The surface of the lake’s tributaries equals to 475 square kilometres. The inflow of water in the lake reaches 240 cubic meters per second, while the outflow reaches between 40 and 90 cubic meters per second. The difference in the inflow and the outflow of water causes the lake to disappear. The lake usually reaches 550 meters above sea level and covers an area of 20 to 30 square kilometres. During heavy rainfall, the lake fills up in about two to three days while in dry periods it disappears in about three to four weeks (Kranjc 1986).
The lake is fed by the following brooks and streams: Cerkniščica, Stržen, Lipsenjščica, Žerovniščica, Martinjski obrh, Grahovščica, Gorički potok, Laški studenec, Trsenec, Otoški obrh, Mrzlek and many smaller springs, which dry up in dry periods. Worth mentioning are also two periodic springs which emit water during heavy rainfall: Suhadolica and Vranja jama.
The outflow of water from the lake is entirely karstic. The largest sinkholes are on the west part of the lake, near a rocky slope in the Jama inlet: Velika Karlovica, Mala Karlovica, Rakovski mostek, Svinjska jama, Kamnje and Narte. The underground sinkholes in the central part of the lake are: Rešeto, Vodonos, Bečki, Sitarica, Retje, Ponikve, Ajnce jame. There are two sinkholes near the hamlet Otok: Češljenca and Levišča. These two sinkholes dry up the last.
At the bottom of the lake, there are also estavele. Estavele are karstic holes that are springs during rainfall and sinkholes in dry periods. There is the estavela Cemun in the spring of the Stržen Brook and two more in the inlet Zadnj kraj: Gebnu and Kotel (Gospodarič and Habič 1979).
When highest, the lake has three islands: Mala Gorica in the inlet Jama, Velika Gorica (Goričica) and Otok. When the lake is full, the hamlet Otok on the island Otok is the only island village in Slovenia.
Lake Cerknica has had a colourful history. The streams and brooks as well as the disappearance of the lake are a mixture of natural conditions and past efforts of the local people to drain the lake or to preserve water in it.
LAKE CERKNICA THROUGHOUT HISTORY
Lake Cerknica was mentioned for the first time under the name Lugeon palus (the sad lake) already in antiquity, but also several times afterwards. It was early known as a natural landmark. It used to be drawn excessively large on many maps, which showed its great importance (Kranjc, 1986).
The first gentlefolk known to have had rights to fish in the lake were the Patriarchs of Aquilleia. In 1319, Patriarch Pegam granted the right for fishing in the lake and its tributaries to Oldarik Cividale. A hundred years later, Postojna’s countryseat got half of the lake together with the fishing rights. In 1477, Cesar Friderik IV donated among the other things also the right to fish in the lake and its tributaries to the town Lož. The countryseats in possession of fishing rights in Valvasor’s times were: Haasberg, Schneeberg, Lož, Turjak and the Stična Monastery (Žirovnik 1989).
A number of foreigners visited the lake before Valvasor did but no one described it in such detail as did Valvasor in his book The Glory of Duchy Kranjska. He felt that Lake Cerknica was a true nature’s wonder due to its mysterious disappearing characteristic (Valvasor 1689).
Valvasor sent his description of the lake to the Royal Society in London and was consequently elected a member of the most famous science society in the world. His descriptions aroused further interest in the lake. Enthusiastic over the richness of the lake, he wrote about the lake’s double benefit for the people: the agricultural exploitation of the lakebed as well as fishing and hunting for water birds (Kranjc 1986).
Valvasor paid special attention to fishing. When the lake was full of water people weren’t allowed to fish. Peasants from the village Gornje jezero were set as keepers. They informed the parish clerk when the lake started to disappear. Then the parish clerk announced the beginning of fishing by ringing the bell in St Kocijan’s Church. Peasants from this village left their work on the fields, grabbed their fishing nets and rushed to fish in the Veliki and Mali Obrh Brooks up to the demolished old bridge. The peasants gave part of their catch to Prince Eggenberg, the county judge, while they kept the other part as payment. The locals fished naked and they did that with no shame. This custom was so deeply rooted that even the Kartouzians could not eradicate it. At that time each part of the lake and each cave was specifically prearranged. To avoid litigation, it was determined who could fish where, when, for whom and in what way (Valvasor 1689).
The fish catch was especially abundant when the lake had not disappeared for years. In 1655, when the lake disappeared after five years, fish and crabs were everywhere. When Prince Eggenberg towed the fishing net in the area of Rešeto for the first time, he loaded twenty carts full of fish. Count of Turjak caught seventeen carts of fish at the second tow. Princess Eggeberg donated the third tow, which amounted to nine carts, to the Stična Monastery. Afterwards, the towing of fishing nets continued (Valvasor 1689).
When the lords’ fishermen had finished the parish clerk rang the bell in St Janez’s Church to announce that fishing was then open for the inhabitants of Cerknica and surrounding villages. The locals had to pay the parish clerk if they wanted to fish. Naked, they rummaged through the dried lake and gathered what was left. Fishing was not allowed in the Narte and Pijavec Caves since they never completely dried and people believed that that was where the fish spawned (Valvasor 1758).
Nearly seventy years after Valvasor Steinberg wrote a new thorough book about the lake and enriched it with numerous copper engravings. An especially rich catch was registered in the winter of 1714 when the lake disappeared after seven years. There was a true abundance of fish – they could be literally scooped up with fishing nets and loaded on carts. In the severe cold, they sold the frozen fish in Trst (Trieste), Reka (Rijeka) and the Štajerska region (Steinberg, 1758).
In 1844, the locals cleaned up some major sinkholes clogged with reeds, branches and logs, and deepened their entrances since they had only been able to swallow higher waters. In front of the Velika and Mala Karlovica Sinkholes they fitted wooden lattices for easier cleaning. The lake started to disappear more frequently and consequently the number of fish was reduced (Kebe 1860).
During heavier rainfall the lake used to flood the nearby fields and in 1881 it also flooded the village Dolenje Jezero. For five weeks the water in the village was almost one meter high (Žirovnik, 1898).
The water caused problems to the owners of the land around the lake since they could rarely bring home the hay before the water washed it away. Many times, they could only reap the reeds from their boats. The marshy meadows rarely dried insofar as to let them normally cut grass. For that reason they frequently sought the authorities’ help (Kabaj 1925).
In 1888 Putick made a plan for draining the lake. He cleaned and deepened some major sinkholes so that they reached lower waters (Kranjc 1986). He was very successful in his work. The authorities turned down further locals’ requests since the lake would thus endanger the lower lying Planina Polje (Kabaj 1925).
In 1910 the locals under the guidance of their fellow villager Martinčič, thoroughly cleaned the Mala Karlovica Sinkhole and mined some water barriers. Since then the water ran off more rapidly and the lake disappeared regularly each year. The success gave them the confidence for further work that was later interrupted by the war (Kabaj 1925).
Žirovnik wrote that before the major draining interventions all the water of the Stržen and Žerovniščica Brook that hadn’t flowed into the area of Beček and Retje in the dry periods flowed into the area Vodonos. The Vodonos only rarely dried up. The Cerkniščica Brook, which never completely dried up in its lower current, flowed into the Rešeto Pond (Žirovnik 1898).
In 1921 a water company was founded in Cerknica and its objective was to drain the lake. Up until World War II Hočevar led the project in which they deepened the lakebed from the Narte Sinkhole to the Mala and Velika Karlovica Sinkholes. They lowered the entrances of the Velika and Mala Karlovica, Kamnje and Narte Sinkholes, and regulated the flow to Svinjska Cave. They cleaned the sinkholes and fitted a lattice in front of Velika Karlovica to ease its cleaning. They mined narrower cave tracts and siphons (Kranjc 1986). Kunaver (1922) thought that the water didn’t disappear because of the siphons, so they should be removed at any cost.
The water company dug up the naturally meandering channels Stržen, Žirovnišica, Lipsenjščica, Trsenec and Goriški potok and made them straight. They also dug draining ditches near the Lipsenj area. These draining interventions caused the lake to disappear a month earlier while the water level of the lake lowered only slightly (Kranjc 1986).
All draining interventions after the end of WW II altered the lake so much that the west part of the lake had no water in dry periods. Something had to be done to retain at least some water also in that part of the lake and to avoid the annual death of fish. For this purpose the Cerknica Fishing Association built a dam in front of the Rešeto Caves in the fifties and thus created a small permanent lake. Each year when the lake starts to disappear the fishermen from Cerknica try to save the fish by taking them out of the sinkholes and putting them into the man-made Rešeto Pond. Since this pond lies on permeable karstic terrain and it could dry up in the drought the fishermen wanted to connect it to the Strženo and Žerovniščia Brooks. For this purpose, they built dams also in front of Ponikve, Retje and Sitarica Caves in the following years (Korošec 1996).
At the request of the Cerknica Municipal Assembly at the end of the sixties Jenko designed an outline scheme to permanently dam the lake. The purpose of the scheme was to improve water conditions at a low cost. These improvements would help the development of tourism and fishing and level the water regime of the Sava River Basin. On the basis of this project they dug a man-made tunnel near the Rakovski mostek Sinkhole which led to Blatna dvorana (the mud hall) in the Velika Karlovica Sinkhole and placed a movable gate in front of it. They stuffed the Velika and Mala Karlovica and Narte Sinkholes (Kranjc 1986). By damming the main sinkholes, the runoff of the lake lowered to high and middle waters. In regular conditions the lake lasted longer but it still disappeared in dry periods (Habič 1974).
At the end of the eighties, the man-made gate was constantly open and the channel leading up to it was dug out. The lake began disappearing several times a year (Korošec 1993). With the pretext that “natural condition” must be restored, the company Area from Cerknica which managed the planning of the Notranjska Regional Park removed the stopper in the Mala Karlovica Sinkhole and the dam near the Ponikve Cave in 1992. The planners didn’t know that up till then the peasants had been draining the lake for more than one hundred years (Korošec 1998).
Each year there is less and less water in the lake. Nowadays it is very rare for the lake to last until summer. Reeds have widely expanded over the lake probably because the locals have abandoned the less profitable mowing that used to take place on the lakebed. The shortage of water and its hasty runoff threaten the birds while nesting. The predators easily reach nests that remain ashore and the nests are destroyed. In dry periods there is no appropriate water area which would serve as the living space for birds, fish, amphibians and other animals. All draining interventions that took place in the past contributed to this present state. In dry periods there is also great risk of fire (Vovk 1979).
Spawning is much dependent on the lake’s water level. The fish spawn in shallow waters on the banks. If the water runs off too quickly, the fish eggs and the offspring stay ashore and the spawning fails. If the fish spawn in the low waters of the channels, the eggs are eaten by other fish (Vovk 1979).
Metod Korošec, univ. dipl. soc.
Literature: Gospodarič, Rado in Habič, Peter (1979): Kraški pojavi Cerkniškega polja. Krasoslovni zbornik VIII/1. Slovenska akademija znanosti in umetnosti, Ljubljana, str. 11 -158. Kabaj, Matej ((1925): Cerkniško jezero in okolica. Učiteljska tiskarna, Ljubljana. Habič, Peter (1974): Tesnenje požiralnikov in presihanje Cerkniškega jezera. Krasoslovni zbornik VI. Slovenska akademija znanosti in umetnosti, Ljubljana, str. 37 - 56. Kebe, Gregor (1860): Popis Cerkniškega jezera. Novice gospodarske, obertniške in narodne, 18. Steinberg, Franz Anton (1758): Grundliche Nachricht von dem in dem Inner-Krain gelegenen Czirknitzer See. A.E. Reichbardtin, Laibach. Korošec, Metod (1993): Cerkniško jezero nekoč, kaj pa danes?! Ribič, LII, 12, str. 332 - 333. Korošec, Metod (1996): Ponosni na svoje delo. Ribič, LV, 12, str. 310 - 311. Korošec, Metod (1998): Projektanti parka so predlagali ribolov z mrežami. Ribič, LVII, 1 - 2, str. 5. Kranjc, Andrej (1986): Cerkniško jezero in njegove poplave. Geografski zbornik XXV/2. Slovenska akademija znanosti in umetnosti, Ljubljana, str. 75 -113. Kunaver, Pavel (1922): Kraški svet in njegovi pojavi. Učiteljska tiskarna, Ljubljana. Trontelj, Peter (1993): O naravovarstvenem konceptu Cerkniškega jezera s poudarkom na varstvu ptic. Acrocephalus, 14, št. 56 - 57, str. 63 - 80. Valvasor, Janez Vajkard (1689 - 1984): Slava vojvodine Kranjske: izbrana poglavja. Mladinska knjiga, Ljubljana. Vovk, Joža (1979): Ihtiološke raziskave Cerkniškega jezera. Krasoslovni zbornik VIII/6. Slovenska akademija znanosti in umetnosti, Ljubljana. Žirovnik, Jožef (1898): Cerkniško jezero. Slovenska matica, Ljubljana.