What a difference a month of rain makes.

Two months ago, Folsom Lake stood at its lowest depth in history, and federal officials were engineering a special pumping system to ensure drinking water would keep flowing to Sacramento suburbs.

Following a month of persistent rain and snow in Northern California, lake levels are triple what they were in early December, and the reservoir contains more water than average for early February.

Lake levels have rebounded so fast, in fact, that after four years of drought, officials are talking about releasing water downstream in the near future to mitigate flood risks caused by a wet winter and an increasingly full lake. Such releases happen routinely in a typical Northern California winter – flood control is a primary purpose of Folsom Dam. But it’s been years since the reservoir has reached flood-control stage.

Since early December, a series of storms has added about 393,000 acre-feet, or 128 billion gallons, to Folsom reservoir. On Saturday alone, the lake gained about 71,000 acre-feet, the largest single-day increase in a decade. As of Monday, Folsom Lake was at 104 percent of average for this time of year and at 54 percent of total capacity. Two months earlier, it was at 14 percent of capacity.

Much of the rebound stems from runoff from Sierra storms. But the striking rise also reflects how little water federal operators have released from the lake in recent months. Daily water releases from Folsom averaged just 600 cubic feet per second in December and January, state data shows. During the last wet winter, five years ago, average daily releases were more than 13 times as high.

And the return to more typical snowfall in the northern Sierra means Folsom Lake levels likely won’t plummet again soon. The mountain snowpack that supplies the lake in spring and summer is an estimated 20 percent above average for early February.

Still, lots of water in Folsom Lake doesn’t mean the drought is over. Folsom is the smallest of Northern California’s major reservoirs, and so it fills up more quickly than others. No other major reservoir in the state has reached normal levels or is near flood-control stage. Most reservoirs aren’t even close.

State regulators say at least one of three things would need to happen for the drought to end: statewide reservoir storage would need to be at 90 percent of average levels; runoff forecasts for the state’s water year, which runs from October through September, would need to be 110 percent of average; or reservoirs on the four major rivers in the Sacramento River basin would have to reach flood-control stage.

Shasta Lake is at 76 percent of average depth for this time of year; Lake Oroville is at 66 percent; and Trinity Lake is at 40 percent. Combined, those lakes have about 10 times the storage capacity as Folsom Lake.

“It will take them more than one wet year to recover from their condition,” said Tom Gohring, executive director of the Sacramento Water Forum.

Given continued concerns, the State Water Resources Control Board is expected Tuesday to extend through October the mandatory water cuts it ordered last year for urban residents across California. The agency has proposed relaxing the mandates slightly for many local water agencies to account for the difference in climate between coastal and inland regions. But most communities in the Sacramento region still will be required to cut usage by 25 percent or more compared to 2013.

Monday brought other sobering reminders about the lingering impacts of California’s historic drought. The National Marine Fisheries Service released data confirming an alarming decline in the population of the winter-run Chinook salmon, an endangered species native to the Sacramento River.

Drought has diminished the supply of cold river water the salmon need to spawn and survive. Last year, federal officials held more water behind Shasta Dam, at the expense of farmers and cities, in hopes of creating deep cold-water pools to aid the fish. Simultaneously, they released more water than normal from Folsom Dam to keep ocean salt water from overwhelming the Sacramento-San Joaquin Delta, which serves as the hub of the state’s water-supply network.

The plan failed. The river heated up anyway, and the National Marine Fisheries Service said Monday that only 3 percent of the juvenile salmon survived last year. It marked the second straight year that the vast majority of juvenile winter-run Chinook salmon were cooked to death in the Sacramento River; in 2014, only 5 percent of the juveniles survived.

Already, farmers, fishermen and environmentalists are fighting over what to do this year about the salmon. Because Chinook have a three-year spawning cycle, the 2016 season is considered critical to keeping the salmon from heading to the brink of extinction. The state is considering storing even more water at Shasta this spring. For farmers, that would mean another year of severely diminished supplies.

The best tonic for winter-run Chinook – and Folsom Lake – would be continued steady precipitation this winter, particularly in the form of snow in the Sierra. But a bountiful snowpack is not assured. After a wet start to 2016, there is no heavy precipitation forecast for Sacramento or the Sierra over the next seven days.

For more coverage of the California drought and water issues, please visit the Sacramento Bee.

Top image: Young Chinook salmon move through a plastic pipe as they are loaded into a tanker truck at the Nimbus Fish Hatchery in Rancho Cordova, California, in 2014. This week, scientists reported critically low numbers of winter-run Chinook salmon survived last year. (Rich Pedroncelli, Associated Press)