Selective Withdrawal from Density-Stratified Reservoirs

TLDR: In lakes and reservoirs stable density stratification is caused primarily by the temperature variation with depth, and secondarily by a variable concentration of dissolved and suspended solids as mentioned in this paper, which is caused mainly by the surface layers are heated strongly by the sun, while wind and surface evaporation cause mixing in a surface zone, called the epilimnion.

Abstract: In lakes and reservoirs stable density stratification is caused primarily by the temperature variation with depth, and secondarily by a variable concentration of dissolved and suspended solids. In the summer the surface layers are heated strongly by the sun, while wind and surface evaporation cause mixing in a surface zone, called the epilimnion, which is fairly uniform in temperature. Below this is the thermocline through which the temperature decreases rapidly to that of the lower zone, the hypolimnion, which feels negligible effect of the surface heating. During the fall and winter, the epilimnion cools, often to such an extent that the whole water column overturns and becomes fairly well mixed. In the spring the stratification is gradually reestablished. This is briefly the annual regime of lakes with little inflow or outflow. Man-made reservoirs have thermal regimes similar to lakes, but with the added effects if substantial inflows and outflows, which contribute to the heat and mass balance. Incoming river flows may sometimes spread out on the lake surface, if they are less dense than reservoir water, or may plunge to the bottom or to some intermediate level of neutral buoyancy, where an internal density current is formed. The river water density depends on its concentration of dissolved salts and suspended sediments as well as its temperature.