Lake Mead, which is located east of Las Vegas, has reached milestone low levels of water to due drought conditions in the western U.S.
In the 78 years since the dam was filled in May 1937, the water line has not dipped below 1,080.18 feet above sea level until recently.
On Tuesday, it reached 1,079.76 feet which means the lake is at just 38 percent of its holding capacity.
These falling levels could affect the millions of Americans for whom Lake Mead is a major water source which includes most of the growing Las Vegas Valley.
It also threatens electricity production, as turbines at Hoover Dam churn the water to generate power.
With the hot summer months approaching, experts acknowledge the possibility that levels could sink to as low as 1,073 feet this year.
Once the lake reaches 1,075 feet or below, a water shortage declaration goes into effect, triggering restricted allotments to Nevada and Arizona.
Environmentalists are advocating major reductions in water use, as the downward trend is projected to continue.