Reservoir Stratification, Thermoclines, and Turnover

Posted by admin on January 5, 2022

By Todd Driscoll
Texas Parks and Wildlife Department

Most anglers have likely heard the words "thermocline" and "turnover" associated with local fishing reports and dock talk at tackle shops. Both result from thermal stratification, which is an annual occurrence at a majority of southern reservoirs. As part of the stratification process, reservoirs develop pronounced temperature barriers (thermoclines) and anoxic zones (water without oxygen) in the summer and “turnover” or mix during the fall. Many bait fish and sport fish relate to the thermocline during summer, as it contains the coldest water with adequate oxygen levels. Anglers can put more fish in the boat by better understanding the predictable process of stratification.

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. Have you ever wondered why ice floats instead of sinks? Obviously, ice is the solid form of water, but it floats instead of sinking because ice is less dense than water. 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 and this warm water layer expands into deeper water. As this process continues to accelerate into June, the reservoir becomes stratified into three layers: 1) the upper layer of warm, less dense water of similar temperature, 2) the thermocline (small middle layer where the temperature cools dramatically between the upper and lower layers), and 3) the lower layer of colder, more dense water below the thermocline (usually has no oxygen). At Sam Rayburn and Toledo Bend reservoirs, the thermocline will typically begin to form at a water depth of 25 feet and can increase in depth to approximately 40 feet at mid-summer.

Once thermal stratification develops, the top water layer does not mix with the lower layer due to the strong water density/temperature barrier at the thermocline – it literally functions as a wall. Due to the lack of mixing, the lower layer becomes anoxic (has no oxygen) within three to four weeks after stratification from decomposition of organic matter. 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 gas, which primarily persists during anoxic conditions present in the lower layer.

Usually, anglers can find the thermocline with their electronics, as the density of water abruptly increases at this layer. It may take manual increases of sonar 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 actual 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 (typically 20 - 30 feet at Sam Rayburn and Toledo Bend). Sport fish, especially black bass and crappie, will relate to these bait fish throughout the summer. Finding structure and cover at 20 - 30 feet will maximize catch rates when fishing deep during summer.

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 upper layer. When the upper layer temperature is equal to or lower than that of the thermocline, 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 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 upper layer mixing with the poor water quality of the lower layer, which causes pockets of anoxic water. Fishing can also be tough for a few days during turnover due to the sudden change in water quality.

Contact us by phone (409-698-9114) or email (todd.driscoll@tpwd.texas.gov) with questions. Good luck and good fishing!