| Summary: | The processes that form intraglacial lakes are poorly understood because of the difficulty in detecting and assessing such hidden lakes. Extensive geophysical surveys were performed between 2007 and 2010 in order to reassess the risk of an outburst flood from Glacier de Tete Rousse, French Alps, where outburst flooding from an intraglacial lake caused 175 fatalities in 1892. Our geophysical survey combined ground-penetrating radar measurements and nuclear magnetic resonance imaging. We found a subglacial water-filled reservoir with a volume of 55 000 m(3). Artesian outpourings occurred when the subglacial cavity was reached by two borehole drillings, indicating that the hydrostatic water pressure exceeded the ice pressure at the bottom of the cavity. On the basis of these geophysical and glaciological findings, we warned the public authorities in July 2010 of the risk facing the 3000 inhabitants downstream of the glacier. The subglacial reservoir was drained artificially. This example demonstrates how geophysical surveys can be used to detect this type of hazard when it is suspected, in particular when no hydraulic outlet from the snout exists. Numerous ice temperature measurements have shown that the tongue of this glacier is cold-based. This thermal regime could explain the accumulation of water in this glacier.
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