Rosemary Lowe scoops up a shovel of dirt and dumps it into a hole around the base of a slender cottonwood tree.

One down, thousands more to go.

Lowe and dozens of volunteers spent a recent day planting native trees along a half-mile stretch of the Santa Fe River that has been reduced to a dry, sandy wash.

"We've got to do something, and this is one little place we can do it," Lowe says, wiping sweat from her brow near Caja del Rio in Agua Fría. "And if we multiply that by thousands of other places around the world, think of what we can do."

Federal agencies, states, tribes and concerned citizens are spending millions of dollars and thousands of hours on waterway restoration projects to reverse decades of poor management and combat the mounting threats of population and climate change.

Nationally, there are more than 37,000 river restoration projects under way, costing more than $1 billion annually, according to a study released this month by Colorado College.

Andrew Fahlund, vice president for conservation for American Rivers, said every region of the country will eventually be affected either by water pollution or overconsumption. "Look at the southeastern United States right now, and you would think you were in the midst of the Colorado River basin," he said. "They're having good old fashion water wars in Georgia and most people associate Georgia with verdant hills and full streams."

The U.S. Bureau of Land Management has spent close to $15 million in the last couple of years on its Restore New Mexico program, which includes oilfield restoration as well as work on the rivers and streams that flow through BLM land.

The U.S. Forest Service spent about $500,000 on watershed work in New Mexico and Arizona last year and plans to spend just as much this year, said Penny Luehring, watershed improvement program manager for the agency's southwest region.

Just weeks ago, the agency and its partners finished planting willow trees along the Centerfire Creek in Western New Mexico as part of a comprehensive plan that included removing cattle and building culverts for a road that crosses the creek.

Land managers agree that cooperation has been essential in trying to treat entire river systems rather than just a stretch at a time.

"We've been very successful in telling the story to all different kinds of groups — industry groups, conservation groups, other agencies — and they've all been very willing to join with us to try and fix some of these past mistakes," said Linda Rundell, state director for the BLM in New Mexico.

The work has resulted in more wildlife habitat, fewer invasive species, less erosion and the recharging of the aquifer in many areas. And managers say those benefits can't be realized soon enough.

Federal researchers at Sandia National Laboratories in Albuquerque predict the fresh water supplies of more than half of the nations in the world will be stressed in less than 20 years, and by 2050, three-quarters of the world could face fresh water scarcity.

The U.S. is no exception, said Michael Hightower of the lab's Energy Systems Analysis Department. Groundwater pumping will likely have to be reduced in the next five to 10 years to prevent the depletion of many of the nation's aquifers, he said. "We've been overpumping those aquifers for the last 50 years and it's beginning to catch up with us," Hightower said.

John Horning, the executive director of WildEarth Guardians, the environmental group sponsoring the recent Santa Fe River planting day, said rivers throughout the Southwest need to be made resilient so they can withstand reduced flows.

"Most rivers in the Southwest have been damaged in one way or another. This one," he said, standing in the middle of the sandy Santa Fe, "has had a dam on it for over 100 years so we don't have the perennial flows that we used to have. As a result, what was once a pretty lush, rich corridor for wildlife and for humans isn't that anymore."

"It's pretty much an open wound, and we're trying to heal it," he said.