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Scientists Plan for Grand Experiment in the Colorado River Delta

 The dry Colorado River Delta against the Cucapá Mountains. Photo by Erik Rochner/National Geographic

The dry Colorado River Delta against the Cucapá Mountains. Photo by Erik Rochner/National Geographic
 
Once written off as dying of thirst and beyond revival, the delta of the Colorado River is slated to get a rejuvenating flood that for scientists offers a unique opportunity: the chance to study how plants, trees, birds, fisheries, and the vast delta ecosystem as a whole respond to an experimental pulse of river water.
 
This flood is made possible by Minute 319, the add-on to the 1944 treaty between the United States and Mexico that divides the Colorado River between the two neighboring nations.
 
Signed in late 2012, Minute 319 allows Mexico to store some water in Lake Mead, the giant reservoir behind Hoover Dam, establishes new rules for sharing shortages in times of drought, and commits the two nations to return some flow to the delta as part of a five-year pilot project.
 
The Colorado Delta was once one of the planet’s great desert aquatic ecosystems, boasting 2 million acres of lush wetland habitat.  For millions of years, it received a huge spring flood as the winter snows melted in the Rocky Mountains and the resulting flows coursed south.  The flood waters spread across the delta before emptying into the upper Gulf of California.
 
That yearly flood cleansed the river channel and floodplain, recharged groundwater, aided the reproduction of native cottonwoods and willows, and sustained the overall delta ecosystem and its extraordinary bird and wildlife habitat.  It also connected the Colorado River to the sea, where fisheries depend on the mixing of saltwater with fresh water for their spawning and rearing grounds.
 
But the construction of big dams and river diversions during the 20th century siphoned off the river’s flow, leaving little or none for the delta in most years of the last half century.  Wetlands have shrunk to about 10 percent of their former area, and fish, birds and wildlife have declined dramatically.  The native Cucapá, who fished and farmed in the delta for at least a thousand years, have lost their way of life.
 
The last time the delta enjoyed a significant influx of fresh water was in the late 1990s, a period of unusually high precipitation in the Colorado watershed that resulted in “surplus” water passing through the basin’s dams, across the international border, and on to the thirsty delta in northwestern Mexico.
 
Writing in this week’s issue of Eos, a publication of the American Geophysical Union, seven scientists who have collectively clocked decades of research in the Colorado Delta note that those 1990s flood pulses “demonstrated the resilience of the riparian zone and gave hope for its potential restoration, should a regular supply of water be found.”
 
Minute 319 calls for a flood pulse of 105,392 acre-feet (130 million cubic meters).  The water will be released from Lake Mead at Hoover Dam, and then, mimicking the historic natural flood, will flow south to the delta.
 
Compared with the pre-dam spring flood of some 15 million acre-feet, this pulse appears paltry. But the delta scientists expect it to be sufficient to flood low terraces and backwaters, move channel sediments, recharge groundwater, and promote the germination of native cottonwoods and willows, which create prime habitat for birds.
 
Last February, a National Geographic team and I traversed parts of the delta with Osvel Hinojosa Huerta, an ecologist with the Mexican conservation organization ProNatura Noroeste and a co-author of the Eos article.  We visited a local nursery that was growing young cottonwoods, willows and mesquite, native trees that are being planted along the river channel and in wetland restoration sites.  The anticipated spring flood will help them get established and reproduce.
 
Teams of scientists from both sides of the border are collecting pre-flood baseline data and, after the flood, they will track the immediate and longer-term effects on the delta’s vegetation, birds, wildlife, and other ecosystem attributes.
 
The hope is that beneficial results, and the potential for more, will lead the two countries to expand the restoration effort beyond this pilot period and establish a longer-term commitment to revitalizing the delta.
 
The experimental spring flood offers a rare scientific opportunity, to be sure.
 
But even more important, it is a crucial step toward restoring a great aquatic ecosystem, and shows the power of political and scientific cooperation in solving our water problems.

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