a river in Yugoslavia, part of the Ljubljanica River (a right tributary of the Sava). It is 85 km long to the juncture of the Ljubljanica, with 20 km of its length underground; it drains an area of approximately 2,000 sq km. The Pivka originates in and flows through karst terrain, cutting through Postojna Cavern. The river creates underground rapids and siphons. The volume of water is high from January through May, with slow flow from August through October. The city of Ljubljana is situated on the lower Pivka.