Oulujärvi (Finnish pronunciation: [ˈouluˌjærʋi], Swedish: Ule träsk; also known as Lake Oulu) is a large lake in the Kainuu region of Finland. With an area of 928 square kilometres (358 sq mi) it is the fifth largest lake in the country. The lake is drained by the Oulu River, which flows northwestward from the lake into the Gulf of Bothnia. Its nickname is the "Kainuu Sea", and it is bordered by the three municipalities of Vaala, Paltamo and Kajaani. About 40 percent of the lake is in Vaala.