Subglacial stream
[2] Surface meltwater flows downward through millimeter-sized channels that join together in a network of tributaries, growing in size until reaching the bedrock.
[5] Channelized drainage systems are characterized by water flowing predominantly through tunnels along the bed of the glacier that take meltwater rapidly and directly to the glacial terminus.
[2] Channels that maintain long-term stability in water flow and location can erode the bedrock over time, resulting in tunnels that cut into the bed rather than the ice above.
[2] Distributed drainage systems can consist of a network of linked cavities, porous flow and canals in the sediment, and a thin film between the ice and bed.
[2] When the bed is deformable, wide, shallow canals up to 10 cm in width can form in the surface of the sediment, topped by the glacial ice.
[5] Both of these sources involve small amounts of water released relatively uniformly throughout the bed of the glacier, making them unlikely to form large drainage channels.
[5] As temperatures rise and surface melting increases water flux to the bed in late spring, the winter stream system is disrupted.
[2] Distributed flow channels, lacking the capacity for increased volumes of meltwater, experience a rise in water pressure and are destabilized.
[7] This change can happen gradually or can be triggered by events that rapidly increase meltwater flow, such as consecutive days of high melting or a large rainstorm.
[5] The discharge of subglacial stream systems of marine-terminating glaciers into the ocean has a significant impact on the volume and distribution of glacial melt at the terminus.
[8] In temperate glaciers, which are characterized by the presence of liquid water at their base and are able to slide, subglacial streams have a significant impact on glacial movement.
The water pressure and friction experienced at the base of a glacier depends in part on whether the subglacial hydrological system is channelized or distributed.
[7] Sustained high levels of meltwater input result in a shift from a distributed network of subglacial streams to a more channelized system as larger passages through the ice develop.
[4] Though the concentration of dissolved organic matter in glacial meltwater is low, the sheer amount of freshwater discharge from glaciers makes glacially-sourced DOC an important source of bioavailable carbon to marine ecosystems.
Geological processes, including the grinding of glaciers on the bedrock below and water-rock interaction, ensure that minerals are continuously fed into the subglacial system.
[3] Biological processes also provide nutrients to subglacial streams, with nitrification and denitrification by microbes affecting downstream communities during periods of melt.
[12] The permanent formation of eskers is more common in retreating glaciers and ice sheets, as their termini are thinning, which favors the deposition of sediment.
[12] Advancing glaciers and ice sheets exhibit steepening termini, which increases shear stresses and, consequently, water pressure, which favors the flushing of deposited sediment out of stream channels.
[11] Greater water input from surface melting may affect the hydrology of subglacial systems, changing the timing of seasonal variations.
[14] This would cause the transition from winter distributed subglacial drainage to summer channelized streams to occur earlier in the year.
[14] However, short-term fluctuations in meltwater volume and pressure, which may become more intense as runoff increases, could offset this decrease in sliding by causing localized speedups.
