Origin myth

Local mythology asserts that the lake was created by a cow called Urkon or Sommakoa. The lake basin was carved by Urkon with her hooves during an outburst of wrath; subsequently the basin was filled with water which became the lake.^[8] Following this, the wizard Somme from Tullerum locked Urkon in a cave, Urkons Grotta. It was said that she would escape when a crowned king visited the district of Ydre, where the lake lies, and kill him. According to folklore this was the fate of the legendary king Frode. As Frode was passing through Ydre, Urkon escaped and chased him; when he thought he had left Ydre and was thus safe, the cow killed him.^[8] At present there are two commemorative raised stones at Fruhammar, the location of this purported event.^[8]

In Urkons Grotta, the cow is said to rest on a bed of mineral coal and fur. Each Christmas Urkon eats one hair straw from the fur, and when she has eaten all straws it is said that she will come out and bring about the end of the world.^[8]

Danish crossing on the frozen lake

In 1568, amidst the Northern Seven Years' War, a Danish raiding party led by Daniel Rantzau managed to escape back to Denmark by crossing over the frozen lake, avoiding Swedish defences to the west and east. The escape made it possible for the raiding party to burn down Eksjö further south along their retreat.^[9][10]

Age of log driving and barges

During much of the 19th and 20th century the lake was used for log driving. This activity peaked around World War I and declined as the road network around the lake improved.^[11][12] Wood and coal from various charcoal piles near the lakeshore were also transported in large wooden barges. By 1945 fifteen barges were operating in the lake as were the steamboats Boxholm I and Boxholm II that served as tugboats.^[13] At present only one barge survives at the naturum Sommen museum.^[14] In 1973 log driving ceased for good.^[11][12][13]

Sommaskep revival

The lake is known for its unique wooden boats called sommaskep. Sommaskep are for rowing, and differ from ordinary small traditional wooden boats in Sweden for having a longer stem. An explanation for this design is that it makes boats easier to navigate in the typically short-wavelength waves of Sommen.^[15] The tradition of building sommaskep has been kept alive as Gunnar Gustavsson, a local carpenter, was tasked by the local history society of Torpa with building sommaskep in the early 1990s; he made a copy of an antique sommaskep in Malexander.^[15] In recent times sommaskep boats have been built in Malexander (2007, 2016).^[15][16]

Deglaciation

Late Baltic Ice Lake around 10,300 B.C., with a channel near Mount Billingen through the Central Swedish lowland.

Sommen

Following its maximum extent about 20,000–17,000 years ago the Fennoscandian ice sheet began to shrink gradually. The ice sheet's northward retreat brought the ice-front to the area of Sommen about 12,000 years ago.^[28] At this point the ice remained locally stagnant as it melted, with ice persisting longer in valleys than in hills and ridges.^[26] Thus the ice-front during deglaciation is thought to have been highly sinuous.^[26] Stagnation of the ice led to the formation of hummocky moraine and glacifluvial deposits.^[26] With its water surface at 146 metres above sea level, Sommen is slightly above the highest postglacial coastline (Swedish: högsta kustlinjen, HK) at 137 metres. This ancient coastline corresponds to the coastline of the Baltic Ice Lake during the Younger Dryas.^[29] These are near the lake's eastern shores with patterned ground which presumably formed during the periglaciation of the area during the Younger Dryas.^[30] The biogeography of various aquatic species deemed glacial relicts that are found in Sommen is likely related to a different geography during the early history of the lake. One theory claims that aquatic species were transferred from the Baltic Ice Lake through a natural lock system in connection with a temporary advance of the ice-front during the Younger Dryas.^[29] On land, the unusual occurrence of dwarf birch near Sund is also judged to be a leftover from a cold geological past.^[20]

At present the post-glacial rebound of the crust in the lake area is uneven. At the lake's outlet in the northwest it is 2.36mm per year while in the eastern Svanaviken it is 2.05mm. This means the lake is being slowly tilted and the southeastern shores drowned while near the outlet land is being dried up.^[31]

Birds

Large birds that breed in the area include fish hawk, European herring gull, heron and black-throated loon.^[32] Inventories of these four birds were taken in 1974, 1980, 1994 and 2012.^[32] According to this data, populations of European herring gulls and heron appear to have declined.^[32] Other water birds that breed in the area include oystercatcher, common gull, black-headed gull, common tern, common sandpiper, goosander, great crested grebe, mute swan and Canada goose, together with two duck species; mallard and common goldeneye.^[32] Various pairs of peregrine falcon nest in the lake area; this species was almost extinct in Sweden in the 1970s.^[33]

An inventory made in 2018 showed that during summer about 1,000 cormorants live in the lake area. As cormorants feed on fish, their population is thought to impact on the amount of fish, possibly producing an ecological disturbance.^[34] Hunting of cormorants is allowed under certain conditions from 2019 to 2023.^[35]

Fish

Twenty-two species of fish are found in Sommen.^[36] Common fish in the lake are Eurasian perch, northern pike, brown trout, common whitefish, Sommen charr, smelt, burbot, European eel, ruffe and vendace.^[36] Less common fishes include common roach, common bream, tench, vimba and common bleak.^[36]

Crustaceans

The two large crustaceans of the lake, the signal crayfish and the noble crayfish, are not as abundant as fish.^[36] The signal crayfish is an introduced species. The lake stands out for having three crustacean species that are relicts from the time of the Fennoscandian Ice Sheet deglaciation about 12,000 years ago.^[28][29] These species are Pallasea quadrispinosa, Mysis relicta and Limnocalanus macrurus. However, the lake lacks the most common relict crustacean found in the lakes of southern Sweden, the Mysis relicta.^[29]