Numerous factors affect streamflow, and therefore the potential for
flooding. Most important are the amount and type of precipitation, the
nature and condition of the drainage basin, and climate.
All
rivers are subject to fluctuations in flow. During a rainstorm, the
amount, intensity, duration, area of storm, and path of the storm all
influence the runoff reaching the stream. The amount, intensity, and
duration of storms affect the ability of the land to absorb the
precipitation and therefore affect the rate of runoff. The area and path
of the storm relate to the area of the basin receiving rainfall, which
in turn, represents the area contributing runoff. The area and the
runoff rate determine the volume of water that will pass a given point
downstream.
The shape, size, soil type,
and topography of the drainage basin are other factors affecting the
quantity of water reaching the stream. These factors are usually
constant. However, the absorptive or shedding properties of the soil
vary with vegetation cover, season, and previous rainfall.
In the
case of reforestation and revegetation, the rate at which surface water
flows to the main channel may be slowed, and hence the runoff is spread
over a longer period. In addition, the passage of water tends to be
retarded in basins with many natural storage areas, such as lakes and
wetlands, and even those with artificially created storage.
Consequently, smaller peak flows are produced than in basins without
these modifying influences. The best North American example of a basin
with large natural storage is that of the St. Lawrence River, which has
the Great Lakes in its headwaters.
Climate has an
important influence on the relationship between precipitation and
runoff. Frost makes most soil impenetrable if the soil contains
moisture. In northern latitudes, the winter season, during which a large
part of the year’s precipitation is stored in the form of snow, is
often followed by sudden melting, with much of the runoff flowing
quickly over frozen ground to reach streams. Also, the heavy ice
formation on rivers influences flooding, particularly on the northward
flowing rivers. Climate controls the strength of an ice cover in winter,
and its manner of breakup, which governs the severity of ice jamming.
Flash
floods can be extremely dangerous. Unanticipated, they usually happen
on small watersheds as a result of a torrential downpour, often caused
by heavy thunderstorm activity.
A flash flood is characterized by
the occurrence of the peak of the flood within six hours of the onset of
rainfall. The flood conditions develop rapidly because the rainfall is
so heavy the ground is incapable of absorbing the water quickly enough,
resulting in a very high runoff rate. The events are generally locally
intense and damage is usually restricted to a limited area. Large rivers
remain unaffected, while smaller streams can overtop their banks, even
in a drought year.
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