Geology Reference
In-Depth Information
snow-cover depth is often infl uenced by the presence or absence of vegetation. North of
treeline and above timberline snow is blown clear of exposed surfaces and accumulates
in lee-slope positions, hollows, and other topographic irregularities. Snow distribution is
further complicated because, in the northern boreal forest, it may also depend on arboreal
species present. For example, the Siberian taiga is composed predominantly of pine (
Pinus
silvestris
) and tamarack (
Larix dahurica
) whereas the spruce (
Picea glauca
,
Picea mariana
)
is more common in the North American boreal forest. Because more snow remains on
spruce foliage than on more delicate larch branches, the snow cover is typically thinner
beneath a spruce canopy than a larch canopy.
The pattern of snow melt varies from year to year and from locality to locality, depend-
ing upon climate (i.e. solar radiation, aspect, wind direction, etc.) and other site-specifi c
details. Nevertheless, a few generalizations about snow melt are possible. To illustrate, the
pattern of snow melt over a three-week period in a small drainage basin near Resolute,
in the Canadian Arctic (Figure 9.14), permits the following observations. First, the primary
control over snowmelt is clearly solar radiation. This is especially the case in high latitudes
because of the near-continuous daylight in the summer months. There are also marked
diurnal variations in runoff. These are related to solar radiation inputs. Second, as the
snow pack ablates, water percolates downwards through the pack and refreezes. Thus, the
formation of basal ice layers complicates the snowmelt-runoff relationship and prolongs
the snowmelt season. Third, as the snow bank continues to ablate and progressively shrink
in size, exposed ground starts to thaw. In its fi nal stage, the snow bank consists almost
entirely of basal ice.
9.6.2. Surface and Subsurface Wash
Depending upon the volume of snow, the rapidity of thaw, and the nature of the substrate,
runoff can be both overland (surface) and subsurface in nature. Both are concentrated at
the downslope edge of an ablating snow bank.
One of the earliest quantitative measurements of slopewash is provided by A. Jahn
(1961) from Spitsbergen. Using simple sediment trays, it was calculated that approximately
12-18 g m
−2
year
−1
of sediment was being washed downslope beneath large perennial
snow banks. This corresponds to a surface denudation rate of approximately 7 m/1000
years, a value considerably lower than comparable rates from non-permafrost regions
(Young, 1974).
Subsequent studies by A. G. Lewkowicz and colleagues (Lewkowicz, 1983; Lewkowicz
and French, 1982a, b; Lewkowicz and Kokelj, 2002) indicate that surface denudation due
to suspended sediment removal varies between
∼
0.4 and
∼
2.6 mm/1000 years. By contrast,
solute removal is 8-30 times greater (between
74.0 mm/1000 years). It has also
been found that most surface wash is derived primarily from snowmelt and that summer
rain rarely produces overland fl ow. Finally, average values of suspended sediment that are
removed by slopewash processes can be as high as 1200 g m
−2
year
−1
on disturbed slopes
(experiencing active-layer detachments) but, on undisturbed slopes, erosion rates are low
in comparison to global rates.
Relatively little is known about subsurface wash and the role of seepage in slope modi-
fi cation. Solute concentrations probably increase throughout the summer in response to a
higher residence time and a progressive desaturation of the active layer. Denudation from
subsurface wash probably approximates that from surface wash. The Russian literature
suggests that “thermo-erosional wash” exists, in which small mineral particles are liber-
ated by melt of frozen ground. This may contribute to what is termed “thermo-planation”
∼
2.0 and
∼
