Macrofaunal and isotopic estimates of the former extent of the Colorado River estuary, upper Gulf of California, México

Faunal and isotopic evidence can be used to reconstruct the zone of fresh water influence of the Colorado River prior to its diversion for agricultural and domestic uses. The beaches and islands of the Colorado delta are predominantly composed of shells of the bivalve mollusk Mulinia coloradoensis. The shells date from before the construction of upstream dams, and δ18O values from the shells are significantly more negative thanδ18 O values from species living in the delta today. Both faunal and isotopic evidence indicate thatM. coloradoensis is a brackish water species that thrived when the river flowed into the Gulf. The proportion of empty shells of M. coloradoensis ranges from 80–95% near the riverʹs mouth to only 25% 65 km to the south. Shells of the species are rare to absent 80 km south of the mouth of the river. Macrofaunal evidence indicates a mixing zone extending as far as 65 km along the western shore of the upper Gulf of California. Averageδ18 O values in shells of M. coloradoensis become more positive with increasing distance from the riverʹs mouth, reflecting the greater dilution of river water with normal salinity Gulf water. Averageδ18 O values in the fossil shells approach values in live bivalve mollusks at a distance 65 km south of the mouth of the river indicating that the mixing zone of the former Colorado River extended at least 65 km from its mouth. The effect of virgin Colorado River flow in the upper Gulf of California was geographically extensive.