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Silence of the Clams: Reconstructing the history of the Colorado River

by Dr. Peter D. Roopnarine
Asst. Curator, Department of Invertebrate Zoology & Geology
California Academy of Sciences

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The Colorado River is the major source of freshwater today for the arid southwestern United States and northwestern Mexico. Since the 19301s the river has been utilized for irrigation, flood control and to supply water to the growing cities in the southwest. No water from the river reaches the ocean anymore. Prior to damming and diversion, the river flowed into the northern Gulf of California, creating a tremendous delta system in the process.

Plans for long-term development and international water agreements with Mexico are based partly on a historical record of Colorado River flow. The historical record, however, spans less than 100 years, calling into question its long-term reliability. The focus of our research project has been to develop a method for reconstructing the flow of the Colorado on a much longer time-scale, namely 2,000 years.

We have been analyzing the stable isotopic variation of oxygen 16 and 18 in fossil and live-collected clam shells from the northern Gulf of California. Clam shells grow by adding increments, leading each shell to be a record of its own growth environment. Stable oxygen isotopes are incorporated into the calcium carbonate shells during growth. The amount of each isotope incorporated in to the shell is a function of the water1s temperature and isotopic composition during growth.

In the past, the isotopic composition of marine waters in the northern Gulf of California depended both on temperature variation, and on the mixing of oxygen 18 poor river water with oxygen 18 rich marine waters. Today, because the river no longer flows into the Gulf, isotopic variation in shells is due solely to temperature variation. By comparing the isotopic compositions of fossil(pre-diversion) shells with those of live shells, we can estimate the annual volumetric flow of the river in the past. Preliminary results suggest that the historical record of flow is not typical of long-term variation and that the southwest is in general drier than the past century has been.

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