Holocene sediment deposition in contrasting Belgian catchments

peer reviewedHolocene alluvial sediment deposition was studied in several contrasting Belgian catchments: the Geul (380 km2) and its tributary the Gulp (47 km2), the Amblève (1070 km2) and its tributary the Lienne (148 km²), and the Dijle (750 km2). These catchments have a comparable Holocene climatic evolution, but they differ largely in land use history. Detailed total Holocene sediments deposition was assessed for each catchment using more then 1000 hand augerings. Detailed radiocarbon dating of fluvial deposits was performed in the Dijle catchment, while iron slag was used as a tracer for sediments deposited after 1350 AD in the Lienne catchment. For the Geul River the presence of lead contamination, originating from 19th century mining activities, were used as a tracer. Results show that sediment deposition is much largest in the Dijle catchment (4.6 Mg/ha catchment area) then in the Gulp catchment (1.3 Mg/ha catchment area), while the lowest deposition is found in the Amblève catchment (0.2 Mg/ha catchment area). It is clear that these differences are mainly due to historical land use differences, although it can not be excluded that part of these differences are due to physical factors, especially for the Amblève catchment. For the latter catchment, the distribution of iron slags in the floodplain sediments showed that large parts of the alluvial plane were eroded after 1350 AD, and thus that there is no net aggradation as for the Dijle and Gulp floodplain. Remobilization of floodplain sediment in the Amblève makes it also difficult to estimate Holocene erosion rates. Nevertheless it is clear that an important part of the sedimentation took place after the Middle Ages, and that during Medieval times the river changed from a system with two or more channels to a system with one single channel. This is probably related to increasing deforestation after 1350 AD. Dating results of the Dijle floodplain show that the sedimentation rates are largely influenced by land use with main deposition after the Middle Ages. There is a gradual increase in sedimentation rate since Neolithic times and each new cultural period is recognized by an increase in sedimentation rate related to the intensification of land use. Several sites show that sedimentation rates have recently decreased, probably due to a reduction in cropland area. 12 Dating of the Geul River floodplain suggests a high sedimentation rate during the 19th century and lower rates for more recent times, which can be related to the more intense 19th century land combined with mobilization of sediments by the mining activities, which ended at the end of that century. From the data collected in these catchments it is clear that land use changes are the main driving factors for accelerated alluvial sediment deposition rates, although climate fluctuations can have influenced the impact of land use changes