Lakecaster Online Archives - Sep, 2004

Reservoir Stratification, Thermoclines, and Turnover

By Todd Driscoll
Texas Parks and Wildlife Department
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Thermal stratification is an annual occurrence at a majority of southern reservoirs. As part of the stratification process, reservoirs develop pronounced thermal barriers (thermoclines) and anoxic zones (water without oxygen) in the summer and "turnover" or mix during the fall. Although many anglers are familiar with these terms, I find that few fully understand the mechanisms surrounding this predictable process.

Thermal stratification is a direct response to the unique relationship of water density and temperature. The maximum density of water occurs at 39°F. As water cools below this temperature, it becomes lighter. As a result, even though ice is the solid form of water, it floats because it is less dense. However, in this part of the country, water temperatures in our reservoirs seldom fall below 45°F, so to simplify, water becomes less dense as it warms. This is an important point relative to the rest of the discussion of thermal stratification.

During the spring season, the first several feet of water begin to warm. Due to the density relationship described above, this warm water layer floats, as it is less dense than the cooler water below. However, at this time this thin surface layer of warm water is subject to mixing from wind and wave action. As spring progresses into summer, the surface waters begin to warm faster than the heat is distributed by mixing and this warm water layer expands into deeper water. As this process continues to accelerate into July, the reservoir becomes stratified into three layers: the epilimnion (the upper layer of warm, less dense water of similar temperature), the hypolimnion (the lower layer of cold, more dense water below the epilimnion - usually has no oxygen), and the metalimnion (the small zone where the temperature cools dramatically between the epilimnion and hypolimnion). The thermocline is the point in the metalimnion where the temperature change is most drastic. At Sam Rayburn and Toledo Bend reservoirs, the thermocline will typically begin to form at a water depth of 25 feet and increase in depth to approximately 40 feet at mid-summer.

Once thermal stratification develops, the top layer (epilimnion) does not mix with the lower layer (hypolimnion), due to the strong water density/temperature barrier at the metalimnion. Due to the lack of mixing, the lower layer becomes anoxic (has no oxygen) within three to four weeks after stratification. A positive indication of summer stratification is the smell of rotten eggs at the powerhouse of Sam Rayburn and Toledo Bend during water releases. This odor is hydrogen sulfide, which primarily persists during anoxic conditions present in the hypolimnion.

Usually, anglers can find the thermocline with their electronics, as the density of water abruptly increases at this layer. It may take some adjustment to sensitivity settings, but the thermocline will show up as a straight line of increased clutter, similar to a scattered school of baitfish. Although baitfish and gamefish can suspend in relation to the thermocline, oxygen levels in productive lakes like Sam Rayburn and Toledo Bend often prevent this. During mid-summer when the thermocline may be in 40 feet of water, oxygen levels typically fall to stressful levels below depths of 25 feet. This reduction in oxygen is caused by the bacterial decay of nutrients and organic matter in the water. In this mid-summer situation, baitfish will typically suspend at the lowest depth of adequate oxygen instead of the thermocline. However, in extremely clear, unfertile lakes with visibilities exceeding 10-15 feet, oxygen levels are often adequate below the thermocline, as there are less nutrients decaying and consuming oxygen.

As the fall season approaches, water temperatures at the surface begin to cool and due to increasing density, the cooler water sinks. Wave action assists with distributing the cooler water throughout the epilimnion. When the epilimnion temperature is equal to or lower than that of the metalimnion, the entire water column is subject to mixing. The mixing process, referred to as fall turnover, can be relatively sudden, with an entire reservoir turning over in less than a week during windy conditions. Typically, this occurs sometime in October at Sam Rayburn and Toledo Bend. Although rare, isolated fish kills can be observed during turnover, due to the well-oxygenated water in the epilimnion mixing with the poor water quality of the hypolimnion, which causes pockets of anoxic water.

If you have questions concerning the fisheries of our area lakes, stop by the Inland Fisheries office at the Jasper State Fish Hatchery or contact us by phone (409-384-9572) or email (todd.driscoll@tpwd.state.tx.us). Good luck and good fishing!