The city of Phoenix this week outlined how it will voluntarily contribute water to a regional plan to shore up the country’s largest reservoir that delivers Colorado River water to three states and Mexico.

The river cannot provide seven Western states the water they were promised a century ago because of less snow, warmer temperatures and water lost to evaporation. Water managers repeatedly have had to pivot to develop plans to sustain it for the long-term.

Phoenix, the nation’s fifth-largest city, is among entities in the river’s lower basin that are part of the “500+ Plan” meant to delay further mandatory shortages. All pieces of the plan haven’t been finalized, but farmers and Native American tribes are expected to play a big role.

The Colorado River serves more than 40 million people in Arizona, Nevada, New Mexico, Colorado, California, Wyoming, Utah and Mexico. Lake Mead and Lake Powell store the water and are used to gauge the river’s health.