Did you know that the Tennessee Valley region averages 51 inches of rain a year? The Gulf of Mexico is a source of this moisture, as are the dissipating hurricanes and tropical storms moving across the Southeast.
December through early May is the major flooding season in the Tennessee Valley. Winter storms provide the most rainfall because they are generally more numerous, last longer and cover the largest areas. Average rainfall ranges from 3.0 to 5.5 inches per month. Runoff, the amount of water that ends up in the river system, is about 23 inches, or 44 percent of average rainfall.
Generally, runoff is heaviest in winter and early spring when vegetation is dormant and the ground is saturated. As a result, heavy storms moving across the Tennessee Valley between December and early May become potential causes of floods— necessitating a winter drawdown to provide flood storage.
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The amount of water in the Tennessee River system and its reservoirs depends on rainfall and runoff. Runoff is the portion of rainfall that doesn’t soak into the ground but drains from the surrounding lands into streams and reservoirs.
In early spring, we need plenty of rain to fill the tributary reservoirs, and summer storms help keep reservoir levels up and adequate water flowing through the system. However, meeting flow requirements can be a challenge during the summer. Summer storms generally bring less rainfall than winter ones, and more rainfall is absorbed by the roots of green plants before it can drain into streams, rivers, and reservoirs.
Even in summer, however, flooding is still a concern. The higher reservoir levels that enhance summer recreation also mean that less flood-storage space is available. In the winter, when more runoff reaches the river system, reservoir levels are kept lower so there’s room to store the water from heavy rains.
Above is a sample operating guide that illustrates the cycle of water levels in TVA’s ten largest tributary reservoirs through the course of a year. The shaded gray area indicates the expected rage based on historical record. The blue line, the flood guide, illustrates the ideal level for flood control—when water levels exceed this capacity, TVA's River Forecasting Center will move water through the system to prevent flooding. And the green line shows the system minimum flow operating guide, that level necessary to maintain water quality and sustain aquatic life. If water levels dip below the green line at any point throughout the river system, TVA will conservatively release water from the deep tributary reservoirs to maintain minimum flows downstream. The red line represents reality, or the amount of storage currently available in our tributary reservoir system.