Ottawa River - Geology

Geology

The Ottawa River lies in the Ottawa-Bonnechere Graben, which is a Mesozoic rift valley that formed 175 million years ago. Much of the river flows through the Canadian Shield, although lower areas flow through limestone plains and glacial deposits.

Following the retreat of the glaciers from the area at the end of the last ice age, the valley was flooded by an arm of the Atlantic Ocean known as the Champlain Sea. Fossil remains of marine life have been found in marine clay formed during that time. Sand deposits from this era have produced vast plains, often dominated by pine forests, as well as localized areas of sand dunes, such as Westmeath and Constance Bay. Clay deposits from this period have resulted in areas of poor drainage, large swamps, and peat bogs in some ancient channels of this river. Hence, the distribution of forests and wetlands is very much a product of these past glacial events.

Another consequence was the formations of large deposits of a material commonly known as Leda clay; these deposits become highly unstable after heavy rains. Numerous landslides have occurred as a result. The former site of the town of Lemieux, Ontario collapsed into the South Nation River in 1993; however, the residents had already been relocated because of the suspected instability of the earth in that location.