The paper presents hydrological analyses of the source region of the Gacka River, one of the most famous sinking rivers in the world. Its course flows through karst field Gacko polje, which is located in the central part of Lika-Senj County and belongs to the part of the Dinaric karst. The analyses were obtained on the basis of discharges at the stations Gacka source, Sinac –Tonković vrelo source, Ličko Lešće – Pećina source, and Čovići - Gacka in the 1997-2016 period. The hydrological analyses were carried out based on annual, monthly and daily discharges. According to the statistical analysis performed, the average annual discharge value at the Gacka source station is 2.92 m3/s, Sinac (Tonković vrelo source) 3.06 m3/s, Ličko Lešće (Pećina source) 1.57 m3/s and at the Čovići station 12.05 m3/s. The hydrological analysis also included the analyses of recession, frequency and duration curves and the occurrence of low and high water. The Visual Basic macro tool was used for the recession analyses and estimate of groundwater recharge between two recession curves. At the end, the modelling of the discharge data was carried out using the Weka software, where by using the machine learning methodology two hydrological models were developed with different input variables of characteristic discharges.