A LAKE THAT APPEARS AND DISAPPEARS
An expert associate: Prof.dr.sc.Čedomir Benac
Find out how nature perform this magical trick!
Ponikve is a rare karst phenomenon of periodic lake which appears and dissapears.This karst depression of 2km lenght, maximun width 1,5km and 200m deep is one of the largest in Dinaric karst.
In the period of heavy rain or when snows melt, the water begins to accumulate in the karst subterranean and with hail, noise and fog gets out to the surface. The green valley you are surrounded by is filled with water while houses in Ponikve in a couple of hours get a lake view. Several times in the past, water reached the thresholds of the house.
The lake which can be over 20m deep is divers’ magic paradise. In spring, when the snow melts divers dive among the flourish fruit trees, as well as hunting high stands, electric poles, trails and dry stones. This is a short-lived nature
phenomenon because Bura (a local wind) throws water of the lake back to the sources in Bakar bay.
WHAT IS KARST?
Over millions of years due to the process of karstification or corrosion effect of surface and ground water on sedimentary rocks rich in mineral calcite, a karst, a field of specific morphology was created.
WHAT IS DINARIC KARST?
The Dinaric karst is known in the world as a classic type of karst and covers almost half of the land part of the territory of the Republic of Croatia. Submerged karst along the coasts and islands of the Adriatic Sea is a special feature.
WHAT ARE PONIKVE?
Ponikva, a sinkhole or karst valley, is a naturally round or elliptical funnel, often covered with red soil, a common occurrence in the karst. Ponikve were created by the corrosion of carbonate rocks and sometimes by their collapsing.
WHEN WERE PONIKVE FORMED?
The area around a sinkhole is shaped in sedimentary rocks from the geological Upper Cretaceous-Palaeogene period, such as; limestone, dolomitic limestone and Breccia containing up to 99% calcium minerals (CaCO3). During the evolution of karst, specific and relief forms on the surface were formed – a sinkholes.
VELI I MALI LUG /big and small grove/
When joining two of the eponymous sinkholes Mali i Veli lug, a karst depression is created -Ponikve. The bottom of both depressions is filled up and covered with red soil (terra rossa).
HOW DOES THE WATER FLOW?
Karst areas have a large permeability. Therefore, the surface water rapidly penetrates into the underground. Surface water from the wider area of Ponikve flows through cracks and caverns to the karst aquifer. From there it flows to the shore of the sea in Bakar Bay. The usual subterranean level of water is more than 100m below the surface area. However, after a long period of intense precipitation it can suddenly rise.
Sometimes the groundwater level rise is so strong that the bottom of Ponikve can be easily flood.
When the water supply is smaller, the water appears at the bottom of Mali Lug, and at larger flow the water is poured into Veli lug. At extremely high water inlet, the level of water is raised in Veli lug. In this way a periodic lake is created, as a rare phenomenon in Kvarner and Gorski kotar karst.