Akerselva flows out of Maridalsvannet, which is the most important source of drinking water in Oslo, and through the districts of Nordre Aker, Sagene, Grünerløkka and Gamle Oslo. The river finally flows into the Oslo Fjord at Paulsenkaia in Bjørvika. The river is 9.8 km long and has a drop of 149 meters. There is a rich flora and fauna around, and a lot of fish such as salmon and trout.
In the Middle Ages, this river was called Frysja, interpreted as “the cold” after the verb “freeze”. “The inhabitants of Vatterland, Greenland and Piipervigen” were in a royal rescript of 17 October 1636 ordered to build in Christiania (old name of Oslo), if they could afford it. Those who did not have enough money were to move to the area by “Aggershuses elv” between Sagene and the current estuary. Because the name of the river was too cumbersome, it was shortened to Ager’s river, which later became Akerselva.