Lake Slapy is a multipurpose reservoir built on the Vltava River about 40 km south of Praha, the capital of Czechoslovakia. It represents the third major step of the Vltava Cascade of Reservoirs (Lipno, Orlik and Slapy) built primarily for power generation. Orlik Reservoir (volume 0.722 km3, 75 km long, 70 m deep, completed in 1960) with its re-regulation step (volume 0.013 km3) is located immediately upstream of Slapy. After its completion in 1954, the Slapy area became recreational and, because of the lack of natural lakes in the country, is heavily visited during summer.

The underlying geological formation is a plutonic massif, the bed being formed by granodiorite from the younger Paleozoic with the inclusion of Algonkian paleo-volcanites. The reservoir is riverine in its shape, with an average width of about 300 m and a length of 44.5 km, and is rather through-flowing. Due to the peak power generation at both Slapy and Orlik reservoirs, the water level fluctuates daily in addition to the seasonal variation with winter minima. Characteristic is the brown water color resulting from the paper mill industry in the upper reaches of the Vltava (some 200 km upstream from Slapy) and also from natural fulvic acids supplied by bogs in the Black Forest Mountains. Other sources of pollution are municipal sewage containing wastewater from food industry and agricultural non.point runoff.

During its history the reservoir water quality has changed several times: due to "reservoir aging" in 1954-58 when heavy water blooms of blue-greens were observed, in 1960 due to construction of Orlik Reservoir immediately upstream, and in 1966 due to stopping the operation of old paper mills in the upper reaches which reduced the load of resistant organic matter and decreased the brown water colour. Also, the use of motor boats and houseboats has been prohibited since 1970. Most profound effects were due to the construction of the upstream reservoir, which resulted in lowering the load of phosphorus due to its retention by Orlik Reservoir and resultant shifting of the blooms to that reservoir. Also, the stratification structure in Slapy changed: until the construction of Orlik, Slapy were fed by a river with a natural temperature regime, whereas afterwards the inflow is mainly hypolimnic water of Orlik. A continuous water quality trend is related to the development of agriculture in the watershed (about 50% of the watershed is agricultural land). During the last thirty years the amount of fertilizers applied to the drainage area has been gradually increasing and field meliorations were carried on in the upper reaches of the river. This is mainly reflected in the concentration of nitrogen compounds (annual mean values up to 5 mg l-1 N), but also of chlorides and sulphates. Concerning limnological research Slapy Reservoir is, on a long term basis, one of the most intensively studied reservoirs (Q).