The Colorado River, which provides drinking water to 40 million people in seven U.S. states, is drying up, straining a water distribution pact amid the worst drought in 12 centuries, exacerbated by climate change.

California split from the six states of Arizona, Colorado, Nevada, New Mexico, Utah and Wyoming on Tuesday in the face of a U.S. government deadline to negotiate their own supply cuts or face possible mandatory cutbacks by the federal government.

Reservoirs fed by the mighty river, which supplies water to tens of millions of people in the western United States, are perilously low after more than 20 years of drought.

Officials from six of the states submitted a “consensus-based" model on Monday to the Bureau of Reclamation, the government agency in charge of managing water resources, but California’s absence from the agreement means the deadline was missed.

When the states struck their agreement 100 years ago, it envisaged the river could provide 20 million acre-feet of water a year. An acre-foot (1,233 cubic meters) of water is generally considered enough to supply two urban households per year.

Officials from six of the states submitted a “consensus-based" model on Monday to the Bureau of Reclamation, the government agency in charge of managing water resources, but California’s absence from the agreement means the deadline was missed.

Many experts see its decision to sit out the agreement as fuelling the chances that the water fight will end up in the nation’s highest courts.

Although California was deluged for weeks from late in December by seven atmospheric rivers that dumped up to 30 inches (76 cm) of rain over some areas, little of that reached the Colorado River basin.