Lake Mälaren (historically occasionally referred to as Lake Malar in English) is the third-largest lake in Sweden, after Lakes Vänern and Vättern. Its area is 1,140 km² and its greatest depth is 64 m. The lake drains, from southwest to northeast, into the Baltic Sea through Södertälje kanal, Hammarbyslussen, Karl Johanslussen and Norrström. The easternmost bay of Mälaren, in central Stockholm, is called Riddarfjärden. The lake is located in Svealand and bounded by the provinces of Uppland, Södermanland, Närke, and Västmanland. The two largest islands in Mälaren are Selaön (91 km²) and Svartsjölandet (79 km²).
The viking age settlement Birka on the island of Björkö and Hovgården (on the neighbouring island Adelsö) has been a UNESCO World Heritage Site since 1993, as has Drottningholm Palace on the island of Lovön.
origin of the name Mälaren
stems from the Old Norse
appearing in historical records in the 1320s and meaning gravel
. The lake was previously known as Løgrinn
, which comes from the Norse word lögr
, meaning a lake or liquid. This is etymologically similar to the aged verb löga
, "to bathe", arguably related to lördag
By the end of the last ice age about 11,000 years ago, much of northern Europe and North America was covered by ice sheets
up to 3 km thick. 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
. Initially the rebound was rapid, proceeding at about 7.5 cm/year. This phase lasted for about 2,000 years, and took place as the ice was being unloaded. Once deglaciation was complete, uplift slowed to about 2.5 cm/year, and decreased exponentially after that. Today, typical uplift rates are of the order of 1 cm/year or less, and studies suggest that rebound will continue for about another 10,000 years. The total uplift from the end of deglaciation can be up to 400 m.
As recently as the Viking Age Mälaren was a bay of the Baltic Sea, and seagoing vessels using it were able to sail far into the interior of Sweden. Birka had an auspicious location due to its proximity to the trade routes through Södertälje kanal. Due to the post-glacial rebound, Södertälje kanal and the mouth of Riddarfjärden bay had become so shallow by about 1200 that ships had to unload their cargoes near the entrances, and progressively the bay became a lake. 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. The lake's surface currently averages 0.7 meters above sea level.
According to Norse mythology
, the lake was created by the goddess Gefjun
when she tricked Gylfi
, the Swedish king of Gylfaginning
. The land thusly removed by Gefjun was transported to Denmark
, becoming the island of Zealand
A selection, in alphabetical order:
The most common nesting
birds on the skerries of Mälaren are also the most common in the Baltic Sea. After a survey in 2005, the ten most common species were found to be common tern
, herring gull
, black-headed gull
, common gull
, tufted duck
, Canada Goose
, common goldeneye
, lesser black-backed gull
and common sandpiper
. White-tailed eagle
, Greylag goose
, barnacle goose
, black-throated diver
, red-breasted merganser
are less common, and some of these latter are endangered in the Mälaren area. Since 1994 a subspecies of great cormorant Phalacrocorax carbo sinensis
, nests there as well. The 2005 survey tallied 23 breeding colonies with 2178 nests, of which the largest colony had 235 netsts. Most experts believe the great cormorant population has peaked and will stabilize at around 2000 nests.
One of the characteristic species is the osprey which has one of its strongest presences in lake Mälaren. The osprey nests in almost all bays of the lake.
The Zebra mussel is considered an invasive species and is causing some problems in lake Mälaren.