Seasonal Issue: Low Dissolved Oxygen and Restricted Flows

[Phil Lilley]

Many issues to talk about, as well as fishing, in the fall. Dissolved oxygen, water temperature, restricted flows...... confusing details to understand when fishing tailwaters. Wish we didn't have to consider them when JUST fishing, but they do affect the way trout move, hold, feed-- and don't feed-- and that affects our fishing strategy.

I know this is old hat to some of you, but I'd like to go over the basics. Lakes change at different seasons of the year. As spring and summer pass, surface water warms and separations or layers form. Because water density changes when it differs in temperature, these layers become very defined as summer wears on. If you look at the Lake Profile - http://www.swl-wc.usace.army.mil/pages/reports/remote/profiles/tabpro.htm you will see a profile, kinda of a photograph, of the water at the dam at Table Rock. See where the temp drops, as well as the DO level, as it gets deeper. And the big drop near the top - this is called the thermocline and where, generally, a large number of fish will hold. As the water warms on the surface, the thermocline drops lower in the lake. We get our water at 130 feet deep and is marked by an asterisk to the side. As you see, the water temperature is colder at that level than the surface but the DO is very low and gets lower as you drop down. As colder weathers rolls in-- November and December-- surface temperatures drop, literally. Cold water is heavier than hot water and thus drops and "turns" the lake over at some point in the game. It's like a tilting table: when the load on top gets heavier than the load on the bottom, it tilts and turns over, leaving the heavier on the bottom. This happens generally from around Thanksgiving into December. Until then, we're stuck with low DO levels entering into LT.

Dissolved Oxygen

Water contains oxygen, H2O - O stands for Oxygen. Oxygen is measure by parts per million. On a scales of 0 to 12, 12 is about the highest you'll find in lake water- usually in the top layers where sunlight, wind and rain adds oxygen. In any lake or pond during the seasons, the layers form layers. Each layer has different density and oxygen levels, depending on the season and temperature of the water at the different levels. The layers start forming in late spring when the top levels start to warm. As summer rolls on, temps in the upper layers really rise and becomes lighter then the layers below. Because of the lack of sunlight, oxygen levels drop as you get lower in the lake or pond and later in the fall, DO amounts at the bottom are nile. As it gets closer to winter and the air temps drop and winds pick up, the surface temperatures drop also. Cold water is heavier than warm water thus this cooler water sinks to the bottom. This starts the the turning effect. When alot of water on top become cooler than the bottom- heat rises- the two levels mix and thus- good DO throughout the depths. The the cycle starts all over again.

As far as the different levels- 0 - 12...... where 0 is real bad (no oxygen- things die) and 12 is usually the surface reading on a lake during alot of wind- may be even 13. The State of Missouri has said that anything under 6 parts is considered pollution. If a business or private individual discharges water with a lower reading than 6- they could get in trouble. But since a dam and the water it releases is not considered "point source" discharge, these rules do not apply and cannot be enforced. The Corp's low point is 4 parts- they try and not go below 4 when they release water from Table Rock. Fisheries for MDC has said that 6 parts is a good bottom indicator-- where fish and other water creatures can live, feed and reproduce. They also say anything below 3 parts can and will cause death in most trout, but this depends on water temperature also. Stress is the key. If a rainbow is already in stress because the water temp is above- say- 60 degrees and then he's hit with low DO- say 3 or even 4 parts, he could die. And the bigger the trout is, the more stress all these factors affect it.

What does low DO do to our trout? It slows them down a bit. How do you know when DO levels are too low and threaten the life and health of trout? A high number is 12 parts per million (ppm). A low number is 0 ppm. Generally, fisheries biologist say 3 ppm is the bottom on the scale, and with high water temperatures, could cause death if prolonged. Six ppm is what the State of Missouri Clean Water Act says is the standard for "safe" water. But the Corps, as a federal agency, doesn't have to adhere to state regulations and has set its mark at 4 ppm. Are we happy about this? No. We've appealed for change but to no avail-- yet.

The Corps does put restrictions on flow at Table Rock. These flow restrictions differ as the fall months progress and the water quality drops. This just means that even in peak times, levels will not exceed the ability to add enough DO to the effluent to keep levels above 4 ppm.

There are three ways dissolved oxygen is added to the near-nil levels in September and October. The hatchery outlets are rich in DO. That's why you will see large numbers of trout with their noses in the effluent. The Corps has modified the turbines and added vents at the top of the chambers to allow air to mix with the water, creating a sloshing effect as it enters the lake. This is hard on the turbine blades, causing the surface of the blades to weaken. Corps officials like to reminds us about this -- that they are sacrificing for the good of the trout. When all of the above fails to add adequate DO to the tailwater, the Corps injects liquid oxygen directly into the turbines. Monitors keep track of DO levels as they enter the lake.

One thing you have to realize-- when the federal government build dams, bureaucrats have to promise that the dams won't hurt the fishery in either the lake above or the tailwater below. When they build dams that are high and the water coming out is too cold for warm water species, they have to provide coldwater species for that fishery, such as the Neosho Federal Trout Hatchery producing 200,000 rainbows per year. Water quality standards also have to be maintained at a level the fish can survive and thrive. The definition of thrive is in question here. The Corps' definition is to "just get by." Sportsmen and fishing-related businessmen think thriving means the fish "move and grow respectively, with adequate food supply to reproduce." But the food supply is low, and there is no reproduction. That must change, and I believe it will in time.

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[JCW355]

That link wouldn't work for me.

[Marcus Swartz]

Thanks for the information........ Always looking to learn more. 

This was very helpful. 

[MickinMO]

Would opening a spillway or two just a tad add any amount of usable O2 to the upper part of the lake without inceasing temps too much?