The paper discusses the general characteristics of the Boljunčica river basin and course, and the Boljunčica reservoir in Istria. Boljunčica is a torrent river located in the karst area. Available flow and water level data for measuring stations Brus, Katun, dam Letaj, Letaj output and Čepić were processed, using basic statistical data processing, probability of occurrence, intra-annual distribution, analysis of trends, frequency and durability of daily flows. The paper analyzes the impact of the construction of the Boljunčica reservoir on the water regime downstream of the reservoir or Letaj dam. A comparison of inflows into the reservoir and outflows from it was performed to show the existence of loss from the floodplain. Based on the data from the reservoir and the Čepić station, a trend of decreasing maximum water levels is observed due to the increase in the capacity of the abyss in the floodplain of the reservoir. There is also a problem of sediment accumulation in the floodplain, which reduces the capacity of the reservoir by raising the bottom of the reservoir, and creating pressure on the dam. By comparing inflows into and outflows from the reservoir, the obtained data show that almost 3/4 of the inflowing water is lost due to sinking in the floodplain of the reservoir. The lowest upstream station Čepić, after the construction of the reservoir, has flows of the order of two times smaller than in the state before construction, but the flows in both periods were small. The average flows of watercourses are very small due to the large surface of the basin where there is no abstraction and drainage of water outside the basin.