The Krka River, however, flows above the level of fluctuations of the karst underground watercourses, as a floating river that is not lost on such a substrate.
Water that flows then sinks, the intricate and interconnected underground passages sharing these waters, that then spring again – these are the distinctive properties of karst. The Krka River area is no exception, indeed, 72.5 km of river flowing through the waterless karst is a natural phenomenon. The waters of the Krka River are completely fresh for the first 49 km of the river’s course, while in the remaining 23.5 km, the influence of the sea is evident. With its tributaries, the aboveground catchment covers an area of 2450 km², while the hydrological catchment covers an even larger area, 2650 km². The total drop in elevation is 224 m. Karst regions are characterised by a complete lack of water, dominated by the carbonate rock. Their tectonic fracturing means that all atmospheric water quickly sinks underground. The Krka River, however, flows above the level of fluctuations of the karst underground watercourses, as a floating river that is not lost on such a substrate. The dynamic and direct connection between the surface and underground water flows is a special feature of karst, creating different habitats, transferring nutrients, and enabling life both above and below ground. That is why preserving the sensitive karst ecosystem is of the utmost importance.
Complex geological conditions are the reason behind these complicated hydrological relationships. Two megastructure units meet in the area of the Krka River: the Dinaric and Adriatic. The Dinaric carbonate platform is the site of the mountainous part of the Krka River basin, to the tectonic zone of Strmica–Knin–Petrovo Polje field, while the protected section of the canyon lies on the Adriatic carbonate platform. Of all the carbonate deposits, the majority are water permeable, allowing surface water to sink into the karstified relief. Only a smaller portion is water impermeable rock, forming a barrier for the underground watercourses. The zone of impermeable rock is situated in the source area of the river around Knin, and north of this, where we find the springs, as aboveground and underground watercourses that “feed” the canyon section of the Krka River. The southernmost karst spring in this zone is the Čikola spring in the Petrovo Polje field. During dry periods, it has no waters, while during rainy weather, it forms a river. The zone of impermeable deposits, or flysch, extends along the southern border of the park, around Skradinski Buk, to the Torak spring at the confluence of the Čikola and Krka River, and encompassing the lower part of Visovac Lake. The Krka is also fed by underground water sources. A part of those waters flow from the inland areas of Mt. Dinara (Grahovo Polje field), while a part comes from the upper catchment of the Zrmanja River, with which the Krka shares an underground water profile. The most abundant springs along the Krka River Canyon are Miljacka and Jaruga, in the Čikola River canyon, this is the Torak spring, while the brackish spring Litno is found in Zaton in the downstream Prukljan Lake section of the river.
The Krka River catchment receives about 1250 mm of precipitation per year. The most rain falls during the winter, while the summers are very dry. The water levels in the river are highest in November and March, and lowest in August. The mean flow rate in the river at the spring is 12 m³/s while at Skradinski Buk it is 55 m³/s. The highest ever flow rate, 481 m³/s, was recorded on 24 December 1982, while the lowest, 4.99 m³/s, on 5 October 1961. The highest water temperature recorded was 27.5°C at Skradinski Buk, recorded on 25 August 2011, while the lowest was 3.5°C, recorded on 12 January 1985. The highest water level at Skradinski Buk, 223 cm, was recorded on 7 December 2005, while the lowest, just 5 cm, was recorded on 3 October 1990.