Drought and demand are pushing the Colorado River beyond its limits — with the needs of more than 40 million people in seven Western states projected to outstrip dwindling supply over the next 50 years, according to an advocacy group’s report on endangered rivers released on Wednesday.

The annual top-10 list by Washington, D.C.-based American Rivers points to a three-year federal Bureau of Reclamation study that warned last December that the river won’t always be able to serve all the residents, businesses, ranchers, Native Americans and farmers who rely upon it.

Already, the Colorado River is drained of nearly every drop by the time it reaches Mexico, American Rivers spokeswoman Amy Kober said.

The list, produced with Protect the Flows and Nuestro Rio, notes that the Colorado is sometimes called the most controlled and plumbed river on Earth, and has more dams and diversions planned. River water irrigates nearly 4 million acres of farmland, which yield about 15 percent of the nation’s crops, and serves as a primary drinking water supply for cities including Denver, Los Angeles, Las Vegas and Phoenix.

Federal and local officials planned to highlight the threat to the Colorado River at a news conference Wednesday at a marina on the scenic but drought-scarred Lake Mead reservoir behind Hoover Dam. A white “bathtub ring” of minerals stretches more than 100 feet up the rocky shoreline from the current water line to the 1983 high-water mark. Officials say Lake Mead is still about half-full.

The listing drew an endorsement from a top Interior Department official, who praised it for “bringing further visibility to the problems facing this critical resource.”

“The river is the essential foundation to the health and prosperity of the American Southwest.” Assistant Interior Secretary for Water and Science Anne Castle said in a statement. It “provides multiple and diverse benefits including water supplies for agriculture and people, outdoor recreation, clean energy generation, and unparalleled ecosystems.”

Kober noted the Colorado River also topped the America’s Most Endangered Rivers list in 1991 and 2004, and made the list in 1992, 1997, 1998 and 2010. Rankings are determined by nomination from river groups and advocates based on the size of the threat, the significance of the waterway to people and nature and whether it can help influence action in the coming year.

“The bureau report underscores that there’s not enough water to meet demand,” Kober said. “No one is going to fix the Colorado River in one year. But we need people to come together.”

Castle noted that in addition to completing the 163-page Colorado River Basin Water Supply and Demand Study, the federal government signed a recent supplement to a Colorado River water use treaty with Mexico. It provides for international cooperation to ensure that river water reaches the Gulf of California for the first time in decades.