Valley Rainfall

The Tennessee River watershed has one of the highest annual rainfall totals of any watershed in the United States, with an average of 51 inches of rain a year. The Gulf of Mexico, located to the south, provides a major source of moisture. Wintertime storms provide the most rainfall because they generally cover a larger area and are greater in duration and frequency. The basin also is subject to heavy rainfall resulting from dissipating hurricanes moving across the southeastern U.S.

In over 100 years of record-keeping, the annual rainfall has varied between a high of 65 inches in 1973 and a low of 36 inches in 1985. The greatest extremes occur in the eastern half of the Valley. The average annual precipitation at several locations in the mountains of the French Broad, Hiwassee, and Little Tennessee River watersheds exceeds 80 inches. In contrast, other locations in the French Broad and Holston River watersheds have annual averages of less than 40 inches.

Average monthly rainfall/runoff comparison

The monthly average rainfall in the Tennessee Valley ranges from 3.0 to 5.5 inches. March, July, and December are typically the wettest months, and September through November are typically the driest. Of course, in any year, the wettest and driest months may be different.

The average annual runoff in the Valley—the amount of water that ends up in the river system after it rains—is about 23 inches, or 44 percent of the average annual rainfall over the region. The monthly average runoff varies from a high of almost four inches in March to a low of less than one inch in August, September, and October. Generally, runoff is heaviest in the winter and early spring, when the 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 major floods.