Summary:

In this paper an attempt of estimating the self-purification processes in the Smortawa river was undertaken. The investigation was carried out at the natural and altered transects of the river between km 9+676 and km 7+105 in hydrological year 2004. At the examined stretch three measurement sections were located (km 9+676, km 8+030 i km 7+105), where the values of oxygen indicators (biochemical oxygen demand (BOD₅) and chemical oxygen demand (COD_(Mn) and COD_(Cr)) were measured. At the examined stretch of the river, the values of oxygen indicators decreased depending on the river’s course. The reduction of organic pollution of the Smortawa river at investigated transect was observed. Between km 9+676 and km 8+030, where the river bed is natural, wide, and rich in water plants, self-purification processes occurred significantly better than at the altered stretch.