It is the biggest water source in Bela krajina, a meeting point of several underground flows originating from lake Bajer near Rožni Dol, the Rečica near Vrčice, the Reka at the Gorjanci mountain range  and the underground flow from Mirna gora.

The Krupa river springs in the northern part of the low karst plateau of Bela krajina.Its powerful sources appear under a 30-metre-high crag with a typical tectonic structure. This source has the greatest water power in Bela krajina; Its absolute minimum flow is around 400 l/s, its average  minimum flow is 1000 l/s. Most other sources reach 50 l/s. The Krupa carved an impressive, partly canyon-like river bed into the karst plateau. The 2.5 km long Krupa eventually joins the Lahinja. The difference in altitude between the source of the Krupa and its mouth is 6m. This enabled the operation of four mills and two sawmills in the past. The source of the Krupa is the habitat of proteus; it is the only known place in Slovenia where the cave shell Congeria kusceri has been found. Together with the protected area the Krupa was proclaimed a natural monument in 1997.