Have you ever gone to a lake or reservoir in the fall to find that it smells like sulfur, has a unusual color, and decomposing vegetation in the water? What you are witnessing is a lake or reservoir turnover. Lakes and reservoirs in Ohio usually have two turnover periods, one in the fall and another in the spring. Depending upon the lake or reservoir, it is also possible for some waters to experience additional turnovers between these periods if conditions are just right. Turnover is important because it results in mixing the water, allowing oxygen to occur at all depths and releases unwanted gasses that accumulate on the bottom during some times of the year. What causes a lake to turnover?