A tributary is any river or stream that drains into another flowing body of water, and not directly into the sea or a lake.
The Ohio and Missouri Rivers in North America, both large rivers, are actually tributaries of the Mississippi River. The Krka River receives five tributaries in its freshwater part: Krčić (10.5 km long), Kosovčica (12.5 km), Orašnica (5.3 km), Butišnica (39 km) and Čikola with the Vrba River (37.8 km), while in the submerged estuary of the river, it receives the Guduča River (7 km).
The Krka River begins where the Krčić River ends. In the morphogenetic sense, the Krčić is the source of the Krka River, and today is considered its first tributary. It springs in the foothills of the Dinara Mountains, west of the village Kijevo, at an elevation of 375 metres. It flows for 10.5 km. During heavy rains or snow melt in the mountains inland, the Krčić becomes a torrential river, while during the summer months, it dries out completely. In the recent geological past it did not dry out, as the climate was cooler and there was more precipitation, making the Krčić and Krka one flowing waterbody. This can be seen the remnants of the many travertine deposits in the Krčić Valley, high above the present day course of the Krka river.
In the Kninsko Polje and Kosovo Polje fields, the Krka receives several calm tributaries: Butišnica, Orašnica and Kosovčica. On the earliest maps, from the period of establishment of Venetian rule in this region, the area under the Knin fortress, where the Krka and Kosovčica meander, once formed a wide wetland belt that was marked as a paludi (marshland). With time, the need arose to regulate the upper course of the Krka River and to allow for amelioration of the surrounding karst fields. The first significant left tributary of the Krka River is the Kosovčica River, which carries the water of several permanent and temporary springs rising along the eastern edge of the field, while the right tributary is the Orašnica, which receives most of its water from the Crno Vrelo (Black Spring) and other springs in Vrpolje. Downstream of the mouth of the Orašnica, once the Krka leaves the Kninsko Polje field, its largest tributary enters on the right side, Butišnica. The Butišnica River springs from several sources near the village Kaldrma, in the mountainous areas of the northernmost parts of the Krka River catchment. This is the most water-rich tributary of the Krka, with a flow rate of 2 to 20 m³/s and a catchment area of 225 km². During World War I, it was separated from the Krka by an embankment, and so now the two rivers flow parallel until their confluence, such that the “natural” mouth at Bulin Kuk was shifted about 2 kilometres downstream.
The Čikola River runs through the Petrovo Polje field, and this is the final and largest tributary in the freshwater part of the Krka River. It flows from its source in the foothills of Mt. Svilaja to its confluence with the Krka under Skradinski buk, and this river is part lowland and part canyon river. In the literature data, the length of the Čikola varies from 39 to 47 km, while the catchment basin covers an area of 836.8 km². During wet winters, the river is full of water and floods part of the field, while in the summer months, the water levels are very low due to drought and use of the river water to irrigate the fields, and therefore its central section often dries out. Throughout the entire 19th century, the locals faced the issue of how to regulate the flow of the Čikola River due to flooding of the fields, destruction of crops and the spread of malaria.
The final tributary of the Krka, the Guduča River, is an extension of the course of the Bribišnica stream, which springs on the western side of the hill Bribirska Glavica. Near the bridge in Lađevci, the Bribišnica stream becomes the Guduča River, and flows over the next 7 kilometres to Prukljan Lake. The upper course often dries out, while the lower course is a submerged valley. The lower course and mouth of the Guduča River is an area characterised by the wetland vegetation, rich and diverse flora and fauna, and exceptional pleasing landscape.