The reservoir was created from the former sea bay, two rivers flow into it: Dzierżęcinka and Unieść. The average depth of the lake is 1.4 m, the maximum depth is 3.9 m. Jamno is the 9th largest lake in Poland in terms of area, it is connected with the Baltic Sea through a channel called Jamieński Nurt. There are marinas and water equipment rentals at the lake. During the summer holidays, water trams run on Jamno, they take you from Koszalin to Mielno and the other way around. The lake is an ideal place for lovers of water sports and fishing.