Glaciation of the Ottawa Valley
For the last 3 million years or so, North America has been subject to periodic major glaciations, each followed by a relatively brief, warmer, interglacial period. The most recent glacial advance, the Wisconsinan occurred between 35 000 and 10 000 years ago. At its peak, a single blanket of ice covered everything east of the rocky mountains as far south as southern Ohio.
The Wisconsinan glaciation began with ice accumulation in the highlands of Quebec. The ice sheet expanded southwards and westwards and extended into the St. Lawrence Valley and the Lake Champlain region. An ice lobe flowed across the Ottawa Valley and then flowed southwestwards and westwards across the Frontenac Arch and into the Lake Ontario Basin. Ice flow in the Ottawa Valley upstream from Ottawa was originally southeastward, but as the ice sheet thickened it over topped the fault scarp on the south side of the valley. It then flowed southward across the valley and into the Madawaska Highland. Ice flowing across the Ottawa Valley east of Ottawa flowed mainly southward but was deflected to the southwest reaching to Buffalo and beyond.
At the time of deglaciation, the St. Lawrence Lowlands were isostatically depressed below sea level. Proglacial Lakes developed in the Lake Ontario basin to the west and in the Lake Champlain basin to the south. As the ice retreated from the St. Lawrence Valley, marine waters invaded and the Champlain Sea was formed. The Champlain Sea extended up the Ottawa Valley. It is not clear whether it extended into the Lake Ontario basin. There is no evidence that it did, but if not, why not? An ice barrier along the Frontenac Arch would have been required.
At its peak, the Champlain Sea extended up the Ottawa River Valley as far as the mouth of the Dumoine River. However above, the top of Lac Des Allumettes it would have been more of a fiord than a sea. The Champlain Sea occupied the Ottawa Valley between 12 000 and 10 000 years ago. The western limit of the Champlain Sea was approximately along a line running from Brockville to Perth to Pembroke. However it is difficult to trace.
At the time of the Champlain Sea, the ice front extended east-west across Ontario and across the upper Ottawa Valley. Huge volumes of water were impounded against the southern face of the glacial front in the Lake Huron Basin (Lake Algonquin) and the land under the glaciers was significantly depressed. Lake Algonquin drained across central Ontario, through the Lake Simcoe basin, Kirkfield and into Lake Ontario at Kingston.
As the glaciers retreated north a lower drainage path was uncovered at Fossmill. This new outlet dropped the water level in Lake Algonquin by almost 50m. This huge volume of water flowed down the Petawawa River valley. When it reached the still waters of the Champlain Sea it created a huge delta which is perpetuated today as the Petawawa Sand Plain.
As the glaciers retreated farther north the drainage eventually shifted to the Mattawa River valley and the followed the modern course of the Ottawa River through the Upper Ottawa Valley.
The drainage remained down the Ottawa River for over 5 000 years. By the time the drainage switched to its modern course through the Great lake and the St. Lawrence river, the Champlain Sea had long since receded. And while the initial surge of water when the drainage broke through into the Ottawa Valley were massive, so were several later episodes when Lake Agassiz and Lake Objibway also drained through the valley.
The middle and lower portions of the Ottawa River Valley were significantly affected by this history. As mentioned above, a delta developed as the glacial drainage reached the Champlain Sea. The delta emerged and the front migrated eastward as the Champlain Sea regressed. As the sea level fell (that is, the land rose) the Ottawa river cut channels into the emerged part of the delta and into what had been the floor of the Champlain Sea. While the chronology is somewhat uncertain, it appears that the delta moved down river at a speed of a few hundred meters per year.
The Petawawa Sand Plain extends upriver to at least Point Alexander. Whether this sand is due exclusively to the Petawawa drainage is not clear [to me]. The chronology suggest that by the time the Mattawa channel had opened, the Champlain Sea had already retreated down the valley.