Use geopedological methods for the evaluation of sedimentation rates on river floodplains, southern Québec, Canada

This article pertains to the fluvial dynamics of rivers in southern Québec, in particular to the aggradation and pedogenetic processes observed in floodplains affected by periodic floods. The frequency of flood events, notably along the Saint-François River and its main tributaries, leads to fine materials being frequently deposited on floodplains and affected development of alluvial soils. Particle size and physical and chemical analyses have led to a better understanding of the dynamics involved in the formation of floodplains and the development of soils related to this fluvial environment. Also, sedimentological analyses (layer texture and thickness, microstructure) combined with radiocarbon dating (14C) and isotopic methods (210Pb, 226Rd) enabled the determination of sedimentation rates along the floodplains. The radiocarbon dating results obtained from the organic layers buried in alluvial soils show relatively variable ages, i.e. between 2210 ± 60 and 30 ± 70 years BP. The data gathered reveal an active overbank deposition, which shows evidence of the high flood recurrence in many rivers of the basin. It is estimated that the annual floodplain aggradation ranges from 1.0 to 7.6 mm yr− 1 on average, which causes increases in floodplain elevations (vertical accretion). The sedimentation rates obtained using the constant rate supply dating model (210Pb) show average values that range from 2.1 to 10.7 mm yr− 1. Also, the presence of contaminated layers at the lower level (> 100 cm) of the floodplains suggested an active sedimentation rates along the rivers affected by floods.