167: Subglacial Drainage

Subglacial drainage can occur wherever ice at a glacier bed reaches the pressure melting point. The subglacial drainage system is fed from a mixture of surface, englacial, subglacial, and groundwater sources that differ in terms of their spatial distribution and characteristic patterns of temporal variability. Subglacial drainage systems are not readily accessible, and knowledge of their characteristics is derived from a range of indirect methods including radio-echo sounding, the use of artificial tracers, monitoring, and manipulation of subglacial conditions via boreholes, and monitoring of glacial runoff properties. Subglacial water flow is driven by gradients in hydraulic potential, and occurs through either fast/channelized or slow/distributed systems located at the ice-bed interface, or through subglacial aquifers. Water can be stored subglacially in cavities, in the pore space of subglacial sediments, or in subglacial lakes. Drainage system structure evolves continually on various timescales in response to changing water inputs, evolving glacier geometry, and changes in glacier flow dynamics. Major hydrological events in such systems include the spring and fall transitions, outburst floods, and structural changes related to glacier advances and glacier surging. In the absence of variable water inputs from the glacier surface, subglacial drainage systems may be sensitive to forcing by earth, ocean, and atmospheric tides