the origin of Mälaren was probably originally about Vänern: the Swedish king Gylfi promised a woman, Gefjon, as much land as four oxen could plough in a day and a night, but she used oxen from the land of the giants, and moreover uprooted the land and dragged it into the sea, where it became the island of Zealand. The Prose Edda says that 'the inlets in the lake correspond to the headlands in Zealand'; since this is much more true of Vänern, the myth was probably originally about Vänern, not Mälaren. The Battle on the Ice of Lake Vänern was a 6th-century battle recorded in the Norse sagas and referred to in the Old English epic Beowulf. In Beowulf, Vänern is stated to be near the location of the dragon's mound at Earnaness. Geography Vänern covers an area of , approximately twice the surface area of Luxembourg. Its surface is above sea level and it is on average deep. The maximum depth of the lake is The water level of the lake is regulated by the Vargön Hydroelectric Power Station. Geographically, it is situated on the border between the Swedish regions of Götaland and Svealand, divided between several Swedish provinces: The western body of water is known as the Dalbosjön, with its main part belonging to Dalsland; the eastern body is known as Värmlandsjön, its northern part belonging to Värmland and the southern to Västergötland. Its main tributary is Klarälven, which flows into the lake near the city of Karlstad, on the northern shore. Other tributaries include Gullspångsälven, Byälven and Norsälven. It is drained to the south-west by Göta älv, which forms part of the Göta Canal waterway, to Lake Viken into Lake Vättern, southeast across Sweden. The economic opportunities Vänern offers are illustrated by the surrounding towns, which have supported themselves for centuries by fishing and allowing easy transportation to other cities or west by Göta älv to the sea of Kattegat. This directly includes: Karlstad (chartered in 1584), Kristinehamn (1642), Mariestad (1583), Lidköping (1446) Vänersborg (1644), Åmål (1643), Säffle (1951), and indirectly Trollhättan (1916). The Djurö archipelago surrounds the island of Djurö, in the middle of the lake, and has been given national park status as Djurö National Park. The ridge (plateau mountain) Kinnekulle is a popular tourist attraction near the south-eastern shore of Vänern. It has the best view over the lake (about above the lake