Lac de Sylans, Lake in Le Poizat-Lalleyriat, Ain, France

Lac de Sylans lies in the Nantua water gap in the Ain department, between Le Poizat-Lalleyriat and Les Neyrolles. The elongated lake has an irregular shoreline with pale sandbars and shallow banks along its near edge, and its still surface reflects the sky in a flat, milky tone. Wooded slopes border it, with a road curving along the left side.

Lac de Sylans formed when a collapse of the northern cliff of the Nantua water gap dammed the valley between the ice factory and the source of the Doye, above Les Neyrolles. In 1865 Joachim Moinat, a café owner in Nantua, began harvesting the lake's pure ice; by 1880 the site produced about 300,000 tonnes a year. Production ended in 1917, and the buildings were sold in 1925.

Joachim Moinat, a café owner in Nantua, began harvesting the lake's ice in 1865; the ice was pure enough to be sold without purification. Blocks cut in winter were stored in sawdust-insulated buildings and shipped by rail from 1882 to Paris, Lyon, Marseille, Toulon and even Alger. Vestiges of the ice works remain, and restoration has revealed the former warehouses.

Access is via the D1084 from Nantua past Les Neyrolles toward Bellegarde-sur-Valserine, or from the A40 motorway at exit 9 (Nantua, Les Neyrolles). The Haut-Bugey railway line runs along the south shore and the A40 along the north shore. In dry summers the water level can drop by four to five metres, exposing sandbars and shallow banks along the near edge.

In dry summers the lake can drop by four to five metres, and in wet periods the overflow escapes at the far end of the Nantua water gap through the Combet stream, which joins the Semine and then the Valserine, a tributary of the Rhône. The lake therefore drains in two directions, depending on the season.