From the Bjornesfjorden, the main river stretches for over 250 kilometres (160 mi) through the counties of Buskerud and Vestfold and then meeting the ocean at the Larviksfjorden at the town of Larvik.

Numedalslågen is one of Norway's longest rivers, beginning in Eidfjord and then running through the municipalities Nore og Uvdal, Hol, Rollag, Flesberg, Kongsberg, and Larvik.

These municipalities cooperate in the administration and use of resources connected to the river in various projects under the Green Valley (Grønn Dal) umbrella.

Most of the power potential of the main river is developed, except the stretch between Hvittingfoss and Larvik and Godfarfossen in Dagali.

It was completed in 1928 and was designed by Norwegian architects Carl Buch and Lorentz Harboe Ree.