The Colorado River is the most important source of water for the arid southwest and its entire flow is now diverted for human uses before it can reach its delta in Mexico. This lack of freshwater and sediment has changed the environments and biotas of the delta. The mollusk Mulinia coloradoensis was once so abundant that the delta's beaches are made up of its shells. However, its abundance has declined dramatically -- only a tiny population persists in a small area near the mouth of the river. It is likely that M. coloradoensis needs the lower salinity conditions created by river flow. Understanding the paleoecology of this species is a key to understanding the environments of the delta before the impact of human activity.
Comparison of oxygen isotope ratios in the shells of dead Mulinia coloradoensis and the living marine bivalve Chione fluctifraga suggest that Mulinia has greater tolerance for lower salinity. Furthermore, Mulinia shells decrease in abundance with increasing distance from the river's mouth. We can estimate the isotopic composition of the Colorado before diversion using freshwater clam shells collected in 1894. These data allow us to predict the minimum river flow that would be necessary to restore the populations of Mulinia coloradoensis to pre-diversion levels.