As cold weather rolls in from the North, the surfaces of reservoirs and lakes cool, making their water heavy enough to sink and mix with cooler water in the thermocline below. Wind encourages the mixing, and eventually the thermocline narrows, then disappears. As cooled water from the surface sinks to the bottom, debris bubbles up to the surface, accompanied by hydrogen sulfide and other gases produced by disintegrating plants, releasing a musky or sulfurous smell.
Fishing gets difficult under such conditions, but fortunately, Turnover lasts only about a week on any one lake. Once the lake or reservoir has turned over, the fall Coldwater Period begins.