Underground landscape

Observing a  vertical cross section of a karst system it is possible to point out different zones, depending on the presence of water and on how water moves within.
The catchment zone
The catchment zone is the one closest to the surface where surface water and meteoric water sink and seep deep underground. Catchment may take place through a large number of fractures, thus being diffuse, or through concentrated sinking input points, such as dolines, where surface water can collect and concentrate: in this way the corrosive action of water concentrates on a small number of fractures that are then widened preferentially, thus leading rapidly to  the creation of underground karst galleries. At times the entrance of surface water into the deeper layers is characterized by spectacular forms. When water courses flowing on impermeable rocks that cannot be karstified come into contact with karstifiable rocks, such as limestones and dolomite, they are literally swallowed underground, at times with small losses, that gradually dry up the flow of the river, till it disappears,  leaving a dry valley, at times with spectacular sinkholes that capture the flow of the water totally. This is the so called allogenic recharge. A particularly impressive example is the sinking of the Reka river in Slovenia. Through the San Canziano caves the river is swallowed and disappears, to reappear 40 km downstream, from a resurgence near Trieste, with the new name of Timavo.
The transfer or vadose zone
Below the catchment zone, the vadose zone of vertical transfer develops, where water mainly flows downwards and caves have a prevalently vertical trend. Due to the progressive union and concentration of underground water flows, from the vertical transfer zone water gradually passes to the horizontal transfer zone where veritable underground water courses exist, Just like surface water courses, they entrench and erode canyons, gorges, meanders, characterized by a series of  erosion forms similar to potholes and cauldrons at the base of shafts. Here water generally has a strong energy and a high speed,  so that erosion phenomena prevail over corrosion.
This entire zone belongs to the so called vadose zone, a term that indicates the presence of voids filled with both air and water, where water flow is free. In the vertical transfer zone, it is quite rare to find completely flooded galleries or conduits, while in the horizontal transfer zone underground lakes may form, where obstacles to the outflow may cause the formation of small basins, which may often be temporary. At times the water level can rise above the ceiling of the gallery, in which case the lake is turned to a siphon, i.e. a part of a gallery completely flooded, where the ceiling sinks below the water surface. Normally in the zone of horizontal flow , after passing a siphon it is possible to continue explorations of the sub-aerial zone. Siphons of this type generally are not very deep. One of the main causes of the formation of lakes or siphons is the presence of depressions whose bottom is impermeable due to clay deposits. Furthermore, the water level in lakes and siphons may vary remarkably, depending on external input (rainfalls). In very rainy periods, galleries that are normally dry can be completely flooded and vice versa. In some karst systems, water level is known to rise over 100 m, in particularly rainy periods, thus obviously flooding all the galleries below this level.
The phreatic zone
Below the vadose zone of horizontal flow , is the phreatic zone, i.e. in this zone, all cave passages, conduits, galleries, rooms, shafts,fractures, voids of any shape and size are completely flooded. This zone draws the attention of cave-divers. The top of the phreatic zone, also called the water table, is found at sea level, in karst close to coastline, while far from the sea it is at the same level as the main valley floors, close to the so-called base-level, i.e. the level below which all voids are completely full of water.

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