Geology Reference
In-Depth Information
production, and therefore average ground lowering, is at least one order of magnitude
lower than the present yield rate. Thus, a rate of
50 mm/1000 years, as calculated for the
Colville River in northern Alaska (Arnborg et al., 1967), is probably more typical of most
periglacial environments.
∼
10.2.5. Fluvio-Thermal Erosion
The thermal effects of running water have already been commented upon (see Chapter
8). In addition to normal mechanical effects, running water possesses the ability to thaw
permafrost. The most obvious result is the existence of taliks, or unfrozen zones, beneath
the channels of all rivers and water bodies that do not freeze to their bottoms in winter
(see Chapters 5 and 6). Where large river channels are incised within ice-rich and/or rela-
tively unconsolidated sediments, lateral erosion can form thermo-erosional niches often
several tens of meters deep (Figure 10.4A). These sometimes cause bank collapse, often
in large blocks delineated by ice wedges (Figure 10.4B). This process, fi rst described from
the Colville River Delta (Walker and Arnborg, 1966), occurs widely along the banks of
major rivers, such as the Yukon River in western Alaska and the Lena River in central
Siberia, where they traverse ice-rich alluvial lowlands. Coastlines developed in ice-rich
and unconsolidated sediments are also subject to rapid erosion and retreat by this process
(see below). As regards channel dynamics, fl uvio-thermal erosion is important for at least
three reasons: (a) the collapse of river banks provides material for bedload transport (see
above), (b) it promotes the development of wide, fl at-bottomed channels (see below), and
(c) it helps explain the effi ciency of lateral stream migration and the formation of valley
asymmetry in permafrost areas (see below).
10.2.6. Channel Morphology
River channels may be single or multiple, straight or meandering, and large or small. In
periglacial environments, all types of channel pattern exist but undoubtedly the most
common is the multiple, or braided, channel (Figure 10.5).
The dominance of braided channels is the result of at least three factors. First, braiding
requires appreciable sediment load. Second, braiding is related to rapid and large varia-
tions in discharge. Third, bank erodibility is clearly a factor since excessive lateral erosion
will not only lead to wide channels, shoaling, and the development of multiple channels,
but also to the entrainment of large quantities of debris. All these requirements are present,
in varying degrees, in periglacial environments underlain by permafrost. This is because
the discharge of both nival and proglacial streams is subject to rapid and extreme fl uctua-
tion and the presence of loose glacigenic sediment supplies material for transport.
As a result, the fl oor of many stream channels in the high latitudes is covered in alluvial
sediments ranging from coarse gravel to medium sand. The banks are often abrupt, giving
a shallow box-like profi le to the channel (Figure 10.6). At times of peak fl ow, the entire
fl oor becomes covered with a layer of turbulent water, in which bedload transport domi-
nates. As discharge decreases and competence drops, coarser material is deposited, and
the stream assumes a new braided pattern. Thus, given time and repeated adjustments to
the braided channel pattern, the channel fl oor is constantly reworked.
The importance of abundant bedload sediment in producing braided stream channels
is illustrated by the absence of well-developed braided stream channels in those areas
where bank erodibility is restricted and where debris suitable for transport is limited. For
example, it is instructive to consider the 120 km-long Thomsen River on Banks Island, the
largest river in the Canadian arctic islands. For the majority of its middle length, the
channel is single, somewhat sinuous, with only occasional braiding. As such, it contrasts
