RIVER FLOODING

Inundation of a normally dry area along a river (usually the mainstem) due to increased water level or discharge. Because the drainage areas of mainstem rivers are large, they do not Flash Flood as their smaller tributary streams do. River flooding occurs more than a few hours after the causative event (usually widespread heavy precipitation and/or snowmelt) of the flood. In many cases, the end of flash flooding along tributary streams may be followed by or coincident with river flooding (i.e., heavy precipitation which initially causes flash flooding on tributaries goes on to cause river flooding on the larger rivers). Because of the longer time factor, river flooding usually can be forecast with much greater accuracy than flash flooding. The forecasting of river flooding is one of the primary duties of the National Weather Service River Forecast Centers.