What is the most significant reason for turnover in a surface water reservoir?

The most significant reason for turnover in a surface water reservoir is a change in surface water temperature and density. Turnover occurs in water bodies when the temperature differences in layers of water cause denser, cooler water to sink and warmer water to rise, which typically happens seasonally. This process is essential for maintaining oxygen levels in the water and distributing nutrients throughout the reservoir.

As the water at the surface warms during the summer, it becomes less dense than the cooler water below. When the water temperature drops in the fall, the denser water sinks, facilitating a turnover. This phenomenon helps to replenish oxygen in deeper waters and can significantly impact aquatic ecosystems by redistributing nutrients and removing waste products.

While increased evaporation rates, pollution runoff, and seasonal weather changes can influence water quality and quantity in a reservoir, they do not directly facilitate the physical mixing and distribution of water layers as temperature and density changes do during turnover events. Thus, the pivotal role of temperature and density in promoting turnover makes this answer the most relevant to understanding the dynamics within surface water reservoirs.