The Manicouagan cores

Its round shape and central island have earned Lake Manicouagan the nickname 'the Eye of Quebec'.  This  unique  lake  is located within a meteorite crater and is the deepest in Quebec, reaching depths below sea level. The lake is also part of the territory of the Innu people, which was lost in 1970 when the Manic-5 dam was commissioned. Léo Chassiot, a professor in the Department of Environmental Sciences at the Université du Québec à Trois-Rivières, has made it his mission to decipher the history of this now-submerged landscape, which is preserved in the lake's sediments.

A bit of cartography

“If you look at historical maps, there were two large lakes, forming a slight arc, on either side: Lake Manicouagan to the east and Lake Mouchalagane to the west,” explains Léo Chassiot. Before it was flooded, Lake Manicouagan was already 60 km long, between 2 and 3 km wide, and 320 m deep. The dam raised the water level by 130 m and joined the two lakes together. 
At Univerrsité Laval's Department of Geography, Professor Patrick Lajeunesse was interested in this lake and surveyed its depths using a 250 kHz multi-beam echo sounder. At this frequency, the wave is reflected by the sediments, and by knowing the speed of wave propagation in water and the GPS position, it is possible to create a 3D bathymetric map of the lake bottom. This map reveals a maximum depth of 100 m below sea level! However, this depth is only that of the sediment surface. The rock that forms the true bottom of the lake lies even deeper beneath the sediment layer. To penetrate the sediments, Patrick Lajeunesse used acoustic imaging with lower frequency waves of 3 kHz, capable of imaging 20 to 30 meters of sediment below the surface. Using even lower frequencies, his German colleagues were able to reach the bedrock 250 to 280 meters below the sediment surface. However, this depth is achieved at the expense of resolution, whereas 3 kHz imaging provides a more accurate visualization of sediment accumulation over the last few decades, particularly since the filling of the reservoir. While working as a research professional alongside Patrick Lajeunesse, Léo Chassiot analyzed this data to gain an initial insight into the morphology of the lake and its history. The 3 kHz acoustic data already provided clues about the nature of the sediments, but to improve  his understanding of the sedimentation dynamics, Léo Chassiot undertook to core the lake bottom. Using the geophysical data, he identified 20 sites located between 220 and 440 m deep that were likely to bear traces of the dam's flooding and organized a coring campaign.

Coring expedition

In the winter of 2022, Léo Chassiot arrived at the Uapishka–Monts Groulx station, where he received support from the Pessamit community to travel across the reservoir and collect 20 sediment cores. Anthony Bacon, a territorial agent, helped us for 15 days, safely transporting the equipment on snowmobiles across the ice. Sometimes we had to travel 30 km in one direction to reach a site, and then another 25 km back. Each time, we had to set up the tripod, drill the ice, lower the corer, and bring it back up with the core sample. However, with depths of between 300 and 400 metres, the corer couldn't be raised manually. We attached it to the snowmobile and Anthony slowly pulled it up,' says Léo Chassiot. Twenty cores, of about one metre in length, were retrieved from the depths of the lake, ready to reveal 2,000 years of history

What the cores reveal

Examining the sediments shows that there was a drastic change in the sedimentation regime when the dam was filled. Before 1970, the lake resembled a fjord, characterised by steep slopes that contributed mineral sediments. 'Before the dam was built, the sediments were light brown, but after it was built, they became dark and rich in organic matter. Sedimentation was previously linked to the erosion of glacial deposits, but the construction of the dam submerged all of these active sedimentary systems under 130 metres of water, bringing them to a halt. Since then, there have been no additional mineral inputs, but organic sediments come from the decomposition of flooded soils,' says Léo Chassiot. 
The cores also reveal landslide deposits. "During a landslide, a plume of turbid water and debris rushes down the slopes, depositing thick layers of homogeneous sediments. This is followed by fine particles that remain in suspension and settle very slowly to form clear layers. These are good indicators of landslides,” he explains. These landslides, which were numerous before the dam was built, are now rare, although the dam itself has caused a few. One of the past landslides dates back to the mid-17th century and may well have been caused by the magnitude 7 earthquake that shook Charlevoix in 1663. Radiocarbon dating indicates that a major landslide occurred at the same time on the banks of the Saint-Maurice River. Historical records suggest that the earthquake was likely felt as far away as Boston, so it is plausible that the unstable slopes bordering Lake Manicouagan were destabilised.

What the cores don't tell us

The cores contain other clues about the past which Léo Chassiot would like to analyse, such as charcoal and diatoms, which could provide evidence of forest fires and changes in the quality of the lake water over the centuries. The cores would also reveal the effect of the dam on the mercury cycle. Beyond the lake, he would like to study the hydrological activity of rivers. As he says, 'The region is vast; there are plenty of lakes in which to study the history of the Uapishka Mountains.'

Selected publications

• Chassiot, L. et al. Slope failures and event sedimentation in Manicouagan impact crater lake, northeastern Canada: from the 1663 CE Charlevoix earthquake to the large reservoir impoundment. Geological Society of America Bulletin, in  press (2026).
• Chassiot, L., Lajeunesse, P., L’Heureux-Houde, F.-X., Bernier, J.-F., Lenz, K.-F., & Gebhardt, A. C. (2024). Geomorphology and Late Quaternary evolution of Lake Manicouagan, a deep impounded impact crater-fjord-lake of the eastern Canadian Shield hinterland. Geomorphology, 446, 108978 (2024). https://doi.org/10.1016/j.geomorph.2023.108978
• Lenz, K.-F., Gebhardt, A. C., Lajeunesse, P., Lohrberg, A., Gross, F., & Krastel, S. (2023). Valley morphology and Quaternary seismic stratigraphy of the Manicouagan impact crater lake (Eastern Canada). J. Quaternary Sci., 38, 1025-1043. https://doi.org/10.1002/jqs.3529
• Lajeunesse, P. (2014). Buried preglacial fluvial gorges and valleys preserved through Quaternary glaciations beneath the eastern Laurentide Ice Sheet. Geological Society of America Bulletin 126, 447–458. https://doi.org/10.1130/B30911.1

Affiliations

Institut nordique du Québec
Département des sciences de l’environnement, UQTR
Centre de recherche sur les interactions bassins versants écosystèmes aquatiques (RIVE)

Photo credit: Nadine Beaudet, Les Vues du Fleuve