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Subglacial Drainage System
In a glacier, superposing layers are continuously and the deeper snow layers are compressed to high levels in the bottom. With a density of 0.83 g/cm3 the snow becomes impermeable to air, and then we can call it ice (Its density ranges from 0.83 to pure ice density of 0917 g/cm3), a perennial snow with high density is called firn. A glacier can be cut by 3 parts:

- The accumulation area, the net mass balance is positive

- The transportation area, transport of ice accumulation from area to area ablation

- The removal area, the net mass balance is negative

If one summarizes the precipitation as snow accumulates overlapping layers and layers throughout the years, they are transported from the accretion zone to the ablation zone by a gradual drift of the ice gravity. The glacier is a collector and precipitation issuing gradually with years of freshwater. According to Braithwaite, Glacier mass balance is linked by changing atmospheric environment, glacier dynamics and hydrology. Within this glacier, there is formation of regular channels, arborescent networks or nonarborescents networks leading freshwater liquid to the glacier's bottom. The activity of these networks varies both diurnally and seasonally in connection with the cold of night or winter, which causes a network collapse, the network reappears in spring. Thus, there is formation of some freshwater reservoir in the form of channels, or in the form of subglacial lakes. The formation of lakes and canals is determined by the geomorphology of bedrock, its composition (impermeable or not), the thermal conditions of the bedrock and the presence of till (sediment).

Water flow is largely determined by the effective pressure inside the glacier, we have both a vertical stress by the glacier mass and a horizontal stress dependent of the bedrock geomorphology and advancing glacier’s slope. Increase of pressure induces a widening of channels, which in turn causes a discharge of sediment, before the glacier removes a large amount of water to restore some balance. Rainfall through the firn, affects the frequency and amplitude of subglacial drainage.

Furthermore, subglacial lakes induce an increase in the velocity of downstream ice-flows creating a zone of distortion between the upstream and the downstream of the lake. This phenomenon is a result of a heat exchange, the density of water and a virtually no friction in contact with water. Fracture propagation to the base of the Greenland ice sheet during supraglacial lake drainage. In Antarctica, these lakes are sometimes longer than 250 km and thus compromise the integrity of the ice cap. Furthermore, freshwater discharged and headed toward the ocean, or a high intake of freshwater can destabilize ocean currents and thus the climate.

Uncited references
IPCC, Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge Univ. Press, Cambridge and New York, 2007).

Lemke, P., J. Ren, R.B. Alley, I. Allison, J. Carrasco, G. Flato, Y. Fujii, G. Kaser, P. Mote, R.H. Thomas and T. Zhang, 2007: Observations: Changes in Snow, Ice and Frozen Ground. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. 48 s.

UNEP (United Nations Environment Programme), 2007. Global Outlook for Ice & Snow. Division of Early Warning and Assessment (DEWA), UEP. 238 s.

UNEP (United Nations Environment Programme), 2008. Global Glacier Changes: facts and figures. Division of Early Warning and Assessment (DEWA), UNEP. 88 s.