Marine Life Decline in Colorado Delta Studied

A new study of the troubled Colorado River delta suggests that marine life there has declined by 95% since the 1930s, when the United States started damming the river and diverting its water toward the booming cities of the Southwest.

Scientists reported in Friday’s issue of the journal Geology that clams, once so abundant that their shells formed large islands in the Gulf of California, have nearly disappeared there. The scientists estimate that in the 1930s the area held 6 billion clams at a time, or five of the animals in every square foot. Today, they report, there is, on average, one clam in every three square feet--and many areas are wholly devoid of clams.

“The islands we camp on are made of nothing but shells. You go out there today, you don’t see nearly as many,” said Karl W. Flessa, a paleontologist at the University of Arizona who terms his new report “Silence of the Clams.”

The study is important, ecologists say, because it provides scientific documentation to back the claims of local fishermen and environmentalists who say the dearth of river water is killing the desert estuary. The delta is where the river reaches the sea and where fresh water and salt water mix. The area, where Baja California meets the Mexican mainland, is almost the size of Rhode Island.

“Before [the study], it was all anecdotal information--Mexican fishermen saying there used to be fish and shrimp when the river flowed. People dismissed those as old wives’ tales,” said Ed Glenn, a botanist at the University of Arizona who has done extensive studies of the habitat along the edges of the Colorado River.

Flessa and his colleagues in Mexico and Virginia studied the clams because their hard shells leave a record of their existence, unlike the bodies of other animals that decay and are lost to the geological record. The clams, he says, stand as a proxy for other marine organisms, such as fish and shrimp, that may also have once filled delta waters.

“ ‘Teeming with life’ is the phrase that comes to mind,” Flessa said.

Some of Glenn’s studies support Flessa’s contention. Glenn found that shrimp and fish populations rebounded in years after overflow water was released from Hoover Dam and flowed all the way to the sea.

River water carries nutrients and reduces the salt content of the water, which can be crucial to survival of a species. For shrimp larvae, for instance, fresh water serves as a kind of nursery: The young shellfish there are protected from saltwater predators.

The new study, using geological dating techniques, provides a portrait of what the area’s biology might have been like before water was diverted. There is a gap in knowledge about the area because environmental impact statements were not required in the 1930s, when Hoover Dam was built and the river’s fate was changed.

That project provided the water and power for Los Angeles and made it possible for Las Vegas and Phoenix to rise from the desert.

Delta advocate David Hogan applauded the study, calling it “substantial scientific evidence of what has long been common knowledge.”

Horgan, who coordinates the rivers program for the Tucson and Berkeley-based Center for Biological Diversity, is part of a coalition of environmentalists who have sued the federal government over the damaging effects of dams and water diversions on listed species.

Currently, the United States uses about 90% of the river’s water, or about 13.5 million acre-feet per year. Mexico takes the remainder, 1.5 million acre-feet. Unless there is an excess of water, as in an El Nino year, almost no fresh water reaches the Gulf of California, Flessa said.

Environmentalists are urging the government to allocate money, not just for cities, but for species as well. At a minimum, Hogan said, his group would like to see a quarter-million acre-feet of water flowing through the delta every three to four years, representing the occasional flooding that occurred in the natural river system. Environmentalists also would like to see a small amount of water flow through the river each year.

Scientists agree that even a partial restoration of the river--and preservation of the endangered Southwestern willow flycatcher, the Yuma clapper rail and the desert pupfish--could be accomplished with a fraction of the water that once flowed through the delta. Other species that stand to benefit are the totoaba, a 6-foot-long game fish, and the vaquita, the world’s smallest porpoise.

“Not nearly as much water is needed as people thought for some of those species,” Glenn said.

At the very least, he said, he hopes the studies will raise awareness of the need for occasional flooding of the area. He also hopes they remove U.S. Bureau of Reclamation managers’ temptation to capture the extra flow during rainy years and use it to provide cities more than their current allocations.