Research conducted in the own catchment of the Warta river between Oborniki (km 206.3) and Skwierzyna (km 92.2) aimed at determining factors contributing to the self-purification of river water. The following factors were considered: land use, soil type, lake density, precipitation and air temperature over a 10-year period, as well as precipitation chemistry, river-bed morphology, and use of riverside zone. As shown by the results, the small proportion of arable lands with the large proportion of forests and lakes in the catchment, as well as the afforested river banks create favourable conditions for reducing the inflow of nutrients to the Warta water. It was also found that air temperature in the summer half-year considerably affects the concentration of Kjeldahl nitrogen, ammonia nitrogen, total phosphorus and phosphates in river water, while precipitation in the winter half-year contributes to the washout of nitrate nitrogen from the catchment.

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