Because the position of rivers and lakes is governed by the water table, a fundamental link exists between rivers and geology. Channelized bodies of water occur wherever the water table intersects the landscape; both the extent of a drainage basin and the direction in which a river travels are determined by topography, which in turn is controlled by composition, configuration and distribution of the underlying bedrock.

During geologically recent episodes of continental glaciation, when enormous ice sheets scoured southward across the ancient Precambrian heartland of the Canadian Shield, the eventual path of the present-day Ottawa River was modified by discontinuous deposits of unconsolidated glacial till, sand, and gravel – material derived by erosion of the underlying bedrock as a direct result of four major glacial advances.

The interplay between the Ottawa River and its rocky underpinnings has resulted in exposure of features never before seen, as the river carries out its own, more recent process of erosion, revealing vertical sections and near horizontal bedrock platforms along its generally rocky shores. A trip down the Ottawa River reveals remarkable exposures of intrusive plutons, dykes, and sills more than 2.5 billion years old, all well displayed in the upper reaches of the river’s drainage basin north of Lake Timiskaming.

These ancient rocks extend almost to the south end of Lake Timiskaming, where the southern part of the Canadian Shield was raised more than 15 km vertically along a world-famous fault zone known as the Grenville Front. Recent studies have shown that, at this time, the Grenville uplift caused rivers to flow north from the Gatineau region to the Arctic Islands! The vast, now deeply eroded Grenville block, comprised of a wide array of folded and highly contorted metamorphic and igneous rocks more than 1 billion years old, is exposed almost all the way south to Ottawa.

From Pembroke southward, however, discontinuous plates of sandstone, limestone, dolostone, and shale start to take over. From Ottawa south to the St. Lawrence River, the Precambrian rocks are completely covered by these younger Paleozoic rocks. A great variety of fossils are preserved, recording the evolution of organisms that lived in seas that covered much of Canada from about 570 million to 440 million years ago. Throughout the subsequent time during which more advanced life forms developed, including dinosaurs, the land now traversed by the Ottawa River remained above the sea, and so was subjected to erosion. As a result, no strata containing dinosaur fossils occur within the Ottawa drainage basin.

During the more recent period of continental glaciation, the ice sheets became sufficiently thick to depress the land, resulting in marine flooding of most of the present Ottawa River drainage basin. As the ice melted, 10,000 to 7,000 years ago, the Champlain Sea crept well up the Ottawa Valley. Marine animals such as clams and barnacles, and even some whale skeletons, were locally preserved in sandy and muddy deposits left behind as the land once again was elevated above the sea.
Shifting river channels of the post-glacial period

The modern river evolved as the ancestral Ottawa River and its tributaries adjusted to the retreat to the Champlain Sea. Between 10,000 and 8,000 years ago, there was a much larger flow of water through the ancestral Ottawa River than at present. Large glacial lakes in northern Ontario and the prairie provinces, and the upper Great Lakes all drained into the Ottawa River. Several times during this period the Ottawa River shifted into new channels. By about 8,000 years ago, modern drainage had become established.

River Geotrivia

How many are aware of the importance of the early work on the Ottawa River by Sir William Logan, named by a Canada-wide panel as the most important Canadian scientist? Like all early geologists, he was a complete naturalist, collecting information on biology, meteorology and ethnology, as well as geology.

Geologist Clyde Kennedy, author of the History of the Ottawa Valley, was very active in the first ethnological studies on Morrison Island.

Reference: Kennedy, Clyde C., 1970. The Upper Ottawa Valley. Renfrew County Council, Pembroke, Ontario, 256p.