floods". Flash floods normally rise quickly to their peak and then subside rapidly. They Ice accumulations can and cause flooding periods of normal flow. dam a even stream during occur primarily in mountainous areas and along the headwaters of small streams. Floods along larger rivers usually rise much more slowly but may persist for days or even weeks. A stream channel overgrown with vegetation has little flow carrying capacIty so that even a moderate flow results in flooding. Trash, timber and other debris swept downstream can form an almost impenetrable dam at bridges and other constrictions which forces the stream to overflow its banks. These debris dams often collapse suddenly, sending a destructive wall of water downstream. 7