The Yelkoma Lagoon located at Ceyhan River Delta has been significantly deteriorated due to dam construction activities that cause lower freshwater inputs. Compared to the Mediterranean (similar to 40 ppt), high salinity of the Yelkoma Lagoon (up to 116.0 ppt), has adversely affected the ecological life in the lagoon as evidenced by the number of fish deaths. In addition to less freshwater input, high evaporation rates and structure of the inlet channel between the lagoon and the Mediterranean are determined as major causes of high salinities. In this study, we applied End-Member Mixing Analysis to calculate the monthly amount of freshwater that should be contributed from the Ceyhan River to the Yelkoma Lagoon to decrease salinity and ensure sustainability of ecological life and fisheries. As a first stage of the study, a conceptual hydrological model of the Yelkoma Lagoon has been developed based on water volume and its salinity. A bathymetric map of the lagoon was produced allowing measurement of the area and volume of each sector in the lagoon. Monthly salinities were obtained from field surveys for all components of mixing equations. The optimum salinity of the lagoon water has been foreseen to be between the salinity of the Ceyhan River and the Mediterranean. Based on the conceptual model and using a mass balance equation, contributions of saline and freshwater end-members were calculated. As a result of mixing calculations, monthly required flow-rates of freshwater from the Ceyhan River to the Yellcoma Lagoon were determined to be in the range of 0.199 m(3)/s to 3.698 m(3)/s. The calculated freshwater has been given to the lagoon since March 2009, and positive developments have been observed in terms of ecological life.