The Cryosphere www.the-cryosphere.net 1994-0416 1994-0424 4 1 2010 10.5194/tc-4-13-2010 http://www.the-cryosphere.net/4/13/2010/ http://www.the-cryosphere.net/4/13/2010/tc-4-13-2010.html http://www.the-cryosphere.net/4/13/2010/tc-4-13-2010.pdf 13 20 2010-01-15 Reduced glacier sliding caused by persistent drainage from a subglacial lake E. Magnússon eyjolfm@raunvis.hi.is H. Björnsson H. Rott F. Pálsson Institute of Earth Sciences, University of Iceland, Sturlugata 7 – Askja, 101 Reykjavík, Iceland Institute of Meteorology and Geophysics, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria The Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Sturlugata 7 – Askja, 101 Reykjavík, Iceland We present velocity observations of a glacier outlet in Vatnajökull, Iceland, deduced from interferometric SAR (InSAR) data obtained during the ERS1/2 tandem mission in 1995–2000. More than a 50% decrease in glacier velocity was observed subsequent to a large jökulhlaup from the subglacial lake Grímsvötn in 1996. The glacier had not reached its former flow rate in 2000. The jökulhlaup damaged the lake's ice-dam causing persistent drainage from the lake. InSAR based studies of water accumulation within Grímsvötn suggest that a leakage of >3 m³ s^−1 prevailed throughout our study period. We suggest that the lake leakage kept open a tunnel at low water pressure underneath the whole length of the glacier. The tunnel flow drained water from its surroundings, hence lowering the water pressure of a distributed drainage system, underneath the upper and centre parts of the glacier, which prior to the jökulhlaup sustained significant basal sliding. This is in accordance with theoretical prediction that tunnel flow in a steady state may cause slow-down in glacier velocity by reducing the subglacial water pressure. 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