Abstract
The Colorado River of the United States and Mexico is extensively dammed and diverted; only a fraction of its previous flow still reaches its estuary. How has the lack of freshwater inflow affected the estuary's biota? Because no prediversion studies are available for comparison, we examined the composition and geochemistry of subfossil shells dating from before the Colorado's diversion. The bivalve mollusc Mulinia coloradoensis was once the most abundant species of clam inhabiting the Colorado Delta. Today, however, only a small population survives near the mouth of the river. The relative abundance 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 δ 18 O values in shells of subfossil M. coloradoensis are significantly more negative than δ 18 O values in live‐collected shells of Chione fluctifraga , the most common bivalve living on the delta today. This indicates that M. coloradoensis lived in water lower in salinity than is now typical of the delta. The decline in abundance of M. coloradoensis is most likely due to the post–1930 decrease in flow 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.