Taking as an example main great rivers that drain the western flank of the Eastern Carpathians, a study regarding the current fluvial bed sediment has been carried out, in order to tackle, among others, such controversial problems as “downstream fining” and “downstream coarsening”, of river bed deposits’ bimodality, of the effect of the sediment sources in the basin. One of the study’s conclusions is that the river bed deposits’ bimodality can be explained, in the cases we studied, by the superimposing of two grain size distributions, of different origin.
For the East Carpathian Rivers (without the Siret River), the coarse particles are part of a unimodal distribution with a right skewness and a strong “downstream fining”. The source of these materials is autochthone (through the abrasion and hydraulic sorting of the bed material). For the same rivers there has been observed a second distribution with a 0.5-0.25 mm class mode and which generally has a left skewness. The source of the second distribution is allochthone (the quantity of sand that reaches the river bed through the erosion of the hillslope basin fields). The intersection of the two distributions occurs in the area of the 1-8 mm fractions where, in fact, the right skewness (for gravel) and left skewness (for sand) histograms tails meet. The idea that the 1-8 mm fractions would be in penury in the bed deposits, is false because for the rivers that have low sources of fine sediments in the basin the 1-8 mm fractions have a higher proportion that the fractions under 1 mm.For the Siret River, the bed sediment bimodality is highest because the second mode, that of the sands, represents 25 % of the full sample. Unlike the tributaries, the source of the first mode, that of the gravel, is foreign to the Siret (the massive input of coarse sediment through the Carpathian tributaries), while the second mode, of the sands, is local.