In this study, the effects of contaminant elements in the surface water of 4 rivers (Buyuk Melen, Kucuk Melen, Aksu, and Ugursuyu) within the Duzce Watershed, which provides drinking water to Istanbul in the Western Black Sea region of Turkey, were determined. The water quality parameters were measured during all 4 seasons in the months of March, June, September, and December 2001-2006 at these 4 different sites. The parameters included calcium (Ca2+), magnesium (Mg2+), nitrite (NO2-), nitrate (NO3-), ammonium (NH4+), sulfate (SO42-), chloride (Cl-), and phosphate (PO4-3) ions. Seasonal changes in water quality were evaluated using analysis of variance. Ca2+, Mg2+, Cl, NO2-, and SO42- in the Buyuk Melen river, Ca2+, NO3-, and PO4 in Aksu river, Ca2+, Mg2+, Cl-, NO2-, NH4+, PO43-, and SO42- in Kucuk Melen river, and Mg2+ and PO43- in Ugursuyu river were found to vary seasonally (ANOVA, p <0.05). Furthermore, NO3- and PO43- in Buyuk Melen river, Na+ in Kucuk Melen river, as well as Na+ and SO42- in Ugursuyu river showed highly significant seasonal differences (ANOVA, p < 0.001). The maximum values of each chemical parameter in streams within the Duzce Watershed basin were recorded and evaluated according to Turkish Standard (TS) 266 as well as European Union (EU) and World Health Organization (WHO) criteria.