Two people fishing

You may have noticed a strange and offensive odor emerging from ponds, lakes or reservoirs this fall that at times may also be tasted in the treated drinking water. Though the odor can be overwhelming, there’s technically nothing wrong with it. In fact, the smell is the result of an entirely natural process.

Termed “lake destratification” or more commonly lake turnover, the process is a seasonal event that usually takes place in the fall and spring. Though it is harmless to humans causing no more than an assault on the senses, it can affect the health of fish.

Lake water is normally “stratified” or layered due to temperature. Science Encyclopedia explains the process in terms of water circulation, which is the mixing of water in a lake. Water mixes at the surface and within lake’s layers. “When layers mix and change places, a lake is said to turn over. Turnover occurs when water in an upper layer is heavier, or denser than the layer of water underneath it.”

When temperatures either begin to cool off or significantly warm up during a change of seasons, the turnover process takes place.

Temperature layers in the water remain constant during the hot days of summer and early fall. As the surface continues to cool, the water in the top layer becomes denser than that in the depths, so it sinks, which speeds up the mixing process.

Scientific studies show that oxygen in the water also stratifies, with the warmer water near the surface containing dissolved oxygen, and the cooler water found deeper below the surface experiencing oxygen depletion.

Over and above the taste and smell issues, Texas A&M’s AgriLife Extension notes that these turnovers can sometimes cause fish kills.

“The problem arises when this stratification is broken down quickly, causing the two layers to mix or ‘turnover,’” AgriLife Extension reports. “The turnover mixes the oxygen rich surface water with the deep oxygen depleted water. The dissolved oxygen concentration in the mix can be too low to support life in the pond. Both fish and plankton can die from low dissolved oxygen following a turnover.”

The AgriLife report goes on to note that the timing of the event, saying that turnover only happens when the surface water is cooled quickly. That brings the surface water closer to the temperature of the deep water and lets them to mix. Cold rain and wind cools the surface water causing the turnover.

“Usually this happens during thunderstorms,” the report noted. “Many people confuse turnover with an algae die-off; externally they can look very similar but without a cold rain and wind there can be no turnover.”

Turnover is not only natural, it’s a vital part of the ecosystem. According to a Science North report: “The mixing of lake waters is important to move nutrients around.”

A person wearing a bucket hat fishing

During periods of turnover, which has already been noted on portions of Lake Granbury, the chances of bringing home a record catch may drop off a bit.

The Texas Elite Angler website notes that “Early to mid-October is generally the time of the turnover, when the surface temperatures reach the low 70s.  If you’re putting up a blank (catching no fish) during this time period you have a built in excuse. Blame it on the turnover.”

But a slowdown in fishing doesn’t last long.

“Eventually, the fish will recover and begin feeding aggressively again,” said professional bass fisherman Kevin VanDam to Bassmaster magazine. VanDam has won multiple Angler of the Year titles and bass fishing tournaments. “The first day is the toughest, but it’s usually over in a week. Once things settle down, the bass get extremely aggressive and resume feeding up prior to winter.”

Turnover can occur at different times in different bodies of water. A deep lake usually turns over later in fall than a shallow lake in the same region because the water in its depths is colder and more time is needed for the surface water to reach the same temperature.

The good news is that while the process can be irritating to the senses and disruptive fishing enthusiasts, it is temporary.