Several local fishermen recently told me that when Clear Lake turns over the bass fishing will improve. Most fishermen have heard the phase “a lake turning over.” This is especially true during the fall and early winter months, but few actually know what happens when a lake turns over.
Lake turnovers are normally caused by seasonal temperature changes of the lake”s surface water. Turnover usually occurs twice a year, in the spring and then again in the fall. Stratas or layers of water which have different temperatures form in lakes during the warm summer months. While these temperatures can vary by as much as 20 degrees, depending on the time of the year, they are important to the fish ? such as trout. Trout prefer colder water and will seek these stratas because they are cooler than the surface temperature. This is especially true in deep lakes such as Berryessa, Indian Valley Reservoir and Upper Blue Lake.
As the daytime temperatures drop during the fall months, the surface water starts to cool. Cold water is denser than warm water and, as such, heavier. As the surface water cools it gets heavier. This “heavy water” starts to sink, pushing the warmer water to the surface. This is where the term “turnover” comes from. Water reaches its maximum density and weight at 39 degrees. While the surface temperature may drop even more, the lake won”t continue to turnover. It”s rare for the surface temperatures in lakes in Lake County to drop to as low as 39 degrees but it”s not uncommon for the temperature to drop to 45 degrees, which will cause a turnover in lakes such as Indian Valley and Upper Blue Lake.
A good example of what can happen in a turnover occurred in 1991, when Upper Blue Lake experienced a massive turnover. That was the year when the air temperature plummeted to as low as plus-5 degrees in some areas of Lake County. The surface temperature at the lake dropped so fast that the cold surface water rushed to the bottom, forcing the warmer water to the surface. Along with the warm water, decaying algae from the bottom also rose to the surface and for several weeks the normally blue-colored lake turned a dusty brown. While this is common in many of the northern states, it”s rare when it happens so fast in our local lakes.
Whereas bass aren”t normally affected by a lake turning over, trout are. Lake turnovers can have a huge impact on the trout fishing. A lot of fishermen think the reason trout stay on the surface after a turnover is because the water is cooler. Actually, the reason the trout are feeding on the surface is because during a turnover the cold water sinking to the bottom stirs up the silt, which results in nutrients and plankton being forced to the surface. The bait fish go to the surface to feed on the plankton and the trout follow the bait fish. Bass tend to stay in shallow water and near cover where the water temperature is the same from top to bottom, such as at Clear Lake. This is especially true for largemouth bass. However, since smallmouth stay in deeper water they will use turnovers to feed on bait fish near the surface.
Lake Berryessa is a good example. When that lake turns over, the trout can be seen chasing shad on the surface. Another good example is Lake Mendocino. When this lake turns over, threadfin shad by the thousands can be seen dimpling the water. The reason these shad are on the surface is because they”re feeding on the plankton that has been forced from the bottom.
Another reason fish will be near the surface is oxygen. During the fall and winter months, the surface of the lake is constantly being disturbed by winds, which results in more oxygen. The bottom of the lake is often deprived of oxygen during the fall months because as plants die they decompose and use up the oxygen
Clear Lake doesn”t have a turnover in the sense of the word, the reason being that the lake is so shallow the water temperature more or less stays constant from the top to the bottom. I have often taken water temperature readings from the bottom to the surface and have seen only a few degrees difference. There may be some minor turnover in some of the deeper areas in the south end but most of the lake”s depth is less than 15 feet.
