Reduced glacier sliding caused by persistent drainage from a subglacial lake

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 1995–2000. More than a 50% decrease in glacier velocity was observed subsequent to a large j kulhlaup from the subglacial lake Grímsv 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 Grímsv tn 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 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.