Stratification in Reservoirs and Lakes By: Mahmud Achmad Factor Causing Stratification Stratification is a process of generating thermal layers in a reservoir or a lake. When the summer starts, the water temperatures at upper layer become warmer while the bottom layer is colder. This leads to increase the differences of the temperatures between the upper layer and bottom layer (Weitzal 2002). From a geological standpoint, according to Chapman (1996 p.320), “lakes are ephemeral. They originate as a product of geological processes and terminate as a result of the loss of the ponding mechanism, by evaporation caused by changes in the hydrological balance, or by infilling caused by sedimentation”. Hutchinson (1957) in Chapman (1996) classified lakes into 11 mayor types and 76 sub-types. Some of these mayor types are glacial, tectonic, fluvial, shoreline, dammed, volcanic, and solution lakes. Reservoirs are defined as water bodies which formed by embankments or dams to supply water for developing human activities and needs. Characteristics of reservoirs experience similar to lakes, however, reservoirs are mainly controlled by the inflow- outflow process (Chapman 1996). Wetzel (2001) states that lakes and reservoirs can gain heat through a number of ways: direct absorption of solar radiation as a dominant source, transfer heat from the air and sediment to the water, condensation of water vapour at the water surface, and heat transfer from terrestrial sources via precipitation, surface runoff as well as ground water inputs. As a consequence, lakes generate thermal stratification that defined as changes in the temperature profile with depth within a lake system (Chapman 1996). Based on the temperature vertical zone in lakes as described in Figure 1, there are three main strata during the period of stratification (Wetzel 2001): - Epilimnion layer is a surface water that usually warm. The temperature of the layer is influenced by wind and wave circulation. - Hypolimnion layer is the bottom layer that usually much colder and stable.