Effects of inherited resistant layers on channel-bar shape and long-term morphological evolution of the Eems-Dollard Estuary (the Netherlands/Germany)

Erodible or erosion-resistant layers are widely acknowledged to be important for fluvial and estuary morphology and sedimentary architecture, but little is known about their effects on long-term evolution of Holocene estuarine morphology. In this contribution we show how the presence of inherited resistant layers determine the position of confluences and bars along the entire Eems-Dollard estuary. This estuary, located on the Dutch-German border, has served for decades as a prime example for autogenically developed systems worldwide. By comparing detailed historical bathymetry maps of the last two centuries to the recently mapped position of resistant Pleistocene clays and tills, we demonstrate that resistant layers at critical positions (ca. 10-15 m – MSL or two-thirds of the maximum channel depth) cause channels to widen locally where these layers are present. Channel confluences, on the other hand, preferentially form where resistant layers are absent. Estuary widening leads to mid-channel bar formation, and bend curvature subsequently causes the effect to propagate at least a meander wavelength in landward direction. The importance of resistant layers increases when sediment is exported from the system, for example during dredging. Erosion-resistant layers also occur in many other estuaries and deltas, because depth of channels is similar to thickness of Holocene wedges underlain by harder layers until dredging and channel confinement leads to deepening. Our results show how inherited geology controls the evolution of fluvial and tidal sedimentary systems and challenge the view that these are predominantly formed by hydromorphodynamics. The insights on the effects of inherited geology on channel-bar patterns will help to better manage modern estuaries in a sustainable way and to understand and predict channel belt dimensions in the fluvial rock record.