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.
SUMMER: The upper (warmwater) layer may be from 12 to 40 feet thick, while the thermocline may be 2 to 15 feet thick. The lower (coldwater) level usually contains less dissolved oxygen than the upper layer.
POSTSUMMER: The surface of the water radiates heat to the atmosphere at night as water above the thermocline gradually cools. The thermocline remains intact but becomes closer in temperature to the layer above. Oxygen-poor water remains trapped below the thermocline.
IMMINENT TURNOVER: The thermocline shrinks as it approaches the same temperature as the uniform mass of water above.
TURNOVER: The thermocline disintegrates, and water mixes from surface to bottom. The water continues to cool as it circulates, aided by wind. The oxygen level of the water drops for a short time as the oxygen-depleted hypolimnion mixes with the water above.