Precipitation is the principal cause of lake level fluctuation. However, the lag between precipitation and lake level change varies from days to years, depending on the lake. Dams can modify some fluctuations, but varying lake levels are normal. Water is most dense at 39 degrees Fahrenheit. Above and below that temperature, water expands and becomes less dense.
Many lakes stratify in winter because ice covers the lake surface.
Lakes in areas with milder winters do not stratify during the winter. Plant material either decays or sinks to the bottom and accumulates in this stagnant layer. During the fall turnover, surface waters cool until they are as dense as the bottom waters and wind action mixes the lake so that water temperature from surface to bottom is the same. The presence of oxygen in lake water determines where organisms such as fish and zooplankton are found. In spring, when the lake water is well mixed, oxygen is usually present at all depths and organisms may be distributed throughout the lake.