High-resolution multiproxy records of sedimentological changes induced by dams in the Sept-خles area (Gulf of St. Lawrence, Canada)

A multiproxy approach was performed on two sediment box cores sampled in the NW Gulf of St. Lawrence in order to better understand the impacts of recent climatic or land-use changes on the sedimentary regime in the proximal offshore zone. Multibeam and seismic surveys allowed for the careful selection of coring sites. The chronology of the cores was established by 210Pb measurements and validated with coinciding historical weather events recorded in both cores, confirming that the sediments are recent (last 370 years). The lithological variations in the cores were quantified using 1) stratigraphic markers and grain size measurements, 2) high-resolution X-ray micro-fluorescence (XRF), and 3) physical and magnetic tracers. The grain size measurements allowed the identification of a major flood around AD 1844 ± 4 years, whereas a drastic decrease in variations and in the median grain size (d50) occurred around AD ~ 1900, highlighting the offshore impact of the SM1 dam construction on the Sainte-Marguerite River in the early 1900s. Sedimentological variations have been investigated by wavelet analysis using XRF data and the sharp disappearance of high frequencies (< 16 year periods) around AD 1900 in the core facing the dammed river (Sainte-Marguerite River), but not in the reference core, also provides evidence of dam influence on the proximal offshore zone sedimentary regime. Indicators are proposed for a posteriori environmental impact assessment of dams.