Researchers discover Quebec’s deepest lake

Lac Manicouagan was located on the eastern side of the Manicouagan reservoir. Th
Lac Manicouagan was located on the eastern side of the Manicouagan reservoir. The white line marks the former shoreline. Another large body of water, Lac Mouchalagan, was located on the west side of the reservoir. Both lakes disappeared after the reservoir was impounded in the 1960s. - Pierre Markuse
The thick layer of sediment accumulated at the bottom of this lake could be used to study climate change, environmental changes and earthquakes that have occurred over hundreds of thousands of years.

A study conducted by a team from Université Laval has established that the title of deepest lake in Quebec can be claimed by Lac Manicouagan. This new champion has a maximum depth of 320 meters, 40 meters deeper than the previous record. However, there’s no point in hoping to get to its shores to contemplate it or take a photo, as it lies at the bottom of the Manicouagan reservoir.

The Manicouagan reservoir was impounded in the 1960s following the construction of the Daniel-Johnson dam," recalls study leader Patrick Lajeunesse , professor in the Geography Department at Université Laval and researcher with the Quebec-Ocean group. Previously, there were two arc-shaped lakes in this region, each about 60 kilometers long, facing each other: Lake Mouchalagan and, 70 kilometers to the east, Lake Manicouagan."

These two lakes were located on either side of a crater formed 214 million years ago by the fall of a meteorite," continues the researcher. "The rise in water level that followed the construction of the dam meant that the shores of these two lakes are now more than 130 metres below the surface of the reservoir. The water contained in the reservoir reached the beads of the meteorite crater, but the crater’s central rebound was not flooded. This is what formed René-Levasseur Island. In satellite photos, the whole thing looks like a giant eye. Some refer to it as the ’eye of Quebec’."

Lajeunesse’s team used the research boat Louis-Edmond Hamelin to survey the part of the reservoir above Lake Manicouagan. Using data accumulated over several hundred kilometers of transects, they drew up a detailed bathymetric profile of the lake. While we were surveying," says Professor Lajeunesse, "I could see the depth figures climbing all the time. At 452 metres deep - the 320 metres of the original lake and some 130 metres attributable to the presence of the dam - I realized that this corresponded to the height of the Petronas Towers in Kuala Lumpur, the tallest twin towers in the world."

The geological processes that led to the creation of such a deep lake are not known. The hypothesis that seems most likely to me is that this depression was once part of the valley of a steep-sided river that overcrusted the bedrock, forming a deep canyon," says Professor Lajeunesse. The bedrock of Lake Manicouagan is much deeper than 320 metres. At its deepest, it’s 600 metres".

Geomorphologists define the depth of a lake as the distance between the water surface and the top layer of sediment. "Lake Manicouagan’s sediment layer is exceptionally thick. It reaches 280 metres in places. It is thought that, because of the lake’s depth, the sediments would not have been disturbed during the last ice ages. In theory, therefore, we could use these sediments as archives to study climate change, environmental change and earthquakes that have occurred over hundreds of thousands of years."

"You can distinctly see trees and shrubs still standing, streams, beaches, landscapes as they were before impoundment, as if time had stood still."

-- Patrick Lajeunesse, about echo-sounder images of the shores of Lake Manicouagan Surveys taken with a multibeam echo sounder have enabled researchers to visualize what lies at the bottom of the reservoir today. We can clearly see trees and shrubs still standing, streams, beaches and landscapes as they were before the reservoir was filled, as if time had stood still," notes Professor Lajeunesse. If we had the resources, we could do high-resolution mapping and use underwater robots to study the seasonal dwellings that the Innu of Pessamit had established on the shores of Lake Manicouagan. In this way, we could contribute to enriching knowledge of this community’s cultural heritage."

Details of the study have just been published in the journal Geomorphology. The authors of the study are Léo Chassiot, Patrick Lajeunesse, François-Xavier L’Heureux-Houde and Jean-François Bernier, from Université Laval, and their German colleagues Kai-Frederik Lenz and Catalina Gebhardt.