Istanbul, the largest metropolis of Turkey and one of the most crowded metropolises in the world, is facing the risk of water scarcity. Analyses indicated that one of the alternative solutions coping with this problem is water transfer from a watershed that is located outside the administrative boundaries of Istanbul. Due to its water potential and relatively less degraded water quality Buyuk Melen Watershed was considered to be the most feasible option. As the result of analyses, State Hydraulic Works (DSI) has planned construction of a reservoir in the Buyuk Melen Watershed to supply the required amount of water for Istanbul also in low flow seasons. Since the reservoir will be located at the downstream region of the watershed, its water quality will be affected by human-induced activities, thus measures need to be taken to prevent pollution prior to reservoir construction. Quantifying the response of the planned reservoir to external pollution loads is an important step in the planning and management of the watershed. Mathematical modelling is a useful tool for estimating the future water quality and understanding the possible responses of the reservoir to various pollution loads. In this study, a water quality model capable of simulating hydrodynamics, transport and water quality in reservoirs is used for preliminary estimation of possible impacts of several management options in the watershed. Model results indicated that, all of the point sources in the watershed must be controlled by advanced wastewater treatment and should be diverted from the streams and the planned reservoir. Model results also indicated that reduction of agricultural diffuse loads by 30-40% is expected to have a perceptible improvement of reservoir water quality.