The Annual Drawdown Cycle

“At the end of each summer, the TVA (Tennessee Valley Authority) begins the annual drawdown of upstream reservoirs to make room for the rains and snows of winter and spring. All of TVAs reservoirs are part of a single system managed to fulfill seperate but intertwined missions – navigation, flood control, power supply, water quality, recreation and land use. The annual drawdown of upstream reservoirs in North Carolina, East Tennessee, and North Georgia is central to the way the system works. It is important to understand that reservoirs in the TVA system look like natural lakes, but unlike lakes thier water levels change by design not chance. Read on to learn why reservoir levels rise and fall and how they affect the overall working of the Tennessee River System.”

How the Annual Drawdown Cycle Works

“Water in the Tennessee River System rises and falls in annual cycles. For the reservoirs on the Tennessee Rivers this change is relatively minor. These reservoirs are designed to provide for navigation and must have enough water for barge traffic to get through. Reservoirs on the tributary river, such as Douglas Lake, which branch off the main channel upstream serve a different purpose. They serve as an emergency storage system to prevent flooding downstream. They do this by holding an enormous amount of winter and spring precipitation that often falls in higher elevations. To make room for this water, the reservoirs are typically emptied to 3/4 of thier depth by January 1st of each year. Thats why levels of tributary reservoirs may rise and fall from 35 to 90 feet over the course of a year. The diagram below shows how the flood control system works.”