Tufa Barriers, also sinter terraces, are deposits of limestone in a stream or flowing body of water. In contrast to rimstone pools in caves, such sinter terraces form relatively quickly on the earth's surface. In cold water, dams of porous calcareous tufa form; in thermal springs, dams of densely layered calcareous sinter or sometimes even opal, if the spring is very rich in quartz. The calcareous tufa variant is typical for karst areas with very limestone rich karst springs. Branches and twigs that have fallen into the water, as well as algae, mosses and ferns that grow on the dams, are encrusted by limestone and when they have decomposed pores remain. In thermal springs, the water is usually too hot for plant growth, and hot water can also dissolve quartz, so this form of basin can also be made of water-rich quartz. These pools are therefore very similar to sinter pools in caves.
The basins form when water flows down a slope with a certain gradient. At first, small deposits form on obstacles, stones or even plants, and then over time, larger and larger barriers form in the stream. Over time, these barriers develop into smaller pools that cascade the water. In rare cases, smaller pools become larger lakes, sometimes with dams several metres high. The dams keep growing because lime is precipitated at the water surface and transported by waves to the edge, where it is deposited on the rim.
In temperate zones, such sinter basins are usually Quaternary formations; in the interglacial periods, the temperature was higher and the production of the springs was also greater, so the growth of the dams was faster than today. Famous sinter stages are the Plitvice Lakes in the area of Croatia or Pamukkale in Turkey.