CONTRIBUTION TO THE INTERPRETATION OF THE ROLE OF GEOENVIRONMENT IN EXTREME DISCHARGES OF THE ROSOMAČK RIVER - VISOK

нeизучeних aли и хидрoлoшки изучeних вoдoтoкa, a крoз прикaз улoгe гeoсрeдинe. Нa кoнкрeтнoм примeру Рoсoмaчкe рeкe, хидрoлoшки Abstract: Annex to the interpretation of the role of geoenvironment in the formation of ex treme discharges of the Rosomačka River provides exact and empirical evidence of the signif icance and the need for its knowledge. For hydrologically unstudied watercourses, especially for those which do not have continuity of discharges, the knowledge of the role of geoenviron ment in the emergence of their extreme discharges contributes to a more realistic response to practical water management issues, protection against adverse effects of waters, defining the ecological aspect of watercourses under the influence of various changes in the near future (climatic, anthropogenic, hydrological), preserving the quality of river water, meeting the re quirements of the EU Water Framework Directive 2000. The aim of the paper is to understand better the river systems and their runoffs in the form of high and low waters or complete drying up of hydrologically unstudied but also of hydrologically studied watercourses, and through the representation of the role of geoenvironment. On the concrete example of the Rosomač ka River, a hydrologically unstudied watercourse, the significance of geoenvironment in the interpretation of its extreme discharges was emphasized. The contribution of the following elements of geoenvironment of the basin area was analyzed: lithological, neotectonic, hydro geological and morphological. The research method is based on field research, quantitative geomorphological analysis, systematization and synthesis of the obtained results, as well as the analysis of published papers. Additionally, the contribution is reflected in emphasizing the need for communication between experts of various specialties dealing with hydrographic systems. watercourse, the Rosomačka River, Stara planina

Напомена: Истраживање је реализовано у оквиру пројекта евиденциони број 37008, (Програм ТР), финансираног од стране Министарства за науку и технолошки развој. Abstract: Annex to the interpretation of the role of geoenvironment in the formation of extreme discharges of the Rosomačka River provides exact and empirical evidence of the significance and the need for its knowledge. For hydrologically unstudied watercourses, especially for those which do not have continuity of discharges, the knowledge of the role of geoenvironment in the emergence of their extreme discharges contributes to a more realistic response to practical water management issues, protection against adverse effects of waters, defining the ecological aspect of watercourses under the influence of various changes in the near future (climatic, anthropogenic, hydrological), preserving the quality of river water, meeting the requirements of the EU Water Framework Directive 2000. The aim of the paper is to understand better the river systems and their runoffs in the form of high and low waters or complete drying up of hydrologically unstudied but also of hydrologically studied watercourses, and through the representation of the role of geoenvironment. On the concrete example of the Rosomačka River, a hydrologically unstudied watercourse, the significance of geoenvironment in the interpretation of its extreme discharges was emphasized. The contribution of the following elements of geoenvironment of the basin area was analyzed: lithological, neotectonic, hydrogeological and morphological. The research method is based on field research, quantitative geomorphological analysis, systematization and synthesis of the obtained results, as well as the analysis of published papers. Additionally, the contribution is reflected in emphasizing the need for communication between experts of various specialties dealing with hydrographic systems.

INTRODUCTION
River waters are of great importance for the entire range of needs of a modern man (water supply of population and industry, irrigation, electricity production, biodiversity conservation, fish-ery, etc.), but also for the protection against their adverse effects (torrential floods), as well as active control of discharges during periods of low water. Bearing in mind the acute problem of water short-ages in the world, knowledge of the regimes of the rivers, regardless of their size, is gaining importance. Therefore, developed countries tend to know the regime of all watercourses, low, high, perennial and ephemeral, or all that are important for the local community or country.
Watercourses represent complex natural systems. One of the important elements of the regime of each watercourse are its extreme discharges, that is, high and low waters. High and low waters fall into frequent natural disasters both worldwide and in this country. Their intensity and frequency of occurrence in the territory of the Republic of Serbia places them in the category of potential limiting factors in the economic and ecological sphere of further development.
For watercourses where there are systematic and continuous hydrological measurements, precipitation monitoring, monitoring of water quality and other, it is possible to reliably determine the watercourse regime and to respond to numerous tasks. Under the conditions of non-existence of hydrological measurements and other observations, that is, in the case of hydrologically unstudied watercourses, the conclusion on any important issue is a very complex and demanding task. This is a consequence of the reliability of data on hydrologically unstudied watercourses, which in the event of inaccuracy can have side effects for users. A certain part of this delicacy is a consequence of the number of parameters that have influence on the formation of the water regime (N i k i ć , 2003; Ristić and Malošević, 2011). The most common reason for the lack of data is the cost of research work to quantify a number of parameters. In addition, depending on the need for which the analysis is carried out, in order to minimize investments, the type and scope of research work are minimized.
The necessity of interpreting extreme discharges refers to both hydrologically studied and unstudied watercourses. In addition to the precipitation regime and the surface structure of the basin, a significant factor in the interpretation of high and low waters is the geoenvironment of the basin area (J ev đ ev i ć , 1956). High and low waters are part of the hydrological regime of the river runoff, in which high waters are characteristic of the flood period, and low waters of the recession period of the year (McM a hon et al., 1982).
Knowing the hydrological state of the watercourse and the forecast of possible behaviour under the influence of various changes in the near future (hydrological, climatic, anthropogenic and others) is necessary in order to make as efficient, rational and sustainable use as possible. For this reason, research aimed at interpreting high and low waters of hydrologically unstudied watercourses is guided by parameters with a high degree of significance (C h ow, 1964; R i a n n a et al. In this paper, an analysis of some elements of geoenvironment which contribute to the interpretation of extreme discharges was carried out on the example of the Rosomačka River, which is a hydrologically unstudied watercourse, and where the influence of anthropogenic activities can be practically ignored. The following has been analyzed: the geological structure of the terrain, lithological composition, neotectonic circuit, hydrogeological conditions and geomorphological processes. The applied approach has shown that it can contribute to understanding and argumentative qualitative interpretation of the historical phenomenon of extreme discharges, as well as the prediction of future events.

RESEARCH AREA
The subject of this research is the watercourse and the basin area of the Rosomačka River, located in Eastern Serbia, on the southwestern slopes of Stara planina, about 45 km east of the town of Pirot (Figure 1). Administratively, the research area belongs to the Municipality of Pirot, and spatially forms a part of the Visok area and a small part of the "Stara planina" Nature Park. The area of Visok extends in the east from the spring part of the Visočica River (in Bulgaria), to Zavojsko Lake in the west, and from the ridge of Stara planina in the north to the Visočica River in the south. The surface of the topographic basin area of the Roso-mačka River is 23,24 km 2 , and the area of about 100 km 2 is covered by the reconnaissance and the neotectonic analysis. The general fall of the basin area is towards the southwest, which is due to the direction of the course of the Rosomačka River, from the northeast to the southwest. The Rosomačka River is the right tributary of the Visočica River and belongs to the Black Sea Basin.
The area of Visok is characterized by a mountain climate, with sharp winters and a short, relatively warm summer period, where snow is often held on the ridge of Mt.Stara planina by the end of May.
Systematic hydrological measurements on the Rosomačka River and observation of meteorological parameters in its topographic basin area were not performed. The basin area of the Rosomačka River is poorly populated, with only two villages in the basin, Slavinja and Rosomač. There is a small number of aging households in both villages, in which agricultural production is at the level of a garden and a bit of cattle. There is no industrial, tourist or military facility in the basin, and the road infrastructure consists of: a local asphalt road between these two villages, forest roads and some neglected macadam roads. The natural conditions, with an insignificant impact of anthropogenic activities, dominate in the Rosomačka River Basin.
The Rosomačka River belongs to the category of hydrologically complex watercourses due to the following: a) large oscillations of extreme discharges during the year, from powerful torrential streams to drying up, b) in the summer period, with alternating shifts of the discharge sections (perennial discharge) and non-discharge sections (dry). There are attractive geomorphological and hydrogeological objects in its riverbed and valley that are the result of mutual effects of geoenvironment and river waters.

MATERIAL AND RESEARCH METHODOLOGY
The multidisciplinary research was carried out in the basin area of the Rosomačka River, through the following four phases: a) collecting documents and their analysis, b) field research, c) cabinet research, and d) interpretation of the obtained results.
The field research included geological, hydrogeological, geomorphological and hydrological reconnaissance, while detailed mapping was done in some sites. Field work was carried out during various annual seasons. Considering that the Rosomačka River is a hydrologically unstudied watercourse, high waters are interpreted on the basis of hydraulic tracks on the coast, and low waters by recording the location of the submerge, or the place of the discharge renewal in the riverbed. The results of reconnaissance and mapping were displayed on topographic backgrounds with a 1:25,000 scale.
The analysis of the collected publications and fund documentation was done in the cabinet, as well as the analysis and processing of data collected by field research. On the basis of the studied literature and the results of the conducted field research on the Rosomačka River Basin, the extremely complex geological structure of the terrain, complex neotectonic relations, hydrological and hydrogeological conditions were determined. Considering that this is a hydrologically unstudied watercourse, it was not possible to contribute to the interpretation of the role of geoenvironment in the formation of extreme discharges based solely on the results of conventional research and descriptive state of the field situation. Therefore, a neotectonic analysis of a wider area of research was conducted.
In order to gain an insight into the regional rupture characteristics of the research area, the method for remote detection was applied. The rupture structure was reconstructed by a stereoscopic analysis of aerial photographs. Panchromatic black and white shots, high gloss, made during 1968 and 1982 (MGI) were used. The photogeological analysis represented a visual detection of geological structures and composition of the terrain based on the aerial photographs. The obtained results were displayed on topographic maps at the scale of 1:25,000. Then, in order to determine the youngest, neotectonic terrain activities, the method of quantitative geomorphological analysis was applied. For the characteristic analysis parameter, the energy of the relief was chosen. A morphometric-statistical procedure consisting of the following steps was performed: calculating the height difference within each unit field; the value of the height difference was reduced by the value of the reference level which represented the average value of all height differences in individual fields on the studied terrain. The obtained positive and negative energies of the relief were the areas of intensified erosion or accumulation processes; using the "current average values" method, the first trend of anomalies of relief energy was obtained; with the interpolation of the values of the first trend, a map of isolines was obtained which reflected the general effect of the youngest tectonic movements (M a r ko v i ć , 1983). For the observed field of the unit area of the studied area, a square of 1x1 km was taken. The results were shown on 1:25,000 topographic maps by the isolines of the first trend of the negative energy of the relief. The isolines of the first trend of the positive energy of the relief and the saddle area -the reference level.
In the making of the map of a detailed rupture composition and neotectonic active structures, the data presented on the neotectonic map of Serbia at the scale of 1:500,000 (M a ro v i ć et al., 2002) at the scale of 1:2,000,000 were used (Komarnicki et al., 1997).
Determination of the gradient of riverbed fall was carried out from topographic maps at the scale of 1:25,000, using GIS software.
The display of the results was obtained by Rosgenov's classification of watercourses (Rosgen and S i l vey, 1996), but in this case modified in order to interpret extreme discharges. In this way, the variation of the geoenvironmental parameters significant for extreme discharges was reduced, and the integration of similar elements of the geoenvironment for the isolated parts of the course was achieved.
The analysis and synthesis of the results of the research argued (within the limits of the previously described precision) the cause and effect connection of the extreme discharges of the Rosomačka River and the geoenvironment of the basin area.

RESEARCH RESULTS
The Rosomačka River is formed by merging of the Caričin potok and the Zanoška River at the altitude of 1,068 m a.s.l. on the southwestern slopes of Mt.Stara planina. The spring zones of the watercourses from which the Rosomačka River emerges are located at the foot of the ridge of Mt.Stara planina, below the peaks of Begova kula (1,668 m) and Tupanac (1,673 m). The only significant tributary of the Rosomačka River is the stream called Šišarka, of the basin area of 3.45 km 2 , which flows from the left valley side by the village of Rosomač (altitude of 845 m a.s.l.). The length of the basin perimeter by topographic watershed is 28.4 km.
The Rosomačka River has a distinctive rainand-snow discharge regime, characterized by high waters in the spring and late autumn periods, i.e. winter and summer low waters. The occurrence, duration and quantity of low and high waters are dependent on the quantity, intensity and intra-an-nual distribution of precipitation, and partly from the hydrogeological conditions in the basin area and the watercourse itself. There are heavy torrential streams after intense precipitation and snow melting, and then they transport large amounts of load. Low summer waters are usually more scarce than winter, as the existence of accumulated atmospheric precipitation in the form of snow on the ridge and the slopes of Stara planina is important for low winter waters. Summer low waters occur in July and August, and slightly less in September, although torrential streams are frequent after intense summer precipitation (N i k i ć et al., 2016). For the recession period, the characteristic of the Rosomačka River is to dry up at two sections ( Figure 2): the first one is in the middle course, in the area of the karstified Middle Trias limestones, while the other one is in the lower part of the course in the alluvium of the Visočica River (Figure 3).
The confluence of the Rosomačka River and the Visočica River is at the altitude of 720 m a.s.l. and this is the hipsometrically lowest point of the basin area. The hydrographic lenght of the course of the Rosomačka River, from the hypsometrically highest spring point of the watercourse (altitude of 1,570 m a.s.l.) to the confluence of the Visočica River (altitude of 720 m a.s.l.), is 11.75 km, the altitude difference is 850 m, and the slope of the riverbed i=7.24% (Figure 4). The length of the course of the Rosomačka River from the merging of the Caričin potok and the Zanoška River (altitude of 1,068 m a.s.l.) to the confluence (altitude of 720 m a.s.l.) is 8.4 km, the altitude difference is 348 m, with the slope of the riverbed i=4.17%.
The length of the course of the Caričin potok from the highest spring point (altitude of 1,442 m a.s.l.) to the merging with the Zanoška River is 2.01 km, the altitude difference is 374 m and the slope of the riverbed i=18.54%. The length of the course of the Zanoša River from the highest spring point (altitude of 1.570 m a.s.l.) to the merging with the Caričin potok is 3.24 km, the altitude difference is 502 m and the slope of the riverbed i = 15.47%.
The altitude difference between the hypsometrically highest point in the basin (altitude of 1,673 m a.s.l.) and the lowest (altitude of 720 m a.s.l.) is 953 m, with the slope of the terrain i=7.71%, at a distance of 12.32 km.
On the longitudinal profile of the riverbed, the thalweg, of the Rosomačka River, the upper, middle and lower courses are clearly distinguished (Figure 2). The upper part of the course has the highest slope of the riverbed and the lower part of the course is the lowest. The basin area is relatively narrow, elongated in the north-south direction, while in the lower course it is very narrowed, practically to the valley along the riverbed. The watercourse itself, approximately rectilinearly flows from the slopes of Mt. Stara planina and on its way to the Visočica River, administratively cuts across several different geological units.
The upper course of the Rosomačka River (I) was largely built from the shales of the Rhine-Cambrian age (R,Cm), and to a lesser extent from sands, alevrolites, clays and conglomerates of the Lower Triassic (T 1 ). The middle course (II) is of the following formations: a) sandstones, alevrolites, clays and conglomerates of the Lower Triassic (T 1 ) ); e) loose rocky material of the Quaternary age that are formed of alluvium and talus. The lower stream (III) is formed of limestone, marl and sands of the Lower Cretaceous (K 1 1+2 ), through which sedimentary deposits of the Quaternary are represented by the terrace and alluvial (al) formations of the Rosomačka River and the Visočica River ( Figure 3). All the abovementioned sedimentary series are characterized by a slight slope towards the valley of the Visočica River, rarely larger than 20 o (A n đ el kov i ć , 1996). Therefore, the horizontal projections even of the thin geological formations are expressed, that is, on the surface of the terrain, they are more represented and in this way they get on the higher hydrological and hydrogeological significance.
The dominant influence on the formation of the type of drainage network and the Rosomačka River Valley had a neotectonic structure. The endogenous process changed the position of the erosion base during the evolution by the rise and fall of the terrain. In the area of Mt.Stara planina and thus in the studied area, the morphostructured forces performed global and regional changes in the relief, while the exogenous forces processed and shaped it (Dimitrijević, 1978). This connection between endogenous and exogenous processes is clearly visible in the Rosomačka River Basin through registered morphological objects such as: gorge, waterfalls, alluvial plains, ponors, ponor zones, potholes, narrow gorge.
By analyzing the rupture structure by the method for remote detection, fragmentary structures were determined which were formed during the Neogene period, or which later renewed their activity in the Post-Neogene period. By the method of quantitative geomorphological analysis, the position of neotectonic structures and the sign of movement of the separated tectonic blocks were determined. The distinct regional neotectonic active structures extend in the direction southwest-northeast and divide the basin area of the Rosomačka River to the north, which is relatively elevated, and the central and southern parts, which are relatively lowered. Gravitational movement along the isolated regional neotectonic structures led to the tectonic lowering of Block 2-Igin vrtop in relation to Block 1-Tupanac, that is, Block 4-Rudine compared to Block 2 (Figure 3). The position and distribution of the separated neotectonic active structures indicate the block structure of the basin area.
On the basis of the performed analysis, the northern part of the terrain, Blok 1-Tupanac with positive values of anomalies of the relief energy, is clearly distinguished, indicating the relative elevation of the terrain. Therefore, the enhanced process of elution and erosion develops on it. The neg- ative values of the anomalies of the relief energy are shown in the southern parts of the terrain, Blok 2-Igin vrtop and Blok 4-Rudine, which are relatively lower. Therefore, it is especially interesting that at Block 2, at the same time the processes of erosion and accumulation take place, and on Block 4 only the accumulation process (Figure 3).
Reconstructed regional neotectonic active structures between blocks are unique linear geological structures, which consist of systems of the parallel faults of the same direction of extension. The relative elevation of the northern part (Block 1), i.e. the relative lowering of the southern part of the basin (Block 2 and Block 4) along the isolated regional neotectonic structures, does not mean that the height difference between the blocks is in the form of "steps". These tectonic "jumps" have been "processed" over time by exogenous agents, and their recent morphological form is in the form of a very steep terrain, in which the Rosomačka River cut the riverbed. These steep parts of the terrain between the blocks are also visible on the graphic representation of the thalwegs of the Rosomačka River (Figure 2).

THE ROLE OF GEOENVIRONMENT IN THE INTERPRETATION OF EXTREME DISCHARGES OF THE ROSOMAČKA RIVER
Quantitative geomorphological analysis found that the basin area of the Rosomačka River extends to three separate neotectonic blocks: Blok 1-Tupanac, Blok 2-Igin vrtop and Blok 4-Rudine. The upper course of the Rosomačka River largely belongs to the neotectonic Blok 1-Tupanac, and to the smaller part of Block 2-Igin vrtop, the middle course is in its entirety at Block 2-Igin vrtop, and the lower course at Block 4-Rudine (Figure 3). The slope of the Rosomačka riverbed at Block 1 is i=16.54% at the length of 2.30 km, at Block 2 it is i=5.50% at the length of 7.84 km, and at Block 4 it is i=2.48 % at the length of 1.61 km (Figure 4). A qualitative interpretation of the extreme discharges of the Rosomačka River from the aspect of the influence of geoenvironment, i.e. its lithologic, neotectonic, hydrogeologic and geomorphologic elements, was carried out for all sections of the river course and shown in Figure 4. Within the individual sections of the river course, where the role of geoenvironment was multiple, additional division through the unification of the dominant role of geoenvironmental elements was done.

Upper course -I
The rocky masses participating in the upper course of the Rosomačka River (I), at tectonic Block 1 (Paleozoic shales) and at Block 2 (sandstones and conglomerates of the Lower Triassic) have low porosity. This feature is especially important from the aspect of distribution of atmospheric precipitation that is excreted on the ridge of Mt.Stara planina. Due to the lack of significant porosity of the rock masses, the infiltration of atmos- Figure 3. A schematic representation of the geological structure and the neotectonic structure of the basin area of the Rosomačka River (Anđelković et al., 1977, and supplemented) Legend: 1-neotectonic active structure of a regional character with a lowered block label; 2-neotectonic active structure of a local character with a lowered block label; 3-topographic watershed; 4-ephemeral course; 5-perennial course; 6-ponor zone; 7-gorge; 8-narrow gorge; 9-neotectonic block; 10-geological boundary. For stratigraphic labels, the description of lithology is given in the text. pheric waters is modest, and the surface runoff is dominant. After precipitation or snow melting, the filling of soil water capacity and local aquifers of a cracked type is firstly taking place, followed by diffusional water interflow along the slope. Then water concentrates in shallow depressions and riverbeds of ephemeral and perennial streams, which ultimately flow into the Zanoška River or Caričin potok (Figure 3). In addition to water, large volumes of loose rocky material flow into the drainage system from the upper course area. On the basis of quantitative geomorphological analysis, it was established that Block 1 had positive values of anomalies of relief energy, that is, showed the highest relative elevation. Because of this, the erosion process is developing on it and this part of the basin represents a significant source of load that occurs in the Rosomačka River.
In the recession period, the Rosomačka River in the upper course has a perennial discharge. The perenniality of the discharge is maintained due to the significant amounts of precipitation and their favourable intra-annual schedule, as well as underground waters from local cracked aquifers.
However, in the period of floods, snow melting or intense precipitation, due to low infiltration and the dominant surface runoff, the Rosomačka River becomes a torrential stream. During this period, the Zanoška River and the Caričin stream have significant kinetic energy due to the exceptionally steep terrain in the area of the regional neotectonic structure, on the part between neotectonic Blocks 1 and 2. The mentioned influence of neotectonicity is also indicated by a small waterfall, about 4 m high in the riverbed of the Caričin stream at about 1 km upstream from the merger with the Figure 4. The cross-sectional profile of the riverbed and the Rosomačka River plan with the elements of geoenvironment significant for interpreting extreme discharges in certain parts of the river course Legend: 1-slope of the riverbed from the spring to the confluence into the Visočica River; 2-slope of the upper, middle and lower course; 3-slope of the riverbed on tectonic blocks; 4-slope of the riverbed in more detailed separate parts of the river course. Zanoška River. This "jump" in the terrain geologically represents one of the preserved "steps" within the framework of a regional neotectonic structure that separates Block 1 from Block 2.
Thanks to the erosion power of torrential streams and perennial discharges, the Rosomačka River in Block 2 formed a short gorge called the "Reka" in the Lower Triassic sediments. The reasons for the formation of the "Reka" gorge in the area of Block 2, downstream of the neotectonic structure of a regional character, are the following: a) by merging of the Caričin stream and the Zanoška River, the quantity of discharges has been substantially increased, and the perenniality of the discharge of the Rosomačka River was achieved, b) due to the significant riverbed fall at the neotectonic active structure of a regional character, the watercourse gains a great kinetic energy, and c) the Lower Triassic sediments (T1) are more susceptible to destruction and vertical erosion from shales of Rifeo-Cambrian (R,Cm).
At the upper course (I), the gradient of the riverbed fall of the Rosomačka River Basin is i=13.99% at a length of 3.85 km (Figures 2 and 4). The perennial river course and the kinetic energy of the torrential streams with large amounts of load contributed to the formation of a cross-sectional profile typical for the spring part of the watercourse in impermeable rocky masses (Figure 4).

MIDDLE COURSE -II
The middle course of the Rosomačka River (II) is completely located at Block 2. It extends from the "Reka" gorge to the exit from the narrow gorge called "Rosomačko grlo" (Figure 3). The slope of the riverbed in the middle course is i=4.33% at a length of 6.29 km. The regime of extreme discharges of the Rosomačka River in the middle course is very complex. The complexity is reflected in the following: a) in the recession period upstream from the village of Rosomač there is a complete drying up of the watercourse, and downstream of the village there is a renewal of the discharge, i.e. there is a perennial discharge, b) in the period of floods there is a perennial discharge in the entire middle part of the river course Figure 4).
Due to the distinguished specificity of extreme discharges, the middle course is divided into a part upstream of the village of Rosomač -IIa and a part downstream of the village of Rosomač -IIб ( Figure  4). For these parts of the middle course the diametrically opposite effect of the fluvial process is characteristic. In the middle course upstream of the village of Rosomač (IIa), the effect of the fluvial process formed the accumulation form -the alluvial plain, and at the part downstream of the village (IIб) the erosion form -narrow gorge was formed. The cause of this diversity is endogenous i.e. neotectonic activity.

Middle course upstream of the village of Rosomač -IIa
The middle course upstream of the village of Rosomač lies in a small part on the terrain, which is formed of sands, alevrolites, clays and conglomerates of the Lower Triassic (T1), and mostly on the terrain formed of limestones and dolomites of the Middle Triassic (T2) (Figure 3). From the point of view of interpreting extreme low water discharges in this part of the river course, the porosity of the mentioned rocky masses is essential. The formation of the Lower Triassic does not have hydrogeologically significant porosity and is therefore impermeable, while the Middle Triassic limestones are strongly cracked and karstified and therefore permeable.
The middle course of the Rosomačka River upstream of the village of Rosomač is predisposed and thus partly controlled by a neotectonic structure of a local character, of the approximate northsouth direction (Figure 3). The east wing of this local neotectonic structure is subblock 2a-Padež, which is the only one within Block 2 characterized by a relative increase. By the increase of the subblock 2a-Padež, a severe neotectonic landform of the approximate north-south direction was formed. It has a small slope of the bottom to the south and i=4.56% at a length of 3.99 km, with steep and long sides. The small slope of the neotectonic landform bottom is due to the spatial positioning of Block 2 during a gravitational decrease along a separate regional neotectonic structure. In this narrow neotectonic landform, the Rosomačka River formed an alluvial plain of small thickness by depositing the load. Along the alluvial plain by the perimeter of both valley sides, there are numerous taluses and smaller mantled slopes. Practically, in the same area and at the same time, in the narrow neotectonic landform two geomorphological processes, fluvial and colluvial, are taking place. Their products are the accumulation of geomorphological forms, alluvial plain and taluses (Figure 4).
The alluvial plain of the Rosomačka River extends to a lower part of the Lower Triassic (T 1 ) clusters, and largely on limestones and dolomites of the Middle Triassic (T 2 ). Limestones and dolomites of the Middle Triassic (T 2 ) are intensely karstified and cracked, so there are ponors at the bottom of the riverbed. In addition to the ponors, there were numerous ponor zones in the thin alluvium of the Rosomačka River. Alongside these ponor zones, the river waters in the summer recession period completely submerge (Figure 4). The capacity of the "swallowing" of the ponor zones is higher than the amounts of water that in the recession period flow from the upstream, non-karstified part of the basin, so that the Rosomačka riverbed, on a part of the river course to the village of Rosomač, is without discharges (dry) for several months during the year (Ni ki ć, 2003). That is why the Rosomačka River is also the allogeneic watercourse.
The submerge of the Rosomačka River along the ponor zones takes place during the period of floods as well. During this period, due to the inflow, which is much larger than the capacity of the "swallowing" of the ponor zones, the Rosomačka River realizes its discharge in this part. However, due to the small slope of the bottom of the riverbed and the submerge in all the periods of the year there is a certain decrease in the quantity and kinetic energy of the inflowed waters. This results in the selective deposition of the load in a narrow neotectonic landform. By depositing the load hypsometrically high on the slopes of Stara planina, the Rosomačka River in this part of the river course formed a 50-90 m wide, 1-2 m thick, and about 2.2 km long alluvial plain.

Middle course downstream of the village of Rosomač -IIб
Downstream of the village of Rosomač and the confluence of the stream of Šiškarka, the Rosomačka riverbed is cut into the Jurassic formations made of lithologically different components. Most components have no significant porosity, and a smaller number is of a local porosity of the cracked type.
Downstream of the village, the slope of the Rosomačka riverbed is i=3.92% in the length of 2.30 km (Figure 4), its valley gradually narrows, the valley sides become steeper and reach the watershed. On these sides, at various levels of the hypsometry, there are parts of the terrain that are made of limestones or sandstones. These rocks often have cracked porosity within which the local aquifers of cracked type were formed (Figure 4). Groundwaters from the local cracked aquifers from both sides of the valley gravitate and are collected into the Rosomačka riverbed, which is cut into the impermeable rocks. In the recession period, these groundwaters form a quantitatively modest discharge downstream of the village of Rosomač (Nikić, 2003). In this way, on this part of the course, the Rosomačka River has a perennial discharge, i.e. it does not dry up (Figure 4).
In the period of floods, in this part of the course, a quantitatively significant discharge of the Rosomačka River is formed. This is contributed by a significant inflow and torrential streams from the upstream part of the basin. A characteristic example of the intensity and strength of the Rosomačka River torrential streams, on this part of the course, is the narrow gorge of "Rosomačko grlo" ( Figure  5). It was formed at the end of Block 2, by cutting the river into the Upper Jurassic klastic sediments ( Figure 3). The deep cutting of the riverbed and formation of the narrow gorge was contributed by the neotectonic activity and the gravitational lowering of Block 4 in relation to Block 2 along the regional neotectonic structure. By lowering Block 4, there was a significant increase in the fall of the terrain, which led to the intensified cutting of the Rosomačka River. As a consequence, the narrow "Rosomačko grlo" gorge was formed, 0.8 to 1. The narrow gorge of "Rosomačko grlo" ends in a subvertical cliff about 4-5 m high, built of Jurassic conglomerates. The exit from the narrow gorge is the cut at the bottom of which is a riverbed about 1.2 m wide, through which the Rosomačka River flows into Block 4. The vertical rocky cliff represents one of the preserved "steps" within the regional neotectonic structure that separates Block 2 and Block 4.

Lower course -III
Block 4, on which the lower part of the course of the Rosomačka River (III) is formed, starts downstream of the narrow gorge of "Rosomačko grlo». On the basis of quantitative geomorphological analysis, Block 4 shows the negative value of the anomaly of the energy of the relief, i.e. it is characterized by relative lowering where the enhanced accumulation process takes place. In Block 4, the Visočica River formed a wide valley representing the erosion base for all the watercourses in the area of Visok.
Block 4 is built of limestones, marls and sandstones of the Lower Cretaceous (K 1 1+2 ), through which the Quaternary sediments are represented by terraced and alluvial formations (Figure 3). Lower Cretaceous sediments have no significant porosity and therefore belong to the category of impermeable rocks. The alluvium of the Rosomačka River, as well as the Visočica River, has an intergranular type of porosity and belongs to the category of highly permeable rocks. The slope of the Rosomačka riverbed at Block 4 is i=2.48% is 1.61 km long (Figure 4). Due to its hydrological, morphological and hydrogeological specificities, the lower course is divided into two parts. The first part is upstream of the village of Slavinja (IIIa), and the second part is downstream of the village of Slavinja (IIIб) (Figure 4). In the recession period, the lower course upstream of the village of Slavinja (IIIa) has a discharge, while the part downstream of the village of Slavinja (IIIб) is drying up. In the period of floods, the Rosomačka River achieves perennial discharge along the entire lower course. There are rare cases where on this part of the course when there is a flood or the transport of higher quantities of larger load. In the period of floods, the Rosomačka River is of a relatively calm watercourse, usually with a full moistened profile of the riverbed.

Lower course upstream of the village of Slavinja -IIIа
The lower course of the Rosomačka River upstream of the village of Slavinja (IIIa) extends from the exit from the narrow "Rosomačko grlo" gorge to the alluvial plain of the Visočica River. This part of the course extends to the plain terrain or alluvial plain of the Rosomačka River, formed by depositing the load through the structures of the Lower Cretaceous. The width of the alluvial plain of the Rosomačka River is 20-30 m, with a capacity of 0.5-1.5 m, length about 750 m, with a fall in the direction towards the Visočica River. The riverbed in this part is shallowly cut, with depths up to 0.6 m, 2-5 m wide, with a slope of the riverbed of i=2.77% at a length of 1.08 km (Figure 4).
Along the whole course, upstream of the village of Slavinja, there is a gradual decrease in the amount of low water discharge due to the submerge of waters into the alluvium of the Rosomačka River. Infiltrated river waters maintain a wa- Figure 5. The "Rosomačko grlo" narrow gorge (Photo by Z. Nikić, 2002) ter-saturated environment in a thin alluvium where the impermeable formations of the Lower Cretaceous are formed in its basis. Therefore, the possibility of infiltrating river water is limited, so in the recession period there is a perennial discharge in this part (Figure 4). During the period of floods in this area, the discharge is perennial.
Immediately downstream of the exit from the narrow gorge, the Rosomačka River formed a shallow inundate deposite of about 90-110 m in length, about 50 m wide, which slightly falls towards the Visočica River. The inundate deposite was formed by depositing the torrential load after leaving the narrow gorge. Its lithological and granulometric composition is the same as the alluvium, and downstream it ends in a gradual transition into the alluvium of the Rosomačka River. Inundate deposite points to the torrential character, that is, the significant kinetic energy, the amount of waters and loads that the Rosomačka River has before reaching Blok 4. This inundate deposite is clearly shown on the graphic representation of the Rosomačka River profile for the section IIIa, in the form of a subhorizontal riverbed ( Figure 6).

Lower course downstream of the village of Slavinja -IIIб
The lower course of the Rosomačka River downstream of the village of Slavinja (IIIб) is entirely in the alluvial plain of the Visočica River (Figure 3). For this part of the course, it is characteris-tic that the valley of the Rosomačka River is not recognized, but only its riverbed cut into the alluvium of the Visočica River. The riverbed is up to 2 m deep, 2-4 m wide, with a slope of only i=1.89%, at a length of 0.53 km (Figure 4).
In the recession period, the Rosomačka riverbed from the village of Slavinja to the confluence into the Visočica River is usually dry, with no discharges for several months (N i k i ć et al., 2016). Due to the considerable potential and the high permeability of the alluvium of the Visočica River, the submerge of the low waters of the Rosomačka River during the recession period is complete (Figure 4). The infiltrated waters of the Rosomačka River flow into a compact type of the aquifers in the alluvium of the Visočica River in which basis the impermeable formations of the Lower Cretaceous are formed. In the recession period, the level of the compact type of the aquifers in the alluvium of the Visočica River is below the Rosomačka riverbed, which enables the infiltration of river waters. In the period of high waters, the Rosomačka riverbed has a discharge with a full riverbed profile (Figure 4).

CONCLUSION
In addition to precipitation, a significant element in the formation of extreme discharges of the Rosomačka River is the geoenvironment of the basin area. Considering that the hydrological re- The research works have determined that the Rosomačka River is a watercourse: a) which in the summer recession period has an alternate shift of sections with and without discharges, i.e. with discontinuous discharge, b) partly a torrential stream of large destructive force, c) partly allogeneous, d) with attractive geomorphological objects, e) with a composite valley.
On the basis of quantitative geomorphological analysis, it was established that the basin area of the Rosomačka River extends to parts of the three neotectonic blocks: Blok 1-Tupanac, Blok 2-Igin vrtop and Blok 4-Rudine. Tectonic blocks were built by geological formations consisting of numerous units of different lithological composition, of various mechanical and hydrogeological characteristics. Tectonic blocks were separated from each other by neotectonic active structures of a regional character along which the gravitational lowering of the terrain occurred. The Rosomačka River receives great kinetic energy precisely in a part of the course through neotectonic structures of a regional character along which the gravitational lowering of the blocks has been achieved. In the recent morphology of the terrain these neotectonic structures are not a "step", but an exceptionally steep terrain in which the Rosomačka River cut in the riverbed. Formed geomorphological erosion (gorge, narrow gorge, pots) and accumulation (alluvial plain, inundated range) forms point to the strength of the kinetic energy of the Rosomačka River.
In favourable hydrogeological conditions, in the summer recession period, geoenvironment can have a twofold effect on the formation of low waters. On the part of the Rosomačka riverbed in the rocks of karst and intergranular porosity, the process of submerging takes place, which leads to a decrease in discharge and complete drying up. On the other hand, in the part of its riverbed, which is without discharge from the upstream part of the basin in the recession period, the local aquifers on the valley sides provide groundwaters that accumulate in the basin and in this way a low water discharge is formed. In this case, the ground-waters in the summer period provide the perenniality of the Rosomačka River discharges to the place of another submerge.
In the period of floods, in the upper part of the basin, at Block 1, due to the absence of hydrogeologically significant porosity, and after intense precipitation or snow melting, there is a formation of an intense surface runoff and torrential stream waves.
In the middle course of the Rosomačka River, the range of extreme discharges from drying up to heavy torrential streams is an eclectic example of the role of geoenvironment. The elements of geoenvironment that contribute to this condition are lithological (porosity), neotectonic (lowering of terrain, fragmentary structure), hydrogeological (permeability). They also contributed to the formation of accumulation and erosion geomorphological objects.
In addition to the above mentioned, knowledge of the geoenvironment of the basin area has practical significance in making decisions on water use, water protection, the selection of profiles for setting observation hydrological and precipitation stations in the basin, more realistic fulfillment of the requirements of the EU Water Framework Directive 2000, as well as the general principles of environmental protection. Note: The research was carried out as part of the project No. 37008 (Programme TR), financed by the Ministry of Science and Technological Development.