largest Alpine lake in Europe (area 224 square miles 【581 square km】), lying between southwestern Switzerland and Haute-Savoie département, southeastern France. About 134 square miles (347 square km) of the lake's area are Swiss, and 90 square miles (234 square km) are French. Crescent in shape, the lake is formed by the Rhône River, which enters it at the east end between Villeneuve, Switz., and Saint-Gingolph, France, and leaves it at the west end through the city of Geneva. The only important tributaries are the Dranse (south) and the Venoge (north). Lying at an elevation of 1,220 feet (372 m), the lake is 45 miles (72 km) long, with a maximum width of 8.5 miles (13.5 km) and an average width of 5 miles (8 km). The maximum depth is 1,017 feet (310 m), the mean depth 262 feet (80 m). The strait of Promenthoux, or Nernier, separates the lake into two well-marked divisions, the Grand Lac (east) and the Petit Lac, the special Genevese portion. The water, unusually blue and transparent, is subject to remarkable fluctuations of level known as seiches (seiche), in which the whole fluid mass in the lake rhythmically swings from shore to shore. The lake is not as rich in fish as the other Swiss lakes; there are known to be 20 indigenous species and 6 that were introduced in the 19th century.