The Gacka Beauty, 61 km long with its tributaries, springs unexpectedly quiet from below Godača and Venac hills in Sinac. Once out of the karst underground, it shows its liveliness and playfulness after fifty meters already. It rushes down in small cascades, forming a river stream or tributaries. It lazily meanders down the plain to Karlov kanal near Otočac. From this point on it merges with the Lika River.
Although essentially a karst river, the Gacka is extremely slow and quiet with a uniform flow of water. Apart form the uniform abundance of its source, the Gacka is known for its stability and small temperature differences. The average annual water temperature is 9.4°C (7.9°C in winter, 10.8°C in summer), which is particularly suitable to the fish of the salmonid family. The Gacka water is rich in oxygen (9.1 to 13.5 mg/l), even to supersaturated conditions. It is slightly alkaline as well as fairly hard and rich in calcium salts.