The class was designed to be operated as deep sea team sweepers, to combat the threat posed to submarines by Soviet deep-water buoyant moored mines codenamed "Cluster Bay".

The Rivers were also armed with a single 40 mm Bofors gun on the manually operated World War II-era Mark III mounting, and two L7 GPMGs.

The class was designed to operate in deep water and ocean environments, if necessary, for long periods of time without support.

Upon entering service they joined the 10th Mine Countermeasures Squadron based at Rosyth and eleven were assigned to various Royal Naval Reserve (RNR) divisions around the United Kingdom.

Blackwater, Spey, Arun and Itchen were subsequently assigned to the Northern Ireland Squadron where they replaced Ton-class vessels patrolling the province's waterways and participating in counter-terrorist operations in support of the British Army and the Royal Ulster Constabulary (RUC).