Gacka is a river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 located in the Lika

Lika

Lika is a mountainous region in central Croatia, roughly bound by the Velebit mountain from the southwest and the Plješevica mountain from the northeast. On the north-west end Lika is bounded by Ogulin-Plaški basin, and on the south-east by the Malovan pass...

 region in central Croatia

Croatia

Croatia , officially the Republic of Croatia , is a unitary democratic parliamentary republic in Europe at the crossroads of the Mitteleuropa, the Balkans, and the Mediterranean. Its capital and largest city is Zagreb. The country is divided into 20 counties and the city of Zagreb. Croatia covers ...

. Because for a large part of its course it is a subterranean river

Subterranean river

A subterranean river is a river that runs wholly or partly beneath the ground surface – one where the riverbed does not represent the surface of the Earth ....

, estimates on its length vary. The overground part has undergone substantial human intervention - before it was 32 kilometers long, now it is only 11 km. It has been regulated by man to flow to an accumulation in Gusić polje and then into the Adriatic Sea

Adriatic Sea

The Adriatic Sea is a body of water separating the Italian Peninsula from the Balkan peninsula, and the system of the Apennine Mountains from that of the Dinaric Alps and adjacent ranges...

 through a hydroelectric installation HE Senj

Senj Hydroelectric Power Plant

Senj Hydroelectric Power Plant is a large high-pressure diversion power plant harnessing the Lika and Gacka Rivers water in Croatia. Senj HPP has four turbines with a nominal capacity of 55 MW each having a total capacity of 220 MW....

 near Sveti Juraj. HE Senj takes inflow from Gacka and Lika

Lika (river)

Lika River or Rijeka Lika is a stream in Croatia which gives its name to the Lika region. It is known as a sinking river because at the end of its course, it flows into a series of ponors or swallow-holes and disappears from the surface...

, since it was built in 1965. The river passes through karst

KARST

Kilometer-square Area Radio Synthesis Telescope is a Chinese telescope project to which FAST is a forerunner. KARST is a set of large spherical reflectors on karst landforms, which are bowlshaped limestone sinkholes named after the Kras region in Slovenia and Northern Italy. It will consist of...

 fields and most notably the town Otočac

Otocac

Otočac is a town in Lika, Croatia. It lies in the northwestern part of Lika, in the Gacka river valley. The population of the town is 4,354 as of 2001, with a total of 10,411 people within the municipality at large, the majority of whom are Croats ....

, and is known as a superb spot for trout

Trout

Trout is the name for a number of species of freshwater and saltwater fish belonging to the Salmoninae subfamily of the family Salmonidae. Salmon belong to the same family as trout. Most salmon species spend almost all their lives in salt water...

fishing.