A reader recently asked if it's true that lakes "turn," meaning the bottom water swaps places with the top water.

We are several months away, but this in fact happens on the Highland Lakes and most other Central Texas lakes once a year, usually in October and November. During these fall months, the water at the surface cools because of cooler air temperatures and shorter day lengths.

The cooler air temperatures cause surface water temperatures to fall below the temperature of the water below. This cooler water sinks because it is more dense than the warmer water below. Thus, it causes the warmer water below to rise to the surface.

Strong winds behind cold fronts often accelerate this process and help mix the water. So, during the fall and early winter, the surface and bottom water will indeed trade places.

A nasty odor
When the lake turning process begins it often brings up bottom water that is very low in dissolved oxygen and high in hydrogen sulfide. You may notice a bad odor, like the smell of rotten eggs. This nasty odor is associated with this naturally occurring hydrogen sulfide, which is a gas that is dissolved in water. This hydrogen sulfide is formed from decomposing deposits of organic matter, such as decaying plant material. This odor will often remain until the lake waters slow their turning.

Lakes will continue to turn slowly through winter as cold air temperatures cause the surface water to get even colder. The lakes stop turning in late winter when air temperatures begin to warm.

Through the spring and summer months, the upper layers of the lake warm because of the warmer air temperatures and longer day lengths. While the upper layers of the lake get warmer, the lower layers remain cool. This type of water profile is very stable. That's because the cooler, more dense water at the bottom stays there, while the warmer, less dense water stays at the surface. This profile will continue until fall, when cold fronts cause the surface water to cool, and the process starts all over again.

How to check Lake Travis' temperature
Want to know what the water temperature of Lake Travis is today? Check LCRA's daily river report (see bottom of first paragraph), or the University of Texas Applied Research Laboratories' Lake Travis Test Station.

LCRA chief meteorologist Bob Rose writes regularly on weather and water topics. Do you have a question? Contact Bob Rose.