Krupa is a karst river with the sources emerging from beneath a 17 m high limestone cliff, located 3.3 km south-east from the town of Semič. With a total length of 2,580 m, it serves as a major left tributary of the Lahinja River.

The river's source is exceptionally voluminous due to the extensive karst hinterland from the Kočevski Rog area and parts of the Gorjanci Hills, ensuring it never dries up even during severe droughts. The entire area surrounding the river is protected as a monument of nature and is included in the European Natura 2000 network due to its high biodiversity.