of different ways, and their effects can be minimized in several different ways. Perhaps
unsurprisingly, floods tend to affect low-lying areas most severely.
Causes of Floods
Floods usually occur when precipitation occurs more quickly than that water can be ab-
sorbed into the ground or carried away by rivers or streams. Flooding may be sudden and
unexpected, in the case of a flash flood, when very intense rainfall occurs in an area (Figure
13.17). This strong rainfall will fall too fast to be absorbed into the ground, and will overflow
the banks of the streams and rivers. Alternately, floods can occur more slowly, when a long
period of rainfall fills the ground with water and the levels of rivers and streams gradually
rises. Less commonly, floods can occur when a dam breaks along a reservoir — as you might
expect, this type of flooding can be catastrophic. In California, floods commonly occur when
rainfall far exceeds annual averages, such as during an El Nino year. High water levels have
also caused small dams to break, wreaking havoc downstream.
Figure 13.17: A flash flood in England in 2004 was caused by three and a half inches of rain
that fell in just 60 minutes. It devastated two villages. ( 14 )
Vegetation is an important factor in determining whether a flood occurs. Plants tend to
slow down the water that runs over the land, giving it time to enter the ground. Even if
the ground is too wet to absorb more water, plants still slow the water’s passage across the
earth, increasing the time between rainfall and the water’s arrival in a stream. For the same
reason, wetlands also play a key role in minimizing the impacts of floods; they act as a buffer
between land and high water levels. Flooding is therefore less common in areas that are
heavily vegetated, and can be more severe in areas that have been recently logged.