Mandalselva or Mandalsåni or Marna (English: Mandal River, Old Norse: Mǫrn) is a river in Vest-Agder county in Norway. The river has its origins in the mountains between Ose in Setesdal in Aust-Agder county and the Upper Sirdal valley in Vest-Agder county.
The river flows south to its mouth at the North Sea at the town of Mandal. The river is 115 kilometres (71 mi) long and flows through the municipalities Åseral, Audnedal, Marnardal, and Mandal. The largest tributaries are Monn, Logna, Skjerka, Kosåna, Logåna, and Røyselandsbekken. Skjerka, Monn and Logna all flow into the lake Øre in Åseral which is considered the beginning of the main Mandalselva river. The river passes through the villages of Kylland, Bjelland, Laudal, Heddeland, Øyslebø, and Krossen.
Hydropower development of the river on a larger scale started in 1930. At present there are six power plants along the river and its upper tributaries: Logna, Smeland, Skjerka, Håverstad, Bjelland, and Laudal.
The drainage basin covers an area of 1,800 square kilometres (690 sq mi). The mean flow of water in the river is 85 cubic metres per second (3,000 cu ft/s). During flooding periods in the spring and autumn, water flow can be far greater. The highest stream flow in recent times were measured in October 1987, when it passed about 970 cubic metres per second (34,000 cu ft/s) at Kjølemoen in Holum.
Salmon and trout
The original salmon stocks are extinct as a result of acid rain, but extensive liming has led to a better environment in the river, and a new salmon stocks are established. It was caught 8,785 kilograms (19,368 lb) of salmon and sea trout in Mandalselva in 2015.