Adriatic coast reaches deeply into the lowlands where isolated karst patches can be found. The karst phenomena and forms have developed primarily in limestone. Croatia’s karst is characterized also by phenomena and forms in marl and sandstone rocks; or rather flysch series that are rare even in global proportions.
Karst rivers are rich in waterfalls and known for travertine phenomenon resulting in formation of characteristic geomorphologic forms. The penetration of water into stony substratum created caves, abysses, ice-pits including underground springs and estrevelle.
There are many threats to the karst rivers and underground waters as many of the sewers are discharged directly into the rivers. Caves are used as landfills and some are open to tourists without previously conducting biospeleological studies. Intensive agriculture is taking place in the areas where the ground allows for penetration of pesticides into underground rivers.
The longest Croatian rivers are Sava, Drava and Danube. The rivers flow into the Black Sea (62% catchment area) and the Adriatic (38% catchment area). The rivers belonging to the Black Sea catchments are lowland rivers along with numerous flood forests, grasslands, still backwaters with respective habitats can also be found.
The stretch of the river Drava between the mouth of Mura to Barc in Hungary forms steep landslide along its unfortified banks in which sand martins and kingfishers make their nests. In the sandbanks nesting birds such as small tern and little ringed plover can be found.
Due to their limestone base, rivers of the Adriatic catchment areas are short with frequent rapids and waterfalls, including sections of canyons. Numerous sink rivers flow through karst fields creating systems of underground water circulation, that are abundant with endemic fish species.
Lakes in Croatia are few but extremely valuable. The most well known are Plitvice Lakes that are formed by the Korana River flowing into 16 cascading lakes with numerous travertine downstream beds in a vivid biodynamic process. This is one of the most well known Croatian National Parks and the only one included on the UNESCO World Cultural and Natural Heritage list.
Vransko Lake near Pakostane is the greatest natural lake in Croatia with a surface area of 30.7km². Vransko lake on the island of Cres is much smaller but fairly deep (74m) and the Blue and Red lakes near Imotski are unique as well.
Croatia has an outstanding wealth of wetland habitats, four of which are by today included in the Ramsar list: Lonjsko Polje (50.5 hectares), Kopacki Rit (17.7 hectares), Lower Neretva (11.5 hectares), and Crna Mlaka fishponds (625 hectares).
Lonjsko Polje, in the central course of Sava, is a flood area with wide, wet flood forests of common oak, flood meadows and pastures and numerous backwaters.The habitats of numerous threatened species such as the white tailed eagle, lesser spotted eagle, black stork, spoon bill, white stork, spoonbill, white stork, blackbird and others are located here.
Kopacki Rit, situated where the Drava flows into Danube, is full of lakes, ponds and canals including fishponds. It is where the largest population of white eagles resides and the location for 90% of Croatia’s nesting white geese. It is also the only place in Croatia where the great cormorant is known to nest. Kopacki Rit is particularly valuable as a resting and feeding place for migratory waterfowl. During migration, flocks of several tens of thousands ducks and geese may be met there.
Another Ramsar site is the Neretva valley with wide wetland habitats including the largest and richest reed species in entire Mediterranean Croatia.
The biggest threat to the wetland areas are changes in water regulations specifically pollution from discharge and pesticides into the rivers. The construction of hydropower plants and formation of storage lakes have considerably affected the river flows and the karst. The wetlands are often drained and turned into marshy meadows and agricultural land.
One of the major threats to biodiversity remains to be water pollution from urban agricultural and industrial sources. Only 40% of the population has access to sewage systems and only 12% of the water is being treated. Less than 5% receives secondary treatment.
Rivers are not managed for biodiversity. They are managed for flood control only. This is why flood control dams as well as electric hydropower plants continue to have an adverse effect on the ecosystems. Nevertheless, HEP (Croatia Electric Utility) announced the construction of two new hydro power plants including the one on the Dobra river near Lesce, a rich karst area.
For more information on Croatian biodiversity click here.