Thanks to a landmark agreement between the United States and Mexico, the parched Colorado River Delta will get a rejuvenating shot of water this spring for one of the first times in five decades, just in time for World Water Day on March 22.

On March 23, 2014, the gates of Morelos Dam on the Arizona-Mexico border will be lifted to allow a "pulse flow" of water into the final stretch of the Colorado River. Officials and scientists hope the water will help restore a landscape that has long been arid but that once supported a rich diversity of life.

"The pulse flow is about mimicking the way the Colorado River flowed in the springtime, thanks to snowmelt from the Rocky Mountains, before all the dams were built," says Sandra Postel, director of the Global Water Policy Project and a National Geographic Freshwater Fellow. By the early 1960s, dams on the Colorado, such as Glen Canyon and the Hoover Dam, had diverted so much water that there was precious little flow entering the lower Colorado.

Water that did make it to Morelos Dam was diverted into Mexico's Mexicali Valley for crop irrigation, leaving little for the wildlife or indigenous people living in the delta.

View Images This spring, a "pulse flow" of water will be released through Morelos Dam on the U.S.-Mexico border in order to benefit the plants and animals in the parched delta. PHOTOGRAPH BY PETER MCBRIDE, NATIONAL GEOGRAPHIC

Water for the pulse flow is being released from Lake Mead behind Hoover Dam at an unspecified time. It will take a few days to travel some 320 river miles (515 kilometers) to the Morelos Dam. On March 23, the gates of Morelos Dam will be opened by the International Boundary and Water Commission, which operates the structure. That will allow the pulse flow to enter the last 70 miles (113 kilometers) of the Colorado River. Peak flow through the gates is expected around March 27, and then the flow will taper to a lower volume for about eight weeks.

As agreed upon by the U.S. and Mexico, the total amount of flow over the period will be 105,392 acre-feet of water (130 million cubic meters). That represent less than one percent of the pre-dam annual flow through the Colorado, "but in terms of recent flows it is very significant," says Postel.

The outcome of the pulse flow remains somewhat unpredictable. Groundwater "sinks" along the route will trap an unknown amount of the water, and debris could block part of the flow or cause it to reroute. "There's a lot of uncertainty because this is an experiment that hasn't been done before," says Postel. (See "The American Nile.")

If the flow reaches the Sea of Cortez (Gulf of California), as scientists hope, it should happen in about two weeks. Except for a few short periods of heavy precipitation (most recently in the 1990s), the Colorado has not reached the sea since 1960. That has negatively impacted what used to be one of the world's most productive fisheries, which previously benefitted from the nutrients brought by the river. (See "8 Mighty Rivers Run Dry From Overuse.")

View Images Formerly lush, the Colorado's delta in the Sea of Cortez has long been an arid wasteland, thanks to overuse of the river. PHOTOGRAPH BY PETER MCBRIDE, NATIONAL GEOGRAPHIC

Rebirth of a Lush Ecosystem

"We can't wait for the water to come," says Osvel Hinojosa Huerta, a Mexican ecologist with the nonprofit Pronatura Noroeste who has spent years studying the delta. Hinojosa Huerta, who is also a National Geographic Emerging Explorer, says the pulse flow will help restore about 70 miles (113 kilometers) of the river's course and 2,300 acres of floodplain, including freshwater marshes.

The region once boasted two million acres of wetlands that comprised one of the planet's great desert aquatic ecosystems. But decades of scarce water have reduced vegetation in the delta by 90 percent, and recent years of drought have made the situation even more extreme.

The flow will benefit hard-hit cottonwood and willow trees and provide habitat for a host of wildlife, including endangered birds such as Yuma clapper rails, Virginia rails, and California black rails, says Hinojosa Huerta. Migratory birds like warblers and flycatchers will also benefit from restored habitat in the delta, which serves as an important corridor on their journey. The southwestern willow flycatcher is one species of special concern, he notes.

The pulse flow is timed to coincide with maximum seed production of native willows and cottonwoods, says Hinojosa Huerta. Those trees have been dying off in the delta in recent decades, because floodwaters are the primary way they disperse their seeds, he notes.

"The reason the pulse flow ramps up quickly and then has a long tail is because the peak flow is to spread the seeds and the tail is to maintain soil moisture so the seedlings can grow and the roots can follow the water down into the soil," says Hinojosa Huerta.

View Images The mighty Colorado is reduced to a trickle in Mexico. PHOTOGRAPH BY PETER MCBRIDE, NATIONAL GEOGRAPHIC

Monitoring the Flow

For months scientists have been making detailed ecological studies of the lower Colorado River in order to gather baseline data before the dam gates open. Scientists from the U.S. Geological Survey, the Mexican government, the University of Arizona, the Universidad Autónoma de Baja California, Pronatura, and others have been studying the river and the surrounding ecology. Once the pulse flow starts, scientists will be monitoring water flows, salinity, temperature, groundwater recharge, vegetation growth, and impacts on birds, fish, and other wildlife.

A primary goal is to "see how water behaves in this system," says Jennifer Pitt, who works on Colorado River issues for the Environmental Defense Fund in Boulder, Colorado. Pitt and Hinojosa Huerta co-chair a binational working group on the river's restoration.

"We might learn that it would have been better to have less volume of water for more days, or that we got it just right, or maybe that we need twice the volume for one day, and so on," says Pitt. "Osvel [Hinojosa Huerta] did his dissertation on where the best bird diversity exists in the delta and found a strong correlation to open water, and now we'll be able to test his conclusions," she adds.

View Images Instead of wetlands, much of the Colorado River Delta is now covered by dry, salty, cracked earth. PHOTOGRAPH BY PETER MCBRIDE, NATIONAL GEOGRAPHIC

Binational Cooperation

The landmark agreement clearing the way for this spring's water release, known as Minute 319, was signed in November 2012 as an addendum to the 1944 water treaty between the U.S. and Mexico.

In addition to the pulse flow, the agreement allows Mexico to store water in U.S. reservoirs, and it specifies that both countries will share the benefits of water surpluses and the burdens of water shortages. It also promotes cooperation on conservation projects such as removing invasive tamarisk.

Minute 319 provides benefits that are "critically important on both sides of the border," says Anne Castle, the assistant secretary for water and science at the U.S. Department of Interior in Washington, D.C.

The agreement is the first in which two countries have come together to allocate water specifically to benefit the environment in a cross-border setting, Castle says. Governments in other countries are watching the pulse carefully, she added. Kyrgyzstan has already expressed interest in the agreement as a model for how an international river might be shared.

The current agreement between the U.S. and Mexico expires in 2017, but Castle says there is "very significant interest in discussing an extension of Minute 319.

The pulse flow will give us more information to work out the details for future agreements."

Hinojosa Huerta says a key to winning widespread support for the pulse flow along the Colorado has been assuring water users that the event will not affect their own water rights in any way. "Farmers, irrigation districts, and water managers have been very supportive," he says. "They are excited that the river is going to have water again."

The Colorado River Delta may never recover to its former size and glory, "but we know that if you add some water, life does return," says Postel. "We've seen rivers running dry all around the world, from being dammed and diverted, and here's one ecosystem of great significance that two countries are working cooperatively to try to restore. So many others need restoration too."

Help restore water to the Colorado River Basin by joining Change the Course, a project of National Geographic and partners. Sign up online or text "River" to 77177.