Rock beds have been assumed to be impermeable. The classical view is that water will find its way into channels through a water film at the ice-bed interface driven by a pressure gradient that is given by the gradient of ice thickness (Walder, 1982). It has also been demonstrated that flow into major R-channels can take place through complex linked cavity systems that exploit the natural cavities in the lee of bedrock hummocks and can shrink or enlarge diurnally and seasonally to accommodate changing water fluxes (e.g Kamb, 1987). Any discontinuous lenses of till resting on the rock surface will have relatively little effect on the hydraulic system, and will have porewater pressures that are the same as the ambient pressures.
However, impermeable rock is a concept rarely realized in nature. Even the Precambrian rocks of the Fennoscandian shield commonly have hydraulic conductivities in the topmost 100 m or so of about 10-6-10-7ms-1 (largely determined by flow along frequent fractures), giving transmissivities of the order of 10-4-10-5m2s-1 (e.g. Gustafsson et al., 1989). Comparing this with the basal melt fluxes in short valley glaciers (see section 2.3.1) suggests that in some cases groundwater flow can be an effective means of transporting at least the winter flux of basal meltwater towards major subglacial conduits.
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