What causes stratification in lakes and reservoirs?

Stratification in lakes and reservoirs is primarily caused by the formation of separate layers based on differences in temperature and density of water. As sunlight heats the surface layer of water, it becomes less dense compared to the cooler layers beneath it. This creates distinct thermal layers, typically comprising an upper warm layer (epilimnion), a middle cold layer (metalimnion or thermocline), and a lower cold layer (hypolimnion). The stratification process is essential in understanding the water chemistry and biology of these aquatic systems, as it affects the distribution of oxygen and nutrients.

While options such as uniform temperature throughout the body or mixing of water and air could potentially lead to more homogenous conditions, they are not directly related to the formation of distinct layers in stratification. Human intervention can influence stratification through practices like damming or water withdrawal, but it is not the primary natural cause. Therefore, the correct answer reflects the fundamental natural processes that establish the layers within these bodies of water.