The dry Colorado River Delta will receive a resurrecting flow of water this spring, one that Scientific American calls “an unprecedented experiment in ecological engineering” thanks to a historic agreement between the United States and Mexico.

Starting today, the pulse will be released from Morelos Dam, which sits on the international boundary, and will travel 75 miles to the Gulf of California. Below the dam, the Colorado is usually completely dry. This pulse of water will mark the first time that the United States and Mexico have put water back into the parched riverbed for environmental purposes.

From Scientific American:

The mighty Colorado rises on the western slope of the Rocky Mountains and drains seven US and two Mexican states along its 2,300-kilometer course (see ‘River run’). Before the 1930s, when dams began to throttle the river, its water ran unfettered into the Gulf of California. But most was soon diverted to slake the thirst and agricultural fields of millions of people in the American Southwest. A 1944 international treaty granted Mexico just 10% of the river’s original flow; vast delta wetlands shrivelled to patches of vegetation clinging to sandy plains.

In 2012, officials drew up an addendum to the original water treaty. Known as Minute 319, the agreement lays out how the United States and Mexico will share water surpluses and shortages until the end of 2017. But it also mandates the experimental release of what it calls “water for the environment”.

That was the call to action for a group of researchers from universities, government agencies and non-governmental organizations who had been working for years on delta conservation (E. P. Glenn, K. W. Flessa and J. Pitt Ecol. Eng. 59, 1–6; 2013). They calculated how much water should be released, and over what period of time. As a result of their recommendations, over nearly eight weeks, dam operators will allow some 130 million cubic meters of water to travel downstream.

A few days ago, water managers began releasing the pulse from Lake Mead in Nevada to make its way more than 300 river miles to Morelos. Although releases begin today, with flows starting around 700 cubic feet per second, peak flow from Morelos is expected on March 27 at about 4,200 cubic feet per second with most of the flow coming between March 27-29 mimicking a spring runoff flood. Flows will then slowly decrease for about eight weeks, releasing a total of 105,392 acre-feet of water. From National Geographic:

“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.

This pulse is part of a five-year project designed to reinvigorate the delta. From the LA Times:

Experts from both countries will study the effects of the release. It’s unlikely the water will reach the Gulf of California and unclear whether it will all soak into the soil or be left standing in parts of the channel.

The pulse is allowed under an amendment approved two years ago to a 1944 treaty governing water use by the two countries. The amendment established new rules for sharing water in times of drought and committed both nations, often at odds over water, to conducting the pulse experiment.

The release is intended to revitalize the delta, help Mexican farmers grow their crops, recharge the aquifer and bring back hundreds of bird species native to the region.