However, in many California lakes, water temperatures will not drop this low. As water above this layer continues to cool, the lake may begin to freeze, leaving a remaining layer of ice on the surface, because ice is less dense than water. Once the surface water temperature drops to 4℃, the temperature at which fresh water is most dense, the top layer sinks to the bottom of the lake. In lakes where temperatures drop below freezing, the separation of layers is profound, with warmer water near the bottom and colder water on top. In winter, the layers may separate again. In autumn, when cool weather lowers the temperature of the epilimnion, the thermocline begins to break down, and the epilimnion and the hypolimnion layers mix, a process that is often assisted by increasing winds and rain. This process of lake separation, known as stratification, typically only occurs in deep lakes greater than 20 ft deep and over the course of the summer months. The thermocline, a thin transition layer of water, separates the epilimnion and hypolimnion and prevents the mixing of these two layers. Cold water is dense and sinks to the bottom, forming a layer known as the hypolimnion. The foul, rotten-egg odor that many notice near lakes, reservoirs, and rivers during the fall is the result of a natural cycle of decay, a process dubbed “lake turnover.” In the summer, less dense warm water creates a surface layer referred to as the epilimnion. Water and air get colder, and sometimes smelly – and not only from decaying salmon carcasses.

As fall sets in and the leaves begin to change, so does the water chemistry in local lakes and reservoirs.

Lake turnover oxygen