Taking as an example six main rivers that drain the western flank of the Eastern Carpathians, a conceptual model has been developed, according to which fluvial bed sediment bimodality can be explained by the overlapping of two grain size distribution curves of different origins.Thus, for Carpathian tributaries of the Siret, coarse gravel joins an unimodal distribution presenting a right skewness with enhanced downstream fining. The source of the coarse material distributions is autohtonous (by abrasion and hydraulic sorting mechanisms). A second distribution with a sandy mode is, in general, skewed to the left. The source of the second distribution is allohtonous (the quantity of sand that reaches the river-bed through the erosion of the hillslope basin terrains). The intersection of the two distributions occurs in the area of the 0·5-8 mm fractions, where, in fact, the right skewness (for gravel) and left skewness (for sand) histogram tails meet. This also explains the lack of particles in the 0·5-8 mm interval. For rivers where fine sediment sources are low, the 0·5-8 mm fractions have a higher proportion than the fractions under 1 mm.For the Siret River itself, bed sediment bimodality is greatly enhanced due to the fact that the second mode is more than 25% of the full sample. As opposed to its tributaries, the source of the first mode, of gravel, is allohtonous to the Siret river, generated by the massive input of coarse sediment through the Carpathian tributaries, while the second mode, of the sands, is local. In this case we can also observe that the two distributions of particles of different origins overlap in the 0·5-8 mm fraction domain, creating the illusion of 'particle lack' in the fluvial bed sediments.acquisition of a comprehensive database in order to better understand the diversity of situations in the field that may involve the process of river-bed material diminution. This opinion is shared by many authors (Sambrook Smith and Ferguson, 1996;Rice, 1998;Gomez et al., 2001) and we also sustain it.For 10 years we have focused our attention on rivers in the drainage basin of the Siret, an important affluent of the Danube in the Romanian territory. We took as examples the experience of many authors (Brierley and Hickin) in their research on downstream variation in grain size on a single river or a river sector; we also thought that a spatial approach of the variability of the river-bed material on many rivers in a river system of over 43 000 km 2 would bring an important understanding in this field. A similar approach has been taken by authors such as Yatsu (1955), Knighton (1980), Ibbeken and Schleyer (1991) and many others. This method proves to be difficult due to the fact that volumetric sampling in river gravel-beds is a significant stumbling block for those that study the phenomena. For instance, in the higher part of the Carpathian rivers that we have sampled, the weight of the sample in situ was more than 1000 kg, which implied an extraordinary effort for the team (see Figure 3 below).In conc...