Nidelva

Nidelva is the main river in the Arendal drainage basin in Aust-Agder county, Norway. The 221.7-kilometre long river begins at the confluence of two rivers: Nisserelva and Fyreselv. The confluence is located by the village of Haugsjåsund in Nissedal municipality in southern Telemark county. The river flows south into the Skagerrak at the city of Arendal. The watershed covers 4,011 square kilometres and has an average waterflow of 114.4 cubic metres per second at Rykene near the mouth. The highest waterflow ever recorded on the river was 1,200 cubic metres per second in the autumn of 1987.

The river flows through Nissedal in Telemark county briefly before entering Aust-Agder county. It then flows through Åmli municipality. The river Gjøv joins the Nidelva there. The large lake Nelaug is a man-made lake along the river that is used for power generation. It continues through Froland municipality and then Arendal municipality. For a while, the river forms the border between Grimstad and Arendal municipalities. The river empties into the Skaggerak just northeast of Rykene in the town of Arendal between the mainland and the island of Hisøya.

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