Nidelva is the main river in the Arendal drainage basin in Agder county, Norway. The 221.7-kilometre (137.8 mi) long river begins at the confluence of two rivers: and . The confluence is located by the village of in Nissedal municipality in southern Vestfold og Telemark county. The river flows south into the Skagerrak at the city of Arendal. The watershed covers 4,011 square kilometres (1,549 sq mi) and has an average waterflow of 114.4 cubic metres per second (4,040 cu ft/s) at Rykene near the mouth. The highest waterflow ever recorded on the river was 1,200 cubic metres per second (42,000 cu ft/s) in the autumn of 1987.