What is turnover in a reservoir?

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Turnover is the vertical mixing of the water column that happens when the layered, or stratified, conditions in a reservoir break down. In many reservoirs, the upper layer is warmer and lighter while the deeper layer is cooler and denser, creating distinct layers that resist mixing. When seasonal changes (like spring warming and wind-driven mixing or fall cooling) reduce the density difference, the layers begin to mix, distributing oxygen and nutrients throughout the water body and often leading to a more uniform temperature profile.

This process is important for water quality because it oxygenates deeper waters and redistributes nutrients, but it can also bring contaminants or low-oxygen bottom water to the surface. It differs from simply the rate water leaves the reservoir or the practice of removing algae by dosing, and it’s not the residence time—the time water spends in the reservoir. Turnover specifically describes the mixing event when stratification breaks down.