In the following I am going to try to change that
Lake Nipissing is unique in many respects as it the biggest waterbody with the Ottawa Bonnechere Graben which is realy nothing more that a giagantic ditch:
click for a pdf
this in combination with glacial isotatic depression directed first the meltwaters from the retreating glaciers to flow over Lake Nipissing down the Petawawa valley for a couple of thousand years then then 10,000 years ago the ice dams blocking the Ottawa River burst and the waters of the upper Great Lakes went down over 122 metres (400 feet)
and those waters gouged out the Mattawa and Ottawa Rivers all the way to Pembroke where it took most of the previous delta and placed this sands downstream from Ottawa as islands like Petrie Island. This probably took less than a year.
and then current Lake Nipissing was just the east bay of Lake Huron and the outlet for all the Great Lakes (excepting Ontario.) This was called the Nipissing Great Lakes phase. This continued until 4,700 years ago
the following image attempts to show what that looked like.
click for a PDF
At this point the currents produced by flows that would rival or greatly exceed the Saint Lawrence River forming underwaterways where the large and violent main river would before it went down the Mattawa. I believe that those channels are still there today! We ran into them while scuba diving and now, I understand that the OPP marine unit and the commercial fishermen now know where they are.
Around 4,700 years ago the isotatic rebound brought up a watershed and the currents decreased in strenght but there is very little relief from the winds velocity and enormous waves because these winds follow the northern shores of Lakes Michigan and Huron deflected by the Penokean and La Cloche mountains and because there is little relief (read - no hills) between there and Lake Nipissing there would be reduced but still awsome waves on Lake Nipissing which was still a bay of Lake Huron!
According to Lumsden around 2,500 years ago Lake Nipissing became a lake as it's south shores came out of the water.