Geology Reference
In-Depth Information
In extreme high latitudes, rivers are generally small, and river ice and river icings are
rare. Instead, the stream valleys are usually infi lled with snow that accumulated by wind-
drifting during the winter months (Pissart, 1976b). The break-up sequence occurs when
the snow reaches saturation point through the arrival of runoff from snowmelt on adjacent
slopes. As ripening of the pack continues, slushfl ow may briefl y be initiated, quickly fol-
lowed by stream fl ow, which rapidly carves unstable channels or tunnels in the snow. In
places, large snow drifts may dam sections of small valleys to impound water up valley,
only for this to be released when the snow jam breaks (Woo and Sauriol, 1980). Thus, the
drainage network often opens up in segments, and until all segments are linked, the basin
is incapable of coherent transport of meltwater runoff (Woo, 1986).
10.2.3. Basin Hydrology
Because permafrost restricts downward percolation, runoff in basins underlain by perma-
frost responds quickly to snowmelt and rainfall events. Vegetation, if present, promotes a
longer recession period, especially in wetlands.
One of the fi rst basin hydrology studies in Arctic Canada was undertaken by F. A.
Cook (1967), who measured the fl ow of a small stream on Cornwallis Island, NWT. This
was shortly followed by a seminal study by M. Church (1972, 1974), in which the different
hydrologic regimes of northern rivers were summarized (Figure 10.3).
To varying extents, all runoff regimes are dominated by rapid melt of snow and ice
in the short winter-summer transition period. In the Canadian Arctic, for example,
this occurs in late June or early July. Then, during the rest of the summer, the runoff
steadily decreases as less and less snow remains to be melted. This progressive decrease
in runoff is periodically interrupted by subsidiary runoff peaks related to summer
storms and direct surface runoff. Such single-peak runoff regimes are termed nival.
They may be either sub-arctic or Arctic in nature depending upon whether or not base
fl ow is maintained throughout the winter. A proglacial regime occurs in watersheds
where permanent snow or ice fi elds exist. Here, melt occurs throughout the summer
whenever warm and/or overcast conditions develop. Peak runoff is often delayed until
late July or early August, and the nival peak is not so important. A third regime is the
wetland type. Because of the water-retaining capacity of organic terrain and tundra,
and the relatively high resistance to runoff presented by such terrain, peak fl ows are
attenuated in such basins. A fi nal category is the spring-fed regime, often associated
with basins underlain by carbonate rocks in the discontinuous permafrost zone. These
streams have relatively stable discharges because the primary source of water is
groundwater.
The runoff season in high latitudes can be divided into four seasons: (1) break-up, (2)
the snowmelt period (the “nival fl ood”), (3) late summer, and (4) freeze-back. In detail,
however, each season is determined by the pattern of local summer weather for each year.
Usually, break-up begins in late spring. Although air temperatures are still below 0 °C at
this time, local snowmelt is brought about by direct solar radiation. Meltwater percolates
to the base of the snowpack and into snow-choked stream channels, where it refreezes. It
is not until two or three weeks later that suffi cient melt has occurred for fl owage of satu-
rated snow to occur. The river channels turn to slush and runoff begins over the snow and
ice on the stream bed. Usually after two or three days of intense runoff, the winter ice on
the stream bottom has been eroded and runoff continues on the stream bed proper. Some-
times, a trigger, commonly a period of warm weather or a heavy storm which fl ushes the
snow out of the stream channel, initiates runoff.
