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: 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 (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.

The river flows through Nissedal in Telemark county briefly before entering 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.

There are sixteen hydro-electric power stations are built along the river, making this one of Norway's most controlled drainage systems. The largest on the river are at Rygene in Grimstad, Evenstad in Froland, and Jørundland in Åmli. Salmon can be found from the mouth to as far as 22 kilometres (14 mi) up the river, where the Evenstad power station is located, effectively blocking their path further upstream.

The name of the river possibly comes from the Old Norse word niðr which means "murmuring, humming". It may also come from the Latin word nitere. The name of the river also may lend itself to several other local names along the river such as Nisser, Nedenes, and Nelaug.

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  View of the River Nid by John William Edy
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  Froland Church located along the river Nidelva
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  View of the river in Arendal
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  View of the river near Gjermundsnes in Åmli
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  Åmfossen power station along the river in Åmli
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  Furre bridge over the river in Froland

• Media related to Nidelva at Wikimedia Commons
• Evenstad power station (in Norwegian)