Variation in surface bed material along a mountain river modified by gravel extraction and channelization, the Czarny Dunajec, Polish Carpathians

Longitudinal changes in the grain size of bar sediments of the mountain Czarny Dunajec, southern Poland, were investigated to determine the impact of human activity in the river on depositional conditions in its channel. The grain size of surface bed material was established on 47 gravel bars along an 18-km-long river reach without tributaries, in which some sections were modified over the past few decades by channelization or gravel extraction and the resultant channel incision. A downstream fining trend of bar sediments was determined from samples taken at the sites with close-to-average river width and a vertically stable channel and used as reference for the other samples. In the deeply incised, upper part of the reach, bar gravels are markedly coarser than the reference grain size. In the narrow, channelized section in the middle part of the reach, bar sediments exhibit better-than-average sorting and change from coarser gradation to one similar to that of the reference grain size along the section. In the wide, multithread channel in the lower part of the reach, bar gravels are distinctly finer than the reference grain size. A similar pattern of downstream variation in the study reach was determined for the river competence on the basis of critical bed shear stress required to entrain D95 particles of the bar gravels. traction of larger particles from the channel bed in the upper part of the study reach facilitated entrainment of exposed finer grains, hence inducing rapid bed degradation. At the same time, the concentration of flood flows in an increasingly narrow and deep channel must have increased their competence, enabling a delivery of the coarse particles previously typical of the upstream reach. The middle section has been channelized to prevent sediment delivery to a downstream-located dam reservoir. However, it actually operates as a conveyor belt, transferring downstream the bed material flushed out from the upstream, incising river section. As low unit stream power and high channel-form roughness facilitate sediment deposition in the unmanaged, lower part of the reach, it functions as a sediment sink and may reduce bed material delivery to the dam reservoir.