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Variation in human activities has greatly impacted the processes and intensities of erosion, sediment transport and storage throughout the Late Holocene, and many lowland rivers around the world have responded to these variations. Although this long-term process–response relationship has been established before, the effects of short-term (c.200-year) changes in human impact on lowland rivers are less well studied. Here, we followed an integrated approach whereby observations of floodplain changes are evaluated against detailed data on human impact for three lowland rivers in the Belgian loess belt: Dijle, Mombeek and Gete rivers. Pollen data were used to reconstruct changes in local and regional vegetation and to calculate human impact scores. Corings along transects and a database of c.160 radiocarbon ages were used to reconstruct geomorphic changes in the river valleys. Our results show a decrease in human impact between 200 and 800 AD, which can be related to the decreased population density in Europe during the first millennium AD. During this period, forests in the studied catchments regenerated, soil erosion decreased, hillslope–floodplain connectivity decreased due to the regeneration of valley-side vegetation barriers, and sediment input in the floodplain decreased. A reaction to this decreased human impact can be observed in the river valleys during the first millennium AD, with a regrowth of the alder carr forest and an increase in the organic matter content of the alluvial deposits with a local reactivation of peat growth. The observed trajectories of Belgian river valleys during the first millennium AD provide more insight into the sensitivity of these river valleys to short-term variations in human impact. These results can in turn be used to better estimate the effects of future changes in the catchments on the fluvial system.

The two-step Landscape Reconstruction Algorithm (LRA) -comprising the models REVEALS and LOVE -translates fossil pollen assemblages into quantitative reconstructions of past vegetation cover. The REVEALS model has been applied to lakes and bogs of varied sizes, but application to records from alluvial sites is currently lacking and could further extend these reconstructions to areas where lakes and bogs are absent.In addition, since alluvial sites are often located in areas that experienced early anthropogenic impact, such data will provide more insight into land cover change in the context of agricultural development. In this paper, we test the performance of the REVEALS model using pollen records from multiple alluvial sites obtained from the floodplains of the Belgian Dijle catchment. The modelled vegetation cover is compared to the observed vegetation cover based on a historical land cover map of ca. AD 1775 (ca.175 cal BP). The discrepancy between the modelled and the observed regional vegetation cover is relatively small and can largely be explained by local differences in land cover surrounding the sites, based on the results of the LOVE model. This study concludes that the LRA approach accurately reconstructs the regional vegetation cover of the river catchment based on pollen obtained from alluvial floodplains, and shows that it is able to reflect major local differences in land cover as well. Including pollen data obtained from alluvial floodplains could lead to a substantial increase in spatial coverage of future LRAbased land cover reconstructions.

<p>Rivers and alluvial floodplains are dynamic environments facing natural and anthropogenic impacts. A thorough knowledge of the functioning of alluvial floodplains and their sensitivity to changes in internal and external driving forces is required for sustainable management of these ecosystems.</p><p>During the Early and Middle Holocene, most floodplains in northern and central Belgium were stable environments with limited floodplain aggradation resulting in the formation of peat. During these times, floodplains consisted mainly of large marshes where peat accumulated and river channels were absent or small. During the Late Holocene, these environments changed completely towards single channel meandering rivers with overbank deposits, impeding peat accumulation, largely as a result of increasing anthropogenic impact. However, this evolution in floodplain geoecology is diachronous as some river valleys transform a few thousand years before others.</p><p>Previous research already showed that river systems respond non-linearly to changes in land-use and land-cover in their catchments, as land-use intensity and slope-channel coupling need to cross a certain threshold to result in significant change. Hence, the differences in timing of floodplain response can to some extent be related to different land-use trajectories in the river catchments. Based on previous qualitative and semi-quantitative research the exact land-cover threshold, i.e. the land-use intensity required to result in transformation of the fluvial system, as well as the timing at which this threshold is crossed, could not be detected. Hence, a quantitative assessment of the resilience of floodplain environments to regional land-use changes is needed. A successful pilot REVEALS-based reconstruction of the Dijle catchment, showed a decrease in forest cover from the Bronze Age onwards, accompanied by an increase in the proportion of cereals.</p><p>In this study, we constructed a database of pollen-records collected in the eastern part of Flanders, mainly retrieved from river floodplains, as deposits from large lakes are not available in the area. We selected sites with varying soil properties, topographies, and histories of human impact in their catchments, to uncover regional differences in land-cover evolution through the application of the REVEALS model. To assess the applicability of this model to alluvial deposits, modern pollen data will be included and outcomes will be compared to modern vegetation maps. In addition, vegetation reconstructions will be compared with historical maps (available from 1778 AD onwards).</p><p>Results will help to answer questions regarding the sensitivity of Flanders to (future) environmental changes. Our study contributes to the understanding of Holocene land-cover change and its drivers, by providing quantitative vegetation cover reconstructions for Belgium that are currently lacking in the European REVEALS reconstructions. Moreover, it extends the application of the REVEALS model to pollen-records retrieved from alluvial deposits.</p>