The biological productivity of the Colorado River Delta is only 5 percent of what it was before the mighty Colorado's water was diverted for human uses. Since the 1930s, an environment that supported billions of clams and other life has disappeared because dams and irrigation projects have reduced the flow of nutrient-laden fresh water to the tidal flats of the delta.

Gleaming white clam shells cover the lower reaches of the Colorado Delta, where the river empties into the Gulf of California between the Baja peninsula and mainland Mexico. Satellite images and field data indicate that at least two trillion (2x 1012) clam shells make up the area’s beaches and islands. The shells form miles of sun-bleached ridges, originally shaped by spring floods, tides, and the passing of generations of abundant shellfish. In the past seven decades, after the river's flow virtually stopped, the clams have become sparse.

A new approach for measuring life on the delta over the past thousand years, pioneered in a study by researchers from four universities, can be used to estimate the prehistoric productivity of coastal ecosystems in other parts of the world. Such estimates will be especially valuable in areas where no biological surveys were made before humans modified the habitat.

“We used a variety of tools to combine paleontological, biological, geochemical, and satellite image data. It was exciting to find out that we can use fossils to address environmental issues that have direct societal relevance,” says Michal Kowalewski, geological scientist at Virginia Tech.

The research report by Kowalewski, Guillermo Avila of the Universidad Autonoma de Baja California, Karl Flessa of the University of Arizona, and Glenn Goodfriend of George Washington University was published in Geology (December 2000). The research was funded by the Eppley Foundation for Research, the National Science Foundation, and the U.S. Geological Survey.

“The satellite images told us the area covered by shells, but that doesn’t provide a full story,” explains Kowalewski. “For instance, do the shells represent a short interval of high productivity, or a long interval of low productivity? Combining satellite data with radiocarbon and amino acid dating shows the changes on the delta.”

By dating 125 shells collected throughout the inter-tidal zone, the researchers learned that essentially all specimens came from the past 1,000 years. Interestingly, every 50-year time between 950 AD and 1950 was represented among dated specimens, showing that shells were continuously produced in the delta before humans altered the river. Thus, the two trillion clam shells washed up onto the delta tidal flats represent 1,000 years of biological productivity. Because shells grow as the animal secretes new layers of material, the scientists could measure life span of long dead clams by counting the seasonal oxygen isotope cycles in the shell (water from spring snow melt has fewer isotopes). They learned that the average clam lived three years; so in 1,000 years, there were 333 generations of clams. That means that at any given time there were six billion clams (2x1012/333) living on the delta, with estimated density of 50 clams per square meter.

"The mud flats of the Colorado River Delta have one of the highest tidal ranges in the world — up to 10 meters. This macro-tidal system was greatly influenced by the river, but now, at its mouth, the Colorado is more like a channel than a river,” says Kowalewski.

The researchers found that where there were 50 specimens per square meter in the past (~five per square foot), today there are only three per square meter (0.3 per square foot). “These estimates indicate a 20-fold drop in the shellfish productivity in the delta since the river has been diverted by humans,” says Kowalewski.

The river's resources are in demand by two nations; thus coordinated efforts have been made since 1981 to release more water to the delta, as rain and snow melt allow. As a result, there has been partial revival of riparian habitats in the upper delta. But benthic (sediment-water interface) ecosystems of the lower delta remain decimated. Only 12 live specimens of the mollusk Mulinia coloradoensis were found, for instance. Yet Mulinia made up about 90 percent of the two trillion shells. Even after adding all small mollusks and all shelled nonmollusk macrofauna, the current density of the benthos (18 per square meter) is only 36 percent of the density during the times when the river flowed to the delta.

Since shellfish form a vital part of the food chain in the area and are an important food supply for migratory waterfowl, the revival of benthos is critical to the entire ecosystem.

The techniques used in the study — the integration paleontological, geomorphologic, geochemical, and geochronological data — can be used to determine the original productivity of other waterways. “Almost all big deltas, such as the Nile or Mississippi, suffer the effects of human management of the river water,” says Kowalewski. “To restore delta ecosystems to their original state, we have to know what that state was. Our approach can be used to gain insights into the original conditions of the delta and assess the negative consequences of water management.

“The approach can be used almost anywhere because shell-fish are common and their shells survive for hundreds of years,” he says. “We can now at least approximate original productivity and look at changes in ecosystem diversity. We can use these methods in any system where we have a fossil record containing shell material from the last centuries or millennia — rivers, deltas, coastal zones, or marine shelves — to get some idea of the pre-human state of aquatic ecosystems.”

For more information, contact Michal Kowalewski at michalk@vt.edu.