The Rak is a

stream A stream is a continuous body of water, body of surface water Current (stream), flowing within the stream bed, bed and bank (geography), banks of a channel (geography), channel. Depending on its location or certain characteristics, a strea ...

Inner Carniola Inner Carniola ( ; ) is a traditional region of Slovenia, the southwestern part of the larger Carniola region. It comprises the Hrušica (plateau), Hrušica karst plateau up to Postojna Gate, bordering the Slovenian Littoral (the Goriška, Gor ...

, a traditional region of southeastern

Slovenia Slovenia, officially the Republic of Slovenia, is a country in Central Europe. It borders Italy to the west, Austria to the north, Hungary to the northeast, Croatia to the south and southeast, and a short (46.6 km) coastline within the Adriati ...

. It sources in

Zelše Caves Zelše (; in older sources also ''Želše'',''Leksikon občin kraljestev in dežel zastopanih v državnem zboru,'' vol. 6: ''Kranjsko''. 1906. Vienna: C. Kr. Dvorna in Državna Tiskarna, p. 122. ) is a village along the road linking Cerknica an ...

() west of the village of

Zelše Zelše (; in older sources also ''Želše'',''Leksikon občin kraljestev in dežel zastopanih v državnem zboru,'' vol. 6: ''Kranjsko''. 1906. Vienna: C. Kr. Dvorna in Državna Tiskarna, p. 122. ) is a village along the road linking Cerknica an ...

, flows across the

Rak Škocjan Rak Škocjan (; ) is a valley and a landscape park (protected area), landscape park, part of Inner Carniola Regional Park in southwestern Slovenia. Administratively, it belongs to the settlement of Rakov Škocjan, Cerknica, Rakov Škocjan. Rak � ...

karst valley for and enters Weaver's Cave (), where it continues for and merges in Planina Cave (), about from its entrance, with the

Pivka Pivka (, , ) is a small town in Slovenia in the Pivka Basin in the Karst region. It is the seat of the Municipality of Pivka. It belongs to the traditional region of Inner Carniola. Name Pivka was first mentioned in 1300 as ''villa Sancti Petri ...

River to form the Unica. The confluence of the Rak and the Pivka is one of the largest subterranean confluences in Europe.

Description

Rising from the karst springs of the

, the Rak at once enters the collapsed valley of

, a

UNESCO The United Nations Educational, Scientific and Cultural Organization (UNESCO ) is a List of specialized agencies of the United Nations, specialized agency of the United Nations (UN) with the aim of promoting world peace and International secur ...

-recognised

karst Karst () is a topography formed from the dissolution of soluble carbonate rocks such as limestone and Dolomite (rock), dolomite. It is characterized by features like poljes above and drainage systems with sinkholes and caves underground. Ther ...

window noted for its two natural

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

bridges. After

meander A meander is one of a series of regular sinuous curves in the Channel (geography), channel of a river or other watercourse. It is produced as a watercourse erosion, erodes the sediments of an outer, concave bank (cut bank, cut bank or river cl ...

ing for barely 2 km, the stream plunges underground at Weaver Cave, then runs a further 3 km through flooded galleries to merge with the sinking Pivka River inside Planina Cave. That subterranean

confluence In geography, a confluence (also ''conflux'') occurs where two or more watercourses join to form a single channel (geography), channel. A confluence can occur in several configurations: at the point where a tributary joins a larger river (main ...

—one of Europe's largest—creates the Unica River and connects the Rak to the 800 km²

Ljubljanica The Ljubljanica (), known in the Middle Ages as the ''Leybach'', is a river in the southern part of the Ljubljana Basin in Slovenia. The capital of Slovenia, Ljubljana, lies on the river. The Ljubljanica rises south of the town of Vrhnika and ...

aquifer, hydraulically linking

Lake Cerknica Lake Cerknica (; ) is an intermittent lake in the southern part of the Cerknica Polje, a karst polje in Inner Carniola, a region in southwestern Slovenia. Description The lake, oriented in the Dinaric direction from north-west to south-east, ...

and the Planina Karst Field. Tracer dye tests show that high-water pulses flow through the Rak–Planina conduit at 90 to 640 cubic metres per hour ( m h⁻¹), whereas summer recession flows slow to under 25 m h⁻¹ and may stall in the pool-and-

riffle A riffle is a shallow landform in a flowing channel. Colloquially, it is a shallow place in a river where water flows quickly past rocks. However, in geology a riffle has specific characteristics. Topographic, sedimentary and hydraulic indica ...

reaches of Weaver Cave. These swings explain why heavy rain on the plateau can raise water in Rak Škocjan by more than ten metres—submerging the natural bridges—while prolonged drought can dry the surface channel entirely. Continuous

logging Logging is the process of cutting, processing, and moving trees to a location for transport. It may include skidder, skidding, on-site processing, and loading of trees or trunk (botany), logs onto logging truck, trucksflysch Flysch () is a sequence of sedimentary rock layers that progress from deep-water and turbidity flow deposits to shallow-water shales and sandstones. It is deposited when a deep basin forms rapidly on the continental side of a mountain building ep ...

rim, whereas low-stage flows are fed almost solely by autogenic recharge from the surrounding limestone

massif A massif () is a principal mountain mass, such as a compact portion of a mountain range, containing one or more summits (e.g. France's Massif Central). In mountaineering literature, ''massif'' is frequently used to denote the main mass of an ...

. Despite its brevity, the Rak supports a varied cold-water

fauna Fauna (: faunae or faunas) is all of the animal life present in a particular region or time. The corresponding terms for plants and fungi are ''flora'' and '' funga'', respectively. Flora, fauna, funga and other forms of life are collectively ...

. Annual surveys list

brown trout The brown trout (''Salmo trutta'') is a species of salmonid ray-finned fish and the most widely distributed species of the genus ''Salmo'', endemic to most of Europe, West Asia and parts of North Africa, and has been widely introduced globally ...

, pike,

European chub ''Squalius cephalus'', the common chub, European chub or simply chub, is a species of freshwater ray-finned fish belonging to the family Leuciscidae, the daces, Eurasian minnows and related fishes. This species is found in Europe where it freq ...

tench The tench or doctor fish (''Tinca tinca'') is a freshwater, fresh- and brackish water, brackish-water fish of the order Cypriniformes found throughout Eurasia from Western Europe including Great Britain, Britain and Ireland east into Asia as far ...

, and

stone loach The stone loach (''Barbatula barbatula'') is a European species of fresh water ray-finned fish in the family Nemacheilidae. It is one of nineteen species in the genus '' Barbatula''. Stone loaches live amongst the gravel and stones of fast flowi ...

, with occasional grayling and perch spread across just 4.8 ha of open water. The valley is protected as a landscape park:

angling Angling (from Old English ''angol'', meaning "hook") is a fishing technique that uses a fish hook attached to a fishing line to tether individual fish in the mouth. The fishing line is usually manipulated with a fishing rod, although rodless te ...

is suspended during heatwaves, and floods—which

scour Scour may refer to: Hydrodynamic processes * Hydrodynamic scour, the removal of sediment such as sand and silt from around an object by water flow ** Bridge scour, erosion of soil around at the base of a bridge pier or abutments via the flow ...

fine

sediments Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

from the doline floor—are left unimpeded to maintain spawning gravels and flush nutrients into the subterranean reaches inhabited by rare cave

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s. Together these

geomorphic Geomorphology () is the scientific study of the origin and evolution of topography, topographic and bathymetry, bathymetric features generated by physical, chemical or biological processes operating at or near Earth#Surface, Earth's surface. Ge ...

hydrological Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and drainage basin sustainability. A practitioner of hydrology is called a hydro ...

and

ecological Ecology () is the natural science of the relationships among living organisms and their environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere levels. Ecology overlaps with the closely re ...

traits make the short-lived Rak an ideal natural laboratory for studying how temperate karst rivers respond to both Mediterranean

cloudburst Cloudburst in New Orleans A cloudburst is an enormous amount of precipitation in a short period of time, sometimes accompanied by hail and thunder, which is capable of creating flood conditions. Cloudbursts can quickly dump large amounts of wat ...

s and Alpine droughts.

References

External links

{{Hydrology of Slovenia Rivers of Inner Carniola Sinking rivers Articles containing video clips