Dead delta's former productivity: Two trillion shells at the mouth of the Colorado River

The diversion of the Colorado River by dams and irrigation projects, starte d in the 1930s, triggered the collapse of the Colorado delta ecosystem, Pal eontological, ecological, geochronological, stable isotope, field, and sate llite image data provide estimates of the delta's benthic productivity duri ng the 1 k.y. directly preceding the artificial shutdown of the river. At l east 2 x 10(12) shells of bivalve mollusks make up the current beaches and islands of the delta. The 125 individual valves dated using C-14-calibrated amino acid racemization indicate that these shells range in age from A.D. 950 to 1950. Seasonal intrashell cycles in delta O-18 values indicate that average-sized bivalves lived at least 3 yr. The most conservative calculati on based on these numbers indicates that during the time of natural river f low, an average standing population of similar to6 x 10(9) bivalve mollusks (population density similar to 50/m(2)) thrived on the delta. In contrast, the present abundance of shelly benthic macroinvertebrates is similar to 9 4% lower (3/m(2) in 1999-2000). The dramatic decrease in abundance testifie s to the severe loss of benthic productivity resulting from diversion of th e river's flow and the inadequacy of its partial resumption (1981 to presen t). An integration of paleontological records with geomorphological, geoche mical, and geochronological data can provide quantitative insights into hum an impact on coastal ecosystems.