The impact of the construction and management of a reservoir on a polluted stream in a semi-arid region has been highlighted using physico-chemical indicators. The Tafna river (Algeria) collects heavily polluted wastewater from the town of Maghnia and from several factories. Organic pollution comes from domestic sewage and from food industries. Ore treatment industries discharge very acidic wastewater. Temperature, pH, conductivity, chloride, dissolved oxygen, biological oxygen demand (BOD), and nitrogen compounds have been surveyed between 1996 and 2001, before and after the construction of the dam. The change in each parameter is compared between sites located upstream and downstream from the dam.The spatio-temporal evolution of the organic pollution indicators was highlighted by ANOVA and principal component analysis. By enhancing sedimentation of the organic matter, the reservoir makes possible a significant decline in the BOD downstream of the dam, but increases NH4 concentration at the downstream sites. The pollution is diluted during high flows and dam releases. Conversely, during low flow periods, mineralization of organic matter is higher in the lower Tafna. Copyright © 2004 John Wiley & Sons, Ltd.