Only 7 kilometers long, river Krupa springs out and flows with its entire length underneath Crnopac (1404), a peak of one of Velebit's slopes. It springs out of two springs not far from the hamlet Mandići, and receiving
Its first tributary Orovaća, it enters a short canyon to the Krupa monastery. After Manastirske luke, Krupa flows along the canyon towards the west to its mouth in Zrmanja. On the half of its flow, uder a right angle from the right it receives its second tributary Krnjeza. Karst spring of Krnjeza is placed in a cave on the bottom of a canyon beneath Krnjeska glava (408 m).

Travertization - travertine barriers formation

Zrmanja, and especially Krupa, are typical karst rivers, and along with other karst phenomenon they are decorated with travertine falls.
Rainfalls are, on their way toward the stream, enriched with carbon dioxide, when going through the pedosphere. With CO2 carbon acid is formed (H2CO3) which has the capability of dissolving limestone, so a big part of carbon acid is tied to calcium carbonate making calcium bicarbonate soluble in water. The other part of CO2 stays free in a shape of free CO2 Which is in a certain chemical balance with the appropriate quantity of calcium bicarbonate. In conditions of spraying water CO2 can move to the area of lower pressure which causes balance disturbances between CO2 - bicarbonate, and breaking of the carbon molecule occurs

Ca(HCO3)2 " CaCO3 + H2O + CO2 Ca(HCO3)2 " CaCO3 + H2O + CO2

One of the products of breaking the carbon molecule is calcium carbonate, which isn't soluble in water.
A further role in sedimenting calcium carbonate is being taken by travertization flora and fauna of the travertine barrier. Extracted in spraying conditions, and flow of water, particles of calcium carbonate could not withhold in places where they were extracted. Travertization moss and algae inhabit such places and hold the extracted particles and crust them on their surface and roots. Such material is called travertine. In time these plants build up on each other and these process is repeated so we say that falls grow. The growth of travertine on Pltivice lakes is about 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).

It is simple to connect the geographic position of Zrmanja with the position of mountain Velebit. Zrmanja is many times defined as the south-east border of Velebit. With its spring beneath the peak Poštak and mouth in Novigrad sea, the river is with its 69 kilometers of flow, almost encompassed south-east slopes of this mountain, uniting them in one unit.
But, the geographic position of the river was much different in the past than it is today. In its geologic past, 40000 years ago Zrmanja with its fossil bed, ran into river Krka at Mokro polje. Today there is only an underground connection between these two rivers. Underground waters of Zrmanja which spring out of several springs near Miljacka falls on the river Krka, have significant influence on the water regime on the middle part of Zrmanja. During summer droughts part of the flow from Mokro polje to The Crni bunar (Black well), and even downstream toward Žegarsko polje is completely without water.
From the spring and approximately four kilometers downstream from village Pađane, Zrmanja flows toward the south-east along the road which connects Gračac and Knin. Here it suddenly changes its direction and under Kravlji bridge in a big arc turns toward the west, winding through small karst fields and canyons. It goes through Mokro polje, Ervenik and Žegarsko polje, after which it still goes toward the west, and receives its tributary Krupa. Downstream from Obrovac, Zrmanja gradually turns toward the south-west and runs into Novigrad sea.

The spring of Zrmanja is on 395 m above sea level, and the mouth in Novigrad sea regarding its entire flow of 69 km means it has an average fall of 5,7 m per kilometer.
The basin of Zrmanja is pretty wide. Although the river receives most water from the mountain Poštak, there is significant influence by waters which fall into pits on edges of these fields near Gračac, and come as springs and tributaries which go into Zrmanja.
The average flow of the river Zrmanja in its spring part near Palanka is 5,2 m/sec, and in the lower part of the flow near Jankovića buk upstream from Obrovac 40 m/sec what is influenced by the tributary Krupa and numerous springs.

From the geologic aspect Zrmanja is also very interesting. The basin of Zrmanja is according to geologic researches built mostly of limestone and dolomite. Waterproof rocks are built of limestone from the geologic era of mezozoic, that is trias, jura and crede (220 to 140 million years ago). Waterproof rocks are made of dolomite and limestone plates from the geologic era of jura, and non-waterproof rocks are clastic, made of sandstone, slate, marl and dolomite. As we said before, about 40000 years ago Zrmanja was flowing into Krka. That means that biggest geological changes occurred in the recent 40000 years. Geologists assume that in the era of 40000 years back to 8000 years ago there was strong erosion which resulted in the creation of todays Zrmanja bed. Today's mouth in Novigrad sea was made by the rise of sea level, which occurred after the latest ice age.