What "full pool" means on a reservoir

Full pool is the elevation a reservoir is designed and managed to hold when everything is normal. On a dam operated by the U.S. Army Corps of Engineers it is usually the top of the conservation pool, the band of water kept for water supply, hydropower, navigation and recreation. Power companies and water districts use the same idea under names like normal pool, summer pool or full pond.

Full pool is written as an elevation, such as 1,071 ft for Lake Lanier. That number is measured against a vertical datum, usually NGVD29 on older projects. A lake “8 ft below full pool” has a surface 8 ft lower than that line, not 8 ft less water everywhere.

Why a lake is rarely exactly at full pool

• Seasonal rule curves. Many reservoirs are held at a higher summer pool and drawn down to a lower winter pool to make room for spring floods. A lake 6 ft below summer pool in January may be right on schedule.
• Drought. When inflow drops, operators still have to release water downstream, so the level falls.
• Flood storage. After heavy rain the level can climb above full pool into the flood pool. Operators then release the extra water over days or weeks.
• Maintenance. Dam repairs, shoreline work and weed control sometimes require a planned drawdown.

Why it matters on the water

Ramps, docks and swim beaches are built around full pool. A few feet below it, the ends of ramps can drop off into mud, stumps and old fences appear near the surface, and coves that were navigable in June become too shallow. Above full pool, courtesy docks flood, debris floats off the banks, and operators may close ramps or post no-wake zones.

Lakeglance does not always know the official full pool for every lake. Instead, on each water level page we compare today’s reading with the normal level for the same month, built from years of USGS records. For managed reservoirs that comparison is often more useful, because it already accounts for the seasonal rule curve.