Bierzwnik Lake lies in the Dobiegniew Lake District in the Choszczno County, Bierzwnik Commune. It is situated around 2 km to the south west of a village with the same name.
Bierzwnik Lake is a large ribbon lake located in the woods. It covers an area of 202.93 ha and it is 5.815 km long and 0.9 km wide. It is not very deep – its average depth is 5.4 m, however, in some places the recorded depth reached 12.4 m.
The shoreline is 13 km long and one should note that the shore is very varied – the lake is characterized by an irregular shape – there are numerous bays and promontories. The west shore is very steep and it is significantly higher than the water level, which makes the lake even more interesting.
The lake lies in the watershed of the Mięrzecka Struga River. On the lake there is an island overgrown with old trees, a place where many rare bird species live. In 1977 the place was established as a nature reserve called ‘Wyspa na Jeziorze Bierzwnik’ [The Island on Bierzwnik Lake].
In the past there was a pier which ran across the middle of the lake. Today, only the piles are left and the locals call them ‘the burnt bridge’. The lake is very popular among tourists. There are many tourist trails along the shoreline with display boards (e.g. on the nature reserve on the island) along the road. In the village of Ostromęcko there is a camping pitch and a small sandy beach.