Arizona became the seventh state, plus Mexico, to join a Colorado River drought plan Thursday, agreeing to help preserve the river's reservoirs, which are dropping to critically low levels.

Arizona Gov. Doug Ducey, a Republican, signed the pact on the last day possible, The Associated Press reported. U.S. Bureau of Reclamation Director Brenda Burman gave the states a Jan. 31 deadline to voluntarily cut back on the amount of water they take from river before she created her own plan.

"The Drought Contingency Plan is the most significant water legislation passed in nearly 40 years — and it was done by putting party labels aside and putting Arizona first," Ducey tweeted.

Under the agreement, the Grand Canyon State must reduce its use of the river's water by up to 700,000 acre-feet by 2026. For perspective, one acre-foot can supply water for one to two households per year.

Arizona was the only state to require legislation to decide whether it would participate in the agreement. Among those who opposed the plan was a group Democrats who called the agreement unsustainable.

State Sen. Juan Mendez told the AP the state should be discussing ideas that longer term and address concerns about Arizona's future water supply.

Farmers in Pinal County, who have the lowest-priority access to the river water, also opposed the Colorado River drought plan. But the state passed another piece of legislation allocating $9 million – $7 million of which will come from taxpayers – for the farmers to build infrastructure to use groundwater instead of water from the river, according to T ucson.com .

Some lawmakers have voiced concern over this return to groundwater, but Republican state Rep. David Cook said there is a 70,000 acre-feet annual limit.