Akerselva or Akerselven () is a river which flows through Oslo. It starts at Maridalsvannet in Oslomarka, and traverses the boroughs of Nordre Aker, Sagene, Grünerløkka, central Oslo and Grønland, whereby it finally ends at Paulsenkaien and Oset in Bjørvika. The river is considered to be a part of the Nordmarkvassdraget, and has the Norwegian watercourse number 006.Z. The entire river is about long, and has a difference in elevation between source and mouth of approximately . In the past, the river was utilized as a source of energy for local industry, and along the river there are many old industrial buildings. In the 1970s, the river was heavily contaminated after 150 years of industrial and sewage discharge. In the 1980s, a local initiative to limit emissions and revive the flora and fauna of the river began. Akerselva has largely been rehabilitated is now "Oslo’s green lung" and along the bank of the entire river from Grønland to Maridalsvannet there are park areas. Salmon run and spawn in the upper part of the river. Early in March 2011, Oslo's water and wastewater department accidentally discharged of chlorine into the river near its headwaters, rendering the river virtually dead as of 11 March 2011. According to Norwegian Broadcasting Corporation, the discharge occurred at the water treatment plant Oset. Name The first element is the genitive case of Aker, the name of the old farm and churchsite that has named several places in Oslo. The last element is the finite form of the noun elv ('river'). The river is considered to be the border between the eastern and the western part of Oslo. In the eastern part people treat the word elv as a feminine noun, so they say elva ('the river'). The westside dialect is influenced by Danish (considered a more appropriate language during the union with Denmark), where the feminine and masculine genders have merged into a common gender inflected like Norwegian masculine nouns. Therefore, people in the western part often treat elv as a masculine noun, causing them to say elven ('the river'). The Old Norse name of the river was Frysja, and this old name has been revived as the name of a neighborhood along the river. The name is probably derived from the verb frusa 'froth', referring to the many waterfalls in the river. Geology Akerselva flows from Maridalsvannet north of the city to Bjørvika, 9.8 km