The Rak is a stream in Inner Carniola, a traditional region of southeastern Slovenia. It sources in Zelà ¡e Caves () west of the village of Zelà ¡e, flows across the Rak à  kocjan karst valley for and enters Weaver's Cave (), where it continues for and merges in Planina Cave (), about from its entrance, with the Pivka River to form the Unica. The confluence of the Rak and the Pivka is one of the largest subterranean confluences in Europe.
Rising from the karst springs of the Zelà ¡e Caves, the Rak at once enters the collapsed valley of Rak à  kocjan, a UNESCO-recognised karst window noted for its two natural limestone bridges. After meandering 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 confluenceâÂÂone of Europe's largestâÂÂcreates the Unica River and connects the Rak to the 800 km<sup>2</sup> Ljubljanica aquifer, hydraulically linking Lake Cerknica and the Planina Karst Field.
Tracer dye tests show that high-water pulses flow through the RakâÂÂPlanina conduit at 90 to 640 cubic metres per hour ( m h<sup>âÂÂ1</sup>), whereas summer recession flows slow to under 25 m h<sup>âÂÂ1</sup> and may stall in the pool-and-riffle reaches of Weaver Cave. These swings explain why heavy rain on the plateau can raise water in Rak à  kocjan by more than ten metresâÂÂsubmerging the natural bridgesâÂÂwhile prolonged drought can dry the surface channel entirely. Continuous logging during the 2008âÂÂ2011 campaign recorded distinct chemical and thermal signatures: winter floods carry cool, allogenic runoff from Lake Cerknica's flysch rim, whereas low-stage flows are fed almost solely by autogenic recharge from the surrounding limestone massif.
Despite its brevity, the Rak supports a varied cold-water fauna. Annual surveys list brown trout, pike, European chub, tench, and stone loach, with occasional grayling and perch spread across just 4.8 ha of open water. The valley is protected as a landscape park: angling is suspended during heatwaves, and floodsâÂÂwhich scour fine sediments from the doline floorâÂÂare left unimpeded to maintain spawning gravels and flush nutrients into the subterranean reaches inhabited by rare cave invertebrates. Together these geomorphic, hydrological and ecological traits make the short-lived Rak an ideal natural laboratory for studying how temperate karst rivers respond to both Mediterranean cloudbursts and Alpine droughts.