Geography Reference
In-Depth Information
FIGURE 4.10 A scanning electron microscope image of a melt-refreeze grain. (Image courtesy of E.
Erbe.)
The Mountain Snowpack as a Water Resource
The implications of mountain snow for human existence are discussed in Chapters 10
and 12, but the importance of meltwater cannot be overstressed. Numerous estimates
indicate that 66 to 80 percent of all water resources used in the western United States
originates as snowfall (e.g., Flaschka et al. 1987). The Pacific Northwest of the United
States is largely dependent upon hydroelectric power from streams that head in the
Cascade and Rocky Mountains, and California's bountiful farm production is derived
largely from meltwater from the Sierra Nevada. In fact, it is safe to say the economy of
the entire western United States is dependent upon meltwater from mountains (Serreze
et al. 1999). Unfortunately, it appears that, as the climate warms, the average annu-
al snowpack accumulation in the Western United States is on the decline (Mote et al.
2005).
From the point of view of snow metamorphism, snow-melt runoff is an extension of
the melt-freeze process. The melting snowpack cannot deliver water to the river sys-
tem until the available pore space is filled to field capacity with liquid water. This means
that there is a lag time from the onset of melt until the snowpack fills up, becomes ripe,
and can transfer water to the stream channel. During the ripening process, water can
flow horizontally along dense layers; or, in cases where there are significant differences
in grain size, between adjacent layers in the snowpack; or vertically through conduits
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