Gacka is a
river located in the
Lika region in central
Croatia. Because for a large part of its course it is a
subterranean river, 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 into an aqueduct in Gusic polje (known as Tunel Gacka-Gusic Jezero) and then into the
Adriatic Sea through a
hydroelectric installation HE Senj near
Sveti Juraj. HE Senj takes inflow from Gacka and
Lika, since it was built in 1965. The river passes through
karst fields and most notably the town
Otocac, and its headwaters are known as a superb spot for
trout fishing.