Subglacial meltwater originates from a range of sources, mainly from melting of ice by geothermal heat trapped at the glacier sole and by the frictional heat caused by ice movement past the substratum. These two sources yield up to some 100 mmyr-1 of water. Close to the ice margin, in the area where englacial conduits extend to the bed, surface ablation water may reach the ice sole with recharge several orders of magnitude greater than the basal meltwater alone. It is difficult to estimate how wide this area is, but as deeper conduits tend to close under cryogenic pressure or they bend horizontally towards the ice margin, it is probable that ablation water would reach the bed only where the ice thickness is less than about 100-200m (Reynaud, 1987; Fountain & Walder, 1998). Under polythermal conditions, such as those that characterized continental Pleistocene ice sheets, the outermost marginal zone and the ice divide zone may be cold-based and do not contribute to the basal water recharge.
Subglacial water can be evacuated to the ice margin through high-discharge drainage systems consisting of channels incised into the glacier sole (R-channels; Rothlisberger, 1972) and channels carved into the bed (N-channels; Nye, 1973). R-channels typically form on bedrock, whereas N-channels tend to occur on soft beds. This is shown by the distribution of landforms across different substrata overridden by the Fennoscandian and Laurentide ice sheets where on bedrock areas eskers dominate, whereas in areas of sedimentary basins away from glaciation centres meltwa-ter channels and tunnel valleys prevail (Clark & Walder, 1994).
Another type of subglacial drainage is the low-discharge, distributed system operating at the ice-bed interface or in the rocks below. A linked-cavity network consists of broad and shallow water lenses connected by orifices, favoured by rapid sliding and high bed roughness (Nye, 1970), which originally was suggested for hard bedrock areas by Lliboutry (1976) and Kamb (1987). A modification of this drainage mechanism was proposed by Walder & Fowler (1994) and Clark & Walder (1994) for soft, deformable beds. It consists of broad, shallow 'canals' interconnected in a non-arborescent system without orifices, capable of evacuating more water than the linked cavities. Yet another mechanism is the subglacial water film, typically about 1 mm thick, generated by regelation at the ice-rock interface (Weertman, 1972). Under special circumstances, parts of a glacier may be lifted by pressurized water leading to short-lasting subglacial sheet-floods of high magnitude.
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