The Slapy reservoir is a part of the so called Vltava Cascade. The dam was constructed at the 91.5 km of the Vltava River in years 1949–1955 and the reservoir impounded in 1955. The dam is 60 m high and 260 m long. Reservoir volume is 269×106 m3. Just below the Slapy dam, there is the end of impoundment of the lower small Štěchovice reservoir (built in 1938–1944). Only several small brooks enter the impoundment of the Slapy reservoir, the mean yearly flow through the Slapy reservoir is 85.2 m3 s-1 and the mean hydraulic retention time is 36 days. Until 1961, another large Orlík reservoir was built above the Slapy reservoir, a large Lipno reservoir was constructed in the upper part of the Vltava River, and several small reservoirs later on. The total catchment area of the Slapy reservoir is 12,968 km2.
The Slapy reservoir is a canyon shaped water body mostly with steep banks. An electric power station, working in peak hours (morning and evening), is located in the dam. As a consequence, periodic seiches occurred, water level fluctuates, and a littoral zone with aquatic plants is not developed. The reservoir is intensely used for recreation and locally as a drinking water source.
In 1959, regular investigations in the lower "lacustrine" part of the reservoir (10 km above the dam) were organized by Assoc. Prof. Jaroslav Hrbáček – the head of the hydrobiological department in the Biological Institute of Czechoslovak Academy of Sciences in Prague. The investigations have continued up to now at regular 3-week intervals, recently organized by the Institute of Hydrobiology (Biology Centre AS CR) in České Budějovice. Since 1996, the Slapy reservoir has been an LTER site. Regular investigations on physical and chemical parameters as well as on pelagic organisms (bacteria, protists, phytoplankton and zooplankton) in the reservoir are carried on. Long-term research in the reservoir already yielded valuable results explaining effects of population and land use changes in the catchment, such as construction of the upper reservoir, changes in fertilizers application, new sewage treatment plants etc.