U radu su analizirani nizovi karakterističnih (minimalnih, srednjih i maksimalnih) godišnjih vodostaja izmjerenih na dvije vodomjerne postaje na Prošćanskom i Kozjak jezeru, dakle na dva najveća jezera u sustavu Plitvičkih jezera. Utvrđeno je izrazito različito ponašanje vodostaja do sredine devedesetih godina i poslije 1997. godine. U prvom podrazdoblju karakteristični su vodostaji bili u stalnom porastu, dok se u drugom nalaze u stalnom opadanju. Točno vrijeme kada je došlo do ovih naglih promjena nije bilo moguće utvrditi pošto su zbog Domovinskog rata bila prekinuta motrenja i uništena oprema od sredine 1991. do početka 1997. na Prošćanskom jezeru i 2001. na jezeru Kozjak. Analiza nizova karakterističnih (minimalnih, srednjih i maksimalnih) godišnjih protoka na vodomjernoj postaji Kozjak most ukazala je na postojanje dugogodišnjeg i zabrinjavajućeg trenda opadanja protoka iz jezera Kozjak. Trendovi opadanja karakterističnih protoka utvrđeni su i na profili Luketići na rijeci Korani. U radu su izračunati i srednji godišnji gubitci vode na potezu Korane od ušća Plitvice do vodomjerne postaje Luketići. Utvrđeno je da na tom profilu sve češće dolazi do sve dugotrajnijih presušivanja. Sve analize iznesene u članku ukazuju na potencijalno opasne hidrološke procese koji se zbivaju na jezerima unutar Nacionalnog parka..; The paper analyses a series of characteristic (minimum, mean and maximum) annual water levels measured at two hydrological stations on the Prošćansko and Kozjak lakes, i.e. on the two largest lakes in system of the Plitvice lakes. Between the periods until mid-1990s and after 1997, a marked difference in the behaviour of water level was determined. The first subperiod was characterized by a continuous increase in water levels, whereas there has been a continuous decrease in the second period. The accurate time when these sudden changes occurred could not be determined due to interrupted monitoring and destroyed equipment during the Homeland War from mid-1991 until the begin of 1997 on the Prošćansko lake, and until 2001 on the Kozjak lake. Analysis of the series of characteristic (minimum, mean and maximum) annual discharges at the hydrological station Kozjak Most indicated the existence of a long, alarming downward trend in the discharges from the Kozjak lake. Downward trends in the characteristic discharges were also determined for the section Luketići on the Korana river. Mean annual water losses for the section of the Korana river from the Plitvice mouth to the hydrological station Luketići were also calculated in the paper. Increasing occurrences of protracted drying up in this section were determined as well. All analyses presented in the paper indicate that potentially dangerous hydrological processes have been occurring in the lakes within the National Park.