On the location Križ potok near the village Lokve is planned construction of dam and reservoir which could be used for the purpose of water supply of Gorski Kotar and part of Istria and coastal areas. Also reservoir can be used for the reception of high water waves to protect Lokve village located below the dam. Planned reservoir could be used also for hydroelectric purposes and connecting to the system of hydroelectric power plant Tribalj. In this thesis is made preliminary design of the dam and associated accumulation at the site Križ potok near the Lokve village. Based on the certain factors which determine the type of dam, on a given location is selected a concrete gravity dam. This conceptual design gives specific location of the dam, type of dam with all necessary facilities. Also, design gives disposition of accumulation in space and its size. Conceptual design contains all the necessary stability calculations for concrete gravity dam that had been made in the traditional way and with use of the software package CADAM. In thesis are also performed calculations for related facilities which are necessary for the proper functioning of the dam, such as outlet with a lock chamber and spillway of practical profile on the dam. The dimensions of the tunnel used for evacuation of the water during the construction of the dam are also defined within the designs. The thesis also describes the geological and seismic conditions prevailing on the location of the construction of the dam, while the hydrological data required for the calculations are taken from the study on the possibilities of water usage of Križ potok for the purpose of water supply.