Buyukcekmece Lake located north of the Marmara Sea coast of Turkey is the third largest water resources among the six main reservoirs of a megacity Istanbul, providing 17% water demand. This study aims to investigate the water quality changes in Buyukcekmece Lake and to classify it in accordance to Turkish Water Pollution Control Regulation for inland surface waters, and assess the impacts of the land-use practices, population density, and settlement in the Buyukcekmece watershed. Physical, chemical, and biological quality of Büyükçekmece Lake has been investigated between December 2007 and June 2008 by collecting samples from the Lake and four streams that feed Büyükçekmece Lake. Land use profile maps of the water basin were created by using ARC-GIS to ensure the quality of drinking water. It was determined that the water quality of Büyükçekmece Lake was still of overall second class, implying that it can be used as a safe drinking water resource following a conventional water treatment scheme; however, it is moving toward the third class. Remarkable portion of the pollution in water basin was attributed to domestic wastewater discharges. Agricultural areas that occupy more than half of the water basin have also adversely affected the water quality.