In deep lakes such as Lake Biwa, the water temperature tends to fall into two layers: an upper layer with water temperatures of up to 20°C, and a lower layer with temperatures of 10°C or cooler. Because of the great difference in density between these two layers, little vertical mixture occurs between them. Between the upper and lower layes lies a boundary layer called the thermocline, where the temperature can change between 3°C and 10°C within the space of 1m. In the vertical distribution of temperatures in Lake Biwa shown in the figure below, the thermocline is found at a depth of about 20m.

In Lake Biwa, the upper water layer begins to cool in the autumn, causing it to grow dense and sink. This sinking generates a convection current in which the entire body of water becomes mixed. In the spring the upper layer warms again, separating the lake into two layers once more. In the winter, when the temperature in the upper layer hovers around 0°C, the deep layer has a temperature in the vicinity of 4°C, so that the lake is completely circulated twice a year.
In tropical regions, strong sunlight ensures that the upper layer of lakes is kept warm throughout the year while the lower layer stays cool, effectively blocking circulation. In reservoirs created by human-made dams, methods of adjustment of water intake and water level and the proportion of total inflow and total outflow are critical to the level of circulation in the reservoir. In reservoirs where water throughput is low relative to the volume of water stored, the water temperature distribution is close to that of natural lakes, but where throughput is higher they tend to resemble the water temperature variation profile of rivers. This eliminates the separation of upper and lower waters, so that no vertical distribution is observed even in the summer.

In this way the measurement of the vertical distribution of water temperatures offers a clearer understanding of water circulation and the spread of pollution.