On mobile, the guide is best viewed in landscape (horizontal) mode. To view or export the data, click on the menu in the top-right corner of the chart.

What Is an Operating Guide?

Operating guides were designed and developed for every reservoir and are used in making critical decisions regarding the storing and/or releasing of water from dams throughout the entire Tennessee reservoir system. These operating guides are centered on decades of operating experience, including more than 100 years of rain data and seasonal variation.

Factors in determining the storing and/or releasing of water include the following:

• A reservoir’s size, shape, surface area and storage capability

• The landscape of the surrounding watershed (Is it widespread and flat as in the western Tennessee Valley or is it mountainous and unyielding as in the eastern Valley?)

• The average rain and runoff

• Industrial, agricultural and municipal needs for water

Midnight Elevations

Recorded midnight headwater elevations above the dam are shown on the graph for two years—last year (black line) and this year (red line). Gauges located at the dam record the elevations in feet above mean sea level.

Operating Range

Based on computer simulations and more than 100 years of rain and runoff data, the expected operating range represents a reservoir’s likelihood of falling within this range of elevation (the gray band on the graph) on average in 8 out of every 10 years on any given day. This is known as the 80-percent probability band.

Comparing Other Reservoir Operating Guides

When comparing reservoir operating guides of other reservoir operating guides, know that tributary reservoir elevations fluctuate significantly throughout the year and those graphs reflect an expansive vertical axis. On the other hand, main-stem reservoir elevations fluctuate very little since those reservoirs have far less storage capacity, so those graphs reveal a much tighter vertical axis.