The flow of Mrežnica runs along the borderline of the northern Dinara area of Croatia in Kordun. Mrežnica's spring is placed 7 kilometers west of the town Slunj, under Popovića peak (479 m). Although in some parts of the flow the river winds, most of its flow is directed from the south toward the north. The mouth in Korana river is in Turanj near Karlovac.
The river Tounjčica springs out of a cave called Tounjčica under the hill Krpela. The first part of the flow, from the spring to the mouth of Rudnica in Tounjčica it runs from the northwest toward the southeast, and at the mouth of Rudnice (village Ožanići), in a sharp curve it changes its direction toward the north, almost horizontal with Mrežnica. Tounjčica runs into Mrežnica near the village Mrežnica.
Mrežnica is characterized by a relatively large number of springs and caves, as well as falls. Besides its strongest tributary Tounjčica, Mrežnica also has tributaries Suvača and Svetojurac. Majority of the water which springs out comes from a pit river Dretulja. Dretulja springs up underneath Kapela, and falls in a pit 7 km after its surface flow through Plaščansko field. A subterranean connection was established between rivers Dretulja and Mrežnica, because of the cellulose factory in Plaški which threw waste waters into Dretulja, and such pollution was also noticed in Mrežnica. All these characteristics are typical for flows in the karst, therefore we can say that Mrežnica is a typical karst river.

The spring of Mrežnica is on an altitude of 270 m, and its mouth on 112 m above sea level, which means the fall of the river from its spring to the mouth is 148 m. The overall length of Mrežnica from spring to mouth is 64 km, average fall per kilometer is 2,3 m.

The Mrežnica basin today is much different than it used to be (picture 1). The reason for that is construction of a dam on the river Zagorska Mrežnica near Ogulin in 1959. Waters of Zagorska Mrežnica are accumulated in the lake Sabljaci where they are through lake Bukovnik, below the surface, led into the hydro-electric power plant Gojak on the river Dobra, and in that way Mrežnica is cut short of 40% of its system. Luckily the flow of Mrežnica is still strong enough to ensure enough humidity for travertine dams, even in dry and hot summer days. Average flow of Mrežnica is 34 m3/sec, but it varies through the year. In its springing part a minimum flow of 5,8 m3/sec was recorded, and the maximum of 67,4 m3/sec. At its mouth the minimum is 13,8 m3/sec, and maximum 122,4 m3/sec.
The canyon through which Mrežnica (90%) runs is a result of deep erosion in the geological past. The other, lower part of the flow was formed through side erosion. The cause of stopping or diminishing the depth erosion was probably, development of travertine dams in the Pleistocene era.

Travertization-formation process of travertine dams

The phenomenon of travertine formation is the main characteristic of the river Mrežnica. Mrežnica has, 93 travertine falls and numerous travertine dams and other shapes in which travertine appears.
Rains on their way toward the stream, become enriched with carbon dioxide when going through the atmosphere. When attaching carbon dioxide, carbonic acid is formed (H2CO3) which has the capability of dissolving limestone, and therefore a part of carbonic acid is tied to calcium carbonate, forming calcium bicarbonate which can be dissolved in water. The other part of CO2 which is not tied stays in the shape of free CO2 which in a certain chemical balance with the appropriate amount of calcium bicarbonate. In the condition of water dispersion CO2 can get in the area of low atmosphere pressure what causes balance disturbances between CO2 - bicarbonate and the result is breaking of the carbonate molecule

Ca(HCO3)2"CaCO3 + H2O + CO2

One of the products of carbonate molecule breaking is calcium carbonate, which isn't soluble in water.
The further role in calcium carbonate sedimentation is played by travertization plants and animals of the travertine dam communities. Particles of calcium carbonate, extracted in spraying conditions, and water currant, could not hold in places of extraction. Travertizing moss and weed inhabit such places and hold the extracted particles and crust them on their surface and their roots. Such a material is called travertine. In time these plants grow on each other and this process is repeated so we say falls grow. Travertine on Plitvice lakes grows at an average rate of 1-3 cm a year. For the purpose of determining the age of travertine dams, tests were made on Plitvice lakes and the river Krka. On Plitivice lakes the age of travertine falls was determined at 4000 years. But older travertine creations were found, which date back 40000 years and were formed in the interglacial period when conditions for travertization were convenient.
Besides the necessary temperature conditions, which are about 14°C, for the creation of travertine we need oversaturation with calcium carbonate (saturation index larger than 3), alkalinity (pH 8,0 to 8,4), certain water clarity (organic matter concentration less than 10 mg/l).