Anyone who has taken a summer swim and gone through the warm surface water to feel the icy water a few feet underneath, has felt the most obvious effect of thermal stratification. This stratification is a natural occurrence, in any static body of water. It occurs when the surface layer of water, warmed by the sun, becomes less dense than the water underneath it.
The surface layer remains on top and the lower layer, deprived of surface contact and insulated from the sun, continues to get colder. This increases the difference in density between the two layers and makes it even more difficult for them to mix together. Once strongly established, this stratification persists until falling temperatures in autumn breaks down the density difference between the two layers allowing them to mix together again. In a river, thermal stratification only occurs in the deepest pools if summer flows are insufficient to mix the water in the bottom of the pools. If present it only breaks down when flow increases.
The significance of thermal stratification to anglers is that the lower layer of water, deprived of surface contact, slowly loses its dissolved oxygen and become less able to support aquatic life. In deep lakes and reservoirs, this has the effect of confining coldwater species, like trout, to a narrow zone below the high temperature surface water but above the bottom layer of cold water lacking oxygen. A good echo sounder will sometimes show this prominent layer of fish, with nowhere to go and very little to eat, and the angler who can accurately get a lure or bait into this "fish zone" can be extremely successful.
Sometimes, the fish will be stressed and not interested in feeding. The smart angler will then target warm water species or another body of water until conditions improve. The aerator at Lake Bullen Merri is an example of a technique used to prevent thermal stratification. It works not by directly aerating the water, but by pushing the cold water up to the surface and generating a water current in the lake to break down the barrier between the two layers (the thermocline). The circulation of the water not only increases the volume and depth of water available to the fish, but makes the substrate of the lake accessible to fish and increases food production in the lake.