Sediment deficit and morphological change of the Rhine–Meuse river mouth attributed to multi-millennial anthropogenic impacts

Many delta systems worldwide are becoming increasingly urbanized following a variety of processes, including land reclamation, embanking, major engineering and port constructions, dredging and more. Here, we trace the development of one system, the Rhine–Meuse delta in the Netherlands (RMD) from two natural estuaries (the RME fed by the Rhine river and the HVL fed by the Meuse river) to a densely urbanized delta and the effect human activities have had on its morphology through time. Estuary outlines determined from palaeogeographical and old maps and tidal range at the estuary mouth were used to reconstruct basic estuary parameters. Depth distribution was predicted with a morphological tool. We have determined that the northern estuary, where port activities dominate, shows a stepwise deepening due to dredging for navigation. The southern port shows stepwise shallowing as humans closed off this branch from tidal action. Both estuaries show narrowing and loss of intertidal width over the past five centuries. The total loss of water volume has been -5.5 m³×10⁹ since 1500 AD coinciding with major human intervention in the system, driven by the rapid economic boom during the mid 16th century. This has led to a reduction of intertidal areas and floodplains and long-term sediment shortage resulting in a myriad of problems, including increased flood risk, threats to bank protection and infrastructure and loss of nature areas. These problems will be exacerbated in coming centuries by predicted sea-level rise. Other urban deltas, unrestricted by engineering and dredging innovations which took place more gradually in earlier centuries, are now undergoing rapid changes in mere decades, changes which took hundreds of years in the RMD. Future predictions indicate that by 2050 the RMD will experience the highest loss of sediment in the 3500 year history of the system, despite these changes only occurring vertically (deepening) and not laterally (narrowing or embanking). This shows an urgent need to reconsider sediment management and spatial planning of port expansion in urbanized systems and the fate of such systems under climate change and rising sea-levels.