A ponor is a natural opening where surface water enters into underground passages; they may be found in

karst Karst is a topography formed from the dissolution of soluble rocks such as limestone, Dolomite (rock), dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathe ...

landscapes where the geology and the geomorphology is typically dominated by porous

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

rock. Ponors can drain stream or lake water continuously or can at times work as springs, similar to

estavelle In karst geology, estavelle or inversac is a ground orifice which, depending on weather conditions and season, can serve either as a sink or as a source of fresh water. It is a type of ponor A ponor is a natural opening where surface water enter ...

s. Morphologically, ponors come in forms of large pits and

cave A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can refer to smaller openings such as sea ...

s, large

fissure A fissure is a long, narrow crack opening along the surface of Earth. The term is derived from the Latin word , which means 'cleft' or 'crack'. Fissures emerge in Earth's crust, on ice sheets and glaciers, and on volcanoes. Ground fissure ...

s and caverns, networks of smaller cracks, and sedimentary,

alluvial Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. ...

drains.

Etymology

The name for the karst formation ponor comes from Croatian and Slovene. It derives from the proto-Slavic word ''nora'', meaning ''pit'', ''hole'', ''abyss''; the English word ''narrow'' probably has the same origin. Several places in southeast Europe (Bulgaria, Croatia, Czech Republic, Hungary, Romania, Montenegro, Slovenia) bear the name ''Ponor'' due to associated karst openings.

Description

Whereas a

sinkhole A sinkhole is a depression or hole in the ground caused by some form of collapse of the surface layer. The term is sometimes used to refer to doline, enclosed depressions that are locally also known as ''vrtače'' and shakeholes, and to openi ...

(doline) is a depression of surface topography with a pit or cavity directly underneath, a ponor is kind of a portal where a surface stream or lake flows either partially or completely underground into a karst

groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit ...

system. Steady water erosion may have formed or enlarged the portal in (mainly

) rock, in a

conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** ...

, or in looser materials. Karst terrains are known for surface water losses through small ponors and its resurgence after having traveled through vast underground systems.

Prevalence

Ponors are found worldwide, but only in karst regions. The entire Adriatic watershed within

Bosnia and Herzegovina Bosnia and Herzegovina ( sh, / , ), abbreviated BiH () or B&H, sometimes called Bosnia–Herzegovina and often known informally as Bosnia, is a country at the crossroads of south and southeast Europe, located in the Balkans. Bosnia and ...

sits on Dinaric karst, with numerous explored and probably many more unexplored ponors and underground flows. There are significant geological ponors in the

Carpathian Mountains The Carpathian Mountains or Carpathians () are a range of mountains forming an arc across Central Europe. Roughly long, it is the third-longest European mountain range after the Urals at and the Scandinavian Mountains at . The range stretche ...

, the Dinaric Alps, Greece, Turkey, and parts of the southern United States. File:Katavothra-Feneos ponor karst-depression Peloponnese Feneos mit katavothre.jpg, Polje of Feneos,

Greece Greece,, or , romanized: ', officially the Hellenic Republic, is a country in Southeast Europe. It is situated on the southern tip of the Balkans, and is located at the crossroads of Europe, Asia, and Africa. Greece shares land borders wi ...

, lake until late 19. Century. Rain showers flood large parts even today File:Avenul campeneasca.jpg, Closeup of ponor in Câmpeneasca cave near Izbuc village, Romania File:Skocjanske-Ponor3.jpg, A ponor where the

Reka River The Reka (literally, 'river' in Slovene), also the Inner Carniola Reka ( sl, Notranjska Reka), is a river that starts as Big Creek ( hr, Vela voda) in Croatia, on the southern side of Mount Snežnik, and flows through western Slovenia, where it ...

disappears into

Škocjan Caves Škocjan Caves (; sl, Škocjanske jame, it, Grotte di San Canziano) is a cave system in Slovenia. Due to its exceptional significance, Škocjan Caves was entered on UNESCO’s list of natural and cultural World Heritage Sites in 1986. Internati ...

, Slovenia File:Ponor, Stubaier Alpen.jpg, A Stubai Alps stream terminates in an alluvial ponor File:Risle ponor.jpg, The

Risle The Risle (; less common: ''Rille'') is a long river in Normandy, left tributary of the Seine. The river begins in the Orne department west of L'Aigle, crosses the western part of the department of Eure flowing from south to north and out i ...

River disappears into a ponor between

Ajou Ajou () is a former commune in the Department of Eure in Normandy. It is located in northern France. On 1 January 2016, it was merged into the new commune of Mesnil-en-Ouche.La Houssaye La Houssaye () is a commune in the Eure department in northern France. Population See also *Communes of the Eure department The following is a list of the 585 communes of the Eure department of France. The communes cooperate in the follo ...

(

Eure Eure () is a department in Normandy in Northwestern France, named after the river Eure. Its prefecture is Évreux. In 2019, Eure had a population of 599,507.Slovenia Slovenia ( ; sl, Slovenija ), officially the Republic of Slovenia (Slovene: , abbr.: ''RS''), is a country in Central Europe. It is bordered by Italy to the west, Austria to the north, Hungary to the northeast, Croatia to the southeast, and ...

File:Katavothra ponor Nestani-Polje Peloponnes Greece.jpg, In

Polje A polje, also karst polje or karst field, is a large flat plain found in karstic geological regions of the world, with areas usually . The name derives from the Slavic languages and literally means 'field', whereas in English ''polje'' specif ...

„ Argon Pedion“ (untilled plain). Plain and ponor documented since 2nd century AD by Pausanias

Dams and reservoirs in karst

Reservoir A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation. Reservoirs can be created in a number of ways, including contr ...

s in karst are prone to losses due to leakage through ponors . The construction of dams to capture water in karst terrains may pose a great financial risk despite initial investigations and thorough sealing treatments. It wasn't until the twentieth century that the first dams in karst were built, some of which famously failed.

References

{{Rivers, streams and springs . Karst formations Dinaric Alps Dinaric karst formations Sinkholes