Water Management Report for Cumberland River Basin

Cordell Hull

Cordell Hull is a main stem reservoir that is fed almost entirely by releases from Wolf Creek and Dale Hollow dams. Under normal circumstances, Cordell Hull has a rapid flow through of cold water that warms minimally before being discharged into the upper reaches of Old Hickory Lake, the next reservoir downstream. Thermal layering or stratification of the main channel portion of Cordell Hull is usually limited to the lower end of the lake in normal years. This year is anything but normal and instead, due to severe flow restrictions from Wolf Creek Dam, Cordell Hull Lake is seeing very dramatic and direct effects. The upper half of Cordell Hull Lake remains in relatively good condition due to inputs of water from Dale Hollow. However below the Gainesboro area, current velocity slows and with the reduced quantity of water, dramatic thermal stratification occurs.

Theoretical retention times for water passing through Cordell Hull have increased from about 8 to 40 days. As a result of this, dissolved oxygen below the near surface water rapidly depletes to levels that are problematic. One of the main issues with lower lake levels system-wide, is the reduced abilities to move water through the system. This slowed water traveling through the dams exacerbates dissolved oxygen loss. A standard rule of thumb is that still water loses 1/10 milliliter. of oxygen per day. So if water is taking 40 days to move through the system, as it is in some areas, water can change from holding 7.5 ml of oxygen (a healthy amount) to 3.5 ml of oxygen (pretty darn low). Water must continue to move through the system for the oxygen, the temperature and navigation.

Amazingly, by mid-June 2007, dissolved oxygen levels were already hovering in the range of 2-3 parts per million (ppm), well below the 5 ppm minimum for maintaining warm water fisheries. Turbine releases from Cordell Hull transferred this low dissolved oxygen water into the upper reaches of Old Hickory Lake. Following the June 2007 measurements, releases were stepped up from Dale Hollow Dam partly to improve conditions for the TVA's Gallatin Fossil Plant. This change has modestly improved the water quality in lower Cordell Hull; however dissolved oxygen levels are still well below normal. Since this situation was likely to continue and perhaps worsen as the hot weather intensifies, the Corps opted to begin continuous spilling of approximately 1500 cubic feet per second (cfs) of water beginning June 18 to produce a flow blended with the periodic turbine discharge.

The spilling operation has greatly improved the flow conditions in the upper portions of Old Hickory Lake and may have improved conditions in Cordell Hull Lake by providing constant movement of the water through the main channel. Measurements made during a recent sampling trip found DO levels of about 8 ppm in the spillway discharge and about 4 ppm in the turbine discharge. This produced an acceptable blended flow downstream in Old Hickory Lake.

As water quality conditions deteriorated in Cordell Hull Lake, the Corps elected to begin spilling water to aerate the tailwater. Water being spilled mixes with the turbine release.

Ironically, despite lowered inflows, the Cordell Hull Lake has been held about 0.5 feet above the normal headwater elevation of 504.0 in order to provide some buffer for downstream projects and needs. This extra water would be available relatively quickly in an emergency situation, such as the drop of a lake pool which might cause concerns for navigation interests.

At present, barring any further restrictions in flows from Wolf Creek or Dale Hollow, the water quality status quo can probably be maintained in Cordell Hull through the end of the critical late summer/fall season.