Great Falls Operating Guide

 Current Year Observed Midnight Elevations
 Previous Year Observed Midnight Elevations
 Guide Curve


What is an operating guide?

TVA uses operating guides for each reservoir to make decisions about moving water through its dams. These guides are based on many decades of operating experience. They are built around the idea of allowing seasonal variations in the use of flood storage space.

Operating guides take into consideration all the features that influence a reservoir’s ability to store and release water—the size and shape of the surrounding watershed, the reservoir’s surface area and the average rainfall and runoff—as well as historical demands for water use and flood storage space at different times of the year.

Observed midnight elevation

Observed midnight elevations are shown for two years: last year and the current year to date. These are the actual elevations of the reservoir immediately upstream of the dam, measured at midnight of each day. The previous night’s elevation is added to the graph by 6 a.m. the next morning.

Guide curve

The line depicts the elevation immediately upstream of the dam which TVA targets throughout the year.


If you compare the operating guides for different reservoirs, please note that the scale for the vertical “Elevation” axis varies. To keep the size of the charts consistent, a larger scale is used for reservoirs that fluctuate significantly; a smaller scale is used for reservoirs that typically fluctuate only a few feet.