Along the Santa Fe River, N.M. —- Rosemary Lowe dumps a shovel full of dirt into a hole around a slender cottonwood tree. One down, thousands to go.

Lowe and dozens of other 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 her brow. "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 last month by Colorado College.

Andrew Fahlund, vice president for conservation for American Rivers, said every region eventually will be affected either by 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-fashioned water wars in Georgia, and most people associate Georgia with verdant hills and full streams."

The 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 rivers and streams.

The U.S. Forest Service spent about $500,000 on watershed work in New Mexico and Arizona last year and plans to spend that 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. Managers say those benefits can't be realized soon enough.

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

The United States 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 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.