The reduction of water hardness, which occurs at waterfalls on rivers in karst areas, is considered to be a result of the waterfall effects. These consist of aeration, jet-flow and low-pressure effects. Waterfall effects bring about two physical changes in river water: an increase in the air-water interface and turbulence. A series of experiments was designed and implemented in order to investigate whether these effects and associated physical changes may cause a reduction of water hardness. From an experiment involving the enlargement of interface area, the plot of air-water interface areas against conductivity revealed that the higher the air-water interface, the more rapidly conductance declines (and Ca 2+ is precipitated). A bubble producer was designed and used to simulate bubbles that are produced by aeration and low-pressure effects and a faster decline of water hardness was observed at the location with bubbles in this experiment. When a supersaturated solution was passed through a jet-stream producer, a rapid reduction of water hardness and an increase ofpH appeared. Field measurements were used to support the laboratory experiments. Work on the Ya He River and at the Dishuiyan Waterfalls revealed that places with aeration had the quickest hardness reduction and the highest average rate of calcite deposition.