AbstractWe observed the dynamic changes in the hyporheic zone, both in the vertical (0‐80 cm) and horizontal directions. Two stations with different hydrological conditions were compared, and subsequently related to two basic processes: infiltration of surface water into the sediments of the river bottom and repeated entry back to the surface stream. By comparing these two locations we attempted to show the existence of a continuous exchange of water between the surface of the river and the hyporheic zone in a process which we call ‘small bottom circulation’. At the same time, we also attempted to demonstrate that this process involves the self‐purification of the infiltrated water. We presume that self‐purification is realized both by means of physical processes (filtration, sedimentation and deposition) and by the biological decomposition of organic matter. Self‐purification of the infiltrated water between the stations which were compared are proved, above all, by the values of O2, BOD, COD, FPOM + CPOM, pH and conductivity. There are also remarkable differences in the qualitative composition in the macroinvertebrat