Vegetation and peat accumulation steer Holocene tidal–fluvial basin filling and overbank sedimentation along the Old Rhine River, The Netherlands

In the transformation from tidal systems to freshwater coastal landscapes, plants act as eco-engineering species that reduce hydrodynamics and trap sediment, but nature and timing of the mechanisms of land creation along estuaries remains unclear. This article focuses on the Old Rhine estuary (The Netherlands) to show the importance of vegetation in coastal landscape evolution, predominantly regarding tidal basin filling and overbank morphology. This estuary hosted the main outflow channel of the river Rhine between ca 6500 to 2000 cal bp, and was constrained by peat during most of its existence. This study reconstructs its geological evolution, by correlating newly integrated geological data and new field records to varying conditions. Numerical modelling was performed to test the inferred mechanisms. It was found that floodbasin vegetation and resulting organic accumulation strongly accelerated back-barrier infill, by minimizing tidal influence. After tidal and wave transport had already sufficiently filled the back-barrier basin, reed rapidly expanded from its edges under brackish conditions, as shown by diatom analysis and datings. Reed growth provided a positive infilling feedback by reducing tidal flow and tidal prism, accelerating basin infilling. New radiocarbon dates show that large-scale crevassing along the Old Rhine River – driven by tidal backwater effect – only started as nutrient-rich river water transformed the floodbasin into an Alder carr in a next phase of estuary evolution. Such less dense vegetation promotes crevassing as sediments are more easily transported into the floodbasin. As river discharge increased and estuary mouth infilling progressed, crevasse activity diminished around 3800 to 3000 cal bp, likely due to a reduced tidal backwater effect. The insights from this data-rich Holocene study showcase the dominant role that vegetation may have in the long-term evolution of coastal wetlands. It provides clues for effective use of vegetation in vulnerable wetland landscapes to steer sedimentation patterns to strategically adapt to rising water levels

Appendix B Van Dinter 2017

<p>Appendix B to Van Dinter 2017. All sources used for the construction of the palaeogeographical map and the archaeological sites of Appendix A.</p><p></p><p><i>This is part of the manuscript:</i>: Van Dinter, M., 2013. The Roman Limes in the Netherlands: how a delta landscape determined the location of the military structures. <i>Netherlands Journal of Geosciences – Geologie en Mijnbouw </i>92-1, 11-32.</p

Appendix A: Palaeogeographical map of western Lower Rhine, The Netherlands (Van Dinter, 2017)

Palaeogeographical map of Limes zone along western Lower Rhine, The Netherlands (scale 1 : 50,000

Appendix D - Van Dinter 2017

<a>Appendix D Van Dinter 2017. </a><p><i>List of scources used for palaeogeographical reconstruction of Utrecht case study, detailed maps and description of channel belts.</i></p><p><i>This is part of the manuscript: </i>Van Dinter, M., K.M. Cohen, W.Z. Hoek, E. Stouthamer, E. Jansma and H. Middelkoop, 2017. Late Holocene lowland fluvial archives and geoarchaeology: Utrecht’s case study of Rhine river abandonment under Roman and Medieval occupation, and its international relevance. 20-yr FLAG special issue on Fluvial Archives, Quaternary Science Reviews 1-39.</p

Scources used for palaeogeographical development of Utrecht case study, detailed maps and extended description of channel belts

<a>Appendix D van Dinter 2017. </a>- Section A; Data. Section B: Reconstruction.<div><p><i>This is part of the manuscript: </i>Van Dinter, M., K.M. Cohen, W.Z. Hoek, E. Stouthamer, E. Jansma and H. Middelkoop, 2017. Late Holocene lowland fluvial archives and geoarchaeology: Utrecht’s case study of Rhine river abandonment under Roman and Medieval occupation, and its international relevance. 20-yr FLAG special issue on Fluvial Archives, Quaternary Science Reviews 1-39.</p></div

Appendix C - Van Dinter 2017

<p>Appendix C - Van Dinter 2017. Parameter values and underlying assumptions used in the modelling of the carrying capacity of the Lower Rhine delta (Chapter 5).</p><p></p><p><i>This is part of the manuscript: </i>Van Dinter, M., L.I. Kooistra, M.K. Dütting, P. van Rijn (†), and C. Cavallo, 2013. Could the local population of the Lower Rhine delta supply the Roman army? Part 2: Modelling the carrying capacity of the delta using archaeological, palaeo-ecological and geomorphological data. Journal of Archaeology in the Low Countries 5-1, 5-50.</p

Vegetation and peat accumulation steer Holocene tidal–fluvial basin filling and overbank sedimentation along the Old Rhine River, The Netherlands

In the transformation from tidal systems to freshwater coastal landscapes, plants act as eco-engineering species that reduce hydrodynamics and trap sediment, but nature and timing of the mechanisms of land creation along estuaries remains unclear. This article focuses on the Old Rhine estuary (The Netherlands) to show the importance of vegetation in coastal landscape evolution, predominantly regarding tidal basin filling and overbank morphology. This estuary hosted the main outflow channel of the river Rhine between ca 6500 to 2000 cal bp, and was constrained by peat during most of its existence. This study reconstructs its geological evolution, by correlating newly integrated geological data and new field records to varying conditions. Numerical modelling was performed to test the inferred mechanisms. It was found that floodbasin vegetation and resulting organic accumulation strongly accelerated back-barrier infill, by minimizing tidal influence. After tidal and wave transport had already sufficiently filled the back-barrier basin, reed rapidly expanded from its edges under brackish conditions, as shown by diatom analysis and datings. Reed growth provided a positive infilling feedback by reducing tidal flow and tidal prism, accelerating basin infilling. New radiocarbon dates show that large-scale crevassing along the Old Rhine River – driven by tidal backwater effect – only started as nutrient-rich river water transformed the floodbasin into an Alder carr in a next phase of estuary evolution. Such less dense vegetation promotes crevassing as sediments are more easily transported into the floodbasin. As river discharge increased and estuary mouth infilling progressed, crevasse activity diminished around 3800 to 3000 cal bp, likely due to a reduced tidal backwater effect. The insights from this data-rich Holocene study showcase the dominant role that vegetation may have in the long-term evolution of coastal wetlands. It provides clues for effective use of vegetation in vulnerable wetland landscapes to steer sedimentation patterns to strategically adapt to rising water levels

Roman impact on the landscape near castellum Fectio, The Netherlands

Castellum Fectio was one of the largest fortifications along the Limes, the northern border of the Roman Empire. The castellum, situated 5 km southeast of Utrecht, the Netherlands, was occupied from around the start of our Era to ca. A.D. 260. It was situated along a river bend of the Rhine that was cut off from the main stream during the occupation of the Roman fort. A 6 m long sediment sequence was recovered from the infill of the residual channel and pieces of Roman wall plaster, glume bases of Triticum spelta and radiocarbon dates indicate that the sediments were deposited during the period of Roman occupation. The combined palaeoecological analyses— palynological, macrobotanical, entomological and geochemical— allow a detailed reconstruction of changing environmental conditions as a consequence of the Roman occupation. The pollen record reveals a dramatic decrease in arboreal pollen, suggesting that the Romans were involved in large-scale deforestation, transforming semiopen parkland to a landscape of meadows and agricultural fields. Non-pollen palynomorphs, botanical macrofossils and insect remains support this conclusion. The recorded mycoflora shows a shift from assemblages characterised by the tree pathogen Kretzschmaria deusta to assemblages dominated by spores of fungi associated with herbaceous plants, concurrent with the decrease in arboreal pollen. The presence of masticated bran fragments of cereals, clover remains, eggs of intestinal parasites and entomological and geochemical data in the upper part of the sequence indicates that these sediments largely consist of faeces that were dumped into the former channel. Surprisingly, seeds of salt tolerant species are encountered in the sediments of this inland site, which was situated outside the influence of the sea. Horses may have brought these seeds to Fectio in their intestinal tracts after grazing in coastal meadows