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Obesity: Science weighs in Contact us Science
Drained of life

The Colorado River's delta ecosystem has shriveled because of man-made changes to water flow, scientists contend


By Alexandra Witze / The Dallas Morning News

SAN FRANCISCO – Trillions of dead clams, so numerous that their shells form entire islands in the Gulf of California, bear mute witness to the draining of the American West's mightiest river.

Colorado River Delta facts

Length of the river: 1,400 miles

Drainage basin: 244,000 square miles

Interrupted by: More than 10 major dams and more than 80 major water diversions

Irrigates: More than 3.7 million acres of farmland

Population served: More than 30 million

States that share water rights: Seven in the United States and two in Mexico

Area of wetlands in the delta historically: 1,930,000 acres

Area of wetlands in the delta today: 150,000 acres

SOURCES: Environmental Defense Fund; Colorado River Water Users Association

The clams – and the once-thriving ecosystem they represent – fell prey to water diversions from the Colorado River, scientists have found. At the river's delta, where fresh water once fed an estuary the size of Rhode Island, only a trickle now creeps. The water instead nourishes Phoenix, Las Vegas, Los Angeles and other cities.

Biological productivity in the Colorado delta – a measure of the environment's health – is just 5 percent of what it was before water diversions began, a new study shows.

Piles of dead clams have revealed, for the first time, just how profoundly the water diversions have changed the river's delta, says Karl Flessa, a University of Arizona paleontologist who led the research.

"How do you know what it was like before humans started affecting the environment, if you don't have any records of what it was like?" he asks. In using the clams, he says, "we are putting the dead to work."

Conservationists may use the research to justify releasing some of the Colorado's water to the delta it once nourished. A coalition of environmental groups is suing the U.S. government over its plan to conserve endangered species on the lower Colorado, saying the plan doesn't take into account the health of the entire delta.

Until the clam research, scientists had no "before" baseline against which to compare the "after" effects of water diversions, says paleontologist Michal Kowalewski, another member of the research team.

"People always said that this ecosystem has been gravely altered by damming of the river and by agricultural activities," he says. "But it was always very anecdotal and qualitative. ... We made some measurements and got some numbers."

In a series of recent research papers, and in a talk this month in San Francisco at a meeting of the American Association for the Advancement of Science, Dr. Flessa laid out the dramatic findings.

"This work is really like opening a window and being able to see back in time," said Ed Glenn, a biologist at the University of Arizona. "What Karl's doing is really innovative and informative."

Colorado River
Virtual tour

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Dr. Flessa's team has been visiting the delta since 1992, gathering data on the numbers and distribution of dead clams. The scientists estimate that 6 billion living clams inhabited the delta in its heyday, packing every square foot with 5 clams on average. Today, each square foot contains an average of only 0.3 live clams, most of a different species.

Mulinia coloradoensis, the clam species that once was so abundant that its shells form islands, has nearly vanished today. The researchers found just 12 live clams of that species.

The dead M. coloradoensis provide more damning evidence: Scientists analyzed the shells for clues to the clams' age. Of 125 clams studied, virtually all were found to date between A.D. 950 and 1950, says Dr. Flessa. Almost none of the dated clams were from after 1950.

Dr. Flessa points out that this timing roughly coincides with the completion in the 1930s of Hoover Dam, the first big water-diversion project along the Colorado.

"We cannot show that the youngest shells are exactly from the 1930s, but a decrease in shell frequency in the age class A.D. 1900-1950 is consistent with that interpretation," the researchers wrote in a report in Geology.

The ecosystem declined even further after Hoover Dam was finished. Since the 1960s, when Glen Canyon Dam was built, virtually no freshwater has reached the river's delta, says Dr. Flessa.

More than 30 million people, and 3.7 million acres of farmland, rely on Colorado River water. By treaty, Mexico must receive 1.5 million acre-feet per year. The United States is entitled to 7.5 million acre-feet for the upper basin states of Colorado, Wyoming, Utah and New Mexico, and another 7.5 million acre-feet for the lower basin states of California, Arizona and Nevada. California, which has been routinely using more than its annual share of 4.4 million acre-feet, is being forced under new guidelines to cut back.

Returning some of that flow to the Gulf of California might partially revive the delta ecosystem, researchers say.

"Obviously, having some water getting to the delta would be very useful," says Dr. Kowalewski, of Virginia Polytechnic Institute and State University in Blacksburg.

But scientists don't know just how much water is required to truly help the delta.

In earlier studies, Dr. Glenn found that shrimp catches in the northern Gulf of California improve any year after high rainfall causes more fresh water than usual to reach the delta. That finding suggests that the river still carries critical nutrients to the delta, just as it did a thousand years earlier.

The Colorado delta is home to two endangered species: the vaquita, a type of porpoise, and the totoaba, a fish. Both species have dropped in numbers in recent decades. But other factors, such as hunting or overfishing, could account for that drop, Dr. Flessa says. M. coloradoensis, by comparison, tastes so bad that no one harvests the clams for food, making them a good way to monitor environmental change.

In another study, Dr. Flessa's team has found that clam shells preserve a record of declining freshwater flows. While alive, the clams incorporate into their shells different forms of oxygen, depending on the saltiness of the water in which they live. Chemical analysis of the oxygen in the shells showed that clams living before 1930 incorporated more of the freshwater oxygen than saltwater oxygen.

With further research, Dr. Flessa hopes to find out whether the delta has weathered such dramatic shifts in the past. After all, it's possible that flows of fresh water into the delta changed over time because of climate or other factors. That information, in turn, could help scientists better understand whether the delta is experiencing unprecedented changes today.

"If we find that the systems are very volatile, then perhaps the fact that we are shutting down the river is not such a critical issue," says Dr. Kowalewski.

"But if we find that over 1,000 years we have changes on the scale of only 10 percent," he says, "then a drop of 90 percent is alarming."

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