We present velocity observations of a glacier outlet in Vatnaj??kull, Iceland, deduced from interferometric SAR (InSAR) data obtained during the ERS1/2 tandem mission in 1995a??2000. More than a 50% decrease in glacier velocity was observed subsequent to a large j??kulhlaup from the subglacial lake Gr?-msv??tn in 1996. The glacier had not reached its former flow rate in 2000. The j??kulhlaup damaged the lake's ice-dam causing persistent drainage from the lake. InSAR based studies of water accumulation within Gr?-msv??tn suggest that a leakage of &3 msup3/sup ssup−1/sup 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 j??kulhlaup 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. The width of the affected areas was ~5 km on the upper part of the glacier and ~8 km on the centre part of the glacier. This indicates that the water pressure reduction propagates laterally from the tunnel over a distance of a few km.
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