Arizona's move comes after the Interior Department announced restrictions this summer that could slash water supplied to the lower basin states — Arizona, California and Nevada — over the next two years, depending on the river's condition. The river's major reservoirs, Lake Mead and Lake Powell, have fallen to historically low levels due to overuse and climate change.
The states sharing the Colorado River have been at an impasse, with upper basin states arguing that their downstream counterparts are responsible for the shortages and should face the largest cuts. The federal government stepped in after the states missed several deadlines for a consensus.
The upper basin states — Wyoming, Colorado, Utah and New Mexico — were exempted from the cuts, an exemption that has drawn sharp criticism from lower basin leaders.
"We are drawing a line in the sand," Arizona Governor Katie Hobbs said in a statement Friday. "I am taking this legal action alongside Nevada because the federal government refuses to hold the Upper Basin accountable for the water deliveries they owe Arizona, and is threatening to force draconian cuts that go well beyond the Lower Basin Agreement."
The Lower Basin Agreement is a two-year voluntary water conservation pact among Arizona, California and Nevada.
Tom Buschatzke, director of Arizona's water department, said intervention in the Nevada lawsuit is necessary for Arizona to protect its legal rights to Colorado River water while preserving a path to a collaborative outcome.
An Interior Department spokesperson did not immediately respond to a request for comment. If reservoir levels plunge far enough, the river could effectively be unable to flow downstream, a condition known as deadpool.