Nidelva

Nidelva is the main river in Norway's Arendal drainage basin, located in the county of Agder. The river begins at the confluence of two rivers, the Nisserelva and the Fyreselv, and runs for 221.7 kilometers (137.8 miles). The confluence is near the village of Haugsjåsund in the municipality of Nissedal in the county of Vestfold og Telemark in southern Vestfold og Telemark. The river flows south into the Skagerrak near Arendal. The watershed covers 4,011 square kilometers (1,549 square miles) and has an average waterflow of 114.4 cubic metres per second (4,040 cu ft/s) near the mouth at Rykene. The river's highest recorded waterflow was 1,200 cubic metres per second (42,000 cu ft/s) in the autumn of 1987.

Before entering Agder county, the river briefly flows through Nissedal in Vestfold og Telemark county. It then flows through the municipality of Åmli. There, the river Gjøv joins the Nidelva. The large lake Nelaug is a man-made lake along the river that is used to generate electricity. It then passes through the municipalities of Froland and Arendal. For a time, the river served as a dividing line between the municipalities of Grimstad and Arendal. The river flows into the Skaggerak just northeast of Rykene in Arendal, between the mainland and the island of Hisøya.

The river is lined with sixteen hydroelectric power stations, making it one of Norway's most controlled drainage systems. The largest on the river are in Grimstad at Rygene, Froland at Evenstad, and Åmli at Jørundland. Salmon can be found from the mouth to the Evenstad power station, which is located 22 kilometers (14 miles) upstream and effectively blocks their path further upstream.

Length: 221.7 km (137.8 mi)