This paper describes the hydrological changes caused by inter-basin water transfer and the reservoir development on the hydrological regimes of two rivers. The Sabljaki Reservoir in the Zagorska Mrežnica River and the Bukovik Reservoir in the upper Dobra River began operation in 1959. Both are part of the hydroelectric power plant (HEPP) Gojak, whose installed capacity is 50 m3/s. Their water volumes at the spillway altitudes of 320·10 and 320·15 m a. s. l. are 3·3 × 106 and 0·24 × 106 m3 respectively. Both the Dobra and Mrežnica Rivers are losing, sinking and underground karst rivers. A 9376-m-long tunnel provides water from the Sabljaki Reservoir to the HEPP Gojak, which was constructed in the Lower Dobra River. The Sabljaki Reservoir is located in the Plaški karst polje, while the Bukovik Reservoir is located in the neighbouring Ogulin karst polje. The consequences of the inter-basin water transfer are strong and have caused abrupt changes in the hydrological regimes of the downstream sections of both rivers. At the same time, the construction and development of both the reservoirs have also caused hydrological changes to the upstream section of the Upper Dobra River. Copyright © 2010 John Wiley & Sons, Ltd.