Summary:

Over a few past decades, some sections of the fifth-order, mountain Czarny Dunajec River, southern Poland, have been considerably modified by channelization or channel incision induced by gravel extraction. As a result, the contemporary river represents a variety of morphologies ranging from single-thread, incised or regulated channel to unmanaged, multi-thread channel. For twelve crosssections with 1 to 4 flow threads, hydromorphological river quality was assessed by four surveyors from the fields of fluvial geomorphology, river engineering and hydrobiology and compared with the abundance and diversity of fish fauna determined by electrofishing. Moreover, the variation in depth, velocity and bed material size was determined for each cross-section on the basis of measurements made at 1 m intervals. Average values of hydromorphological quality for the surveyed cross-sections ranged between 1.08 and 3.96, with the cross-sections with heavily island-braided morphology classified as representing high status (reference) conditions and those located in channelized river sections falling into Class 4. The increase in the number of low-flow channels within a cross-section was associated with increasing variation in depth, velocity and bed material size. Single-thread cross-sections hosted only 2 fish species and 13 specimens caught on average, whereas 3-4 species and 82 specimens on average were recorded in the crosssections with four low-flow channels. The reduced abundance and diversity of fish fauna characterised single-thread river sections located both upstream and downstream of the multi-thread channel reach. Results from regression analysis indicated the number of both fish species and specimens to increase linearly with increasing number of low-flow channels and variation in depth within a crosssection and exponentially with improving hydromorphological quality. This study shows that the simplification of flow pattern and the resultant aggravation of hydromorphological quality in some sections of the Czarny Dunajec, caused by human disturbances, is manifested in the remarkable impoverishment of fish communities. This indicates a need for increasing morphological complexity of the flow pattern and improving hydromorphological quality of the river that would be reflected in a restored high abundance and diversity of its fish fauna.