Watershed models, that enable the quantification of current and future pollution loading impacts, are essential tools to address the functions and conflicts faced in watershed planning and management. In this study, the Watershed Modeling System (WMS) version 7.1 was used for the delineation of boundaries of Koycegiz Lake-Dalyan Lagoon watershed located in the southwest of Turkey at the Mediterranean Sea coast. A Digital Elevation Model (DEM) was created for one of the major streams of the watershed, namely, Kargicak Creek by using WMS, and DEM data were further used to extract stream networks and delineate the watershed boundaries. Typical properties like drainage areas, characteristic length and slope of sub-drainage areas have also been determined to be used as model inputs in hydrological and diffuse pollution modeling. Besides, run-off hydrographs for the sub-drainages have been calculated using the Rational Method, which produces valuable data for calculating the time variable inflow and input pollution loads to be further utilized in the future water quality models of the Creek. Application of WMS in the study has shown that, it is capable to visualize the results in establishing watershed management strategies.