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

The extraction 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.