TUFA

How it all began

The fundamental phenomenon of Krka National Park is tufa, limestone that has deposited out of the water and formed the tufa barriers.

Travertine 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.

Physicochemical processes alone would not be able to create travertine deposits or barriers in the river.

This also requires the living organisms that are able to retain the deposited calcium carbonate crystals, in order to create the travertine deposits. In the splash zones, where water is aerated, these organisms find suitable places to colonise the substrata, as they require wet calcific habitats to survive. Most often, these organisms are calcific mosses (bryophytes have a significant role in trapping the materials for travertine, so they are called travertine-building mosses).

All biological and ecological discussions about the living world of karst waters refer to the travertine-building communities and travertine-builders. The living world at the bottom of the river is called benthos. In the ecological sense, travertine-building communities are based on three fundamental categories: primary producers (plants), consumers (animals), and the decomposers (bacteria, fungi and animals that feed on dead organic matter). The travertine-building mosses inhabit virtually all parts of the waterfall in large quantities, but primarily on surfaces covered by water, at the water’s edge or in wet habitats, and are not found in places of calm water. The Krka waterfalls are inhabited by 41 moss species, of which 33 are porophyte species and the rest are aporophyte species, i.e. those that do not participate in travertine-building.

Based on the plant or animal organisms that participate in the building of limestone deposits, we can differentiate the different forms of travertine: cratoneurumic, briumic, didimodonic, all named after the travertine-building mosses (Cratoneurum commutatum, Brium vebtricosum and Didimodon tophaceus), and chironomidic, gastropodic and trichopteric, named after the travertine-building animal groups, the lake flies (Chironomida), snails (Gastropoda) and caddisflies (Trichoptera).

The aquatic mosses are most abundant in the splash zones, and this is where the most calcium carbonate is deposited. The sedimented calcium carbonate crystals catch onto the moss tufts, with the help of algae, and are built into the moss. This is how travertine forms in so many different shapes. As the moss grows, the deposition surface increases, and so the travertine grows along with the moss. In addition to the travertine-building mosses, different species of algae and cyanobacteria also play important roles in trapping the calcite crystals. These organisms inhabit the moss shoots and secrete a sticky substance that traps the crystals. New crystals accumulate around these trapped crystals, thus beginning to create a travertine deposit. Over time, these deposits grow to form barriers consisting of limestone full of the remains of microscopic algae and petrified aquatic mosses.Cratoneurum commutatumBrium vebtricosum i Didimodon tophaceus), te hironomidnu, gastropodnu i trihoptersku, koje su dobile ime po sedrotvornim životinjskim skupinama trzaljaca (Chironomida), puževa (Gastropoda) i tulara (Trichoptera).

In the travertine-building communities, the primary producers are the mosses, filamentous algae and periphyton (algae growing on plants and solid surfaces). The filamentous algae and mosses often form a mosaic structure of vegetation cover of the travertine-building biotope.

The periphyton growing on moss clumps and filamentous algae on the rocky substrates is made up of diatomaceous algae.

The periphyton, algae growing on tufts of moss and on rocky surfaces, consists of diatomaceous algae, also called diatoms

The presence of algae and mosses is an important requirement for the creation of travertine, especially in determining the form it will take. The periphyton, algae growing on tufts of moss and on rocky surfaces, consists of diatomaceous algae, also called diatoms. A total of 276 taxa of diatoms, belonging to 69 families, have been recorded at studied sites in the Krka River. Due to their sensitivity and rapid response to the physicochemical and biological changes in the river, they are excellent biological indicators. The types of microhabitats found in the Krka River are in fact a hotspot of diversity for freshwater algae. hotspot raznolikosti slatkovodnih algi.

Diatoms are single-celled, autotrophic organisms characterised by their cell walls that contain silica. Diatoms make up to 70% of the total autotrophic components in periphyton communities, and play an important role in carbon fixation. They can be found in virtually every aquatic environment, and they are the dominant form of plant life in ice ecosystems. With 12,000 described species, this is the largest group of algae, though it has been assumed that there may be up to 100,000 different species. Diatoms create deposits called diatomaceous earth. These species are often used in monitoring water quality and monitoring toxic and invasive species, in climate change studies, and in studies on speciation and evolution. Diatoms are algae that have a transparent cell wall made of silicon dioxide. Silica is the main component of glass, and the hydrated form creates opaline glass, so diatoms are often described as living in a glass or opaline house.

The most common diatom genera in the Krka River are the Navicula, Gomphonema and Cymbella. At the Krka source, 25 diatom taxa have been found, 52 species in the upper course, 105 species in the lake-like expansion under the Krka monastery, 150 at Roški slap, 126 in Visovac Lake, 93 along the transect from Lozovac to Visovac Lake, and 147 at Skradinski buk. The highest diversity of species (81) and genera (40) was found at the marshy lake-like expansion near Roški slap and in the Marasovića Lake near Skradinski buk. NaviculaGomphonema i Cymbella. Na izvoru Krke zabilježeno je 25 svojti, u gornjem toku 52, na području ujezerenja podno manastira Krka 105, na Roškome slapu 150, u Visovačkom jezeru 126, na transektu od pristaništa Lozovac do Visovačkog jezera 93 a na Skradinskom buku 147. Najveća raznolikost svojti (81) i rodova (40) zabilježena je na lokalitetu močvarnog ujezerenja efemeralnog karaktera kod Roškoga slapa i u Marasovića jezeru kod Skradinskoga buka.

The project Diatoms of the Krka River – from the source to the mouth aims to compile a systematic list of the diatoms present in the Krka River, and create permanent slides of these species. This in turn led to the idea of establishing the Croatian National Diatom Collection. This is a collection of permanent slides of diatom samples collected in Croatia, and is the first institutional collection of diatomaceous algae in Croatia. The collection currently includes more than 6000 slides, which are in the process of being categorised, with ongoing collections of new slides. Among them are the type materials for species new to science, that are being described by the scientists at the Faculty of Science, University of Zagreb, and now they are no longer being sent to other European collections, but remaining in Croatia, as a national treasure of our species diversity. Collections such as these are a priceless treasure, as in addition to providing information about biological research, they provide insight into the diversity of diatom species, the biodiversity of a certain area, the distribution of diversity, and changes in time and space. This makes it possible to implement effective project of the priceless treasures of Croatia’s biodiversity.

The travertine-building process is a distinctive feature of karst rivers. Plants, especially mosses, serve as the substrate upon which calcite can settle once it dissolves out of the water.

Though this process is fundamentally a physicochemical process, travertine building is largely dependent also on cyanobacteria, diatomaceous algae, and other algae groups, as previously explained. Mosses are the ideal substrate for this process, due to their numerous, branching stems covered with tiny leaves, creating a large surface area with a multitude of micro-spaces. Later, when the entire stem is covered in travertine, this serves as a reinforced structure upon which new travertine can grow in three dimensions. Travertine can also form on vascular plants (those with leaves, stems and roots), creating specific plant communities of the travertine barriers.

During the 20th century, many researchers recognised the important role of mosses and algae in the travertine-building process and the creation of travertine formations. The vegetation at the travertine barriers is the most visible part of the living world on these barriers, and in terms of biomass makes up its greatest part. The last systematic research on the macrophyte vegetation (mosses, vascular plants and algae) at the barriers was conducted in the mid-1950s. Zlatko Pavletić was the first to start systematic research on the flora, vegetation and ecology of mosses on travertine barriers. He performed detailed field studies on the moss flora, but also examined other vegetation and their ecological relationships. More than half a century later, the Croatian Association of Freshwater Ecologists conducted a new study in 2018, and gave the first comprehensive overview of the flora and vegetation of the travertine barriers, at all seven waterfalls on the Krka.

The results of this recent study showed that the flora and vegetation of the travertine barriers of the Krka River are well structured and have high species richness, indicating their natural state and high level of preservation. In comparison with the results of Pavletić’s research, it is evident that the waterfalls are more shaded by woody vegetation now, and that the surface area of the open waterfalls has been reduced.

The overgrowth of the barriers with microvegetation is a problem: vegetation changes the direction of river flow, slowing or halting the travertine building process, until the flow is completely stopped in certain areas. The roots of microvegetation also negatively affect the survival and development of the travertine barriers, as they erode the stability of the barriers themselves and cut off the flow of water as channels are closed up. This negative influence of microvegetation is particularly evident when invasive plant species appear, such as the Tree of Heaven (Ailanthus altissima). With the aim of ensuring the long-term, sustainable management of the travertine barriers at Skradinski Buk, in 2017, the Public Institute of Krka National Park started to implement the interdisciplinary scientific project “Managing and maintaining the macrovegetation at Skradinski buk – developing a multi-criteria model of sustainable management”.

The conditions such as the water flow rate, slope and structure of the river bed differ at every waterfall location, thus creating different waterfalls, and even creating different sites within the same barrier.

Najčešći oblici sedre u krškim tekućicama jesu podvodni pokrivači, pragovi i barijere, koje nalazimo na dnu krških brzica, zastori i konzole, koji nastaju na mjestima gdje se voda slijeva niz okomitu stijenu na koju se posredstvom zajednice taloži sedra, te brade, žljebovi i cijevi, koje nalazimo na mjestima gdje se veće količine vode obrušavaju tvoreći slapove.

The most common forms of travertine in karst watercourses are the underwater sheaths, thresholds and barriers found at the base of the karst rapids, curtains and consoles. These structures are formed at the sites where water runs down vertical cliffs and travertine is precipitates due to the action of the living community, while the grooves, fissures and pipes are formed at sites where larger quantities of water fall, creating waterfalls.

The Krka River has exceptionally suitable physicochemical conditions and the perfect hydrological state for the deposition of travertine. That is why seven majestic waterfalls have formed along its course. However, though on the same river, these travertine barriers and waterfalls differ, as at different sites, the rate of travertine deposition can vary. Moreover, the deposition of travertine on the river began in different periods of time. The conditions such as the water flow rate, slope and structure of the river bed differ at every waterfall location, thus creating different waterfalls, and even creating different sites within the same barrier.

The development of travertine barriers is such that the travertine thresholds are formed first, on slightly raised surfaces in the river, under the water’s surface. These thresholds then grow vertically, slowing the flow of water and creating a cascade, making upstream flow slower and downstream flow faster. This in turn creates a range of different forms: thresholds, curtains, covers, beards, undercuts, semi-caves, cones, pipes, wells, caves, etc.

The travertine formations develop gradually, from the simplest (thresholds, travertine barriers) to more complex forms and series of forms (barriers with caves, semi-caves, travertine curtains, consoles, etc.). The appearance of travertine that grows on the falls is not compact. Under the falls, hollows are formed, creating completely new microspaces that cannot be easily seen. Based on the structure and size of the travertine barriers, we know that the majority of travertine is deposited near the end of the river’s flow, where the water flows slower. Skradinski buk on the Krka River is the longest travertine barrier in Europe.

Outside the present-day course of the Krka River, in the area of the river's previous course near Knin that which was active during the time of the river’s geological development, we find travertine about 125,000 years old – fossil travertine.

The process of travertine building began in the Pleistocene, the last ice age, and with occasionally interruptions has lasted until the present day. After the end of the last glacial period, call the Würm period, the climate began warming and the flow of water slowed, creating the conditions for the settlement of mosses, algae and other organisms that take part in the travertine building process. The Pleistocene travertine in the Krka River Valley and its tributaries is up to 20 metres higher than the present-day riverbed, indicating the height at which the river flowed during that period. This travertine is called dead travertine, or fossil travertine.

Most of the travertine at the waterfalls is less than 10,000 years old. The creation of travertine deposits in the post-Würm period began at Skradinski buk waterfall, Roški slap waterfall and other falls along the river’s course, creating Visovac Lake and other accumulations in the canyon sections of the present-day Krka. The active travertine falls of the river, as sections of the barrier that lie within the current watercourse, are biodynamic formations that continue to grow under the travertine building processes, and so they are called “live” travertine.

Travertine is important for understanding the creation of the present-day appearance of the Krka River. It is represented in two variations – the hard, hollow travertine at the barriers, and the dusty-pelite travertine on the lake bottom. The remnants of travertine barriers are also found at different relative heights in comparison to the current riverbed, which is also important in explaining the rate at which the Krka River cut through the landscape.

The former flow of water over time is best seen in the example of the Krčić River. The Krčić is the source of the Krka River, in the morphogenetic sense, and today represents its first tributary. It springs in the foothills of Mt. Dinara at an elevation of 375 metres, and runs over a length of 10,5 km. During periods of heavy rain or snowmelt in the mountains inland, the Krčić is a torrential river, while it dries out during the summer. If it did not dry out during summer, the Krčić would not exist as its current concept and the source of the Krka would be where the source of the Krčić is today, in the foothills of Mt. Dinara, which legend says is the place where the Krka is born. However, people recognised the seasonal water regime of the Krčić River and gave it is name. In the recent geological past, due to the colder climate and higher levels of precipitation, it did not dry out, and the Krka and Krčić formed the same course. Evidence of its previous regime is visible in the remnants of the vast travertine deposits found within the Krčić valley, high above the present-day flow.

Preserving the good ecological condition of water at the barriers is a top priority task of ecological protection, as the development and survival of the living communities on the barriers depends on it.

Global climate change and changes in water quality are the primary causes of change in sensitive ecosystems. Long-term monitoring of the ecological factors that most influence the development and survival of the living community in the Krka River enables a timely reaction to any observed changes.

The Krka River is a typical karst watercourse, which in the hydrological sense, is marked by pronounced seasonal fluctuations. Due to the reduced abundance of the source, in the dry summer period, the water levels drop. This significantly changes the hydrological conditions at the barriers, as the living world of the travertine builders begin to die off at the dried up barriers. Once these habitats are again under water, they begin to regenerate.

It has long been known that reduced flow rates negatively impact the travertine building process. Rising air temperatures and the stagnation trend in precipitation levels have resulted in a reduction of the mean annual flows of the Krka River, and with that a reduction in mean annual water levels. It can be expected that these trends will also result in an increase in the mean annual water temperature. The quantity of dissolved oxygen in the water, essential for all organisms living within, is directly associated with both temperature and flow rates, as cold water is better able to retain dissolved oxygen and therefore is more oxygen rich. As water temperatures rise, the dissolved oxygen levels drop, and the direct consequence is a decline in the number of living organisms in the water. The quantity of dissolved oxygen also depends on the rate of water flow: faster water is richer in oxygen and tends to warm more slowly than “calm” waters.

The travertine building communities are very sensitive to pollution and reduced water flow rates. Since the building of new travertine barriers depend on the specific communities of organisms involved in their construction and that inhabit them, these communities must be preserved in their original form, to ensure that the travertine building process continues on these barriers. All these changes to the quality of water at the travertine barriers leads to changes in the composition of the living world. The appearance of unfavourable conditions for the development of mosses, the building communities at the barriers, would jeopardise the entire process of travertine deposition and barrier growth. For that reason, preserving the quality of the water in the Krka River and the travertine deposition process is a long-term goal in the management of Krka National Park.

 

Only with the constant growth of travertine it is possible to preserve the survival of waterfalls, which are the backbone of the hydrogeological and landscape image and the foundation of the biological diversity of the Krka National Park.

The travertine waterfalls of the Krka River are a natural and karst phenomenon with appropriate values of world scientific interest. With a total flow drop of 224 m, from the source are Bilušića buk (22.4 m), Brljan (15.5 m), Manojlovac (59.6 m), Rošnjak (8.4 m), Miljacka (23, 8 m), Roški slap (25.5 m) and Skradinski buk waterfall (45.7 m). Waterfalls have a very delicate structure and a dynamic purposeful complex sensitive to natural changes and all human activities. Only with the constant growth of travertine it is possible to preserve the survival of waterfalls, which are the backbone of the hydrogeological and landscape image and the foundation of the biological diversity of the Krka National Park.

Bilušića buk is the first of seven cascades along the course of the Krka River. The falls are found wedged in a canyon t an altitude of 214 m above sea level. Near today's riverbed, well-maintained and attractive “dead travertine” can be seen. There are several speleological objects in the travertine barrier of the waterfall, the most important of which is the travertine cave behind the mill.

The waters of the picturesque Ćorić Lake (Brljan Lake) overflow through Brljan. The waterfall is about 300 m long, about 180 m wide, with a drop of 15.5 m. It is surrounded by sub-Mediterranean vegetation and rocky pastures. Due to the needs of HPP Miljacka, specific hydrological conditions of alternating drying and wetting prevail on the travertine barrier.

Manojlovac waterfall is is the largest and is considered by many to be the most beautiful waterfall of the Krka River. The waterfall is made up of a series of travertine barriers with a total height of 59.6 metres; the largest barrier is 32.2 metres high at the place where where the river makes a sharp turn. The canyon surrounding the waterfall is overgrown with sub-Mediterranean vegetation.

The fourth from the spring is the smallest Krka's waterfall Rošnjak, Sondovjel or Šundovil. It is located thirteen and a half kilometers downstream from the source and one kilometre downstream from the Manojlovac waterfall, squeezed between nearly 200 metre tall cliffs. It is made up of one step and due to its primordial simplicity and mystical inaccessibility, the locals called it the Altar. Oltarom.

Miljacka slap is made up of three larger and numerous smaller travertine steps with a total height of 23.8 metres. In the vicinity of the waterfall there are five speleological objects, a spring-caves. The largest among them is Miljacka 2, also the largest speleological object in the area of the Krka National Park Krka.

Roški slap waterfall is the sixth and second to last waterfall on the Krka River. It is found in an expansion of the Krka Canyon, which in its lower course forms a lake-like section of the river, and in the upper course a stretch of cascades, with numerous backwaters and overgrown cascades that the locals have called Ogrlice, the Necklaces, due to the flashes of silvery colour. The travertine barrier is 650 m long, about 450 m wide with a total height of 22.5 m. The main waterfall is found at the end of the barrier, where the Krka crashes in a wide fan-shaped waterfall into Visovac Lake 15 metres below. Ogrlice. Sedrena barijera duga je 650 m, široka oko 450 m, ukupne visine 22,5 m. Glavni slap nalazi se na kraju barijere, gdje se Krka s petnaest metara visine, u širokoj lepezi, duboko ruši u Visovačko jezero.

Skradinski buk is the last, seventh, longest travertine barrier on the river Krka and in Europe. The combined waters of the Krka and Čikola rivers flow over it. Skradinski buk consists of seventeen barriers spread over 800 m in length. Travertine builders in the upper part of the waterfall, with a network structure of watercourses, lakes and islets overgrown with lush vegetation, still build thresholds and draperies, and in the lower part high barriers with recognizable beards, semi-caves and caves.

Travertine cave behind the mill in the Bilušića buk travertine barrier is the longest travertine cave in Croatia.

Caves and pits are one of the phenomena of karst. These are underground hollows created in limestone sediments thanks to cracks which expanded over time due to the corrosion and erosion effects of water. There are about a hundred caves and pits along the Krka River, with 65 within the boundaries of Krka National Park. The longest caves are located at the Miljacka spring. Once they appeared, the caves and pits became hiding places and living areas for many animal species.

Hollowness and fissures are the main features of the rock of karst landscapes. This leads to the formation of numerous pits and caves. The karstification process is also responsible for the formation of the ornamentation within these structures. Under special conditions, it is possible that this process is reversed: where calcium carbonate is again secreted, and instead of wearing, the rock grows. That results in the formation of stalagmites and stalactites, as well as travertine (tufa), the fundamental phenomenon of Krka National Park.

Travertine cave behind the mill is situated on the right bank of the Krka River in the Bilušića buk travertine barrier, at an elevation of 198 m. It was created in fossil (dead) travertine. It is branching. This is the longest travertine cave in Croatia, 124 m in length. It has four entrances, one of which is closed with low gates, and one is found in the mill. The width of the channel does not exceed 2 m. It is ornamented with a variety of travertine ornaments of unusual form. The cave has a mixture of classical cave and travertine dripstones, and therefore is a suitable place for studying their origins.

Air temperature is 9.3 °C and relative humidity 100 % (3 May, 2010).

For centuries prior to the proclamation of Krka National Park, travertine (tufa) was used as a building material in the areas along the Krka River. Even the Church of St. Nicholas at Skradinski buk waterfall was constructed entirely of travertine. Fossil travertine is still visible today in Šibenik, along the staircase under the monument to King Petar Krešimir IV, and the Robert Visiani Gardens. There are also living “pieces” of travertine to be found in Šibenik. Part of the travertine from the Krka River has been built into the fountain in front of the Church of Our Lady Outside the City. It was placed in a sunny and wet location, and has continued to grow. Today it is overgrown by mosses and is several times larger than it was when it was placed here. This is a rare and valuable specimen of the many ways of cohabitation of the people with the river, which was so important in the past. Since the proclamation of Krka National Park, the extraction of travertine is strictly prohibited, as this rock represents the fundamental phenomenon of the Park.

Travertine is an excellent building material, and was used extensively prior to the prohibition of its use. Due to its moisture content, it is easy to shape, and upon drying out, the rock becomes very hard, due to the secretion of calcium carbonate from the calcium bicarbonate contained in the moisture. It has a specific hollow spongy appearance, and is usually beige to yellowish in colour. Even the ancient Romans used travertine to construct their temples, waterworks, monuments, baths and amphitheatres. The Roman Colosseum is the world’s largest structure built of travertine rock. Travertine is still used in construction today, though most often in interior design, where it is used as stone slabs for lining terraces and garden paths. The travertine from the Krka River is not used.

Sand was also previously excavated from the Krka for use in construction. Sand pits still visible today are the St. Jera pit and the islets Veliki Busen and Mali Busen just upstream from Skradinski buk. The St. Jera pit contains a sand deposit, and is much smaller today than it was 50 years ago. The islet became smaller due to the previous extraction of sand for housing construction. Veliki Busen was one of the larger and more abundant sand “mines” and due to the extensive exploitation of this resource, it is now much smaller than Mali Busen.