Geographical Characteristics and Dimensions

The Cerknica polje (Cerkniško polje) is situated in southwestern Slovenia within the Notranjska region. It is a classic karst polje, representing a major surface karst formation developed in a heavily karstified landscape. When floodwaters submerge the polje basin, Lake Cerknica (Cerkniško jezero) is born—one of the world's most renowned intermittent lakes owing to its dramatic fluctuations in water level and pronounced dry cycles.

During typical high-water phases, the lake stretches approximately 7.4 km in length and 3.3 km in width. Under full inundation, it encompasses an area of around 26 km², while during extraordinary catastrophic floods the water covers the entirety of the basin floor, expanding up to 38 km². Within its theoretical maximum basin perimeter, it spans 10.5 km by 5 km, ranking as the second-largest lake in Slovenia by surface area and the largest intermittent karst lake. The water level fluctuates between 546 m and 551 m above sea level, reaching a maximum depth of approximately 10 metres.

Hydrology and Subterranean Drainage

The hydrographic network of the Cerknica polje is extraordinarily intricate and intimately bound to the regional karst hinterland. On the adjacent Lož polje (Loško polje), the Obrh stream sinks into the ground and resurfaces underground across the Cerknica polje primarily as the Štržen stream and through other major karst spring groups (such as Cemun). Inflow also arrives through the subterranean conduits of Križna jama—one of the world's most pristine water caves—while surface and groundwater runoff from the slopes of Mount Javorniki and the Cerkniščica stream enter along the western and northern margins.

All accumulated water traverses the polje and drains through swallow holes (ponors) along three primary underground axes:

  • Rakov Škocjan Axis: A portion of the drainage flows into the Rakov Škocjan valley as the Rak stream, flowing through karst conduits toward Planina Cave (Planinska jama); there it unites with the Pivka River issuing from Postojna Cave to form the Unica River.

  • Planina Polje Axis: Another direct branch percolates through subterranean fissures directly toward the Planina polje.

  • Bistra / Vrhnika Axis: The third and longest subterranean trajectory navigates deep karst aquifer strata to emerge as powerful karst resurgences near Bistra on the southwestern rim of the Ljubljana Marsh (Ljubljansko barje).

Along the margins, the lake is fed by perennial karst springs, whereas during periods of intense hydrostatic pressure, water bursts forth directly from bottom fissures and shafts across the polje floor. These openings function as estavelles—operating as swallow holes (ponors) during low water conditions and turning into high-pressure fountains during major inundation events.

Inundation Dynamics and Geological Formation

Lake Cerknica typically fills following heavy autumnal downpours and during the early spring thaw on the surrounding karst plateaus. Water typically retreats in May or June; however, during severely arid winter periods, the lakebed may dry out completely in midwinter as well. As the waters recede into the underworld, the floor transforms into expansive hayfields, pastures, and marshy grasslands.

The drainage of the lake proceeds through more than 27 formally named swallow hole zones and hundreds of individual bed fissures and estavelles. Among these, the most iconic is the Rešeto swallow hole system, situated on the northern to northwestern margin of the lakebed, very close to the village of Dolenje Jezero (in the direction of Velika Gorica island). Rešeto is a rocky limestone area punctured by dozens of closely spaced holes and vertical shafts in the bedrock, whose perforated appearance remarkably resembles a giant kitchen strainer or rustic sieve (Slovene: rešeto)—from which its historical name derives. When the lake level drops, water in Rešeto cascades and swirls violently down into the subterranean voids, trapping thousands of fish across the perforated rock floor. In the immediate vicinity of Rešeto lies another key ground swallow hole, Vodonos, while further south and in the central part of the lake basin lie the swallow hole clusters of Retje, Bečki, Sitarica, and Mala, Srednja, and Velika Ponikva. At the far northwestern tip of the polje, within the Jamski Zaliv bay beneath towering cliffs, water plunges into the colossal swallow caves Velika and Mala Karlovica, through which subterranean flows head directly toward Rakov Škocjan.

The origin of the Notranjska karst poljes is genetically tied to the major regional tectonic fault line: Idrija – Hotedršica – Planina polje – Cerknica polje – Lož polje. Prolonged tectonic downfaulting along these zones created closed structural basins, which advanced karstification and subterranean dissolution subsequently sculpted into these remarkable karst poljes.

Importance for Migratory Birds During Inundation

Due to its dynamic mosaic of open waters, seasonally flooded meadows, mudflats, and reedbeds, the Cerknica polje is an internationally recognized wetland (Ramsar Site) and an EU Natura 2000 protected habitat. More than 270 bird species have been recorded here to date. During the spring and autumn flooding phases, the lake serves as an indispensable stopover and foraging hub along the Adriatic Flyway:

  • Waterfowl (Ducks and Geese): Flocks of dabbling and diving ducks rest and feed on the vast surface, prominently including the Garganey (Spatula querquedula), Northern Shoveler (Spatula clypeata), Gadwall (Mareca strepera), Eurasian Wigeon (Mareca penelope), Northern Pintail (Anas acuta), and Tufted Duck (Aythya fuligula). Greylag Goose (Anser anser) and Bean Goose (Anser fabalis) are regular passage visitors.

  • Waders and Shorebirds: As the water levels drop to expose nutrient-rich mudflats and shallow shallows, migrating waders congregate in large numbers, such as the Common Sandpiper (Actitis hypoleucos), Wood Sandpiper (Tringa glareola), Black-tailed Godwit (Limosa limosa), Ruff (Calidris pugnax), and Eurasian Curlew (Numenius arquata).

  • Herons, Spoonbills, and Cranes: The shallows and channels provide rich hunting grounds for the Great Egret (Ardea alba), Purple Heron (Ardea purpurea), Black Stork (Ciconia nigra), and Eurasian Spoonbill (Platalea leucorodia). In recent years, staging flocks of Common Cranes (Grus grus) have also become a frequent sight during migration.

  • Grebes and Divers: Across the deeper water bodies, the Red-necked Grebe (Podiceps grisegena), Black-necked Grebe (Podiceps nigricollis), and occasionally the Black-throated Diver (Gavia arctica) can be seen diving.

  • Migratory Birds of Prey: The seasonal abundance of fish, amphibians, and waterbirds attracts raptors on passage, notably the Western Osprey (Pandion haliaetus) and Western Marsh Harrier (Circus aeruginosus), while the resident White-tailed Eagle (Haliaeetus albicilla) hunts regularly over the expanse.

Customs, Traditions, Myths, and Legends

The extraordinary rhythm of this disappearing lake—where the same soil supports boating and fishing in one season and haymaking or ice skating in the next—has over centuries fostered a rich cultural landscape characterized by peasant ingenuity, time-honoured customs, and mystical folklore.

Traditional Fish Harvesting during Recession: One of the most famous and distinctive customs was the communal fish harvesting as the lake began to dry, meticulously documented in the 17th century by the polymath Johann Weikhard von Valvasor. As the water drained into the underworld, teeming schools of fish (principally pike, tench, chub, and perch) rushed frantically toward the swallowing shafts and cave portals (such as Vodonos, Rešeto, and Karlovica). Villagers signaled the start of the harvest with church bells or horn blasts: with nets, specialized baskets (sakalnice), scoops, and bare hands, crowds gathered at the gaping mouths of ponors and in vanishing puddles. The boldest fishermen ventured deep into dark cavernous shafts with burning pine torches to retrieve fish trapped in rock crevices before the lake vanished completely.

Mount Slivnica – The Mountain of Witches: Overlooking the polje rises the prominent Mount Slivnica (1,114 m), celebrated in Slovenian folklore as the preeminent gathering site of witches (coprnice). According to local tradition, they lived within the steaming Witch Cave (Coprniška jama) just beneath the summit, from which warm vapor plumes drift in winter. Led by Mother Ursula, they were believed to brew thunderstorms, torrential downpours, and devastating hail in their underground cauldrons, riding broomsticks to midnight dances on Midsummer Eve. This vibrant mythological heritage is commemorated annually in the famous Cerknica Carnival (Cerkniški pust), when figures of the witches, the water sprite Jezerko, the giant pike, and the dragon take over the town.

The Legend of the Lake's Creation: Folk imagination attributed the sudden emergence of the lake to a tragic love tale between two opposing fortresses. According to legend, a young nobleman ruled from Šteberk Castle on one side of the polje, while across the dry valley stood Karlovec Castle, whose lord had a beautiful daughter. Due to a bitter family feud, the lord of Karlovec mockingly pledged his daughter’s hand only if the suitor could arrive for her by boat across the dry valley. Desperate, the young knight sought counsel from a cave-dwelling wizard, who instructed him how to plug the underground swallow holes and breach the subterranean springs. A vast lake flooded the valley. However, as the knight rowed across the stormy waters at night to claim his bride, a squall capsized his boat and he drowned; grief-stricken, the maiden plunged into the dark waves to join him.

Jezerko – The Karst Water Sprite: Beyond the witch lore, the untamed subterranean powers were personified by the water sprite Jezerko. Locals believed that he guarded the subterranean water gates within the abysses, deciding arbitrarily when to lock or open them, thus dictating whether the basin would bloom as fertile pastures or submerge as a vast inland sea.

Note:

Sources and bibliography:

  1. DOPPS – BirdLife Slovenia (Society for the Observation and Study of Birds of Slovenia):

    • Ornithological journal Acrocephalus (annual ornithological records and migration surveys).

    • IWC (International Waterbird Census) count data collected across winter and migratory peaks.

    • National project records and distribution maps (Breeding Bird Atlas of Slovenia and passage surveys).

  2. Notranjska Regional Park (Notranjski regijski park - NRP):

    • Official conservation fauna records of Cerknica Polje, documenting over 276 verified bird species.

    • Interpretive visitor guides and cultural heritage guidelines (wildlife observatories, educational trails, and ethnological records).

  3. Ramsar Convention – Ramsar Information Sheet (RIS):

    • Lake Cerknica and surrounding areas designated as a Ramsar Wetland of International Importance in 2006 (Ramsar Site no. 1600).

    • Official RIS listing key migratory, nesting, and passage taxa (including Garganey, Marsh Harrier, Cranes, Spoonbills, and migrating waders).

  4. Natura 2000 (Institute of the Republic of Slovenia for Nature Conservation – ZRSVN):

    • Special Protection Area (SPA) under the EU Birds Directive (SPA Cerkniško jezero, site code SI5000015).

    • Target species conservation plans covering Platalea leucorodia, Haliaeetus albicilla, Circus aeruginosus, Tringa glareola, Grus grus, and associated wetland species.

  5. Historical, Ethnological, and Mythological Sources:

    • Johann Weikhard von Valvasor: The Glory of the Duchy of Carniola (Die Ehre deß Herzogthums Crain, 1689) – primary historical documentation of intermittent hydrology, fish harvests at ponors, and legends of Mount Slivnica witches.

    • Institute of Slovenian Ethnology ZRC SAZU – ethnological collections on karst customs, witch trials, folk meteorology, and carnival traditions.

    • Slovenian folk tales and fairy tales (myths of water sprite Jezerko and the castles of Šteberk and Karlovec).

  6. Scientific Monograph on Lake Cerknica:

    • Cerkniško jezero (ed. Prof. Dr. Franc Lovrenčak et al.) including ornithological chapters by Dr. Iztok Geister and researchers monitoring the Adriatic Flyway across the Notranjska karst poljes.