This invention has to do with the treatment of the tailings stream from a hot water extraction plant for recovering bitumen from tar sand. This stream comprises water and coarse and fine solids. The stream is fed into a mined-out cavity to settle the coarse solids. Some of the decanted water from the cavity is transferred to a large settling pond for clarification. The remainder of the decanted water is returned to the plant and added to the tailings stream to fluidize it and render it pumpable. The process has the following advantages: some of the fines become trapped in the beach of coarse sand formed in the cavity, thereby reducing the volume of sludge which is generated in the settling pond. The use of decanted water to fluidize the tailings issuing from the extraction process reduces the amount of water needed for the extraction process itself, thereby reducing the amount of clarified water required for the plant.