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Newsletter and Technical Publications
Lakes and Reservoirs vol. 2
The Watershed: Water from the
Mountains into the Sea
Ice And Snow: Water In a Frozen Form
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| Photo 16: North Pole ice sheet and Greenland. Satellite image. |
Humans generally do not extensively use water in the form of ice as a water
supply. Ironically, however, about three-fourths of the world ’s fresh water
exists in polar ice caps and massive, slow-moving glaciers. Frozen lakes and
rivers comprise a tiny additional fraction of fresh water. The existence of
water in frozen form has the effect of withdrawing vast quantities of water from
its movement through the hydrologic cycle. With some variability, most of the
world’s ice remains in frozen form over very long periods of time. This is
significant within the context of predicted global warming, which can
potentially give a significant rise in sea level.
Permanent ice cover exists as polar ice caps in the Arctic and Antarctic, and
in Greenland (Photo 16). It also exists at high altitudes and latitudes in the
form of glaciers. It is estimated that the Antarctic ice cap is equivalent to
about 30 million cubic kilometres of water. Ironically, the polar regions
receive extremely small quantities of precipitation annually. Thus, even though
polar ice caps contain a massive quantity of frozen water, the areas are
characterized as arid because of their very limited precipitation. |
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| Photo 17: Mountain glacier in the Andean region. |
Glaciers are slow-moving ice masses in areas where snow or ice has
accumulated in large quantities, typically in mountainous areas (Photo 17). Some
glaciers contain massive quantities of ice, ranging in size up to continental
ice sheets. Many lake basins were formed by the slow movement of huge glacial
ice masses (“glacial scour ”) (Photo 18), over the land surface over geologic
time scales. The Laurentian Great Lakes of North America were formed in this
manner, and provide an excellent example of the almost unbelievable power of
water in frozen form fundamentally to change the character of the land surface.
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| Photo 18: Glacier valley carved by ice flow during the last
ice age, Argentina |
Another form of
perennial ice is
permafrost
(Photo 19), particularly in the northern, sub-Arctic region of the Northern
Hemisphere. Although perennially-frozen soils can interfere with both land use
and land-based activities, this type of frozen water is of no major significance
in regard to human water supplies.
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| Photo 19: Permafrost in the Russian tundra. |
Snow represents a transient form of water. Although it can begin its descent
from the atmosphere in the form of raindrops or droplets, it can freeze into
snow if the air temperature is sufficiently low. At the same time, snow is more
sensitive to elevated temperature than are massive ice caps and glaciers.
Accordingly, although snow falls in many temperate regions, it usually does not
stay on the ground for any appreciable time. In fact, streamflow in some regions
consists mainly of snow that has subsequently melted as the temperature
increased in the spring, resulting in significant runoff from the land surface,
so called snowmelt. In some parts of the world, the winter snow cover in a
watershed can fundamentally control both the quantity and quality of the runoff
likely to be expected during the subsequent spring and summer period. In fact,
some watersheds can exhibit wide variations in water supply, as a function of
the precipitation and snowfall variability from season to season and from year
to year.
Mountainous areas are characterized by limited or no human settlement or
land-based activities. The water originating from snowmelt in mountainous
regions, therefore, is typically the water of best quality within the context of
the hydrologic cycle. Because their high altitudes define the physical
boundaries of some watersheds, some have suggested that mountain-tops can be
said to represent the point from which water begins its journey to the sea.
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