The Pekuakamiulnuatsh, the first people to have lived on the shores of Lac-Saint-Jean, named this huge mass of water the Piékouagami, which means "shallow lake" or "flat lake" in the Montagnais language.
The first European explorer to reach Lac-Saint-Jean was Father Jean de Quen, a Jesuit, who in 1647, crossed the boundaries despite warnings from First Nations peoples of the dangers that could beset the navigators on the way. It turns out that the Montagnais people jealously defended their rivers by exaggerating the risks of crossing them. Father de Quen renamed Piékouagami Lac-Saint-Jean in honor of his patron saint.
Lac-Saint-Jean is shallow at its edges, to the great pleasure of bathers. However, the depth at its centre is impressive with its 63 meters. The temperature of the water near the beaches can reach 26 degrees Celsius on the hottest summer days.
With a diameter of 32 kilometres, it is often called an inland sea. The lake is not foreign to this role, since it was there for thousands of years it was the bottom of the Laflamme Sea, a huge body of water that covered a large part of the territory of the greater region of Saguenay-Lac-Saint-Jean. As for the sandy beaches bordering the lake, their existence is due to sediment carried by rivers, stopped by its arrival in the lake, and accumulated and deposited near its mouth. Furthermore, the plains of the region benefited from this marine or glacial sediment deposited by streams of water from the melting glaciers.