The Gacka is a river located in the Lika region of central Croatia.

Because for a large part of its course it is a subterranean river, estimates of its length vary.

[1] It has been regulated by man to flow into an aqueduct in Gusić polje (known as Tunel Gacka-Gusić Jezero[2]) and then into the Adriatic Sea through the hydroelectric installation HE Senj near Sveti Juraj.

[3][4] The river passes through karst fields and most notably the town of Otočac, and its headwaters are known as a popular locale for trout fishing.

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