But today, only 10 percent of the water in the Colorado even reaches the border with Mexico, depriving that desert region of an essential ingredient for agricultural and urban prosperity.
In the last couple of decades, federal officials have tried to respond to demands to release more water from the Hoover to protect the downstream environment, and in wet years the river gushes anew. For awhile.
Kowalewski and Flessa say there’s some evidence that riparian habitats north of the delta have benefited from the increased supply, but that apparently hasn’t been the case in the critical habitat where the river meets the gulf. The research shows that clams, for example, are not rebounding.
The scientists believe the method they used may be even more important than their findings, because the same techniques can be used to chart the history of almost any coastal region back to the time before human intervention. Most marine species decay after death, leaving a scant record of their existence, but the hard shell of the clam persists.
By dating 125 clam shells from the delta, the scientists were able to reconstruct the last 1,000 years. They found that the average clam lived about three years, so the period covers 333 generations of clams.
The population remained quite stable throughout that period, Kowalewski says, with an average of about 6 billion at any time.
But the significance of the findings go far beyond the lot of the clams. The huge delta—about the size of Rhode Island -is a critical habitat for many species. It is the region where fresh water mixes with salt water, providing an ideal nursery for all sorts of animals, like shrimp, fish and waterfowl.
So the abundance of clams means the conditions were right for many creatures to flourish. Likewise, the decline of clams suggests that many species suffered from a deteriorating environment. That bolsters the claims of fishermen in the region who insist that the diversion of water from the Colorado has had a profound impact on all sorts of marine creatures.
So, where do we go from here? Water is such a prized commodity in the southwest that it’s not likely anyone is going to give up their share to help a delta in Mexico. Wars have been fought over such issues.
But Flessa points out that a little may do a lot.
“The question for us right now is how much more fresh water would it take to restore some of these habitats,” Flessa says. “The honest answer is we don’t really quite know.”
Further research may show that a little increase in water flowing down the river—maybe just 10 per cent—might do wonders for the ecosystem. But even that water will most likely have to be replaced by water from somewhere else, and it’s hard to find water in the desert. Conservation might help, but the Southwest is one of the most rapidly growing regions in the nation, so every drop is precious.
Without an alternative supply, Kowalewski says, the outlook is quite grim.
“This is not going to improve,” he says. “It is only going to get worse.”
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.