Paleoecological reconstruction of the Colorado River delta and estuary shows that the composition and productivity of its molluscan fauna changed dramatically as a result of the damming and diversion of river water for human uses. We analyzed stable isotopes of oxygen from radiocarbon and amino-acid dated mollusk shells to determine the species' salinity tolerances. We deciphered the mode of formation of shelly deposits and measured their time of formation to estimate the pre-dam density of the delta's mollusk population. Surveys of the live fauna were used to estimate current densities. The oxygen isotope composition of pre-dam shells of the bivalve mollusk Mulinia coloradoensis indicate that this once-dominant species thrived when river water still reached the delta. Since upstream river management began in 1935, the population of this species has been drastically reduced -- live specimens are now very rare. Before the dams, river-borne nutrients supported mollusk densities reaching 50 per square meter; densities today are generally less than 10% of pre-dam levels. Cessation of fresh water flow to the Colorado delta and estuary has profoundly altered the region's salinity, sedimentation rates, water circulation, faunal composition and productivity. Partial resumption of flow would likely restore some key marine habitats. The paleoecological methods applied in this study can be used to estimate the composition and prehistoric productivity of coastal ecosystems elsewhere. Such estimates will be especially valuable in areas where no biological surveys were made before humans modified the habitat.

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