Physical mechanisms of river waterfall tufa (travertine) formation

Waterfall tufa is widely distributed around the world, especially in tropic al and subtropical karst areas. In these areas river water is generally sup ersaturated with respect to calcite, and the precipitation occurs mainly at waterfall and cascade sites. Development of waterfall tufa has been descri bed as simply being the result of water turbulence. We believe, however, th at three physical effects can lead to tufa deposition at waterfall sites: a eration, jet-flow, and low-pressure effects. The three physical effects are induced by two basic changes in the water: an accelerated flow velocity, a nd enlargement of the air-water interface area, These two changes increase the rate of CO2 outgassing and the SIc, so that a high degree of supersatur ation is achieved, which then induces calcite precipitation. These "waterfa ll effects" have been simulated in laboratory and field experiments, and ea ch of them can accelerate, or trigger, calcite precipitation. Field measure ments of river water chemistry also show that tufa deposition occurred only at waterfall sites. In these experiments and observations, waterfall effec ts play the most important role in triggering and accelerating CO2 outgassi ng rates. Field and laboratory observations indicate that plants and evaporation also play important roles in tufa formation. Growth of algae and mosses on tufa surfaces can provide substrates for calcite nucleation and can trap detrit al calcite, accelerating tufa deposition. However, the prerequisite for suc h deposition at waterfall sites is a high degree of supersaturation in rive r water, which is mainly caused by waterfall effects. Evaporation can lead to supersaturation in sprays and thin water films at a waterfall site and c ause the precipitation of dissolved CaCO3, but the amount of such depositio n is relatively small.