Nidelva in Aust-Agder is the main river in the Arendal drainage basin. It is formed by the confluence of two rivers, Nisserelva and Fyreselv. The river flows into the Skagerrak at Arendal. The total length of the system is, the watershed covers. Sixteen hydro-electric power stations are built along the river, making this one of Norway's most controlled drainage systems.Salmon can be found up the river, where the Evenstad power station is located. At Rygene, the average waterflow is 110 cubic meters per second (m³/s), but the highest recorded was in the autumn of 1987, with 1200 m³/s.