Sweden’s third largest lake, born in the 12th century AD

Lake Mälaren (Mälaren in Swedish) is quite a curious lake, situated immediately west of Stockholm. “West of” is strictly speaking a misnomer –- the Western half of the glittering Stockholm city waterways are actually nothing but branches of the many-fingered Lake Mälaren itself (the Eastern half is comprised of a series of bays and coves of the Baltic Sea). The lake is middle-sized, with an area of 1140 square kilometers. That makes Mälaren the third largest lake in Sweden. Still, this would be far from impressive, if the water surface alone would be the criterion.

System of natural waterways

What makes Lake Mälaren interesting and important is that it consists almost entirely of narrow arms and even narrower fingers of fresh water, stretching deep into the country –- some 150 kilometers westward from Stockholm and considerable distances toward northwest and southwest as well. The lake is in fact a huge natural network of channels and waterways and is consequently studded with a whole string of sizeable towns along it’s shores -– Södertälje, Mariefred, Strängnäs, Kungsör, Köping, Västerås, Enköping, Sigtuna, and Uppsala (this oldest of Swedish university towns is connected to Lake Mälaren by a short stretch of river). Some of these are important industrial centers –- Västerås (ABB), Köping (Volvo), Södertälje (Scania, AstraZeneca).

The whole region is frequently called the “Mälar valley” (“Mälaren” means “The Mälar”, i.e. it is the definite form of “Mälar”) and has, thanks to the easy communications along the Mälaren waterways, been a densely populated (well, according to sparse Fennoscandian standards), busy area since the times of the Vikings. Lately a new high-speed railway and good motorways have contributed to knitting the Mälar valley towns together with each other and with Stockholm into an important economic unit.

In the Middle Ages, a lake is born

However, Lake Mälaren is a lot more peculiar than just being an elaborate system of natural waterways. One surprising fact is that Sweden’s third largest lake didn’t even exist until the 12th century! Before that time this maritime area was a system of salt-water bays and coves, constituting parts of the Baltic Sea. Here the effect of the inland ice comes in. During the Ice Age the Mälar valley was weighted down by a huge coat of ice, kilometers in height. When the ice melted, then the land begun to rebound at a slow but steady rate, around 0.4 cm per year. Finally, in the 12th century, the Mälar valley had risen so much, that water started flowing out of the system – the water level had become higher than of the sea outside. Rivers replenished the salt water with fresh water and the system was soon washed out of salt. Lake Mälaren was born.

A bottleneck and two corks make a capital

Today the water level of Lake Mälaren is about 70 cm higher than the Baltic Sea level. The remarkable thing is that this fair-sized body of water only has one narrow natural outlet of water. This outlet could be described as two loosely fitting corks in a bottleneck -– a small island, accompanied by a minuscule islet. The small island -– the main cork -– is none other than the original City of Stockholm, today called the Old Town (Gamla Stan). The auxiliary cork is an islet so small that it only has one house –- the building of the Swedish Parliament. Around these two islands flows the entire river-supplied excess water of Lake Mälaren. In springtime the flow grows to spectacular foaming torrents, admired by townspeople standing on the quays of busy central Stockholm. Stockholm actually sits astride the fresh waters of Lake Mälaren and the salt waters (well, actually brackish waters, with a salt content of merely 0.7%) of the Baltic Sea. So if you are lost in the city, you can taste the water of a nearby channel to get your bearings -– sweet is West, salt is East.

Viking legend

According to legend, the city name “Stockholm” ( “stock” = log, “holm” = small island, hence “Stockholm” = Log Island) derives from a group of Vikings who wished to found a new capital. The previous capital, Birka, was built on an island in the Lake Mälaren and had to be abandoned. So the Vikings dragged a log into the lake and decided to build a new capital where the log happened to drift ashore. And that was naturally the Stockholm Old Town island, in view of Lake Mälaren flow patterns.

Environmental success

Lake Mälaren is also a rare environmental success story. In the middle of the 20th century the many towns around Lake Mälaren let their sewage flow into the lake untreated. The water around Stockholm was heavily polluted. An elderly gentleman told me that when he was a child, his nanny had warned him against poking his finger into the water. He disobeyed and promptly got a seriously infected finger, almost killing him with blood poisoning.

Consequently the Mälar valley towns and the city of Stockholm started a cooperative effort, building a large number of sewage treatment plants. In little more than ten years the water around Stockholm had become so clean, that you can now enjoy swimming in the middle of the city (a yearly Stockholm swimming marathon is arranged in the Lake Mälaren part of town), drink the water and catch salmon in the downtown of a bustling European city.

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