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How it all began: tufa

Tufa is frequently found in the surface watercourses of the Dinaric karst, though rarely does it build waterfalls like those on the Krka.

Travertine or tufa is the name for calcium carbonate (limestone) rock that is formed in rivers when minerals from the water settle onto different types of surfaces. Travertine is the fundamental phenomenon of Krka National Park: the creation and development of travertine barriers has shaped the present day appearance of the river bed and the characteristics of the living world of the Krka River. Travertine is frequently found in the surface watercourses of the Dinaric karst, though rarely does it build waterfalls like those on the Krka.

The travertine-building process is constant and dynamic and is based on the mutual action of physicochemical conditions and the living organisms in the water. The knowledge about this process has also evolved over time. Initially, the travertine-building process was explained as a by-product of the work of living organisms, then the result of the work of non-living matter and physicochemical processes, and now, it is known that this is the interaction of the non-living and living world, including plant and animal communities.

Travertine can only be formed in water containing sufficient quantities of dissolved calcium bicarbonate. As the water filters through the surface layer of soil, it reacts with carbon dioxide. This water is slightly acidic, so it dissolves limestone, and is then enriched with calcium bicarbonate.

The landscape the Krka flows through is constructed of limestone and its associated rock, which together are called carbonate rock. This type of rock is easily dissolved and creates interesting forms on the surface and underground. The river water, containing much carbon dioxide, dissolves the limestone rock and becomes enriched with calcium bicarbonate. At splash sites, the water becomes chemically imbalanced, and the calcium bicarbonate is transformed into the lesser soluble calcium carbonate or calcite (CaCOɜ), which then settles out of the water, and deposited on the substrate in the form of tiny crystals.

But calcium carbonate dissolved in the water is not enough to result in the formation of travertine (limestone). Temperature is also an important factor, especially in the summer (vegetative) period. Other importance factors are water aeration, splashing of water particles, pH value, speed of water flow and the substrate of the riverbed. It is also important to know when the erosive power ends, and when the depository force (accumulation or sedimentation) of the watercourse begins.

Research on the isotope characteristics of the deposited CaCOɜ and determination of the influence of hydrodynamics on the transfer of carbon and deposition of CaCOɜ, have established the conditions that control the formation of travertine barriers on the Krka River. The calculation of the saturation indices for calcite, aragonite and dolomite have confirmed that the Krka River is primarily saturated with calcite year round. On the other hand, during periods of high water levels, there is intensive sedimentation of calcite, and then the water is no longer saturated with calcite. Chemical precipitation of calcite occurs due to the degassing process in the splashing water, resulting in the formation of cascades and waterfalls.In this process, the travertine-building organisms (algae and mosses) play a very important role, with the ability to retain precipitated calcium carbonate and to create tufa deposits.