Holocene geoecohydrological floodplain dynamics in NE Belgium: regional drivers of local change

During the Late Holocene, many lowland river systems in temperate Europe transformed from multichannel rivers in densely vegetated peatlands to single-channel, meandering rivers with overbank deposits in more open floodplains. While this transformation in floodplain geomorphology, ecology, and hydrology (i.e. geoecohydrology) is well recognized, its timing varies significantly both within and between different river catchments. To unravel whether the observed differences in floodplain response are due to differences in the timing and nature of the driving forces or to differences in sensitivity to them, we compare long-term and large-scale reconstructions of geoecohydrological floodplain dynamics and of (climatic and anthropogenic driven) land cover change for two contrasting regions: the central Belgian loess belt and the sandy Campine region.

Using a combination of cluster analysis, ordination and Ellenberg indicator scores on a large multi-proxy and multi-site dataset, we identified key trends in the past geoecohydrological evolution of northeastern Belgian floodplains. These trends are largely determined by changes in floodplain wetness, which can in turn be linked to fluctuations in upland forest cover. The Early and Late Holocene floodplain transformations correspond with the respective increases and decreases in upland forest cover in the vicinity of the sites, largely determining the water availability in the river catchments and thereby their local geoecohydrological conditions. Initially, these evolutions were driven by climate, but during the Middle- and especially Late Holocene, they became increasingly influenced by anthropogenic impact, causing the evolutions in the two studied regions to diverge. While floodplains with active peat growth are still found in the sandy Campine region at present, they have become rare in the central Belgian loess belt due to the combination of a higher degree of human impact and a higher soil erodibility in this area. Despite the considerable spatiotemporal variability of the floodplain transformations, we observe a trend toward faster floodplain responses to upland land cover changes over time, likely related to the growing hillslope-floodplain connectivity.