page_title: Ottawa River
section_title: Geology
hierarchical_section_title: Ottawa River / Geology
caption_attribution_description: This is a map of the Ottawa-Bonnechere Graben. Elevation data and U.S. boundaries from USGS, Canadian boundaries and hydrology from Digital Chart of the World. Ottawa-Bonnechere Graben boundaries based on elevation data with [1], [2], [3], and [4] used as references.
context_page_description: The Ottawa River is a river in the Canadian provinces of Ontario and Quebec. It is named in honour of the Algonquin word 'to trade', as it was the major trade route of Eastern Canada at the time. For most of its length, it defines the border between these two provinces. It is a major tributary of the St. Lawrence River and the longest river in Quebec.
context_section_description: 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.
  As the glacial ice sheet began to retreat at the end of the last ice age, the Ottawa River valley, which, along with the St. Lawrence River valley and Lake Champlain, had been depressed to below sea level by the glacier's weight, filled with sea water.  The resulting arm of the ocean is known as the Champlain Sea.  Fossil remains of marine life dating 12 to 10 thousand years ago have been found in marine clay throughout the region. 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.
  Large deposits of a material commonly known as Leda clay also formed.  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.  The town's residents had previously been relocated because of the suspected instability of the earth in that location.
  As the land gradually rose again the sea coast retreated and the fresh water courses of today took shape.  Following the demise of the Champlain Sea the Ottawa River  Valley continued to drain the waters of the emerging Upper Great Lakes basin through Lake Nipissing and the Mattawa River. Owing to the ongoing uplift of the land, the eastward flow became blocked around 4000 years ago. Thereafter Lake Nipissing drained westward, through the French River which later became a link in the historic canoe route to the West.
caption: Map of the Ottawa-Bonnechere Graben
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