The last time Lake Mead was at full capacity was 1983. It’s been in steady decline since. (Ethan Miller/Getty Images)

Lake Mead, once the largest reservoir in the United States, fell below an all-time low water level last week — and had dropped to 1,074.4 feet as of Monday. The lake hasn’t been this shallow since the Hoover Dam was finished in 1936.

The lake is just 37 percent full, said Rose Davis, a spokeswoman for the reclamation bureau. The previous lowest level was 1,074.71 feet above sea level.

“We have passed the historic low of June 25, 2015,” Davis told the Associated Press, “and we expect the lake to continue to drop to levels near 1,070 feet by the end of June.”

If the current low level continued into December, it would be enough for the Bureau of Reclamation to declare a formal water shortage on the Colorado River. But Mead’s water level reaches annual lows in the summer and begins to climb again in the fall. Officials estimate that the level will increase to 1,078.5 feet by December, the Arizona Daily Star reports — just 3½ feet above the bureau’s water shortage threshold.

The carefully monitored reservoir is expected to reach the 1,075-foot threshold at the end of 2017. The last time Lake Mead was at full capacity was 1983, when the level reached a record high of 1,225 feet. Since then, the lake has been in steady decline. The iconic white “bathtub ring” along the shore is evidence of the reservoir’s former expanse, marking the water level from 2000.

The sprawling man-made lake, which straddles the Arizona-Nevada border, has been hit hard in recent decades by prolonged drought and overuse. Mead serves as the water source for tens of millions of people, including residents and visitors of Las Vegas and seven states in the West. The lake is mainly replenished by snowmelt that runs off into the Colorado River, but drought has decreased rain and snow markedly.

Back in 1968, when the water ran free and plenty, an agreement was struck between Arizona, California and Nevada, allowing Arizona to channel 1.6 million acre-feet per year of the Colorado River’s water into Phoenix and Tucson. To sweeten the deal for the other states involved, Arizona agreed that in the case of an official water shortage, it would give up its claim to the majority and allow California to have first dibs, environmental blogger John Fleck writes.

In that situation, Arizona would lose 320,000 acre feet per year, or about 11 percent of its total Colorado River allotment. So officials are trying to come to an agreement in which more strategic usage reductions would prevent the need for such drastic cuts.

But as Fleck points out, the situation could be a lot worse. “We’re now 15 years into a drought deeper than any contemplated when the Lower Colorado River water delivery system was built, and everyone up to now has gotten a full supply,” Fleck writes. “That’s a pretty robust system.”