The foreseen “green” energy transition requires anticipating environmental hazards associated with hydropower and reservoir impoundment. Manicouagan Reservoir, the “Eye of Québec,” is a prominent annular reservoir formed in 1968 CE by the impoundment of a ring-shaped complex impact crater that raised the lake level of a 60-km-long fjord-type lake by 135 m. Sediment cores recovered from the drowned lake document the sedimentary regime shift from organo-clastic to organic-rich rhythmites following the impoundment. The coring dataset highlights numerous lithofacies interpreted as event layers (i.e., mass-transport deposits and turbidites) related to multiple slope failures previously mapped by acoustic swath bathymetry and subbottom imagery of the drowned lake. Multiproxy characterization and dating of event layers allowed us to reconstruct the chronology and spatial distribution of slope failures and underlying triggers over the past 500 years. Lake-wide slope failures were triggered by the strong 1663 CE earthquake with an epicenter located 450 km away in the Charlevoix-Kamouraska intraplate seismic zone. Subsequent event layers dated between the mid-seventeenth and the mid-twentieth century were caused by spontaneous slope failures and/or by regional hydroclimatic changes that modulated sedimentary inputs and the lake level, two factors conditioning lacustrine slope stability. The rapid filling of the reservoir by seasonal river inputs between 1964 CE and 1968 CE caused the failure of deltas in proximal zones and the deposition of a beige-reddish layer across the lake. The drowning of glaciofluvial sediments and forested slopes shut down clastic sedimentation and reduced the lake’s sensitivity to slope hazards, although the onset of organic sedimentation likely fueled hypoxia in deep basins (depth 400 m). Lake Manicouagan provides a unique, high-resolution record of reservoir-induced slope failures, regional environmental changes, and natural hazards. This study further illustrates how earthquakes and water-level changes condition subaqueous slope stability and sedimentary regimes.
Chassiot et al. (Fri,) studied this question.