Twentieth-century channel change on the Dunajec River, southern Poland: Patterns, causes and controls

Abstract

Spatial and temporal patterns of the twentieth-century channel changes of the Dunajec, the second largest river of the Polish Carpathians, are analysed using data from six historical maps, hand-auger drillings in paleochannels and hydrometric data from 10 gauging stations. In its lower course, the river was considerably narrowed, shortened and embanked between the 1880s and the 1920s. Here, bed degradation commenced in the late 19th-century and has so far resulted in 3.1 m of channel incision. In the middle river course, little change occurred in mountain reaches where the Dunajec is confined by valley sides and where bedrock exposures prevent channel incision. In the reaches within intramontane basins, channelization works carried out in the 1950s–1970s considerably narrowed the river and transformed its multi-thread channel into a single-thread, straight channel. Rapid bed degradation induced by the works resulted in up to 2 m of channel incision over the second half of the century. Upstream progression of bed degradation into the adjacent mountain reach was hindered by bedrock exposures at its downstream end. In the upper river course, reaches with the deeply incised channels (up to 3.5 m) prevail over a few past decades in response to gravel exploitation, river channelization and a reduction in catchment sediment supply. More scarce are vertically stable reaches, which owe their stability to low unit stream power in wide, unmanaged reaches and to fixing of the channel bed by drop structures in channelized reaches. This study shows that despite the overall tendency to incision over the 20th century, considerable differences in the timing and course of channel changes occurred between particular river reaches due to variable human impact and local geological and geomorphological conditions. The main phase of channel incision took place progressively later in the upstream direction. However, upstream progression of bed degradation from the already incised downstream reaches was precluded by the Rożnów Dam and bedrock-controlled reaches. Where bed material flushed out from an incising upstream reach had been introduced into an artificially narrowed channel of high transport capacity, it was easily transferred downstream and could not restore the downstream reach to its pre-channelization vertical position.