Numedalslågen stretches for over through the counties of Vestfold and Telemark and Viken_(county), beginning at the Hardangervidda plateau and meeting the ocean at Larvik in Vestfold. Numedalslågen is one of Norway's longest rivers. The river runs through the municipalities Larvik, Lardal, Kongsberg, Flesberg, Rollag and Nore og Uvdal. These municipalities cooperate in the administration and use of resources connected to the river in various projects under the Green Valley (Grønn Dal) umbrella.