The Buyuk Melen, Kucuk Melen. Aksu, and Ugursuyu rivers pass through the important Duzce watershed in northwestern Turkey, and discharge into the Black Sea. The Duzce watershed is projected to provide drinking water for Istanbul until 2040. Surface water was sampled during each season from 2001-2006 at four sites (Buyuk Melen, Kucuk Melen, Aksu and Ugursuyu) and analyzed for temperature (T), pH, electrical conductivity (EC), biological oxygen demand (BOD), dissolved oxygen (DO), total dissolved solids (TDS), total alkalinity (TA), total hardness (TH). and organic matter (pV). Seasonal differences were statistically evaluated according to Turkish Standard (TS) 266, and the European Union (EU) and World Health Organization (WHO) standards. BOD, TA, TDS, TH, and EC of the Buyuk Melen river; BOD, DO, TA, TDS, TH, and EC of the Aksu river, and PV, TDS, TH, and EC of the Kucuk Melen river showed significant seasonal differences. Concerning, these rivers, DO and T of Buyuk Melen; T of Aksu;, DO and T of Kucuk Melen; as well as DO and T of Ugursuyu showed highly significant seasonal differences. Although some of the parameters indicated second-class water quality during some seasons, overall, the water quality of these 4 rivers met the TS 266, EU, and WHO criteria.