In the winter the lake water remains at a fairly consistent temperature from top to bottom. The temperature range is very small, but important. The less dense water freezes on the surface at 0°C (32°F), forming ice. The water actually becomes warmer nearer the lake bottom. In a deep lake like Lake George, the bottom water temperature is 4°C, the densest water.
During spring turnover, several forces are at work, mixing the entire water column. The sun, wind, currents, tributaries and groundwater all join together to mix the huge volume of water. As in fall turnover, nutrients are again mixed throughout the water column. This cycle repeats every year. Without this mixing, a lake can become stagnant, causing water quality to decline.
Thermal layering occurs during the summer months. The warm June sun heats the top layer of water to temperatures of 21°C to 27°C (70°F to 80°F).
Thermal layers, layers of temperature in the Lake, support various animal and plant species and contribute to the ecosystem as a whole. Trout and salmon need cold-water temperatures in order to survive. Bass, perch, and sunfish can live in much warmer waters and need the support of plants and rocky shores for cover and food supply.
Diving into the deeper parts of the Lake during the summer can be a chilling reminder that the warm summer rays of the sun fail to reach the bottom. The cold, dense water remains separated from the warm upper water by a barrier. This barrier, called the thermocline, is where temperatures change rapidly between the warm surface water and the colder deep water. In the summer on Lake George, the thermocline is around 10 meters.
Credits: Text adapted from New Hampshire DES: Interactive Lake Ecology