Kortowskie Lake is the second largest reservoir in Olsztyn (about 90ha). It is situated next to the university campus Kortowo and next to the estate of the detached houses of the Sunny Slope (Słoneczny Stok) and Dajtki districts.

The lake is supplied by the forest creeks (Leśny, Parkowy, Starodworski), and especially by the Kortówka stream, which comes from Ukiel Lake. After the mouth of Kortowskie Lake, it flows into the Łyna River.
 
The coastline of the lake is 4.8km and its west side is forested. The numerous paths in the forest allow leisure activities such as walking and cycling. In the summer, it is best to swim and sunbath on the south side with its separated lido, which is located on the premises of the University of Warmia and Mazury (UWM). Close to the beach there is also a marina with water sports equipment for rent, a restaurant and a pier – students’ favourite spot for evening meetings.

Kortowskie Lake as the world’s first was subjected to the experimental method of reclamation (commonly known as the Kortowski Experiment). Thanks to the cyclic application of this procedure, from 1956 until today, the waters of the reservoir have been largely cleaned at
a relatively low cost.