Restoration flows for the Colorado River estuary, México: estimates from oxygen isotopes in the bivalve mollusk Mulinia coloradoensis (Mactridae: Bivalvia)

Because of competing demands for freshwater, restoration of estuaries requires estimates of inflows to sustain key species. In this study we estimated the pre-dam salinities of the Colorado River estuary by using oxygen isotopes in subfossil shells of the bivalve mollusk Mulinia coloradoensis. Since the construction of upstream dams and water diversions, average salinity in the estuary has increased to 38 practical salinity units (psu) and the population of M. coloradoensis has decreased by ~90%. In the pre-dam estuary, specimens grew when salinity ranged from 22 to 33 psu at the mouth of the river while populations 40 km distant grew at salinities from 30 to 38 psu. The river flow needed to reduce salinities at the mouth of the river to those recorded in the most distant localities (40 km from river’s mouth) ranges from 120 to 290 m3 s−1. If these flows were sustained for a year, they would total 7–16 % of the river’s annual average historical flow (~1.8 × 1010 m3)