The lake drains, from south-west to north-east, into the Baltic Sea through its natural outlets Norrström and Söderström (as it flows around Stadsholmen island) and through the artificial Södertälje Canal and Hammarbyleden waterway.

The etymological origin of the name Mälaren stems from the Old Norse word mælir appearing in historical records in the 1320s and meaning gravel.

At the end of the ice age when the glaciers retreated, the removal of the weight from the depressed land led to a post-glacial rebound.

[citation needed] In the Viking Age, Mälaren was still a bay of the Baltic Sea,[7][8] and seagoing vessels could sail up it far into the interior of Sweden.

[9] The decline of Birka and the subsequent foundation of Stockholm at the choke point of Riddarfjärden were in part due to the post-glacial rebound changing the topography of the Mälaren basin.

According to Norse mythology as contained in the thirteenth-century Icelandic work Prose Edda, the lake was created by the goddess Gefjon when she tricked Gylfi, the Swedish king of Gylfaginning.

The rainbow trout in particular is known to compete with indigenous fish for habitat and food, as they have a faster growth rate and predate on local species.

The plague spread quickly through the lake, exacerbated by boat traffic, which decimated the indigenous noble crayfish (A. astacus) population and caused severe economic losses to the local fishing industry.

Stocking this species is now prohibited within multiple lakes in Sweden in order to protect noble crayfish population.

In 1687, King Charles XI of Sweden commissioned cartographer Carl Gripenhielm to map Lake Mälaren and its surrounding provinces.

The Mälarkarta is unique in that it is surrounded by a frame of 96 small, realistic gouache paintings of cities, castles, mansions, and fairways around Lake Mälaren.

Due to its larger size, it was initially hung in the large viewing room, however in 1931 it was moved to a more secluded wall in a corridor to the map department.

Gripenheim’s Mälarkarta was valid until 1739 when a new, more accurate map of L. Mälaren was completed by the then surveyor and cartographer Jacob Nordencreutz.