Lake Driyviaty belongs to the River Druika system (a left tributary of the River Zapadnaja Dvina in the Baltic Sea basin). The lake was originated by the glacial action in the last ice age (about 12,000 years ago). There are eleven small rivers and springs flowing into the lake and only one outlet, the River Druika. Improvement works carried out in 1930 resulted in the lowering of the lake's normal water level by 3 m and the present level fluctuates by 0.3 1 m in a year. The exchange time of the lake water is 2.6 years.

Lake shores are sandy, and the deepest part of lake bottom is covered by siliceous organic ooze. Thermal stratification does not exist during the ice- free period, when dissolved oxygen is also homogeneously distributed in the entire water column. The annual range of water transparency is 3 4.5 m. The total solid content of lake water is 240 320 mg l-1.

There are 50 species of aquatic macrophytes that cover 17.4% of the lake surface. Hydrilla verticillata, Najas minor and N. flexilis are among the rare species. The species number of planktonic algae amounts to 169, and its total biomass up to 7 g m-3, of which more than 50% are accounted for by blue- green algae. The species number and maximum biomass of zooplankton are 103 and 2 4 g m-3. A marine relict species of amphipod, Pallasea quadrispinosa, that survived from the glacial period was discovered in the lake in 1964, and has been recorded in the Red Data Book of Belarus.

Zoobenthos fauna consists of 75 species and has a total biomass of about 10 g m-2. The fish community is represented by about 20 species; the most important species are Abramis brama, Lucioperca lucioperca and Perca fluviatilis. The nesting of the swan Cygnus olor, also a Red Data Book species, in this lake is remarkable.

Picturesque landscape and good water quality areas of the lake are favorable for people's recreation. The town of Braslav is located on its northern shore (1).