The Otra river is the largest river in the Sørlandet region of Norway. It flows from the Setesdalsheiene mountains in Bykle municipality in Aust-Agder county in the north, down to the municipality of Kristiansand in the south, where it flows into the Skagerrak.
The Otra is 245 kilometres (152 mi) long, making it Norway’s eighth longest river. There are many large lakes along the river including: Åraksfjorden, Byglandsfjorden, Hartevatnet, and Kilefjorden. There are 12 hydroelectric power plants built along the river, which produce much of the electricity for the southern part of Norway.