It depends on the size and draught of the vessel. Speed is left to the discretion of the river pilot but for large ships, I believe the limit is 18 knots when the water level is not high. Most ships travel at between 12 and 14 knots on the river but there are exceptions such as the ferry C.T.M.A. VACANCIER, Oceanex's CABOT and OCEANEX AVALON which moce at 16+ knots. River pilots are responsible for their navigation practices but when they do warships, I believe the ship's CO or flotilla commander (like Stanaforlant) takes full reponsibility for excessive speed practices. In October 2004, Stanaforlant visited Montréal and the six frigates were all doing in excess of 27 knots on the river. I find it excessive and somewhat ridiculous but I guess the terrorist attack threat (or syndrom?) may have had something to do with it.
In winter, ships are required to not exceed 11 knots while downbound and 9 knots upbound between Trois-Rivières and Montréal in order not to break the ice loose outside the main channel.