At the eastern end of Vestfold is the village of Eidsfoss. In this area, water flows into Lake Eikeren from the Bergsvann, Vikevann, Haugestadvann and Hillestadvann. Lake Eikeren also receives waterflow from the Hakavikelva and Steinbruelva Rivers.
In the western part, Eikeren comes together with the Fiskumvannet in a narrow, strait known as Sundet. Here water flows into Lake Fiskum, located at the top of Eikernvassdraget. Through this strait, water flows at an average of 7 m3 per second. From Lake Fiskum, water runs out into Vestfosselva to Vestfossen. The river then go to Hokksund, where it splits into two river that both flow in the Drammen River.
Eikeren, together with Fiskumvannet, is 20 km long, up to 2,5 km wide and up to 158 m deep.
In 2005, Lake Eikeren was developed by the Vestfold Inter Municipal Waterworks, (Vestfold Interkommunale Vannverk), in order to provide extra drinking water for Vestfold. The other source is Farrisvannet, a lake on the border between the counties of Vestfold and Telemark. Vestfold Inter Municipal Waterworks Authority is a company owned by ten municipalities in Vestfold and is responsible for water supply, water treatment, emergency power supply and the main distribution to its members. 
During the summer season, Lake Eikeren is served by tourist boat M/S Eikern between Eidsfoss and Vestfossen. The original M/S Eikern steamer was commissioned in 1861. The steam vessel Cappelen was built in 1880 and was used for timber and passenger traffic. In 1903, the steamship Stadshauptmand Schwartz carried regular traffic and provided a link between the Kongsberg Railway (Kongsbergbanen) and the Tønsberg–Eidsfoss Line. The current vessel M/S Eikern has carried traffic on Lake Eikern since 1997.
- "Eikernvassdraget Interessentlag - EVI". Eikern.no. Retrieved 2013-10-25.
- "Vestfold Vann". vestfoldvann.no. Retrieved October 1, 2017.
- Per Ivar Søbstad (August 10, 2005). "Eikeren". eikerbygda.no. Retrieved October 1, 2017.
- "M/S Eikern - Historikk". Mseikern.no. Retrieved 2013-10-25.
- "Kongsbergbanen". lokalhistoriewiki.no. Retrieved October 1, 2017.
- More about Eikeren
- Map of Eikeren
- Vestfold Intermunicipal Water Works
- Pictures of Eikeren
- M/S Eikern website
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