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.