Top water decision-makers from seven Western states plan to join conservation groups and Native American tribes in San Diego today to begin hammering out rules for squeezing every useable drop from the overtaxed Colorado River.

The work meeting hosted by federal water managers comes amid dire predictions for the waterway. The U.S. Interior Department secretary issued a call to arms five months ago, and declared the river already described as the most plumbed and regulated in the world would be unable to meet demands of a growing regional population during the next 50 years.

“We’re looking at a very significant chance of declaring a shortage in the Colorado River basin in 2016,” Michael Connor, commissioner of the Bureau of Reclamation, said in an interview in advance of the conference.

“We really need to get to specifics, technical liabilities and the political feasibility of projects,” he said.

Connor heads the federal agency responsible for what he called the most litigated and fought-over resource in the country. He said data projects 2013 will be the fourth-driest year in the Colorado River basin during the last 100 years. Last year was the fifth-driest year on record.

The river provides drinking water, power and recreation for about 40 million people in California, Arizona, Colorado, Nevada, Utah, New Mexico and Wyoming. Its largest reservoirs – Lake Mead near Las Vegas and Lake Powell near Page, Ariz. – are projected to drop to 45 percent capacity by September, Connor said.

Mexico also has a stake in the river, and U.S. and Mexican officials signed a pact in November for new rules on sharing Colorado River water, including a deal that lets Mexico store water in Lake Mead. The deal provides for international cooperation to ensure that river water reaches the Gulf of California for the first time in decades.

Anne Castle, assistant secretary for water and science at the Interior Department, called Tuesday’s conference at a U.S. Geological Survey office near San Diego International Airport the start of a “next steps” process.

Castle said she hopes more ideas and practical solutions will surface to deal with shortages predicted by a study released by the bureau in December.

The report looked at supply and demand of Colorado River Basin water. It said that by 2060, with the Southwest’s population expected to swell, the river won’t be able to always serve all the residents, businesses, ranchers, Native Americans and farmers who rely on it.

“This ‘next steps’ process may serve as a template for the way to implement the analysis being done in all these basin studies,” Castle said in a conference call. “Part of that is bringing together all the diverse interests that will be represented.”

Castle said a Ten Tribes Partnership representative of Native American groups and several regional environmental advocates were expected to attend. Plans call for organizing a trio of work groups representing municipal, agricultural and conservation interests.

Jennifer Pitt, head of the Environmental Defense Fund’s Colorado River Project, said groups including Western Resource Advocates, Protect the Flows and Nuestro Rio want to see more water banking, along with more efficient use of existing urban water supplies, the reuse of waste water, better watershed management and improved agricultural techniques.

“Communities that depend on the Colorado River – for water supply or as the foundation of a $26 billion recreation economy – cannot afford to wait,” Pitt said in a statement.