Upstream US dams imperil downstream Mexican clams

provided by University of Arizona

species of clam living in Mexico’s Colorado River Delta is being driven to extinction, because humans use so much river water only a trickle now reaches the sea. Once so abundant that entire islands are built of empty shells, fewer than 30 live Colorado Delta clams (Mulinia colorado-ensis) have been found since 1992. In the early part of the 20th century, Mulinia coloradoensis was the most common clam at the mouth of the Colorado.

    “The change in the environment -- the lack of freshwater -- has caused the demise of these clams. Our work shows that, when these clams were abundant, there was freshwater flowing into the delta,” says paleo-biologist Karl Flessa of the University of Arizona in Tucson. “Now there’s no freshwater coming from the river.”

    By comparing the chemistry of shells from live clams with shells from long-dead clams, he and his colleagues documented the change in salinity of the delta’s water since the 1930s. Researchers Carlie Rod-riguez, Karl Flessa and David Dettman traced the change by testing for a distinctive type of oxygen found more commonly in freshwater. The team also dated the shells using radiocarbon and amino acid analysis.

    Shells of clams that lived before 1930 contained more of the freshwater form of oxygen than shells from clams living on the delta today. The team’s findings are published in the February 2001 issue of Conservation Biology.

    “I think the really cool thing here is that you can reconstruct what things were like before human impact and compare them to afterwards,” Flessa says. “The problem on the delta, like so many other places, is people didn’t start making scientific observations until people had already modified the habitat.”

    The Colorado River once delivered all of its fresh water to its delta in the northern Gulf of California. The river now dries up before getting to the delta because of the nearly complete diversion of river water for irrigation and domestic uses in the United States and Mexico.

    “Turning off the water supply of the Colorado River also turns off the supply of nutrients that reach the northern Gulf of California. And that has probably had a big effect, not just on clams, but on shrimp and fin fish in the area,” Flessa said.

    The reduction in number of shellfish, a vital part of the food chain in the area, has meant a diminished food supply for migratory waterfowl. The delta is a major stopover on the migratory path for many birds, and the reduction of food supply, Flessa and his team suggest, has probably affected some bird populations over the years.

    Flessa says that for the Colorado Delta clam to survive, more river water must be allowed to reach the Gulf of California. He adds that the team’s estimates of the delta’s past salinity can serve as valuable guidelines for restoration efforts.

    This research was supported by the National Science Foundation, the Eppley Foundation for Research and the Center for Biological Diversity.

    Contact: Karl W. Flessa, Department of Geosciences, University of Arizona, Tucson, AZ 85721 USA; (520) 621-7336;