Briançon in the Hautes-Alpes department is located 9.3 mi (15 km) from the Italian border at the crossroads of the catchment areas of no less than five rivers or torrents: the Durance, the Clarée, the Guisane, the Cerveyrette, and the Orceyrette. The site's characteristics determined the constraints and conditions of urbanization of the upper and lower towns. The lower town extends in the trough valley of the Guisane dug by the great glacier descending from the Lautaret pass. As for the upper town, it was built on a glacial lock born from the confluence of the glacier with a small shallower glacier coming from the Montgenèvre pass.