Water Management Report for Cumberland River Basin

Old Hickory

Old Hickory is exhibiting the same signs of much longer retention times due to lowered inflows as have been observed in the upstream Cordell Hull Reservoir. Old Hickory is also being kept about 0.5 feet higher than normal (approximately 445.5) to provide a small amount of extra water in storage should it be needed on an emergency basis for a downstream need, such as navigation. The recent 2 August 2007 water quality monitoring trip revealed that downstream of the TN 109 bridge crossing (CRM 240), Old Hickory Lake is essentially stratified. The lake has generally good dissolved oxygen near the surface, which is attributable to atmospheric mixing and algal activity, but very poor conditions at depths greater than 20 feet.

A towboat approaches Old Hickory Dam. Old Hickory Dam is being operated to maintain water quality in the Cumberland River through Nashville and keep vital navigation traffic moving.

The effect of reduced flows coming from Lake Cumberland was evident early in Old Hickory Lake. Outflows from Old Hickory became problematic with decreasing dissolved oxygen levels reaching critical levels of near 3.0 mg/l by late May. In response to this, spilling from the dam began on 4 June 2007. Various gate manipulations were tried initially in order to provide the desired flows, and at present four gates are in continuous operation. Spilling has proven successful in improving downstream conditions, and helped with the poor dissolved oxygen resulting from turbine discharges. On 2 August the Corps measured the dissolved oxygen in the turbine release at 1.9 ppm while the spillway release was producing dissolved oxygen of 6-7 ppm. Turbine flows continue despite the low dissolved oxygen in order to provide commercial navigation traffic with adequate depths.

This shows spilling underway at Old Hickory Dam on the Cumberland River just above Nashville. Without spilling, dissolved oxygen conditions would be unacceptable for maintaining desirable warm water aquatic life in the river.

It is remarkable, to observe the positive effect spilling has on reaeration. Essentially the spillway crest at Old Hickory Dam is at an elevation of 406 ft., which corresponds to a depth in the lake of about 40 feet. At this depth the dissolved oxygen in the lake is 0.1 ppm. Spilling is having the effect of turning essentially anoxic water with no oxygen into water with fairly good (given the circumstances) dissolved oxygen. Mixing of turbine and spilled water occurs within a couple of miles below the dam to provide acceptable dissolved oxygen levels. The situation in the tailwater is being closely monitored for any signs of deterioration, however for the foreseeable future, operations are likely to continue to follow the same pattern at Old Hickory.