Located near the Black Sea coast of Turkey, Lake Terkos is one of the six main drinking water reservoirs of Metropolitan Istanbul, providing 25% of the water demand. With very limited industrial activities and only 1% of its catchment area being devoted to settlements, it is a relatively less populated and comparatively less destroyed catchment of the metropolis, in contrast to other drinking water reservoirs of Istanbul, the water quality of Lake Terkos is still of overall second class, implying that it can be used as a safe drinking water resource following a conventional water treatment scheme. The most critical group of pollutants was determined to be nutrients, which somewhat exceed Class 2. Presently, phosphorous load is found to originate from point sources of domestic origin, whereas approximately half of the nitrogen load arises from forests and meadows. An appropriate management strategy, concentrating especially on the future population densities, land use and an effective protection practice is needed to maintain the sustainability of the lake as a drinking water resource. (C) 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved.