Water is the foundation and source of life. It is necessary not only for humans but for all plants and animals. Water covers 2/3 of the Earth's surface, but only 1% of the water on the Earth is drinkable. Water-rich areas of the park testify to the density of the hydrographic network or stream network, which amounts to 1 km/km2.
The Kozara National Park is a hydrographical knot of Kozara Mountain. In fact, the sources of almost all the major streams and rivers are on the territory of the Kozara National Park. Deeply incised streams formed steep and deep valleys. Some parts of the watercourses are very colourful, narrowed in canyon and gorges, and leave a lasting aesthetic impression on visitors. (The waterfall at Zečji kamen, Gumline, etc). The Kozara National Park has a dense hydrographic network, which consists mainly of permanent and of the periodic streams to a smaller extent. This network is very extensive and of radial-dendroid type. The formation of hydrographical network in the Park has been significantly influenced by the relief and geological characteristics of the substrate. The areas made of Triassic and marly limestone and marl are waterless, while the terrains made of diabase and shale have opulent water resources. The northern part of the area is in particular rich in streams and springs. The network density in the whole area is 1 km/km2. Water from the northern part of the National Park belongs to the basins of the Sava and Una rivers, and the south-western part belongs to the Sana River Basin.
The most important rivers on the north side of the Park are the Mlječanica, Crna Rijeka and Moštanica Rivers. The Mlječanica River has the largest catchment area and the longest system of tributaries. It is created by merging the Velika and Mala Mljecanica Rivers.
Cut into the south-western slopes of the Kozara National Park the Starenica Stream (the Kozaračka rijeka River) is formed by merging the Bijela voda and Jovovca Rivers, under Gumline. The most significant right tributaries of the Starenica Stream are Uromovac, Kotlovača and Zovik.
The major sources are: the Mala and Velika Mlječanica, Gračanica, Urimovac, Kotlovača, Zovik, Sedra, Vidova voda, Dolinac, Brezovac, Bijela voda, Bukovački točak Rivers, etc.
All water sources are of good quality and can be used for drinking. Hydrological resources in the Kozara National Park are essential for life, especially nowadays when the world is facing the shortage of drinkable water.