The River class was a dissimilar collection of warships, consisting of twelve vessels purchased from the Royal Navy and two built specifically by British yards for the RCN.

Modifications included removing gun mounts to make room for additional depth charge and torpedo systems, as well as adding new communications and radar masts.

The River class were the backbone of the RCN destroyer fleet and served as leaders of the Mid-Ocean Escort Force during the Battle of the Atlantic.

On the night of 25 June 1940, Fraser was tasked to join the destroyer HMCS Restigouche and light cruiser HMS Calcutta on Operation Aerial to rescue 4,000 refugees trapped by the German Army from the coast of Bordeaux, France.

The warships encountered rough seas and poor visibility, forcing the commanding officer of Fraser to close quarters with the other two vessels.

Many of the survivors from Fraser transferred that summer to Margaree and were lost when that vessel sank on 22 October 1940 as a result of a collision with the freighter MV Port Fairy.

While escorting convoy SC-94 on 6 August 1942, Assiniboine achieved her first victory when she rammed, depth charged and shelled U-210 south of Cape Farewell, Greenland.

In October 1943 Saguenay was towed to Digby, Nova Scotia, as a tender assigned to HMCS Cornwallis, the Royal Canadian Navy's training depot for new entries (recruits).

It was initially torpedoed by the British I-class destroyer HMS Icarus and an unsuccessful attempt was made at towing the submarine to port.

On 6 July 1944, Ottawa and Kootenay were detached from a convoy to assist HMS Statice with a submarine contact off Beachy Head, Sussex.

On 7 July 1944, Kootenay, Ottawa, and the corvette HMS Statice depth-charged and sank the German VIIC-class U-boat U-678 in the English Channel south-west of Brighton.