Tuomiojärvi

The name of Tuomiojärvi appears to contain the word tuomio, meaning 'sentence' or 'judgment'. According to linguist Viljo Nissilä, the name of the lake may refer to local court hearings regarding hunting grounds, as minor conflicts such as theft were often settled by the place where they had happened.^[5]

Tuomiojärvi is located in the Kymijoki main catchment area and is part of its second-order Päijänne basin.^[1] The catchment area of Tuomiojärvi covers an area of 55.4 square kilometres (21.4 sq mi), of which 57.5% is dense forest and 15.9% is sparse forest. The catchment area includes a part of central Jyväskylä from Kortepohja to Mäki-Matti, as well as most of western Palokka. It also extends further west into less urbanized parts of Jyväskylä, such as Ruoke and Saarenmaa.^[4]

The largest inflow of Tuomiojärvi is Syväoja, which discharges into the northern part of the lake near Haukkala. It also has the largest basin of all of the lake's inflows, whose main path begins from lake Kaitajärvi in Saarenmaa, flowing through the Ylä-Tuomiojärvi and Myllyjärvi before discharging into Tuomiojärvi.^[6] Other lakes within the basin include Lummelampi and Ruokepuolinen near Ruoke and Hanhijärvi near Hanhiperä.^[7] Other inflows include Eerolanpuro, which begins from the industrial area of Rautpohja, flowing through the pond Vehkalampi in Kortepohja before discharging into the Eerolanlahti bay in the southern part of the lake.^[6]

Tuomiojärvi itself discharges into Palokkajärvi via the short Löylyjoki river, which flows under the national road 4 in Taulumäki. Both lakes are on the same level.^[2] As the exact level of Palokkajärvi may alternate between 93.50 and 94.80 metres, the Löylyjoki may occasionally flow in the opposite direction.^[6] There is a dam under the road bridge that can be used to regulate the level of Tuomiojärvi without affecting Palokkajärvi's level. This dam is rarely used, the level of both lakes is usually regulated by a dam in the upper part of the Tourujoki, the outflow of Palokkajärvi.^[8]

There are seven islands in the lake: Koivusaari, Lehtisaari, Mäntysaari and an unnamed island in the south, as well as Heinäsaari, Lehtosaari and another unnamed island in the north.^[2] Lehtisaari is owned by the parish of Jyväskylä (Jyväskylän seurakunta), which operates a public sauna on the island.^[9]

The Rautpohja (also called Rautpohjanlahti) and Eerolanlahti bays in the southern part of the lake have been designated as Natura 2000 zones, covering a combined area of 43 hectares. The Rautpohja has largely closed up and is mostly surrounded by a type of marsh, as is the Eerolanlahti to a lesser extent. Birds nesting in the bays include whooper swans and black-headed gulls, other notable animals include moor frogs.^[10]

The black-headed gull colony in the area, mostly nesting in the Eerolanlahti, is the largest in the region of Central Finland.^[11] The colony grew significantly in the 2010s as the Killervä [fi] (also called Killerjärvi), a small lake in the middle of the Killeri horse racing track further west was dredged to remove gull nests. In 2013, there were approximately a thousand pairs nesting in the Eerolanlahti bay, 500 in the Rautpohja and only around 40 in the Killervä.^[12] However, the Eerolanlahti colony has declined in the 2020s, while the Killervä colony has grown somewhat and may now be the largest in Jyväskylä.^[13]

There is also another wetland in the northern part of the lake near Palokka called Vasaraisensuo, which consists of a treeless part in the middle surrounded by swampland dominated by birches and willows. Despite having some drainage ditches dug into it, the wetland has mostly remained in its natural state and the ditches appear to be closing naturally. The Vasaraisensuo is not a true conservation area, though its importance for natural diversity has been noted in zoning plans.^[14]