The morpho-sedimentary record of impact crater Lake Manicouagan in northeastern Canada, from an overdeepened valley to a large hydroelectric reservoir

Manicouagan, the ‘Eye of Québec’, is an annular-shaped impact structure located in northeastern Canada (51˚N). The completion of a large hydroelectric dam in 1970 CE flooded two 60-km long lakes that were located on east and west sides of the inner ring. The drowning of the lakes under 135 m of water marked the final stage of the Late Quaternary history of Manicouagan. The morpho-sedimentary record combining sismo-acoustic imagery, swath bathymetry, and sediment cores aims to (1) reconstruct the Late Quaternary history of Manicouagan; and (2) assess the impacts of damming, in this case the drowning of a large boreal lake that used to be an indigenous cultural habitat. At the eastern end, the flooded Lake Manicouagan, has a fjord-like setting characterized by a shelf representing the lowest lake-level, steep walls > 45˚, and deep environments. The glacial-postglacial evolution, linked to the northward retreat of the Laurentian Ice Sheet, is preserved in overdeepened basins reaching 455 m deep, that is 105 m below modern sea-level. To the north, these basins are buried under a postglacial drape resulting from strong river inputs. Downstream, sedimentary units build channel-and-levee systems along a deep canyon. At mid-lake, a sill connects to southern basins displaying a hummocky topography. Postglacial conditions are characterized by a lake-wide record of gravity events originating from the shelf and above. Core-scanning techniques along with dating information (¹⁴C, ²¹⁰Pb, ¹³⁷Cs) on a transect of short-cores highlight the role of natural and human-induced water-level fluctuations on the generation of slope hazards. Ongoing analyses expect to (1) precise the nature, timing, extent and impacts of natural and man-made hazards; (2) reconstruct environment-climate evolution from a remote boreal region where limnogeological records are scarce.