Kentucky Lake and Lake Barkley in Western Kentucky are manmade lakes from the Tennessee and Cumberland Rivers respectively. They are also called reservoirs because of the damns on these rivers that created them. The Tennessee Valley Authority in conjunction with the Federal Government created them for generating electrical power, flood control and navigation.
The TVA along with the Corps of Engineers controls the lakes and their levels. You can find flow rates and lake levels at this TVA website.
The flow of water through the two dams' power plants creates electricity. I understand some of the lowest electric bills in the country are experienced by the southeastern part of the United States because of the inexpensive and "green" generation of electricity from the power plants.
For flood control the lakes are drawn down in the winter to 354 feet above sea level. When the watershed experiences spring rains combined with the snow melt from the north down the Mississippi and the Ohio rivers the Corps of Engineers can hold back the water flowing through the dams and allow the lakes to rise, lessening the potential for flooding downstream. Normal summer pool is 359 feet above sea level or a difference of 5 feet.
Before the dams were created navigation was treacherous. Now with the locks a great amount of product can be shipped by water. Kentucky Lake is in the process of extending their lock to handle longer barges. Currently some barges have to go through the lock in two sections taking a long amount of time and backing up traffic.
The lock project has created many local jobs and the housing market has benefited by those relocating here.