A pair of identical, specially-shaped, and specially-sized flow diverters is provided. In order to concentrate the flow of water passing over a waterfall, such as an ornamental backyard spa or swimming pool waterfall, a first flow diverter is attached adjacent a first sidewall of the waterfall structure, and a second flow diverter is attached adjacent a second sidewall of the waterfall structure. Water passing by the sidewalls is directed inwardly away from the sidewalls, thereby narrowing the flow of water over the fall and reducing overspray and reducing mineral build up and scale problems. In one novel aspect, an assemblage (a saleable product) includes a product container, the pair of flow diverters disposed in the container, a tube of silicone adhesive disposed in the container, and a set of printed usage instructions disposed in the container.