Trehörningen (Sjödalen)

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Trehörningen, also called The Triangle, is a small lake in Huddinge, south of Stockholm. It is part of the Tyresån lake system and gets water from Lake Gömmaren via the streams Solfagradiket and Fullerstaån, then flows into Lake Ågestasjön. The surrounding area, including Orlången and Ågestasjön, shows a long history from prehistoric times to the medieval era. Because this area largely avoided 20th-century suburban development, it is considered to have national cultural value. The landscape mixes open fields, rocky banks, and forests, and it features ancient remains such as grave fields and hillforts.

Today, three-fourths of the catchment area is used for settlements, including central Huddinge, but the lake borders the Orlången Nature Reserve and has cliffs popular for swimming and fishing. Water reaches the lake through open ditches, while motor boats are not allowed and fishing is restricted to protect the environment. In the mid-20th century the lake received wastewater, which caused oxygen shortages and algae blooms. Dredging and other cleanup work in the 1970s helped improve conditions, though some legacy effects remain.

Trehörningen supports a rich bird life, including mallards, coots, grebes, goldeneyes, gulls, herons, mergansers, and osprey, with warblers and other species visiting regularly. Occasional sights include kingfishers and black-throated divers. Forests nearby host long-tailed tits and lesser spotted woodpeckers. Aquatic plants such as reed, club-rush, duckweed, and chickenwort grow in and around the lake. About 30 plant and tree species line the shore, including alder, birch, aspen, spruce, and various wildflowers. Frogs, toads, and bats (northern bat and Daubenton’s bat) are found here. In 2014, observers even noted two dragonflies: a small red one and a large black one.