The rocks beneath the Rideau

I had to ask, why all the little orange and yellow flags stuck in the grass in Bordeleau Park? What were all the men in fluorescent jackets doing with their measuring equipment? Why the floaters in the river and the large working boat laden with all its mysterious equipment? The answer is that a geophysical survey is underway to determine the depth of the bedrock in the riverside parks and underneath the river. They need to replace the ancient sewer that crosses the riverbed and services the districts on either side.

On the Rideau Canal website page I linked above, there's some history of the riverbed:

As the glaciers continued their retreat north, the St. Lawrence and Ottawa river valleys were exposed. The bedrock was depressed below sea level from the weight of the ice. This allowed waters of the Atlantic Ocean to fill the depressed area and mix with river waters and glacial meltwaters. This formed a brackish (partly salty) sea known as the Champlain Sea. This sea in the northern Rideau area lasted from about 12,000 BC to 11,100 BC and extended at least as far south as Rideau Ferry, perhaps to Nobles Bay of Big Rideau Lake. Evidence for this sea can be found in the sediments it deposited, including the Leda clay deposits found in the Ottawa area, many sea mollusc shells and even the bones of Beluga whales which have been found as far south as Smiths Falls. With the weight of the ice gone, the bedrock was rising, a process known as isostatic rebound. The rivers and lakes were establishing themselves.