| Summary: | We used an annual time series (August 2004 until August 2005) of ENVISAT-ASAR data, supported by two ASTER scenes to investigate ice dam fluctuations and changes of the surface area of the marginal lake Grænalón. The lake occupies a lateral valley, dammed by the westernmost flow band of Skeiðarárjökull, one of the largest outlets of Vatnajökull ice cap. During the period of investigation Skeiðarárjökull was affected by three flood events draining subglacially under this outlet: Two jökulhlaups originating from Grímsvötn (29/10 until 07/11/2004 and 04/03-14/03/2005) and a Glacial Lake Outburst Flood (GLOF) of Grænalón (07/08 – 14/08/2005). It was found that changes of the lake surface area and the calving front position are in phase during the two jökulhlaups originating from Grímsvötn. Therefore it seems likely, that lake Grænalón and the western flow band of Skeiðarárjökull are affected by such phenomena. This supports the new theory of sheet flow or coupled sheet and tunnel flow for the propagation of a jökulhlaup under Skeiðarárjökull. Between May and August 2005 the surface area of lake Grænalón increased by 275 ha to a maximum extent of 960 ha. During the GLOF in August 2005, the surface area decreased rapidly by 370 ha to 590 ha. Intense calving during the GLOF lead to an ice front retreat of about 400 m. The maximum extent of Grænalón measured one week before the outburst could be used as a threshold in future SAR acquisitions to estimate the risk of an imminent GLOF.
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