The diversion of the Colorado River by dams and irrigation projects, started in the 1930s, triggered the collapse of the Colorado delta ecosystem. Paleontological, ecological, geochronological, stable isotope, field, and satellite image data provide estimates of the delta's benthic productivity during the 1 k.y. directly preceding the artificial shutdown of the river. At least 2 × 1012 shells of bivalve mollusks make up the current beaches and islands of the delta. The 125 individual valves dated using 14C-calibrated amino acid racemization indicate that these shells range in age from A.D. 950 to 1950. Seasonal intrashell cycles in δ180 values indicate that average-sized bivalves lived at least 3 yr. The most conservative calculation based on these numbers indicates that during the time of natural river flow, an average standing population of ∼6 × 109 bivalve mollusks (population density ∼50/m2) thrived on the delta. In contrast, the present abundance of shelly benthic macroinvertebrates is ∼94% lower (3/m2 in 1999–2000). The dramatic decrease in abundance testifies to the severe loss of benthic productivity resulting from diversion of the river's flow and the inadequacy of its partial resumption (1981 to present). An integration of paleontological records with geomorphological, geochemical, and geochronological data can provide quantitative insights into human impact on coastal ecosystems.