Geology Reference
In-Depth Information
bedload results in the stream becoming overcompetent. To compensate and equalize
energy, it adapts the meandering pattern.
At the macro-scale, the large northern rivers, such as the Mackenzie, Lena, and others,
usually possess single and well-defi ned channels. Despite the fact that these rivers freeze
over during the winter months, fl ow beneath the ice cover is maintained throughout the
year. The thermal effect of the water promotes the development of sub-channel taliks, and
an abrupt shelving of the permafrost table. These factors favor steeply-inclined channel
sides and deep, well-defi ned channels. Multiple channels only develop either where tribu-
taries join or where the rivers exit to large water bodies where high sediment load results
in large deltas (see below).
10.2.7. Valley Asymmetry
The presence of asymmetrical valleys in areas currently underlain by permafrost was fi rst
reported by W. B. Shostakovitch (1927) from Siberia. Today, there is an abundant litera-
ture that describes this phenomenon. To judge from the literature (Table 10.6), the north-
facing slope is usually steeper in the high latitudes. However, steeper slopes of systematic
orientation have also been reported to face south, west, and east.
The most probable explanation for the “normal” north-facing asymmetry involves
greater mass-wasting activity on south-facing slopes and asymmetric lateral stream cor-
rasion (Currey, 1964; Gravis, 1969). This is because south-facing slopes receive greater
solar radiation and, as a consequence, usually have thicker active layers. Asymmetrical
valleys raise problematic questions as to which of the slopes has either declined or steep-
ened in angle and necessitates assumptions as to the manner of slope evolution. There is
Table 10.6.
Some characteristics of valley asymmetry in northern regions.
Area and author
Locality
Valley
Orientation of
alignment
steeper slope
East Greenland:
Poser (1948)
Wollston-Vorland
E-W
N
Malaurie (1952)
Disko
E-W
N
West Spitsbergen
1
:
Dege
Andreeland
E-W
S
Dege
Conwayland
N-S
W
Klimaszewski
Kaffi oya - Ebene
E-W
S
Klimaszewski
Brogger - Halbinsel
N-S
E
Siberia:
Shostakovitch (1927)
Yakutia
E-W
N
Presniakow
2
Yakutia
E-W
N
Gravis (1969)
Yakutia
E-W
N
Northern Canada:
Bronhofer
2
Southampton Island
E-W
N
French (1971b)
Banks Island
NW-SE
SW
Kennedy and Melton
2
Caribou Hills, NWT
E-W
N, S
Alaska:
Hopkins and Taber
2
Central Alaska
E-W
N
Currey (1964)
Northwest Alaska
E-W
N
1. Quoted in Karrasch, 1970.
2. Quoted in French, 1976a.
