Phosphorus is an element responsible for the eutrophication of inland and marine aquatic environments. This global environmental phenomenon represents a current problem causing the decrease in the quality of aquatic environments. Within the frame of resolving this problem, we conducted analyzes of physicochemical parameters (T °, pH, EC, COD, PO4 -3, Pt) on water taken from Oued Bounamoussa (northeastern Algeria) in an attempt to diagnose the eutrophication state of surface water of this watercourse. For this purpose, samples were taken on a seasonal basis during the periods of October 2013, January 2014, April 2014 and July 2014 from ten selected stations. The analysis results show that the temperature values are high in summer with an average of around 30 ° C and low in winter to about 09 ° C. The pH varies between 6.14 and 8.14, with an electrical conductivity content ranging between 266 μs/cm and 4006 μs/cm. An increase in the COD of up to 910 mg/l during July 2014 is witnessed. The eutrophication situation of the environment can be observed by the high values of orthophosphate and total phosphorus that exceed the standards of the surface water’s quality. In general terms, water quality of Oued Bounamoussa that was affected by this phenomenon, is bad which causes an ecosystem disturbance.