Western Bug River is the biggest river in the plain part of Western Ukraine. It takes the beginning in Koltivska valley near Verkhobuzh village of Zolochiv district and Lviv region at an altitude of 303 meters above sea level and belongs to the Baltic Sea basin. The Upper Western Bug river basin covers the territory which administratively belongs to the northern part of Lviv region.
According to the physico-geographicall division of Ukraine (Marynych, Shyshchenko, 2006; Pryroda…, 1972), the investigated territory is situated within Volynske Opillia, Male Polissia and only slightly covers Roztochia, Opillia and Western Podillia region of the Western-Ukrainian land, broad-leaved forests zone and the European plain landscape country. According to the geobotanical division of Ukraine (Didukh, Sheliag-Sosonko, 2003), the investigated territory belongs to the European broad-leaved region, namely to the Central European Province of the broad-leaved forests, South Polish-Western Podillia subprovince of the broad-leaved forests, meadows, meadow steppes and eutrophic bogs and 4 districts, in particular Opilsko-Kremenetskyi district of the beech, hornbeam-oak forests, genuine and steppe meadows and meadow steppes, Roztochia district of the beech, beech-pine, oak-pine, fir and oak forests, floodplain meadows and eutrophic bogs, Malopoliskyi district of the hornbeam-oak and pine forests, floodplain meadows and eutrophic bogs, as well as Liubinsko-Volynskyi district of the hornbeam-oak, oak forests and steppe meadows.
Relief of the investigated region is complex and diverse. It is represented by different for its genesis hills, depressions and low mountains, dissected by river valleys. For relief, the territory belongs to the Volyno-Podilska Elevation (Pryroda…, 1972), which consists of three subregions – Volynska and Podilska Elevations and Male Polissia. Male Polissia subregion is a lowland, covered with sandy loam fluvio-glacial deposits and partially, with loessial rocks and clayey bed of marls weathering. The relief of plain has been formed by the movement of glacier. That is why its height does not exceed 200 metres. There is a developed dense river network on the territory. Areas between the rivers have flat and wavy surface with hills and depressions.
Volynska and Podilska Elevations are the sublime wavy and hilly plains covered with loessial deposits. Podilska Elevation is one of the highest parts of the whole East European plain. It is significantly raised (up to 350-450 meters above sea level) and deeply dissected by numerous river valleys, ravines, gullies, which depth is about 150 meters. The karst phenomena are typical for the most part of its territory. Its most elevated parts are located in Roztochia (up to 397 m). The highest peaks in Holohory are Kamula mountain (471 m) and Voroniaky (to 440 m).
Geologically, the investigated area is located on the main tectonic structures such as Volyno-Podilska plate and Galytsko-Volynska depression [48].
Volyno-Podilska plate is a part of the East European platform located between the Ukrainian shield and the Carpathian geosynclinal region. It is represented by Paleozoic deposits (Cambrian, Ordovik, Silurian, Devonian), in particular sandstones, siltstones, argillites, marls, limestones and dolomites; Mesozoic deposits (Upper Jurassic and Upper Cretaceous) are presented by limestones, dolomites and anhydrites, and Cenozoic deposits include (Upper Miocene) sandstones, limestones, salts, gypsum and anhydrites (Pryroda…, 1972).
Galytsko-Volynska (Lviv) depression – is an extreme western part of the East European platform on the boundary of a young Western European platform where the precambrian foundation is lowered to a depth of 3000-7000 meters. On its surface there is a thick strata of Paleozoic, Mesozoic and Cainozoic deposits (sand, marls, chalk) .
The investigated region is situated in the zone of continental and atlantic air masses influence of temperate latitudes and is characterized by the Atlantic continental climate that is transitional: from moderately warm and humid West European to continental Eastern European. Its characteristic features are significant amount of precipitations (more than 550 mm per year), moderate annual amplitudes of air temperature which doesn’t exceed 24 °C, mild winters (average temperature – 3,5-5 C°) with frequent thaws and unstable snow cover, moderately warm summers with sufficient amount of precipitations during the vegetation period, without persistent droughts and dry winds.
The peculiarity of atmospheric circulation is the presence of continental arctic air in the winter and spring seasons that brings a cold and cloudless weather. Penetration of sea arctic air in summer and autumn seasons can cause a cold, wet weather. Penetration of tropical air is also possible in spring and summer. Continental tropical air masses cause high temperatures and the sea ones – a warm, cloudy weather with fogs. It greatly depends on the cyclones that move all the year, but often recur in winter and spring (Pryroda…, 1972).
Soil cover of the region is diverse, since the investigated area is situated within different geographical zones. There are almost all types of soils, which are typical for the broad-leaved zone. Soddy-podzolised, soddy and boggy soils are prevalent on the plain forest-meadow landscapes. Light gray, gray and dark gray soils, podzolised (forest) soils and chernozems are typical for the watersheds and slopes of the forest-steppe landscapes, while the soddy and boggy soils are common in the river valleys and on bottoms of deep gullies. A small part of the massives are occupied with soddy and leached chernozems. Meadow-boggy soils mainly occur in the floodplains and on the bottoms of deep gullies. Boggy soils are at the foot of the slopes and in the depressions of floodplains. Peat-boggy soils lie down on the periphery of peat bogs, and often cover the central parts of small rivers floodplains and bottoms of deep gullies.
In the Western Bug river basin there is a well developed hydrological network which is characterized by small areas of bogs, small number of lakes and big number of artificial basins of different functions. There are mainly plain rivers with a high density of river network in the region (from 0.2 to 1.2 km/km2; the highest density is in Podilska Elevation, the lowest one is in Male Polissia. The average density of the Western Bug river basin network is 0.35 km/km2 (Pryroda…, 1972).
Rivers have a mixed type of nourishment. They are filled with melted snow waters in spring, with rain precipitation in summer and with ground water the whole year. The highest water level in rivers can be observed in March and April during snowmelt and in early summer, when there are the greatest amount of precipitation. The lowest water level is observed in August-September and December-February. The river stream rate varies, the lowest one is in Polissia region.
The rivers of Male Polissia region are characterized by small tilts, low banks, slow stream, boggy broad and shallow valleys, numerous meanders, as well as hardly noticeable erosive processes, prolonged spring overflows and low summer floods. Density of river network is 0.3-0.5 km/km2. The average annual precipitation is 680-720 mm.
Roztochia-Opillia region is characterized by a high humidity of the climate. The average precipitation is 610-720 mm. River valleys are the most clearly marked relief forms. They are deeply (80-100 m) embedded in the foundation, but usually without of the steep slopes and are almost entirely occupied with boggy floodplains.
Rivers of Western Podillia have often deeply embedded canyonlike valleys with steep slopes and narrow floodplains, combined with flat interfluves. They are characterized by spring overflows and summer rain floods. The density of river network is 0.5-0.7 km/km2. Average precipitation is 560-700 mm. In Podilska Elevation there are karst landforms.
Geographic location, soil and climatic conditions, as well as altitude distribution of the investigated territory promote the formation of rich plant biodiversity. Its special feature is the presence of endemic and relict center, located in the Western Podillia. Different vegetation types such as forest, meadow, steppe, boggy and aquatic ones are typical for the territory (Herenchuk, Koinov, Tsys, 1964).
Forest vegetation, despite the high ploughing degree, occupy up to 25% of the investigated territory. The main forest types are pine (Volynske Polissia), oak-pine (Male Polissia, Roztochia), hornbeam-oak and oak-hornbeam (Volynske Polissia), beech and oak woods (Western Podillia), alder (Volynske and Male Polissia). Meadow vegetation takes up to 13% of the investigated territory. It is represented by genuine floodplains and peaty meadows. Natural meadows are the most common for Polissia while the floodplains occur in Male Polissia. Steppe vegetation occupy a small area, because of the high ploughing degree of the territory. Boggy vegetation is mainly distributed in Male Polissia, rarely in Roztochia. It is mainly represented by eutrophic, less meso-and oligotrophic communities. Floodplain bogs are widespread on the plain. Aquatic vegetation covers the largest area in Male Polissia and Roztoczia but is less distributed in Western Podillia. Air-aquatic vegetation is characterized by the greatest phytocoenotic diversity. The genuine aquatic vegetation occupy a comparatively small area.