Even as Sacramento waits for the soaking El Niño forecast to hit this fall, Folsom Lake continues to lose water and will almost certainly fall Thursday to its lowest level in more than 20 years, government data show.

Folsom Lake provides drinking water to hundreds of thousands of residents in the Sacramento region. Releases from the federal reservoir also serve as a bulwark against Sacramento-San Joaquin Delta saltwater intrusion, and are critical to maintaining the delicate ecosystem of the lower American River.

Folsom Lake became the face of California’s drought early last year when aerial photos of its moonscape lake bed were broadcast nationwide. At its lowest point last year, the lake level was the same as what the reservoir contained Wednesday. By Thursday, the reservoir is expected to fall to levels last seen in 1992, at the tail end of a five-year drought. And by month’s end, the depth likely will approach levels not seen since the great drought of 1977.

Area water officials said they are concerned about the dwindling supply but expressed relief that lake depths are not even lower. The U.S. Bureau of Reclamation, which operates the reservoir, initially warned that the lake could fall to 120,000 acre-feet by the end of September.

“The situation has been so rough,” said bureau spokesman Shane Hunt. “We are doing everything we can to make sure we maintain water supplies to homes.”

Still, he added, “We are better than a worst-case scenario.”