A hotter, drier climate is worsening the imbalance between water supply and rising demand in seven Western states where 40 million people depend on the Colorado River, U.S. Interior Secretary Ken Salazar announced Wednesday after completion of a three-year study.

The study projects a future of falling river flows, shrinking snowpack, wilting crops and an intensifying struggle for wildlife.

Millions of people would be affected by shortages, Salazar said.

“We are in a very troubling trajectory,” Salazar said in a phone conference with journalists and senior officials. “We need to reduce our demand. We also need to look at increasing our water supply through practical, doable, common sense measures such as re-use.”

State water authorities who helped fund and conduct the study have included multi-billion dollar options they are considering to import water into the Colorado River Basin. Examples: diversion of Missouri River water to the Denver area and beyond, and desalination so that oceans could be tapped.

Salazar called the river diversion — as well as ideas submitted to float icebergs south and import water on tankers — politically and technically infeasible. More promising options, he said, are wider re-use of water, conservation programs of the sort that cut consumption in Denver and Albuquerque, and desalination as promising.

“There is no silver bullet that will solve the issue,” he said.

But with collaboration, he saw hope of avoiding legal wars and eventually putting people in sync with their arid environment.

“We can do this,” he said.

The Colorado River Basin Water Supply and Demand Study concluded that climate change will reduce the long-term average of 15 million acre-feet in the river by 9 percent to 13.7 million acre-feet. It found that, within 50 years, the Western states’ annual water deficit will reach 3.2 million acre-feet — but could be as high as 8 million acre feet, depending on population growth. (An acre-foot has been regarded as the amount needed to sustain two families of four for a year.)

Such a deficit adds to the complications resulting from the miscalculation of the average flow of the river by about 1 million acre-feet in the interstate compact that for a century has guided allocation of river water to the states.

Overall, the study reflects growing recognition by the federal Bureau of Reclamation that climate change has serious consequences and confirms ecosystem limits that scientists and people along the river and its headwaters have been observing for years.

Denver residents rely heavily on the Colorado River Basin for water, which is diverted under the Continental Divide through a system of tunnels to the city. Denver Water manager Jim Lochhead, who also chairs the Front Range Water Council, swiftly responded to the federal findings on behalf of metro utilities.

“While this is a critical issue for Colorado, we have time to approach solutions thoughtfully,” Lochhead said. “We don’t need to pursue drastic solutions in the short-term.”

The seven western states, he said, “can work within the framework of existing law and institutions” to ensure sufficient water supplies.

Conservation groups hailed the study as a positive step toward dealing with potentially ruinous problems. Some questioned state estimates of future demand, saying states are motivated to secure federal funding for major development projects.

“It’s critically important that planners start to account for the new normal — a hotter, drier Colorado River Basin,” said Michael Cohen of the Pacific Institute. “Now is the time for Reclamation and the basin states to embrace the proven approaches that have already reduced demand for Colorado River water.”

The conservationists called the proposed Missouri River pipeline ludicrous and construction of five desalination plants far too expensive. The study presents alternative proposals developed by the Environmental Defense Fund, Nature Conservancy, Western Resource Advocates, Pacific Institute and Nuestra Rio that emphasize wider efforts to save water. Average daily use in Albuquerque has been reduced to 70 gallons a day. Denver has reached 85 a day, down from 104 in 2001.

By far the most river water is siphoned for growing food.

EDF regional director Dan Grossman said creation of water banks and better systems for sharing and re-using water appear most promising and feasible.

“The Colorado River is the lifeblood of the dry West and what makes it possible for us to live in this spectacular region,” he said. “We can’t keep bleeding the river.”

But states increasingly are taking an all-of-the-above approach including development of new water sources. “Imbalances of this magnitude are of great concern to us,” said Kay Brothers of the Southern Nevada Water Authority, serving as a leader for state interests in the study.

Bureau of Reclamation Commissioner Mike Connor said he’ll convene states, tribes and others early next year “to take a hard look at some of those proposals.”

Back in 1992, the federal government built a desalination plant near Yuma, Ariz., for $250 million. It was supposed to treat salty agricultural runoff so that water could be pumped back into the Colorado River. But for years this plant has been seen as a white elephant bogged in political and technical muck. Salazar said it is now operational and has increased water supplies.

Colorado Water Conservation Board section chief Ted Kowalski, who led the state’s participation in the report, said Colorado officials “have not taken any of the options off the table at this point.”

But legal and institutional hurdles around the Missouri River proposal “would make that option very difficult,” Kowalski said.

State authorities currently are prioritizing conservation, re-use of water, regional water banks and desalination as primary elements of an emerging common strategy.

Bruce Finley: 303-954-1700, twitter.com/finleybruce or bfinley@denverpost.com