The Pivka basin is a region along the rivers Pivka and Nanoščica that is bounded to the north by high karst plateaus Nanos and Hrušica, to the east by Javorniki, to the west by Slavinski ravnik, and to the south by Snežnik and the valley of the river Reka. The Pivka basin, which includes the Pivka river, is part of the karst Ljubljanica river basin, which merges with the waters flowing from Notranjska (Trbuhovica, Obrh, Stržen, and Rak rivers) in Planina cave.
The northern part of this large basin is made up of impermeable flysch rocks that are subject to surface water runoff. The southern part, Upper Pivka, was formed on limestones and has some karst characteristics.
Pivka gives the basin a name and a unique stamp because this intermittent river carved its bed into the karst plain, which is flooded at high-water levels in the lowest parts, resulting in the formation of intermittent lakes. The basin contains 17 oval hollows, which become temporary lakes as groundwater levels rise in the karst.
The Pivka basin is approximately 25 kilometres long. The Upper Pivka basin is the southern part of the basin, which is about 16 km long and 4-5 km wide. Its surface drops slightly from 620 m above sea level in the southeast near Koritnice, at the foot of the Snežnik massif, to 520 m in the northwest near Prestranek or Rakitnik, where the river Pivka flows into flysch rocks. Pivka is the largest river in the basin. It gets its water from flysch and karst springs in Upper Pivka Basin and sinks in the basin’s north-eastern Postojna Cave.
The basin’s bottom is distinguished by level ground along the Pivka river. Only near the settlement of Pivka is the bottom completely flat; elsewhere, it is slightly undulating and raised a few metres above the bed of the Pivka river; in some places, there are also low cliff peaks. On the west side, the flat bottom gradually rises into slopes, while on the east side, the flat bottom abruptly rises into the steep slopes of the higher terrace, which rises 10–20 m above the basin’s bottom. The rocky bottom is covered in some places by a thin layer of clayey or sandy sediments.
A higher rock terrace on the eastern side of the basin, between the bottom and Javorniki massif, is strongly divided in the northern part. The terrace’s surface is divided by individual mounded peaks and, most notably, numerous sinkholes. Sinkholes are larger along the lower western edge, and there are several large sinkholes and flat-bottomed hollows with the occasional Pivka Lake.
The lake basins’ bottoms are at a height that is still reached by the fluctuation of the karst groundwater level. Water appears in the many karst canals as it rises, and it also appears at the bottom of the valley, where the Pivka springs and flows. The high level of karst groundwater allows water to appear on the surface and the formation of up to 17 lakes.
As a result, the Pivka intermittent lakes are not permanent and only fill up during heavy rains. The water we see as a lake is essentially a surplus of groundwater that the karst world cannot conceal from the living above-ground world.