I got this from Tom Bly earlier today:
Low Dissolved Oxygen in the Greers Ferry Tailwater
We are currently experiencing low dissolved oxygen levels in the Little Red during periods of water
release. The State mandated lower limit of dissolved oxygen concentration for trout is 6.0 mg/l. We are currently seeing values as low as 3.6 mg/l when both units are running. The only
time we are seeing levels at or above 6.0 mg/l is during periods of "0" releases.
Low DO's (dissolved oxygen) have negative affects on the health of fish including reduced feeding, poor
growth and poor body condition. Usually DO levels will pick up relatively quickly after the water exits the
dam. Turbulence caused by the water over the river bottom allows for mixing of atmospheric oxygen thus increasing DO
concentrations. We monitored DO levels under generation in 2008 to determine the distance it takes for this to
occur. When both units are running DO concentrations do not recover until down river of Cow Shoals. DO levels do recover faster at lower flows.
There are a couple of reasons exacerbating the problem we are having.
A reservoir will normally stratify or divide into sections during the summer months. As the lake's surface is
heated by solar radiation it causes the water to be less dense than the colder water below and stratification has begun.
This warm, surface layer is referred to as the epilimnion. The temperature of the epilimnion is fairly
consistent through its depth. Below the epilimnion is a layer of water known as the metalimnion or
thermocline. The temperature of this layer decreases rapidly with depth, about 1.7 degrees per yard or
more. The thermocline may vary in thickness in different lakes and at different times during
stratification. In a large reservoir such as Greers, this is usually the thinnest layer in the stratification
process. The layer of water below this is the hypolimnion and usually shows fairly uniform temperature. In most lakes this layer of water is devoid of oxygen. Though this layer starts off
oxygenated prior to stratification, after stratification there is no oxygen production (no sunlight penetration, no oxygen production). There is however a constant drain on the DO levels due to decay of organic materials and from respiration of bacteria, plankton,
other invertebrates and fish.
Ok, now that the biology lesson is over. Under "normal"
years (non-flood), if there is such a thing, the hypolimnion retains its DO concentrations for a longer period of time.
The flood years exacerbates the DO problem because of the large influx of organic material in the lake thus increasing the biological oxygen
demand. What further complicates this is the large releases of water through the dam to evacuate flood water from the
reservoir. This constant flow of water further evacuates any oxygen rich water left in the hypolimnion.
The only relief from this problem is the de-stratification of the lake as winter progresses. As the epilimnion cools due to colder ambient temperatures the water becomes denser. When
the temperature of the epilimnion equalizes with that of the deeper layers it will mix. This allows oxygenated water to
mix with the anoxic water below. This is known as "turn over" in a lake.
This usually occurs sometime in November on Greers.
The only recourse we have in combating this problem is to restrict stocking to the downstream point where DO
levels recover to an acceptable level. As last year, this will mean curtailing stocking above Cow Shoals during periods
of generation. The good thing is that we are at the time of the year when stocking is reduced and we are not long from
turn over.
We are currently monitoring DO levels on a daily basis through the COE website. Matt Schroeder and I will be on the river Monday to monitor DO levels and find the recovery point.
Hopefully we can catch the river at different flows and determine a recovery distance for the different flows.
In other words, with two units running the recovery point is about Cow Shoals, with one unit running it may occur at Beech Island.
Do not look for this problem to go away even if we have a "normal" year. We are currently entering our third year in a row of high water levels in the reservoir.
The impact of all the organic matter that has entered the lake over this period will be reflected in DO levels for several years to come.
On a different note, we were able to complete the majority of our sampling on the river prior to the lake
rising again. We were not able to conduct the Jon's Pocket or Moss Dam Shoal samples.
If you have any questions regarding this issue please feel free to contact us by e-mail or phone. Have a great day.
Tom Bly
