Struga Siedmiu Jezior (the Seven Lakes Stream) constitutes one of the biggest hydrographic units of the “Bory Tucholskie” National Park. It is a small river (13.9 km), which transfers its waters into the drainage Charzykowskie Lake, thus, as the tributary of the Brda River, it is a Tertiary watercourse in the Vistula riverbed. The area of the Struga River catchment almost in whole corresponds to the area of the Park – its area on the territory of the Park covers 32.6 km2, which constitutes 85% of the total area of the catchment. The remaining 15% is covered by areas located on the territory of the Forestry Management (east and south fragment of the catchment).
A dominating element of water eco-systems refers to lakes, which in the number of 21 represent all the genetic and morphologic types characteristic of lowland areas of Poland. Within close vicinity one may encounter polyhumic, dystrophic, lobelia, mesotrophic and eutrophic lakes.
The lakes of the Park are inhabited by populations of fish characteristic of lowland areas of Poland. In compliance with fishing typology those reservoirs may be classified as four fishing types, namely, the whitefish lakes, the bream lakes, the tench and the pike lakes as well as the crucian lakes. 28 species of fish have been reported in water eco-systems of the Park. Two species are subjected to species protection. They are the bitterling (Rhodeus sericeus amarus) and the spine loach (Cobitis taenia).
Apart from indigenous species, as a result of fish stocking conducted before the Park was established, foreign species appeared such as the silver carp, the big head carp and the common carp.
Constant monitoring of surface and ground waters as well as physical and chemical features of lakes is conducted in order to establish regularities governing water circulation on the territory of the Park and to identify long-lasting changes in physical and chemical conditions in water reservoirs.All the lakes existing in the Park represent habitats of NATURA 2000.