Biržai is one of the oldest towns in Lithuania. Covering 1476 square kilometers, 26 % of Biržai district consists of forests. Biržai was established at the confluence of the Apaščia and Agluona rivers, near the man-made Lake Širvena. Almost all of the Biržai district area lies in the basin of the district’s largest river, the Nemunelis, which is 191 km long. There are 17 lakes in Biržai district, most of them quite small, and 28 rivers and streams longer than 5 km.

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One of the rivers that flows into Širvėna lake is  the Apaščia river. Its lenght is 88 km. From Širvėna lake the Apaščia  flows its waters into another our district river the  Nemunėlis. It is the border between Lithuania and Latvia. Further Nemunelis merges with the river Mūša and becomes the Lielupe river. Going through the Lielupe we can reach the Baltic sea.

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Another river flowing into Širvėna lake is the Agluona. Its lenght is 21 km.

Lake Širvena was created in 1575 after damning the rivers Apaščia and Agluona at their confluence. It is the oldest artificial body of water in Lithuania. The lake was built to protect the Radvila castle in Biržai.