Geography Reference
In-Depth Information
creased, spring runoff and peak flows have occurred earlier, and summer and autumn
flows have been reduced. This may impose a drought stress on floodplain forests (Rood
et al. 2008).
FIGURE 5.18 A hydrograph obtained in 2004 for a stream originating from Green Lake 4. Note the
seasonality of the signal due to mainly snowmelt. (Graph by J. R. Janke using data obtained from
Caine.)
Where alpine glaciers exist, the normal spring runoff is augmented by glacial melt-
ing. In summer, mountains cannot store much water, so thunderstorm rainfall surges
quickly through the drainage network. In the San Juan Mountains of southern Colorado,
erosional rates are three to four times greater in summer than in winter because of
the intensity of summer thunderstorms (Caine 1974). Exceptionally heavy precipitation
can set off flash floods, increasing the potential for erosion or catastrophic jökulhlaups,
Icelandic for outburst floods. Volcanic activity can also melt overlying ice and cause
flooding. These are major environmental hazards to settlements in and near mountains.
For example, Salt Lake City, Utah, where over a million people live in a narrow belt
along the Wasatch Front, is susceptible to flash flooding (Marsell 1972). Sparsely ve-
getated slopes allow rapid runoff, collecting in the major drainage ways. The streams
quickly pick up velocity and become charged with mud and rocky debris. Upon reaching
the base of the mountains, the streams overflow their banks, or the water and debris
disperse in alluvial fans, causing considerable damage. With increasing tourism and
recreation in mountains, human developments may be at severe risk. Although devel-
opment in these regions seems foolhardy, paradoxically, the people most affected are
often those living in established areas at the base of mountains. In 1976, about 145
people died in the Big Thompson Creek flood when a moist air mass pushed up the
Colorado Front Range and stalled. The impervious canyon walls funneled the precip-
itation downstream. The flood destroyed 418 homes, damaged another 138, removed
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