A method is disclosed for clarifying, by flotation, waters contaminated with suspended matter, wherein the clarification treatment takes place in two successive steps implemented in a single installation. The method principally involves flowing contaminated water through a downflow static flocculation step and then down through a lamellar settler separation step positioned directly below the downflow static flocculation step, the settling rates of the heavier particles of the lamellar settler separation step removing heavier particles from the water retained during the static flocculation/lamellar separation step being less than or equal to the settling rates of the floatable particles. The water which has passed through the static flocculation/lamellar settler separation step is then subjected to a flotation step which removes the light particles in the water of which the settling rate is lower than a flotation cutoff threshold.