The Krka and its tributaries Orašnica and Butišnica from the right side and Kosovčica on the left side runs through the Kninsko Polje field. The Čikola River with its tributary Vrba and their occasional small tributaries run through the Petrovo Polje field. The landscape of the Krka River is marked by three different types of relief: the mountainous area to the northeast, the Northern Dalmatian plateau with its canyons, and the lowland expansions or fields. These karst fields, called polje, are the largest karst depressions. They typically have a flattened bottom through which a watercourse usually runs. In open fields, such as the Kninsko Polje field, this water course flows out of the field through a deep canyon, while most karst polje fields, as a rule, are closed in on all sides with water courses springing along the edges, such as the Lika River in the Ličko Polje field. In the Krka River basin, we find three fields: Kninsko Polje, Kosovo Polje and Petrovo Polje. Their valleys lie almost perfectly aligned with the parallels over a length of 41 km. To the east, they are closed off by Mts. Dinara and Svilaja, to the north they continue into the deep Butišnica valley, while to the west is the limestone plateau extending from the southeastern slopes of Mt. Velebit and the Bukovac hills. The Krka and its tributaries Orašnica and Butišnica from the right side and Kosovčica on the left side runs through the Kninsko Polje field. The Čikola River with its tributary Vrba and their occasional small tributaries run through the Petrovo Polje field. Professor Mladen Friganović in his book Polja gornje Krke (Fields of the Upper Krka; Zagreb 1961), gave us a detailed description of all the karst forms found here, and the life contained within. There are two theories about the formation of these fields. One is based on a theory of tectonic genesis, the second on tectonic predisposition, after which the landscape was shaped through external processes. They were shaped in the zone of contact between the Adriatic and Dinaric megastructural units of the karstified Dinaric region. These three fields share a common feature in that the surface water running through them drains into deeply cut canyons, and their watercourses are fully dependant on precipitation. Pluvial or rain river regimes are most prominent in the Petrovo Polje field. In the past these fields were widely flooded in winter, with very little water in summer, making economic development difficult, as the local economy was based on the cultivation of the fields and the many mills driven by water power. These oscillations were particularly felt by the town of Knin. Therefore, the need arose to perform engineering works to regulate the supply of water. The smallest of the fields is the Kninsko Polje. It was first mentioned in the year 1050, as Tenin campum. With the Golubica area, it covers 24,2 km² and has the least defined shape. It is separated from the Kosovo Polje field by the Horse’s Head hill and Burnum ridge, and between these the Kosovčica River has cut its path. The bottom of the field is composed of two floodplain plateaus, the Krka to the east and Butišnica to the west, which are separated by a series of hills. On the southern edge of the field the Spas limestone ridge rises up, and under it is the town of Knin. The Kosovo Polje field is right in the middle of these three fields, in terms of its position, size and elevation. It covers an area of 33.8 km² and is elongated in the direction of the parallels, like the Kninsko Polje field. It stretches over a length of 13.5 km from Burnum to Tepljuh. The largest field is Petrovo Polje. It is shaped like an isosceles triangle, with the base lying along the southeastern edge of Mt. Promina and peak at the confluence of the Vrba and Čikola Rivers. This field is 17 km in length and covers an area of 57 km², making it the largest and most connected alluvial plain. It was previously called Campus Illyricum while the present-day name was first mentioned in the 11st century, likely under the rule of Croatia’s last king, King Petar Svačić, and named after the Petrovac hill fort that once stood on the northern edge of the field, which the king built and occasionally inhabited.