Global altered estuaries with estuarine dams: Pathways for conservation and restoration

Estuarine dams, built between the estuary mouth and tidal limits, provide freshwater storage and storm surge protection but disrupt natural processes, altering hydrodynamics, sediment transport, and ecosystems. These changes affect freshwater discharge, tidal regimes, stratification, and sedimentation, often degrading water quality and obstructing fish migration. Globally, estuarine dams are found in 10% of 2,396 analyzed estuaries and, along with land reclamation, have caused nearly half of estuarine area loss over 30 years. Their construction peaked in mid-income countries during the 20th century, with limited development in low-income countries due to economic constraints and in high-income nations due to stricter environmental regulations. In a recent study of the Nakdong Estuary in Korea, the morphologic equilibrium following dam construction and subsequent restoration was investigated. Long-term numerical modeling revealed that the estuary achieved equilibrium approximately 15 years after restoration. In contrast, human-altered estuaries stabilized more quickly—within about 9 years—due to hydrodynamic adjustments and sediment redistribution that reduced energy dissipation. Model simulations effectively reproduced key morphological changes, including the transition from barrier island formation under wave-dominated conditions after dam construction to sand shoal development under tide-dominated conditions following restoration. Additionally, the model captured shifts in sediment texture: from sand-dominated under pristine conditions, to mud-dominated during the construction phase, and ultimately returning to sand-dominated post-restoration. This study highlights the value of realistic, long-term numerical simulations in understanding estuarine responses to human interventions and restoration efforts. The findings offer valuable insights for developing sustainable management strategies - conservation in low- and mid-income countries and restoration in high-income countries.