Why does this Quebec reservoir form a ring?

In the images of Quebec, the Manicouagan reservoir draws a striking ring. Its shape seems to tell of a single gigantic event. It combines two stories separated by an immense duration.

The first is that of an asteroid impact, about 214 million years old according to NASA’s presentation. The second belongs to the twentieth century: the construction of the Daniel-Johnson dam has raised the water and brought together previously distinct lakes. The central lands were surrounded by the reservoir.

Water has therefore made a structure much older than itself particularly legible. The present circle is neither a form entirely made by the human being, nor a lake that has remained the same since the disaster. It is an encounter between a geological trace and a modern layout: two time scales that photography makes fit in the same drawing.