The Sukhna Lake of Chandigarh City is a man-made lake situated in the foothills of the Shivalik range in the north-east corner of the city. This urban lake is one of the prime tourist places of the 'City Beautiful' and is mainly used for recreational purposes like boating, morning and evening strolls, exercises, food plaza and sightseeing. The catchment of the lake is mainly hilly and erosion prone, with the Sukhna wildlife sanctuary comprising a major portion of it. The water quality index of the lake water and its dependence on catchment characteristics has been studied. By using the National Sanitation Foundation Water Quality Index (NSFWQI) and Overall Index of Pollution (OIP), the results of the water quality assessment have found the lake water as having 'good' and 'acceptable' quality respectively based on past seven years' data. During the last few years, construction activities in the catchment area have speeded up and a few invasive alien plant species have come up in the lake. Inflow of untreated domestic waste water from nearby villages in the catchment, particularly during the rainy season, seems to be the main reason for the weed problem in the lake. Strict enforcement of ban on new construction activities and preventing the release of untreated domestic waste water from the villages located in the catchment are the absolute necessary steps for maintaining and improving the lake water quality.