The fresh river water, mixing with saltwater, created layers in the river. Rendering sonar ineffective at locating U-Boats. At one point, Canada considered closing the river. The U-Boats also deployed a secret weapon to block sonar. It was basically a giant Alka Seltzer table. Anyone know more about this? I guess Canada started the war with only 5 destroyers, capable of anti-sub work. But, the RCN grew quickly and helped with convoys a lot later in the war. I think it was ONS-5, that the RCN was a major factor in the defeat of the Wolf-Packs.
Interesting story. Look at this website for more information about U-Boats in the St. Lawrence. http://www.junobeach.org/canada-in-wwii/articles/submarines-attack-in-the-st-lawrence/ This article from uboat.net is also informative. http://uboat.net/articles/29.html
Didn't Canada finish the war with something like the third largest navy in the world (counting numbers of vessels and not tonnage)?
Indeed they did, although it helped that most of the larger navies were sunk. Most of the ships were Canadian-built, minesweepers and escorts as you say, but producing over 300 of them in a few years in a country that had not previously built warships was very impressive. They also contributed ships to the RN and a few to the USN.
Well, if we are looking at the number of vessels and not tonnage, the US Army was a major player too. https://en.wikipedia.org/wiki/List_of_ships_of_the_United_States_Army
Indeed. I should have said number of warships. At various times the US Army has had more vessels than the USN although I don't think they have ever had more combatants except for possibly during the ACW.