Effects of upstream diversion of Colorado River water on the estuarine bivalve mollusc Mulinia coloradoensis

The Colorado River of the United States and Mexico is extensively dammed an d diverted; only a fraction of its previous flow still reaches its estuary. How has the lack of freshwater inflow affected the estuary's biota? Becaus e no prediversion studies are available for comparison, we examined the com position and geochemistry of subfossil shells dating from before the Colora do's diversion. The bivalve mollusc Mulinia coloradoensis was once the most abundant species of clam inhabiting the Colorado Delta. Today, however, on ly a small population survives near the mouth of the river. The relative ab undance of empty shells of this species decreases with increasing distance from the mouth of the Colorado River, indicating that M. coloradoensis was dependent on the flow of the river. The delta O-18 values in shells of subf ossil M. coloradoensis are significantly more negative than delta O-18 valu es in live-collected shells of Chione fluctifraga, the most common bivalve living on the delta today. This indicates that M. coloradoensis lived in wa ter lower in salinity than is now typical of the delta. The decline in abun dance of M. coloradoensis is most likely due to the post-1930 decrease in f low of Colorado River water to its estuary. Paleontological and geochemical analyses of subfossils can provide environmental baselines for communities that existed prior to human alteration of the habitat.