River frigates and Flower corvettes served different stages of the escort war

How did River-class frigates compare with Flower-class corvettes?

They were different solutions: Flowers provided emergency numbers, while Rivers delivered a larger, more capable later-war escort.

The Flower class answered the urgent need for numerous simple escorts built to basic standards. River-class frigates came later, after anti-submarine warfare had generated new equipment and operational lessons. They formed a more capable halfway house between corvette and sloop while still mixing merchant and naval construction standards. The two types answered different phases of the escort emergency and were never equivalent designs.