The federal government on Tuesday announced new water cuts for Arizona, Nevada and Mexico in response to a climate change-induced drought that is depleting the Colorado River and its reservoirs.

Why it matters: It's the first time in history that the government has triggered a Tier 2 water shortage for Lake Mead, one of the river's key reservoirs, as its levels dip dangerously low.

By the numbers: The Tier 2 designation will begin in January, and means that Arizona, Nevada and Mexico will have to reduce their use of water from the Colorado River.

• Arizona will face the largest cut of 592,000 acre-feet of water, which is approximately 21% of its annual apportionment.
• Nevada will face a reduction of 25,000 acre-feet of water, which is 8% of its annual apportionment.
• Mexico will face a cut of 104,000 acre-feet, or around 7% of its annual allotment from the river.

The U.S. Bureau of Reclamation said Tuesday it will not act upon its June demand that states that rely on the Colorado River agree to a plan to cut up to 25% of their water usage.

• Authorities in California, Arizona and Nevada in the Lower Basin and Colorado, New Mexico, Wyoming and Utah in the Upper Basin have had ongoing debates on how to make cuts. They had until Aug. 15 to come to an agreement.
• Chief Camille Touton said in a statement the bureau is "starting the process" to come up with a plan for them, though she did not announce new deadlines for the states.

What they're saying: “Every sector in every state has a responsibility to ensure that water is used with maximum efficiency," said Tanya Trujillo, the Department of Interior's assistant secretary for water and science.

• "In order to avoid a catastrophic collapse of the Colorado River System and a future of uncertainty and conflict, water use in the Basin must be reduced,” Trujillo added.

The big picture: Much of the American Southwest has been in the grip of a decades-long megadrought from human-caused climate change that, combined with chronic overuse of the Colorado River, has sent it and its reservoirs, Lake Mead and Lake Powell, to historic lows.

• More than 40 million people across seven states and at least 22 tribal nations rely on the river for drinking water, and it's crucial for the Southwest's agricultural and outdoor recreation industries.
• Continued depletion of the river's flow and its reservoirs — the two largest manmade reservoirs in the United States — could further disrupt water delivery and hydropower production for Arizona, Nevada and parts of California within the Lower Colorado River Basin.
• The Mexican states of Baja California and Sonora also rely on the Colorado River for drinking water and agricultural irrigation.

Go deeper: Colorado River at drought tipping point

Editor's note: This story has been updated with new details throughout.