Lacustrine Geomorphology and Sedimentary Processes in the Eastern Arm of Manicouagan Reservoir, Québec

English Abstract:

High resolution multibeam bathymetric and sub bottom profile data were acquired for the first time in 2014 in former Lake Manicouagan, a 60 km long and 3 km wide fjord-lake located in the eastern half of the Manicouagan crater, that is now submerged under 135 m of water in the Manicouagan hydroelectric reservoir. The acquired multibeam echosounder data allowed the production of a complete and full-bottom coverage bathymetric map of the lake area before its flooding at 5m-pixel resolution. This new data provides an invaluable opportunity to analyse the Late Quaternary evolution of this large and deep lacustrine basin and the geomorphological impacts of the recent rapid and high amplitude rise in lake level. Multibeam data allowed the observation of disintegration moraines, sublacustrine fans, gullies, mass movement deposits and crescent-shaped bedforms as well as preserved river beds, fluvial terraces, deltas and strandlines flooded after the construction of the Manic-5 (Daniel-Johnson) Dam in 1970. The multibeam data is complemented by 3.5 and 12 kHz subbottom profiles and short coring samples. Our data suggest that the flooding of the crater basin generated widespread sediment failures in the deglacial and postglacial sedimentary units along the steep slopes of former fjord-lake Manicouagan. The new bathymetric data also allows identifying Lake Manicouagan as one of the deepest lake of eastern North America, which reached 315 m in depth before its flooding.