Sava

The Sava (; , ; sr-cyr, Сава, hu, Száva) is a river in Central and Southeast Europe, a right-bank and the longest tributary of the Danube. It flows through Slovenia, Croatia and along its border with Bosnia and Herzegovina, and finally through Serbia, feeding into the Danube in its capital, Belgrade. The Sava forms the main northern limit of the Balkan Peninsula, and the southern edge of the Pannonian Plain.

The Sava is long, including the Sava Dolinka headwater rising in Zelenci, Slovenia. It is the largest tributary of the Danube by volume of water, and second-largest after the Tisza in terms of catchment area () and length. It drains a significant portion of the Dinaric Alps region, through the major tributaries of Drina, Bosna, Kupa, Una, Vrbas, Lonja, Kolubara, Bosut and Krka. The Sava is one of the longest rivers in Europe and among the longest tributaries of another river.

The population in the Sava River basin is estimated at 8,176,000, and is shared by three capital cities: Ljubljana, Zagreb and Belgrade. The Sava is about -navigable for larger vessels: from the confluence of the Kupa in Sisak a few kilometers below Zagreb.

The name is believed to be derived from the Proto-Indo-European root ''*sewh₁'' ('to take liquid', whence the English word ''sup'') and the ending ''*eh₂'', so that it literally means 'that which waters he ground. The ancient Greeks called it Saos ( grc, Σάος).

Sources

The Sava River is formed from the Sava Dolinka and the Sava Bohinjka headwaters in northwest Slovenia. The drainage basin has other key tributaries, including the Sora, the Tržič Bistrica and the Radovna rivers—flowing into the Sava at confluences as far east downstream as Medvode.

The Sava Dolinka rises at the Zelenci Pools near Kranjska Gora, Slovenia, in a valley separating the Julian Alps from the Karavanke mountain range. The spring is near the Slovene- Italian border at above sea level, in a drainage divide between the Adriatic and Danube basins. The Sava Dolinka spring is fed by groundwater possibly exhibiting bifurcation of source karst aquifer to the Sava and Soča basins. Nadiža creek, a short losing stream flowing nearby, is the source of Zelenci Pools water. The Sava Dolinka is considered the Sava's initial, segment.

The Sava Bohinjka originates in Ribčev Laz, at the confluence of the Jezernica, a short watercourse flowing out from Lake Bohinj and the Mostnica River. Some sources define the Jezernica as a part of the Sava Bohinjka, specifying the latter as flowing directly out of the lake, while another group of sources include Savica, rising at the southern flank of Triglav as the Savica Falls, downstream from Triglav Lakes Valley, and flowing into the lake, as a part of the Sava Bohinjka. The watercourse flows —including the length of the Savica—east to Radovljica, where it discharges into the Sava Dolinka. Downstream from the confluence, the river is referred to as the Sava.

Course

The Sava spans Central- Southeast Europe, flowing through Slovenia, Croatia, Serbia and along the Bosnia-Herzegovina border. Its total length is , including the Sava Dolinka and the Sava proper. As a right tributary of the Danube, the river belongs to the Black Sea drainage basin. The Sava River is the third longest tributary of the Danube, slightly shorter than the Tisza and the Prut—the Danube's two longest tributaries—when the Sava Dolinka headwater is excluded from its course. It is also the largest tributary of the Danube by discharge. The river course is sometimes used to describe the northern boundary of the Balkans, and the southern border of the Central Europe. Before the breakup of Yugoslavia in 1991, the Sava was the longest river lying completely within the country.

From the source to the Sutla

The Sava Dolinka rises in the Zelenci Pools, west of Podkoren in the Upper Carniola region of Slovenia at above sea level (a.s.l.), and flows east, past Kranjska Gora to Jesenice, where it turns southeast. At Žirovnica, the river enters the Ljubljana Basin and encounters the first hydroelectric dam— Moste plant—before proceeding to the east of the glacial Lake Bled towards Radovljica and confluence of the Sava Bohinjka, at a.s.l. Downstream of Radovljica, the Sava proceeds southeast towards Kranj. Between Kranj and Medvode, its course comprises the Lake Trboje and the Lake Zbilje reservoirs, built for the Mavčiče and the Medvode power plants.

The Sava then flows through the capital of Slovenia, Ljubljana, where another reservoir is on the river, adjacent to the Tacen Whitewater Course. There the river course turns east and leaves the Ljubljana Basin via Dolsko, at a.s.l. (at confluence of the Ljubljanica and the Kamnik Bistrica). The course continues through the Sava Hills, where it passes the Litija Basin with the mining and industrial town of Litija, the Central Sava Valley with the mining towns of Zagorje ob Savi, Trbovlje, and Hrastnik, turns to the southeast and runs through the Lower Sava Valley with the towns of Radeče, Sevnica, and Krško. The course through the Sava Hills forms the boundary of traditional regions of Lower Carniola and Styria, At Radeče, the Vrhovo hydroelectric dam reservoir stands. The latter is site of the Krško Nuclear Power Plant, which uses the Sava River water to dissipate excess heat. The easternmost stretch of the Sava River course in Slovenia runs to the south of Brežice, where it is joined by the Krka, and the river ultimately becomes a border river between Slovenia and Croatia, marking of their border near confluence of the Sutla ( sl, Sotla). At that point, the Sava reaches a.s.l. after flowing through Slovenia and along its border.

From the Sutla to the Una

The westernmost part of the Sava River course in Croatia, takes the river east, through the western part of the Zagreb County, between Samobor and Zaprešić. The area encompasses forests interspersed by marshes and lakes formed in gravel pits. As the Sava approaches the capital of Croatia, Zagreb, the marshes give way to urban landscape, but there are surviving examples of the gravel pit lakes, such as the Jarun, and the Bundek within the city. At the western outskirts of Zagreb, there is the western terminus of the Sava–Odra flood-relief canal connecting the Sava to the Odra River plain which is intended to act as flood control retention basin. The canal has been built in response to the most destructive flooding of the river that occurred in Zagreb in 1964, when one third of the city was flooded and 17 people were killed. The city itself marks the western extent of the Sava River basin area especially prone to flooding, spanning from Zagreb to confluence of the river in Belgrade, Serbia.

East of Zagreb, the river turns southeast again further through the Central Croatia, to the Sisak-Moslavina County, the city of Sisak, reaching a.s.l. The city of Sisak marks the westernmost extent of the Sava River navigable to larger vessels. Navigation conditions on the river are poor due to limited draft and fairway width, meandering of the river, bridge clearance restrictions, poor fairway markings as well as presence of sunken vessels and other objects, including unexploded ordnance. The ordnance is left over from various conflicts including the World War II, Croatian War of Independence, Bosnian War, and the 1999 NATO bombing of Yugoslavia. Before reaching confluence of Una at Jasenovac and a.s.l, the Sava River traces Lonjsko polje Nature Park, encompassing marshes frequently flooded by the Sava and its tributaries in the area.

From the Una to the Drina

Downstream of confluence of the Una River, the Sava is once again tracing an international border—between Croatia and Bosnia-Herzegovina. Its meandering course runs generally eastwards along Bosanska Gradiška, and Slavonski Brod to Županja, where it turns south to Brčko. There, the river resumes its predominantly eastward course towards Sremska Rača and confluence of the Drina River. The right bank of the Sava, in this segment of its course, belongs to Bosnia-Herzegovina (with Bosnia's all three administrative entities, Republika Srpska, Federation of Bosnia and Herzegovina and the Brčko District, having gateway to the river), while the opposite bank belongs to Croatia and its Sisak-Moslavina, Brod-Posavina and Vukovar-Srijem counties, except in the area of Jamena and further downstream—which belongs to Serbia and the province of Vojvodina. No cities in this segment of the course span the river. It represents an international frontier, three times seeing adjacent, opposing key settlements: Bosanska Gradiška, Bosanski Brod and Brčko in Bosnia-Herzegovina, opposing Stara Gradiška, Slavonski Brod and Gunja in Croatia.

The segment between the Una and the Drina confluences, corresponding to the Sava flowing along the border of Bosnia-Herzegovina, exhibits small change of elevation, such as from   ASL at Jasenovac to  ASL at Brčko gauges: over of the river between them. The river below Zagreb has a 0.4‰ slope (gradient) on average, much less steep than the course in Slovenia, where the average slope exceeds 0.7‰. This results in the Sava's meandering course running through a wide plain bordered by wetlands.

From the Drina to the Danube

Downstream from the confluence of the Drina, the Sava River changes its eastward course to northeast, until it reaches Sremska Mitrovica, whence it flows southeast and then south to Šabac, before finally turning east towards Belgrade. Most of the river's course in Serbia represents a border between province of Vojvodina, on the left bank, and Central Serbia, on the right bank. Exceptions to that are in area around Sremska Mitrovica, where both banks are in Vojvodina, and downstream of Progar suburb of Belgrade where both banks are in Central Serbia. The river meanders and forms wetlands there as well—the most significant centering on Obedska bara oxbow lake. The Sava River forms several large islands in this segment of the course, with the largest among them— Ada Ciganlija in Belgrade—connected to the right bank by a pair of artificial embankment dams forming Lake Sava since 1967.

The Sava discharges into the Danube, after reaching a.s.l. as its right tributary at the Great War Island off the easternmost tip of Syrmia in Belgrade, away from the Danube's confluence and the Black Sea.

Settlements

Population in the Sava River basin is estimated at 8,176,000, and includes four capitals: Belgrade, Ljubljana, Sarajevo and Zagreb. All except Sarajevo, are on the river banks and represent the three largest settlements found along the river. Belgrade, at the lowest end of the river, is the largest city in the basin with urban population of 1,135,502. Ten municipalities of its outer conurbation have combined population of 1,283,783, taking in many mutual suburbs. The Belgrade metropolitan area has a population of 1,639,121. Zagreb is the second largest city on the river, comprising population of 688,163 living in the city itself, and 802,588 in the city-administered area. Together with the Zagreb County, largely corresponding to various definitions of the city's metropolitan area, it has a combined population of 1,110,517. Ljubljana is the third-largest city on the banks of the Sava, encompassing a population of 258,873 living in the city itself and 265,881 in the city-governed area.

The largest city of Bosnia-Herzegovina on the river is Brčko, whose urban population is estimated at 40,000. Other cities along the river, with populations of 20,000 and larger, are Slavonski Brod (53,473), Šabac (52,822), Sremska Mitrovica (37,586), Kranj (35,587), Sisak (33,049), Obrenovac (24,568), and Bosanska Gradiška (est. 20,000).

Watershed

The Sava River basin covers a total area of making it the second largest Danube tributary catchment by area size, surpassed only by the Tisza basin, and it encompasses 12% of the Danube basin, draining into the Black Sea. The Sava represents the third longest tributary of the Danube and its largest tributary by discharge. The catchment area borders the remainder of the Danube basin to the north and east, and the Adriatic Sea basin to the west and south. The river basin generally consists of parts of Bosnia-Herzegovina, Croatia, Montenegro, Serbia and Slovenia, with a very small part of the catchment area belonging to Albania. Topography of the basin varies significantly. Upstream portion of the basin is more rugged than downstream one, but asymmetry of the basin topography is particularly apparent when comparing right and left bank areas—the former dominated by the Alps and the Dinarides reaching elevations in excess of a.s.l, while the latter is dominated by the Pannonian Plain. The mean elevation of the basin is a.s.l.

Major tributaries

The most important tributaries of the Sava River found in its upper basin are characterized by relatively steep grades of flow, high flow velocities and rapids. Those are left tributaries: the Kokra, the Kamnik Bistrica and the Savinja; and right tributaries: the Sora, the Ljubljanica and the Krka (Sava). Further downstream larger rivers empty into the Sava, as the right bank of the basin grows steadily. Right tributaries in this lower segment of the basin start as fast flowing courses, only to slow down as they enter the Pannonian Basin. They include the Kupa, the Una, the Vrbas, the Ukrina, the Bosna, the Brka, the Tinja, the Drina and the Kolubara. Left tributaries in the lower segment drain plains consequently exhibiting less steep course grades, lower flow rates and meandering. They include the Sutla, the Krapina, the Lonja, the Ilova, the Orljava and the Bosut.

The Drina is the largest tributary of the Sava, flowing in Bosnia-Herzegovina and along border of the country and Serbia. It is formed by the headwaters of the Tara and the Piva at the border of Bosnia-Herzegovina and Montenegro, near Šćepan Polje. Its catchment extends across parts of four countries—reaching as far south as Albania. The Bosna and the Kupa river basins are the second and third largest catchments of the Sava tributaries, each surpassing in size.

Hydrology

The average annual flow rate of the Sava River at Radovljica, immediately downstream of the Sava Dolinka and the Sava Bohinjka confluence, stands at per second. Downstream of the Krka confluence the average flow rate reaches per second, gradually increasing as tributaries discharge along the course— per second downstream of the Sutla, per second following discharge of the Kupa and the Una, per second downstream of the Vrbas confluence, per second after the Bosna river empties into the Sava, and finally of per second at confluence of the Sava in Belgrade. The highest flow rate of per second was recorded by Slavonski Šamac gauging station in May 2014.

Seven out of eight largest reservoirs in the Sava River basin are in the Drina catchment, the largest among them being the Lake Piva on the eponymous river in Montenegro, created after construction of Mratinje Dam. Overall, there are 22 reservoirs holding more than of water in the basin, with four of them ''on'' the Sava, including one on the Sava Dolinka. Most of the reservoirs are used primarily, or even exclusively, for electricity generation, but they are also used as supply of drinking water, industrial water source, for irrigation and food production.

Groundwater is a very important resource in the Sava River basin, generally used for public water supply of potable water, as a source of water for industrial use, but also as the mainstay of aquatic ecosystems. There are 41 identified significant groundwater bodies in the Sava River basin of basin-wide importance, ranging in area size from , as well as numerous minor ground water bodies. Even though most of them are transboundary waters, eleven are considered to be largely in Slovenia, fourteen in Croatia, seven in Bosnia-Herzegovina, five in Serbia and four in Montenegro.

Geology

The course of the Sava River runs through several diverse geological units and orographic regions. The uppermost course of the river and its headwaters in the Karavanke area, is in the Southern Alps, tracing the Sava Fault—itself running parallel to the Periadriatic Seam. Mesozoic and Upper Triassic rocks are exposed in the region. The Ljubljana Basin represents the boundary of the Southern Alps and the Dinarides. Valleys of the Sava Dolinka and the Sava Bohinjka are glacial valleys, carved out by the Sava Dolinka and Bohinj glaciers advancing down Karavanke range to vicinity of present-day Radovljica. In the late Pleistocene, Bohinj Glacier was the largest glacier in the territory of present-day Slovenia, up to thick. Sava Folds, southeast and east of the Ljubljana Basin are thought of as a part of the Dinarides, separating the Ljubljana and Krško Basins, and forming the Sava Hills. The east–west oriented folds are younger than the Miocene and the folding is considered to had taken place in the Pliocene

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