In temperate latitudes, many lakes that become stratified during the summer months de-stratify during cooler windier weather with surface mixing by wind being a significant driver in this process. This is often referred to as autumn turn-over.
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The turnover can cause fish kills in some areas, but it is more common on smaller lakes with little moving water or in backwater creek areas where fish can't escape the water that is lacking oxygen.
All lakes, even the largest, slowly disappear as their basins fill with sediment and plant material. The natural aging of a lake happens very slowly, over the course of hundreds and even thousands of years. But with human influence, it can take only decades. A lake's plants and algae slowly die.
Because warm water sinks in very cold freshwater, fish in these water bodies often gather in groups near the bottom. Some species, like koi and gobies, may burrow into soft sediments and go dormant like frogs and other amphibians, but most fish simply school in the deepest pools and take a winter rest.