For the first time since its construction in the 1930s, the federal government has formally declared a water shortage at Lake Mead, the nation's largest reservoir by volume, on the Colorado River.

Why it matters: The declaration, issued by the Bureau of Reclamation, sets in motion a series of water allocation cuts to downstream states along the Colorado River.

• It also serves as a stark warning to a rapidly growing Southwest population that drought, heat and climate change are major threats to the region.

Driving the news: Lake Mead is at record low levels, having dropped below 1,075 feet above sea level, or 40% of capacity. The cuts come because the forecast lake level for 2022 is below that level as well.

The West has been mired in the worst drought of this century, and when viewed over several decades, scientists have found that the Southwest is locked in the grip of the first climate change-caused megadrought seen in the past 1,200 years.

• The water level of Lake Mead has been on the decline since about 1999.
• Hotter temperatures and a reduction to spring snowmelt has reduced the water flowing into the Colorado River from the Rockies, where the river begins, before winding its way into the Gulf of California. So too has burgeoning water demand from increasing populations and thirsty agricultural interests.
• A series of agreements governs water use from the river, as well as the cuts to be implemented when the water levels dip below a certain threshold.

Details: Lake Powell’s levels also are on the decline, which poses a threat to the electricity generated by the Glen Canyon Dam, threatening the roughly 5 billion kilowatt hours of electricity generated each year at the Glen Canyon Dam.

• This first round of cuts are going to have the greatest impact on Arizona farmers, as the state will lose 18% of its share from the river, which translates to about 8% of the state's total water use, or 512,000 acre-feet. (An acre-foot is about enough water to cover an acre in a foot of water.)
• Farmers in Arizona are likely to experience the brunt of the water cuts, and may be faced with tough choices of letting their fields go fallow or tapping dwindling groundwater supplies or other alternate water sources.
• Under the water allocation cuts, Nevada will lose about 7% of its allocation, or 21,000 acre-feet of water.
• Mexico will see a reduction of roughly 5%, or 80,000 acre-feet.
• According the Bureau of Reclamation, the Upper Colorado River Basin experienced an exceptionally dry spring in 2021, with April to July runoff into Lake Powell totaling just 26% of average despite near-average snowfall last winter.
• The Interior Department agency predicts the amount of water that would flow into Lake Mead without storage behind the dam, is just 32% of average.

What they're saying: "It's clear that the scale and pace of climate change in the Colorado River Basin poses a huge threat to the water supplies on which everything depends," Kevin Moran, who leads the Colorado River program for the Environmental Defense Fund, told Axios.