Observed midnight elevation
Observed midnight elevations are shown for two years: last year and the current year to date. These are the actual elevations of the reservoir immediately upstream of the dam, measured at midnight of each day. The previous night’s elevation is added to the graph by 6 a.m. the next morning.
Normal operating zone
These lines depict the zone of normal operation for power production and summer mosquito control. During high flow periods, the top of the normal operating zone may be exceeded for the regulation of flood flows.
Flowage easement levels
These are elevations to which TVA can raise water levels on Kentucky Reservoir. A flowage  easement is the legal right retained by TVA to flood privately owned shorelands up to a specific elevation as part of its reservoir operations. The summer flowage easement (June 1 to November 30) at Kentucky Dam is elevation 365. The  winter flowage easement (December 1 to May 31) is elevation 375. These flowage easements can vary depending on the location.
Caution
If you compare the operating guides for different reservoirs, please note that the scale for the vertical “Elevation” axis varies. To keep the size of the charts consistent, a larger scale is used for reservoirs that fluctuate significantly; a smaller scale is used for reservoirs that typically fluctuate only a few feet.
 

Reference: http://www.tva.gov/river/lakeinfo/op_guides/kentucky.htm