Mjörn is a lake in

Västergötland Västergötland (), also known as West Gothland or the Latinized version Westrogothia in older literature, is one of the 25 traditional non-administrative provinces of Sweden (''landskap'' in Swedish), situated in the southwest of Sweden. Vä ...

, shared between the municipalities of

Alingsås Alingsås () is a urban areas of Sweden, locality and the seat of Alingsås Municipality in Västra Götaland County, Sweden. It had 24,482 inhabitants in 2010. Geography Geographically the city is situated by the outlet of the small rivulet Säv ...

and

Lerum Lerum is a Urban areas in Sweden, locality and the seat of Lerum Municipality in Västra Götaland County, Sweden. It had 43 570 inhabitants in 2024. Overview Lerum has a station on the Gothenburg commuter rail system and is a suburb of Gothenburg ...

and is part of the main catchment area of the Göta River. The lake is 48 meters deep, covers an area of 54.5 square kilometers and has an elevation of 57.1 meters above sea level.

About the lake

Mjörn is 55 km² in size and has a maximum depth of 48 meters. It is 16 km long and 7 km wide. This makes it the second largest lake in Västergötland that lies entirely within the landscape (after Unden). The lake is 58 m above sea level. There are around 60 named islands in Mjörn.

and Västra Bodarna lie on the lake's eastern shore and

Sjövik Sjövik is a locality situated in Lerum Municipality, Västra Götaland County, Sweden Sweden, formally the Kingdom of Sweden, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. It borders Norw ...

and Björboholm on the western shore. The lake is not fully regulated, so the water level can vary quite considerably. Its ice and winds are also quite unpredictable. Mjörn is drained by Säveån. Its tributaries include Lake Anten and Lake Ålandasjön in the north and Säveån through Alingsås. Its outflow is mainly in Solveden as well as the power plant in Norsesund. It then continues in the lake Sävelången, Säveån through Lerum and Partille municipalities and into the Göta River in Gothenburg. There are several valuable biological, geological and cultural-historical areas around Mjörn.

Fishes

Eighteen different species have been identified so far. Among these are more unusual species such as

alpine bullhead The alpine bullhead or Siberian bullhead (''Cottus poecilopus'') is a species of freshwater fish in the family Cottidae of sculpins. It is found in China, the Czech Republic, Denmark, Finland, Germany, North Korea, Moldova, Norway, Poland, Romani ...

lamprey Lampreys (sometimes inaccurately called lamprey eels) are a group of Agnatha, jawless fish comprising the order (biology), order Petromyzontiformes , sole order in the Class (biology), class Petromyzontida. The adult lamprey is characterize ...

and the special Mjörn trout. The others are

northern pike The northern pike (''Esox lucius'') is a species of carnivorous fish of the genus ''Esox'' (pikes). They are commonly found in brackish water, moderately salty and fresh waters of the Northern Hemisphere (''i.e.'' holarctic in distribution). T ...

perch Perch is a common name for freshwater fish from the genus ''Perca'', which belongs to the family Percidae of the large order Perciformes. The name comes from , meaning the type species of this genus, the European perch (''P. fluviatilis'') ...

zander The zander (''Sander lucioperca''), sander or pikeperch, is a species of ray-finned fish from the Family (biology), family Percidae, which also includes perch, Gymnocephalus, ruffe and Darter (fish), darter. It is found in freshwater and brackis ...

eel Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 20 families, 164 genera, and about 1000 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...

bream Bream (, ) are species of freshwater fish belonging to a variety of genera including '' Abramis'' (e.g., ''A. brama'', the common bream), '' Ballerus'', '' Blicca'', '' Brama'', '' Chilotilapia'', '' Etelis'', '' Lepomis'', '' Gymnocranius'', ...

, roach, ide,

tench The tench or doctor fish (''Tinca tinca'') is a freshwater, fresh- and brackish water, brackish-water fish of the order Cypriniformes found throughout Eurasia from Western Europe including Great Britain, Britain and Ireland east into Asia as far ...

burbot The burbot (''Lota lota''), also known as bubbot, mariah, loche, cusk, freshwater cod, freshwater ling, freshwater cusk, the lawyer, coney-fish, lingcod, or eelpout, is a species of coldwater ray-finned fish native to the subarctic regions of ...

, ninespine stickleblack,

ruffe The ruffe (''Gymnocephalus cernua''), also known as the Eurasian ruffe or pope, is a freshwater fish found in temperate regions of Europe and northern Asia. It has been introduced into the Great Lakes of North America as an invasive species ...

, vendace and smelt. A local rumor in the Alingså area claims that the lake also hosts the

wels catfish The wels catfish ( or ; ''Silurus glanis''), also called sheatfish or just wels, is a large species of catfish native to wide areas of central, southern, and eastern Europe, in the basins of the Baltic, Black and Caspian Seas. It has been intro ...

, but this has never been confirmed. A

fishing license A fishing license (American English, US), fishing licence (British English, UK), or fishing permit is an administrative or legal mechanism employed by state government, state and local governments to regulate fishing activities within their ad ...

is required. In test fishing, perch, bream, northern pike, perch, zander, burbot, roach, ruffe, vendace and tench have been caught in the lake.

Bird life

Mjörn also offers a rich bird life. There are purely maritime species here that do not normally belong in an inland environment, such as

great black-backed gull The great black-backed gull (''Larus marinus'') is the largest member of the gull family. It is a very aggressive hunter, pirate, and scavenger which breeds on the coasts and islands of the North Atlantic in northern Europe and northeastern Nort ...

eurasian oystercatcher The Eurasian oystercatcher (''Haematopus ostralegus'') also known as the common pied oystercatcher, or (in Europe) just oystercatcher, is a wader in the oystercatcher bird family Haematopodidae. It has striking black and white plumage, a long st ...

and

common eider The common eider (pronounced ) (''Somateria mollissima''), also called St. Cuthbert's duck or Cuddy's duck, is a large ( in body length) sea-duck that is distributed over the northern coasts of Europe, North America and eastern Siberia. It breed ...

. During migration times, countless birds pass over Mjörn. Examples of breeding seabirds are the black-throated loon,

osprey The osprey (; ''Pandion haliaetus''), historically known as sea hawk, river hawk, and fish hawk, is a diurnal, fish-eating bird of prey with a cosmopolitan range. It is a large raptor, reaching more than in length and a wingspan of . It ...

little ringed plover The little ringed plover (''Charadrius dubius'') is a small plover. The genus name ''Charadrius'' is a Late Latin word for a yellowish bird mentioned in the fourth-century Vulgate. It derives from Ancient Greek ''kharadrios'' a bird found in river ...

red-breasted merganser The red-breasted merganser (''Mergus serrator'') is a duck species that is native to much of the Northern Hemisphere. The red breast that gives the species its common name is only displayed by males in breeding plumage. Individuals fly rapidly ...

and great black-backed gull.

Bird reserve A bird reserve (also called ornithological reserve) is a wildlife refuge designed to protect bird species. Like other wildlife refuges, the main goal of a reserve is to prevent species from becoming endangered or extinct. Typically, bird species in ...

s have been established in Mjörn.

Islands in Mjörn

All the islands in Mjörn are areas with high nature values. Several small islands are bird protection areas. As late as the 1950s, agriculture was practiced on four of the lake's largest islands. There are now two permanent homes, one on Bokö and one on Kärleken.

Shipping

In 1987, the non-profit association "Mjörns Sjökortsförening" was formed with the aim of creating reliable nautical charts of Mjörn and to manage the marking of a fairway across the lake. Since then, the association has built five new lighthouses on the lake, in addition to the existing "Alingsås angöring" lighthouse, and published seven editions of nautical charts.

Sub-catchment area

Mjörn is part of the sub-basin (642512-129848) that SMHI calls the outlet of Mjörn. The average height is 81 meters above sea level and the area is 155.69 square kilometers. If the 46 upstream catchment areas are included, the accumulated area is 1.115.33 square kilometers. The Säveån river that drains the catchment area has tributary order 2, which means that the water flows through a total of 2 watercourses before reaching the sea after 46 kilometers. The catchment area consists mostly of forest (41%). It also has 55.73 square kilometers of water surface, giving it a lake percentage of 35.8%. Settlements in the area cover an area of 8.65 square kilometers or 6% of the catchment area.

Areas of application

Mjörn is popular as a swimming, boating and fishing lake. Alingsås Segelsällskap, Sjöscoutkår and Mjörns Motorbåtsällskap are based on the lake. Mjörns Ångbåtsförening operates the steam tug S/S Herbert, built in 1905.

Recreational fishing Recreational fishing, also called sport fishing or game fishing, is fishing for leisure, exercise or competition. It can be contrasted with commercial fishing, which is occupational fishing activities done for profit; or subsistence fishing, ...

is controlled by Alingsås Sportfiskeförening. The Mjörn Runt cycle race is organized annually, starting and finishing in Alingsås. Other associations with links to Mjörn include Föreningen Mjörn, Hjällnäs Båt- och Bryggförening, Mjörns Fiskevårdsområdesförening, Föreningen Mjörnbygden and Björboholms Segelsällskap. The attractiveness of the environment around the lake is shown by the increased demand for plots and large house constructions in

. The attraction is the "magnificent view of Lake Mjörn". Also worth mentioning are the old historic large farms of Bryngenäs, Öijared, Östad and Vikaryd. In connection with the advent of the railroad, typical summer homes were built. The villages of Björboholm, Västra Bodarne and Skår, Lövekulle with its large holiday villas, owned by wealthy merchants mostly from Gothenburg, developed alongside both the Western Main Line, whose first sod was turned in Lövekulle outside Alingsås, and the West Gothenburg Line. Several of them still exist along the shores of Lake Mjörn.

The name

The lake's oldest name is Mior, and it is already mentioned in Västgötalagen (1350) as a public lake. According to the county administrative board, Mjörn has also been classified as a lake with "particularly high nature value", meaning the highest possible classification.

History of the lake

There are many ancient remains around the lake and on the islands, and ancient finds have been found on the shores and islands. Several excavations have been made on Risön and objects from the

Stone Age The Stone Age was a broad prehistory, prehistoric period during which Rock (geology), stone was widely used to make stone tools with an edge, a point, or a percussion surface. The period lasted for roughly 3.4 million years and ended b ...

(3000-1800 BC) have been found. At the Brobacka Nature Reserve on the northern shore of Mjörn, Scandinavia's largest giant's kettle field can be found, with the largest one measuring approximately {{conv, 18, m, ft in diameter. It serves as a reminder of the time when the inland ice covered the area. The powerful flow of meltwater through the narrow passage in Brobacka carried large stones, shaping the enormous kettles. Mjörn was once part of the North Sea and remnants from this period can still be found today, including so-called glacial marine relics, caridean shrimp that have adapted to

fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salt (chemistry), salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include ...

and seashells from sea snails.