Influences of Lake Level Changes on Reservoir Water Clarity in Allatoona Lake, Georgia

Abstract

In Allatoona Lake (Georgia USA), secchi transparency (measured every 2 to 4 weeks during a Phase I U.S. EPA Clean Lakes Study) was typically 5- to 8- fold greater in summer than late autumn and winter. The intensity of storms increased in late autumn and winter resulting in high sediment loads from the watershed, but lake level was also drawn down, confounding the influence of external sediment load with that of resuspended lake sediments. For 4 of the 5 years studied, decreases in water clarity were more closely synchronized with lowering of lake level than with storms. Continuous automated sampling of turbidity and other water quality parameters at 15-minute intervals allowed us to assess whether turbidity increases were derived from erosion of the exposed shoreline (i.e., by rain and runoff), or wave-driven resuspension. Regular increases in turbidity and decreases in pH occurred each weekend during the summer, suggesting increased mixing by boat traffic. Increases in turbidity on weekends and after rain events were greater after initiation of drawdown and were particularly strong when lake surface elevation approached that of the summer metalimnion depth. It appears that sediment focused (deposited) in deeper areas during the spring and summer is resuspended by wave action as lake levels drop in autumn.

Key Words:

• water clarity
• lake level
• reservoir
• sediment focusing
• wind
• turbidity