No, this is not another episode in the Bureau of Reclamation’s tragicomic daytime soap opera about Lake Berryessa. This is a report on a phenomenon called “turnover” which is well known to anglers, but not so much to other lake visitors. Some people think that Lake Berryessa turns in both spring and fall. But scientific data shows that the lake only turns once per year - in the fall. Turnover is essentially what it sounds like. The water on the bottom of the lake goes to the top, and the water on the top of the lake goes to the bottom. The bottom layer of water is called the hypolimnion, and the water between the hypolimnion and epilimnion makes up the metalimnion. The metalimnion is also called the thermocline, because a drastic temperature change occurs in that layer. For ease of understanding, I’ll call these the top layer, middle layer, and bottom layer. The diagram below will also help you understand the process for a lake that freezes. When layers mix and change places, a lake is said to turn over. Lakes that turn over once a year are said to be monomictic. Lakes that turn over twice a year, once in spring and once in fall, are called dimictic. Dimictic lakes usually freeze over during the winter. The reasons for both the spring and fall turnover in lakes that freeze are easy to understand so I’ll discuss them first. Although the turning of a lake is partly due to temperature and density differences in the layers, the major cause for the turnover (or mixing) is the wind. The wind causes full mixing of the lake when the temperature of the water is the same at all depths and there are no layers. In late summer lake surface waters of both monomictic and dimictic lakes have reached their annual maximum temperatures. At this time in a sufficiently deep lake, you will find a definite layering of water temperatures. Warmest, and therefore least dense, waters lie on top, and the water temperature decreases with depth, reaching its minimum temperature at the greatest lake depths. How cold the lake bottom water becomes depends on the lake depth and other characteristics but it will never fall below 4 deg C (39 deg F) unless the lake freezes solid. Summer breezes blowing over the lake generally keep the top layer stirred by pushing a quantity of surface water downwind. This draws a flow of deeper water upward (upwelling) along the lee shore to replace the pushed waters. But this upwelling is not coming from the deepest layer, only from the lower part of the top layer. As a result, top layer waters mix, producing generally warm temperatures and high oxygen content (important to fish and other creatures) throughout the top layer. The middle thermocline layer has minimal mixing, and what does occur is slow, thus isolating the bottom waters from the surface zone. In fall, the surface water cools. Its drop in temperature eventually matches the middle layer in temperature. When these top layer waters reach about 10 deg C (50 deg F), they sink into the middle layer waters below, erasing the temperature stratification between the top and middle layers that had built during summer. As fall air temperatures continue to drop, this new upper layer cools to the temperature of the bottom layer. The full water mass of the lake has now reached a uniform temperature, and the surface winds mix the full water body in the “fall turnover”. The water temperatures then continue to decrease into the winter months. For lakes that fall below 39 degrees and finally freeze, the surface becomes colder than the bottom, but this cold water now “floats” on top of the “warmer” water. (See above diagram) The colder surface water is now actually lighter than the warmer water and floats on it until it actually turns to floating ice. The key to this unusual process is how water density varies with water temperature. Water is most dense (heaviest) at 39º F (4º C) and as temperature increases or decreases from 39º F, it becomes increasingly less dense (lighter). Thus, at 39 degrees and below, less dense but colder water is now at the surface and more dense but warmer water is now near the bottom. During spring, the process reverses itself. This time ice melts, and surface waters warm and sink until the water temperature at all depths reaches approximately 39º F. When this occurs, winds blowing over the lake again set up a full circulation system, this mixing known as “spring turnover”. As the warming continues into the summer, the top water layer becomes much warmer and less dense. The warm surface layer now “floats” on the cooler lower layers. Over time into the summer the three water layers again become established, and our cycle has been completed. Since Lake Berryessa water temperatures never get below 39 degrees, it is classified as a warm, monomictice lake. During winter, the surface waters cool to a temperature equal to the bottom waters. But lacking significant thermal stratification, since the water never gets below 39 degrees, much less freeze, these lakes mix thoroughly each winter from top to bottom and continue to mix until spring. This situation is graphically illustrated by the following chart from Alex Rabidoux of the Solano County Water Agency. Alex reports, “SCWA maintains a thermistor chain of sensors to a depth of 70-ft near Monticello Dam. I have attached a little over 1-year’s worth of data, and you can see that Lake Berryessa experiences one mix per year. For last year, mixing occurred in November, 2008 followed by several months where the lake was well mixed, and then stratification began again in March, 2009. For Water Year 2010, you can see a similar pattern, where Lake Berryessa became well mixed a few weeks ago, in mid November.” Alex added that the thermocline is probably somewhere near 40-ft, but there are not enough temperature sensors to say with absolute certainty without finer resolution.
Posted on: Sun, 08 Sep 2013 18:27:19 +0000
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