What "percent full" means for a reservoir

Percent full compares how much water a reservoir is holding with how much it holds at full pool. It is a measure of volume, usually in acre-feet, not of height.

How it is calculated

Operators survey the shape of the lake bed and build a table that converts every water elevation into a stored volume. When the gauge reads an elevation, the table gives the storage. Divide that by the storage at full pool and you get percent full.

Some agencies calculate it against conservation storage, the usable water above the lowest outlet. Others use total storage, including water that can never be released. The same lake can be reported as 75 percent full by one source and 82 percent by another, depending on which definition is used.

Why it is not the same as the level

Reservoirs are shaped like bowls with sloping sides, wider at the top than the bottom. The top few feet hold a large share of the water. A lake 8 ft below full pool might still be 75 percent full, while the last 10 percent of volume can be spread across dozens of feet of depth.

For boaters that means percent full understates the change you see at the shoreline. A drop from 100 to 90 percent can expose wide flats and close shallow ramps.

On Lakeglance

When a USGS gauge also publishes storage, we can show it. For most lakes we focus on the elevation itself and on how it compares with the normal level for this month, which is easier to relate to what you will find at the ramp.