How stable are multiple channels in a delta? : Avulsion and bifurcation in the late Holocene Rhine-Meuse delta, the Netherlands

Deltas have multiple bifurcating channels, that change their position over time (avulsion), to follow the path of the steepest downstream gradient. Gradients are for example affected by sedimentation, subsidence, changing backwater effects, and tidal incursion. In addition, also local meander bend radius at the avulsion point determines which branch is favoured. Given the difference in total hydraulic friction in the channels, a network with multiple channels would be less stable than a single channel. This suggests that evolution towards a single main channel in a delta is most favourable. To test this hypothesis, we focus on late Holocene avulsions in the Rhine-Meuse delta, the Netherlands. From 2500 yr BP onwards, a large scale reorganization of the delta network took place in which a single main Rhine branch silted up in favour of five new branches that eventually evolved into three main branches, when the rivers were embanked around 800 yr BP. This suggests that the delta did not reach the most stable single channel configuration from the preceding period within this timespan. It raises the question what factors determine the most long-term stable channel configuration on a delta scale and how much time does it take to reach this stable situation. To this end, we reconstructed the chronology of these avulsions in high detail and in a 1-D bifurcation model we explored the controls on the stability of these river branches. We show that in the early stage of the avulsion sequence, human reclamation in the lower delta peatland caused the connection between marine ingression and fluvial crevasse channel systems. This formed two new Rhine branches towards the Meuse-estuary tidal inlet around 2000-1500 BP. After these anthropogenically enhanced avulsions, discharge was rerouted into another new branch formed from 1500 BP onwards, which gradually developed towards a large channel in ca. 500 years. Before this channel could grow into the single main channel in the delta, however, other secondary branches were prevented from silting up by humans for navigational purposes and a multi-branch delta remained. These results help to understand the configuration and time control of channel belts in ancient fluvial deposits.