About 15% of Sweden lies north of the Arctic Circle. Southern Sweden is predominantly agricultural, with increasing forest coverage northward. Around 65% of Sweden's total land area is covered with forests. The highest population density is in the Öresund Region in southern Sweden, along the western coast up to central Bohuslän, and in the valley of lake Mälaren and Stockholm. Gotland and Öland are Sweden's largest islands; Vänern and Vättern are its largest lakes. Vänern is the third largest in Europe, after Lake Ladoga and Lake Onega in Russia. Combined with the third- and fourth-largest lakes Mälaren and Hjälmaren, these lakes take up a significant part of southern Sweden's area. Sweden's extensive waterway availability throughout the south was exploited with the building of the Göta Canal in the 19th century, shortening the potential distance between the Baltic Sea south of Norrköping and Gothenburg by using the lake and river network to facilitate the canal.