Englacial Hydrology of Temperate Glaciers

Abstract:

The englacial region of temperate glaciers is generally treated as a passive conveyor of water from the surface to the bed. Consequently, few studies have examined this region and relatively little is known. This is an important issue because englacial processes probably exert a first order control on the distribution of water to the subglacial hydraulic system. Controlling the water distribution probably controls the type of subglacial hydraulic features present and therefore sliding behavior. Certainly, englacial conduits play a major, if not primary, role in conveying water in the ablation zone. In regions of over-deepenings, areas highly crevassed, or in the accumulation zone, the importance of englacial conduits is less clear. Field studies have shown that intersecting englacial passageways in these regions are relatively common, implying that large water fluxes can drain efficiently through a network of fractures. Hypothetically, efficient drainage systems composed of englacial conduits develop in response to point input of large surface water fluxes. Where input is small and distributed, common to highly crevassed areas or the accumulation zone, water is probably routed through a network of englacial fractures. Glacier geometry may also play a role. Conduits may not develop in the over-deepened (closed basins) regions of a glacier requiring another flow pathway. That englacial fractures exist and can convey water presents a promising alternative. Measured rates of flow in fractures strongly suggest laminar conditions and a sufficient fracture density exists to accommodate the estimated water flux generated upstream by surface melt. The slow flow rates do not generate sufficient viscous heat to compensate expected rates of closure by freezing, however field observations and seismic evidence point to spontaneous fracture formation at depth that must regenerate the fracture network. It is unfortunate that englacial investigations are ignored in favor of subglacial observations because both can be achieved in the same drilling campaign with little additional effort.