Early in February the reservoir reached a key threshold: 60 percent full, which is the highest water level allowed during the winter months, according to rules from the U.S. Army Corps of Engineers and the Sacramento Area Flood Control Agency.
Last year, most of the reservoir was a dry, dusty lakebed.
"What reservoir was left was confined to the old river channels before we built the dam," says Drew Lessard of the U.S. Bureau of Reclamation, the federal agency that manages the reservoir.
Just an average-size winter storm can send huge volumes of water down the American River into Folsom Reservoir, boosting the lake by 10 percent or more. A major storm can produce dramatically more than that.
"The watershed is pretty flashy and it responds pretty quickly to storm events," says Lessard. "That's why we need to reserve a space during those winter months in case that happens."
Sitting 40 percent empty allows the reservoir to act as a buffer against floods, gulping the runoff without overflowing. In years where the upstream reservoirs are fuller, Folsom Reservoir is required to remain 60 percent empty.