Temporary lakes dammed by either glacier ice or moraines are common features of mountain environments, and are formed in four main situations (Yamada, 1998; Clague and Evans, 2000):
1. where a glacier emanating from a side-valley blocks the drainage of the trunk valley
2. where a glacier in a trunk valley blocks drainage from side valleys
3. at the junction between two valley glaciers, and
4. behind lateral-frontal moraines and outwash heads.
Cases (1), (2) and (3) form during glacier advance, and case (4) during glacier retreat. In the case of ice-dammed lakes, water level may be controlled either by low bedrock or sediment-floored cols in the lake catchment or by the glacier dam itself. An example of the former case is recorded in Glen Roy, Scotland, where ice advance during the Loch Lomond (Younger Dryas) Stade blocked the drainage of a major valley system (Sissons, 1981). Cols on the watershed controlled water level, and the lake rose and fell through three distinct levels as successive cols were blocked by glacier advance or exposed by retreat. The former water levels are recorded by very prominent shorelines known as the 'Parallel Roads', which remain strikingly clear after approximately 11,000 years of weathering and erosion (Fig. 15.21). Subaqueous fans mark the former glacier terminus, and drapes of laminated sediments cover much of the former lake floor.
In situations where the ice dam controls lake level, lakes are inherently unstable, as high lake levels will tend to destabilize the dam, thus precipitating catastrophic lake drainage (Clarke, 1982). Such lakes will tend to undergo multiple drainage and filling cycles during a single glacial cycle (Benn, 1989b).
The transition from small supraglacial ponds into a large moraine-dammed lake may be quite rapid, occurring within 2—3 decades in some cases (Ageta et al., 2000). Glacier lake outburst floods (GLOFs) from moraine-dammed lakes are currently a significant environmental hazard in high mountain environments in the Himalaya, Andes and North American Cordillera as a result of recent rapid climatic warming and deglaciation (Lliboutry, 1977; Richardson and Reynolds, 2000).
Although short lived, ice- and moraine-dammed lakes can have profound effects on glaciated valleys. High sediment fluxes mean that they infill rapidly with sediment. Different combinations of sedimentary facies are deposited according to locally dominant processes. In
supraglacial moraine-dammed lakes, dumping of the debris cover into the lake produces an ice-cored lake floor where relief inversions and buoyant berg release will occur against a background of fine sedimentation from suspension and iceberg dumping. Backwasting of the ice shorelines and coalescence of neighbouring ponds opens up larger lakes until a calving terminus develops in deep water (Kirkbride, 1993). Retreat of the calving margin causes a transition from an ice-bounded to a predominantly moraine-bounded lake during growth, causing a shift in depositional processes to more distal, fine-grained sedimentation and mass movements from moraine walls. Loss of the subaqueous ice floor produces syndepositional deformation of lake-floor sediments.
Depositional facies in Lateglacial valley lakes in Canada and New Zealand (Shaw, 1977c; Pickrill and Irwin, 1983; Eyles et al, 1987; Ryder et al, 1991; Ashley, 2002; Fig. 15.22) record deposition dominated by subaqueous mass flow deposits affected by numerous deformation structures caused by the melt-out of buried ice. In some areas, valley-side delta and kame terraces are preserved, recording former lake levels or positions of the ice surface. During the wastage of the Cordilleran Ice Sheet (British Columbia), stagnant glacier ice was isolated in deep valley bottoms and broke up into lake basins. Clague and Evans (1994b) have argued that environmental conditions were similar to those occurring today in the St Elias Mountains, where major debris-covered glacier tongues have thinned dramatically during the last hundred years. Useful sedimentological studies have also been conducted at small, younger lakes elsewhere (e.g. Gilbert and Desloges, 1987; Liverman, 1987; Hicks et al, 1990; Bennett et al, 2000).
GLOFs can erode and rework large volumes of sediment. Flood tracks may be preserved in the landscape in the form of channels and boulder fans extending down-valley from the moraine breach (Clague and Evans, 1994a; Coxon et al., 1996).
4O2 GLACIAL LANDSYSTEMS
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