The degradation of river channels may occur downstream of hydraulic structures or may be a consequence of the uncontrolled exploitation of bed material or of the bed levelling. Such a process is especially visible in mountain streams because local degradation causes also local aggradation, which considerably affects the water-flow capacity of their channels. One of the factors that may be used for evaluating the equilibrium conditions of a river channel is the grain-size composition of bedload material. The grain-size composition changes both in the longitudinal profile and the cross-sections of the channel over the whole length of the degraded sector. The present paper addresses this problem using the Targaniczanka stream (Małopolska Province, Poland) as an example. The grain-size composition of the bed material was investigated along a 480 m channel sector and its variability was determined. The samples of bedload were collected by different ways: 9 samples were obtained by conventional method from the surface and/or subsurface layer, and 3 samples were obtained by freezing method using liquid nitrogen. It was found that the sorting of the bed material along the stream differs from that in the cross-sections, which is important for bedload transport or sediment deposition in those sections. The calculations were performed using the Meyer-Peter and Mueller modified formulas.

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