This paper aims to reveal the reciprocal influence of Kurtun Dam and wastewaters from the settlements on the water quality in the stream HarAYit, NE Turkey. Several key water-quality indicators were measured: water temperature (T), pH, dissolved oxygen (DO), electrical conductivity, water hardness, chemical oxygen demand (COD), ammonium nitrogen (NH4 (+)-N), nitrite nitrogen (NO2 (-)-N), nitrate nitrogen (NO3 (-)-N), total Kjeldahl nitrogen (TKN), total nitrogen (TN), orthophosphate phosphorus (PO4 (3-)-P), and methylene blue active substances (MBAS). The monitoring and sampling studies were conducted every 15 days from March 2009 to February 2010 at two stations selected in the upstream and downstream of the Kurtun Dam. It was concluded that the Kurtun Dam Lake had a high-quality water in terms of T, pH, DO, COD, NH4 (+)-N, NO2 (-)-N and NO3 (-)-N values, but slightly polluted water with respect to TKN, PO4 (3-)-P, and MBAS according to the Turkish Water Pollution Control Regulation. The dam improved the stream water quality by increasing the DO concentration, and decreasing the NO2 (-)-N and PO4 (3-)-P concentrations thanks to its hydraulic residence time despite the wastewater discharge by the nearby settlements. However, the wastewater discharge deteriorated the stream water quality increasing the COD, NH4 (+)-N, NO3 (-)-N, and TN concentrations.