Understanding Reservoir Storage and “Percent Full”

Storage is the volume of water held in a reservoir, usually reported in acre-feet (the volume that would cover one acre to a depth of one foot, about 326,000 US gallons or 1,233 cubic meters). Operators calculate storage from elevation using an elevation–capacity table derived from surveys of the reservoir bed.

Why “percent full” varies by operator

“Percent full” is storage divided by a reference capacity, but operators define that reference differently — for example conservation pool, top of active storage, or full pool. Because the bed of a reservoir is not a simple box, a lake can be 90% full while its surface is several feet below full pool.

For that reason this site only shows a percentage when the operating agency publishes one, or when the agency publishes storage and a documented capacity, and we label which it is. We never estimate percent full from elevation alone.