In this study, water quality and pollution status of Buyukcekmece Dam Lake, which is the second largest drinking water resources of Istanbul (Turkey), and its influent streams (Karasu, Izzettin, Eskice, Ahlat, Beylikcayi, Cekmece, Cakmakli and Tahtakopru) were analyzed. For this purpose, some physicochemical parameters, nutrient concentrations and phytoplankton composition were investigated at the lake and its streams. Water samples were collected seasonally from nine sampling sites by using Nansen bottles in May 2017, August 2017, November 2017 and February 2018. Phytoplankton was identified in reference to the literature, including several comprehensive reviews on the subject. Phytoplankton composition of the lake and its feeding streams consists of Bacillariophyta (22), Charophyta (6), Chlorophyta (14), Cryptophyta (2), Cyanobacteria (8), Euglenozoa (8), Miozoa (2) and Ochrophyta (1) division members. As a result of measurements, the minimum and maximum values of some physicochemical parameters and nutrients were as follows; dissolved oxygen (2.01-8.42 mg L-1), pH (7.32-8.85), nitrite (0.014-2.790 mg L-1) and orthophosphate (0.846-69.726 mu g L-1). The results indicated that the basin of Buyukcekmece Dam Lake shows close feature to eutrophic conditions. It is required that, Buyukcekmece Dam Lake and its creeks should be taken under protection continuously for improving its water quality by relevant authorities.