Lake Thrihyrningsvatn is situated at 585 m above sea level in the central highlands quite far away from inhabited areas.
It has and area of 4,3 km² and its greatest depth is 33 m. Its discharge, river Thrihyrningsa, later river Arnardalsa, tributes to the glacial river Jokulsa á Fjollum. The lake is accessible by car from different directions. It abounds in lake char, which used to be quite large when the lake was netted regularly.
The distance from the capital is about 598 km by the Whale Bay tunnel.