The Una (, ) is a border river between Bosnia and Herzegovina and Croatia and a right tributary of the Sava river. It is part of the Black Sea drainage basin, and its watershed has a size of , of which belongs to Bosnia and Herzegovina, and to Croatia. The total length of the river is . The source of the river is located near the village of Donja Suvaja in Croatia, and its mouth is located near the village of Jasenovac, on the border with Bosnia. The largest right tributaries are the Krka, Unac, Krušnica and Sana rivers, and the largest left tributary is the Klokot river. Its longest headwater is the Unac river. The largest and most important city located on the river is Bihać. Other important cities and towns are Bosanska Krupa, Bosanski Novi and Bosanska Kostajnica. The river is characterized by a multitude of waterfalls, rapids, karst springs and relatively untouched nature. A large part of the upper river is part of the Una National Park.
Etymology According to local legends, the river was named by the Romans who, after seeing it for the first time, said una (lat. one, as a reference to its unique beauty) to describe it.
Una is a popular female name even in the areas that are not located in the vicinity of the river. One explanation for this is that people that were displaced during the Yugoslav Wars continued to give the name to their children.
Geography
Sources The source of the Una river is located near the Croatian village of Donja Suvaja and consists of multiple smaller springs. The largest spring by discharge is a large karst spring at an elevation of above sea level, called Vrelo Une, and is considered to be the main spring of the river. The water, originating from great depths, forms a round, calm, bluish green lake, thirty meters by fifteen meters in size, surrounded by forests and steep cliffs of the Lička Plješevica and Stražbenica mountain ranges. The Una river, flowing out of the spring, forms its first travertine cascade roughly 20 meters downstream and another large waterfall 3 – 4 meters in height roughly 100 meters downstream. The spring itself was explored and mapped up to a depth of 248 meters, which makes it the deepest explored spring in the Dinaric karst region and the fifth deepest in the world. Due to its unique aesthetic, ecological, geomorphological and hydrological value, the spring and the surrounding region, totaling an area of , were declared a natural monument in 1968.
The karst aquifer of the Una is located mostly in the territory of Croatia. The aquifer itself takes up a large area from Korana in the northwest to Velika Popina in the southeast. It encompasses the mountainous areas of the Lička Plješevica, Čemernica and Mala Kapela mountains, and the Koreničko, Bijelopolje, Krbavsko and Lapačko polje karst fields. All aforementioned karst fields are directly connected to spring of the river itself, and other nearby springs (Klokot, Vedro Polje, Dobrenica, Ostrovica).
The discharge of the spring varies from a minimal recorded in 1987 to a maximal recorded in 2009, with the average discharge being . The spring seems to have a limited outflow capacity, potentially caused by limited dimensions of the karst outflow channel, water retention in the karst, water flowing under pressure and water overflowing into other watersheds. The total amount of water that flows out of the spring yearly ranges between and . The temperature of the water varies in relatively narrow margins of with an average of , which also leads to the conclusion that the surfacing water spends a large amount of time in the karst underground. The average, minimal and maximal monthly and yearly discharges seemed to have increased in recent times, contrary to a trend of decreasing discharges from other springs in the nearby regions believed to be caused by global warming.
Upper course The upper part of the river spans from its source to the city of Bihać.
After emerging from its source, the river flows for about 5 km before reaching its confluence with the Krka river, after which it forms the border between Bosnia and Croatia for about 7 km and then enters Bosnia completely for the first time, flowing through the village of Martin Brod where it forms a cascade of waterfalls with the total drop being about . Downstream of Martin Bord, the Una more than doubles in size at its confluence with the Unac. After 9 km the river bends west at the town of Kulen Vakuf where it is joined by the small tributary Ostrovica before continuing to flow in a general northward direction. Downstream of Kulen Vakuf, the river meanders through a valley by the village of Klisa before again meeting the Croatian border after about 8 km. This time, the river forms the border for about 18 km and cuts through a gorge, forming a cascade of rapids and waterfalls, the biggest and most notable being the Štrbački buk. After entering Bosnia completely for the second time, the rivers forms the Troslap and Dvoslap waterfalls, after which it exits the national park and flows through the villages of Lohovo and Račić. After that, the river slowly enters the Bihać field, flowing through the villages of Ripač, Golubić and Pritoka, taking in the waters of the Dobrenica and forming multiple river islands and small waterfalls, before finally entering the city of Bihać after about 70 km of total flow. In Bihać, the river forms multiple islands of varying sizes, often surrounded by small waterfalls, some of which are private property while others are public and often popular bathing sites. The core of the city itself is surrounded by the Una river, and a channel carries water from the river to a small hydroelectric power plant. Another small tributary of the Una, the Drobinica, flows through the city in a regulated channel before merging with the Una. In its upper course, the Una is a youthful river with a steep gradient of 0.232% and a large amount of waterfalls. The total drop is over a distance of . The deposition of carbonate minerals and the formation of porous limestone rock (tufa) and pronounced karst influence is characteristic for this section.
Middle course The middle section of the river spans from the city of Bihać to the town of Novi Grad and its confluence with the Sana.
Not long after exiting the city the river takes in the waters from the short Klokot river, after which it meanders, slowly turning eastward before entering the village of Pokoj where it takes a sharp turn north and enters a canyon between Bihać and Bosanska Krupa where it slowly turns eastward again. The Una Kostela hydroelectric power plant is located not long after the entrance into the canyon and is currently one of two hydroelectric power stations on the main course of the river, the other being the small Bihać power plant. The canyon is, on average, around deep with lush vegetation growing on its cliffs. A road that connects Bihać and Bosanska Krupa follows the river through the canyon. After flowing for about 27 km through it, the river exits the canyon and enters the town of Bosanska Krupa where it takes a turn north. Like in Bihać, the river forms multiple islands surrounded by small waterfalls. Downstream of Bosanska Krupa, the river takes in the waters of the short Krušnica river before heading north towards the town of Bosanska Otoka, after which it forms the Inter-Entity Boundary Line and shortly after forms the border with Croatia for the last time, and remains a border river until its mouth. Not long after becoming the border, the river enters the town of Novi Grad, where the confluence with the Sana river is located. The Sana is the longest and most significant tributary of the Una and greatly increases the discharge of the river. Waterfalls and tufa are rarer in this section, albeit still present. The confluence with the Sana marks the end of the middle section of the river and the beginning of the lower section. The total length of the river between Bihać and Novi Grad is around , with a gradient of 0.067% and drop of .
Lower course The lower section of the river spans from the town of Novi Grad to the river mouth.
After exiting the town of Novi Grad, the river flows in a general northeast direction until its confluence with the Sava river near Jasenovac. Major towns and villages on the river in this section are Novi Grad, Kostajnica and Kozarska Dubica on the right bank in Bosnia and Dvor na Uni, Hrvatska Kostajnica and Hrvatska Dubica on the left bank in Croatia. The waterfalls, rapids and tufa rock that were common in previous sections are almost completely nonexistent in the lower section as the Una takes up the characteristics of a mature river, meandering through alluvial plains and having a low gradient. Near its confluence with the Sava exist a couple of oxbow lakes colloquially called Stara Una (lit. "Old Una"). After a total length of about 212 km, the Una flows into the Sava river near the Croatian village of Jasenovac. The total length of the river between Novi Grad and Jasenovac is around , with a gradient of 0.041% and drop of .
Hydrography Watershed The watershed of the Una river has a total area of , of which 79% () belongs to Bosnia and Herzegovina, and 21% () to Croatia. It borders the watersheds of the Korana, Glina and Sunja to the northwest, the Adriatic watersheds of the Lika, Zrmanja and Cetina to the southwest, the watershed of the Cetina to the south and the watersheds of the Vrbas and Jablanica to the east. The shape of the watershed itself is very irregular, but roughly triangular. Due to strong karst influence in the area, the watershed is poorly developed with few large surface streams. The karstification coefficient for the entire watershed is 0.244, which means that 24.4% of the surface area in the watershed lacks proper surface streams and drainage. The karstification coefficient up to Bihać is 0.43, up to Novi Grad 0.28 and up to Kostajnica 0.26. The average air temperature in the area of the watershed is , and the average amount of annual rainfall is .
The watersheds of the Una and the watersheds of the Korana and Glina are often grouped together for analytical purposes in Bosnia and Herzegovina.
Discharge The Una river has an average discharge of at its mouth, with a minimal discharge of . There are relatively small differences in the average monthly discharges (the driest month August has 37% of average yearly discharge while the wettest month April has 155%) and minimal daily and monthly discharges also show a very small difference. Low discharges last a relatively long time and high discharges appear quickly due to sudden rainfall and snowmelt and last a comparatively short amount of time. High discharges and water levels are present in April and December, and low discharges in August and January.
There are multiple measuring stations in Bosnia and Herzegovina on the Una, and their data is show in the tables below +Average discharge of the Una riverMeasuring stationAverage discharge (m3/s)Watershed area (km2)Martin Brod upstream21.0790Martin Brod downstream51.61 950Kulen Vakuf53.22 028Štrbački buk65.02 586Bihać85.603 496Kralje - Bihać100.23 631Bosanska Krupa112.74 092Bosanska Otoka122.04 460Entity line124.44 540Country border126.24 611Novi Grad upstream131.64 798Novi Grad downstream218.58 997Kostajnica232.09 664Dubica238.010 024Mouth240.310 200