The Effects Of Cold Water Discharge On A Downstream Reservoir's Temperature And Oxygen Levels

Data are presented on the effects cold water discharge from Hartwell Dam has on a twenty-mile stretch of river between Hartwell and Clark Hill Reservoirs and its influence on the temperature and oxygen concentrations beneath the thermocline in the lower lake. This study revealed that oxygen concentration increased from 0.7 ppm to between 3.2 and 6.7 ppm when the sub-60 degree F. water was taken from a 105-foot depth in Hartwell Lake and discharged into the tailrace during generation periods. This water further increased its oxygen content as it became agitated while moving over shoal areas between the two reservoirs even though the water temperature sometimes reached 80 degrees F. for short periods of time when the river was low following generation shutdown. Monthly temperature and oxygen collections taken from eight stations between the headwater and dam of the 70,000-surface-acre Clark Hill Reservoir indicated that the entire lake was capable of supporting trout through the month of April. However, this area was reduced to approximately 25% of the total lake surface and 15% of its volume by mid-September. It was also learned that the cold oxygenated water was stratified between two cold low oxygen-bearing layers in the deeper areas nearest the dam and that the suitable trout water (temperatures below 70 degrees F. and containing at least 3.0 ppm oxygen) extended laterally at the same depth but did not contain sufficient oxygen in the larger bays and tributaries after July.

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