DISCOVER Vol. 22 No. 3 (March 2001)
Table of Contents

Silence of the Clams
By Sara Pratt

In 1922 ecologist Aldo Leopold described the delta of the Colorado River as a "milk and honey wilderness" where wild jaguars roamed and melons grew heavy on the vine. Today, after 70 years of damming and diversion, the river barely trickles into Mexico's Gulf of California, its destination for 6 million years. To figure out exactly how the collapse of the delta ecosystem occurred, a group of scientists turned to the delta's most abundant inhabitant - the shells of two trillion dead clams and few dozen live ones. The shells hold markers of environmental change that can be used to reconstruct thousands of years of the delta's history.

Paleobiologist Karl Flessa and his colleagues at the University of Arizona at Tucson examined the carbonate shell beaches and islands at the mouth of the river to determine what the total mollusk population might have been before humans started impounding and diverting the river. Their results show a decline from 50 clams per square meter when the river was at full force, to about 3 per square meter today -- a 94% reduction in the biodiversity of the benthic, or bottom-dwelling, creatures. "This really documents the collapse of the delta ecosystem most dramatically," says Flessa.

Using a technique that measures how long a shell has been dead by examining the ratio of two types of amino acids, the group found that the shell beaches had been deposited over the millennium between 950 and 1950. And based on the age distribution of the shells, they calculated that the 2 trillion shells that exist today represent only 40 percent of the clams that lived in the delta over the millennium. Older shells have been eroded away.

Flessa and his colleagues found the average lifespan of a clam -- about three years -- by measuring oxygen isotope values in the shells' growth rings. This value is the ratio of two different atomic weights of oxygen: oxygen-16 and oxygen-18. The ratios vary with changes in temperature and can therefore indicate the number of seasons a clam lived. "You get a high value when the water is cold, you get a low value when it's warm," says Flessa. "When we see four peaks in isotopic values, we're seeing four cold seasons." The scientists were also able to see that the bulk of the demise occurred after the 1930s, when Hoover Dam was built.

Flessa suggests that one result of the study may be future attempts to restore a portion of the river's historical annual flow for the benefit of the delta ecosystem. "In other river systems a concept has arisen of an allocation for nature," Flessa says. "The idea is that we allocate water for agricultural purposes, we allocate water for domestic purposes, for recreational purposes -- maybe we should allocate some for nature. Then the question becomes, how much?"

That is an answer Flessa thinks he'll be able to answer. Thanks again to oxygen isotope ratios, scientists can use the clam shells to help reconstruct historical flow patterns of the Colorado. "River water is isotopically more light compared to seawater," Flessa says. So the oxygen isotope values can be used to distinguish between growth that occurred in predominantly fresh water, when the river flow was strongest, versus growth in saline water flowing in from the Gulf of California. Correlating this with the number of growth lines on the shells means the scientists can precisely gauge the time of year of each flow rate. "What that allows us to do is actually see how much water was coming down the river in June versus how much water was coming down the river in September," says Flessa.

Aldo Leopold didn't live to see the demise of his cherished delta. He died of a heart attack in 1948 while helping his neighbors fight a grass fire. But unless measures are taken to restore some of the annual flow to the Colorado River, the abundant wildlife and vegetation that Leopold described will soon be completely gone. Flessa has studied one species in particular that has been severely affected by the drastic decrease in flow. The marine invertebrate Mulinia coloradoensis, first described around the last turn-of-the-century, has been so little studied that it was just recently given a common name, the Colorado Delta clam. Only about 30 examples of the species have been found alive since 1992. Though it is not yet on the endangered species list, Flessa intends to see that it is added.


Carlie A. Rodriguez, Karl W. Flessa and David L. Dettman. "Effects of Upstream Diversion of Colorado River Water on the Estuarine Bivalve Mollusc Mulinia coloradoensis." Conservation Biology, February, 2001, pp. 249-258.

Kowalewski, M. et al., "Dead delta's former productivity: Two trillion shells at the mouth of the Colorado River," Geology, v. 28, no. 12. December 2000, pp.1059-1062.

Karl Flessa's University of Arizona home page is at:

See the Delta Project at The Center for the Study of Dead Clams Web site:

© Copyright 2000 The Walt Disney Company. Back to Homepage.