Extended Wet Periods

In many cases flooding is caused by the reception of precipitation over an extended time period, on the order of weeks to months, that leads to the saturation of soils in a large-scale region (Rodda, 1970b; Smith and Ward, 1998). This saturation leads to increased runoff at a time when streams are at capacity (Ward and Robinson, 1990). Additional water introduced to stream channels cannot be conveyed in the channel but is spread across the floodplain. Wet periods are related to synoptic conditions such as the position of the polar front that delivers cyclonic storms in quick succession. Poleward migration of subtropical air masses over continental areas such as the Mississippi River Basin help to supply large amounts of water to be precipitated by frontal activity. For example, see Figure 2. In some locations rainfall may fall on snow-covered or frozen ground (Thomas and Lamke, 1962). These waters are unavailable to the hydrologic cycle as long as they remain in a solid form. In the case of the former, rainfall may accelerate the introduction of water into the stream network as snowmelt augments the precipitation already being received (Kattelman, 1990; Naef and Bezzola, 1990; Caine, 1995). The latter will greatly decrease the infiltration capacity of the soil causing most of the precipitation to quickly enter the stream network (Horton, 1933).

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