Effects of Holocene climate changes and anthropogenic river regulation in the development of a wave-dominated delta: The São Francisco River (eastern Brazil)

The São Francisco River is the fourth longest river in South America and one of the most regulated. Severe coastal erosion has affected the delta shoreline since 1985, leading to the complete destruction of Cabeço village between 1997 and 1999. In this study, we mapped and radiocarbon dated the beach ridge sets occurring on the delta plain and performed a detailed analysis of the delta shoreline changes since 1960. During the Holocene, the delta plain construction was punctuated and took place during episodes of higher river discharges coincident with Bond events 4, 2 and 1 and periods of higher precipitation in the river basin, as reconstructed by δ18O measurements in cave speleothems. The last major episode of delta construction apparently ended at approximately 1.0 ka cal. BP. Since that time, riverine sediment input has been just sufficient to maintain the shoreline. A comparison of historical maps and aerial photographs showed that from 1853 to 1960, the shoreline at the river mouth remained in approximately the same position. A decrease in rainfall in combination with river regulation, particularly after 1985, triggered extensive erosion at the delta shoreline. This erosion was not caused by sediment retention behind the major dams but instead resulted from changes in the backwater/drawdown effects deriving from river regulation. Shoreline erosion mostly affected the river mouth. The mobilized sediments caused progradation of the downdrift shoreline. Updrift of the river mouth, the shoreline remained stable, as it had already reached an equilibrium orientation in which the net longshore transport was zero. •Holocene climate change was the major control in delta plain evolution.•Shoreline progradation took place only during periods of high precipitation in the drainage basin.•Changes in backwater/drawdown effects deriving from river regulation triggered shoreline erosion.•Sediment retention behind dams apparently has not played a role in shoreline erosion.