Downstream variation in bed sediment size along the East Carpathian rivers: evidence of the role of sediment sources

Abstract

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...

Mining soil databases for landscape‐scale patterns in the abundance and size distribution of hillslope rock fragments

“…Visual inspection of cumulative (Figure ) and probability (Figures 7c–7f) distribution plots allowed us to categorize the distributions as unimodal or bimodal, with bimodal distributions containing pronounced plateaux in the cumulative curves, and low points or gaps in the probability curves. Our approach is similar to methods used to detect bimodality in studies of river bedload transport (Wilcock, ; Smith et al , ) and sediment provenance (Sun et al , ; Radoane et al , ). We inspected the original horizon data to avoid designating a distribution as bimodal because of a single large rock fragment; we also confirmed our mode determination using Gradistat (Blott and Pye, ), a grain‐size distribution plotting software that outputs the number of modes in a distribution.…”

“…Visual inspection of cumulative ( Figure 3) and probability (Figures , 7c-7f) distribution plots allowed us to categorize the distributions as unimodal or bimodal, with bimodal distributions containing pronounced plateaux in the cumulative curves, and low points or gaps in the probability curves. Our approach is similar to methods used to detect bimodality in studies of river bedload transport (Wilcock, 1993;Smith et al, 1997) and sediment provenance (Sun et al, 2002;Radoane et al, 2008). We inspected the original…”

Downstream fining in contrasting reaches of the sand‐bedded Yellow River

“…Studies have shown that the bed sediments of most natural rivers become finer grained in their downstream reaches (Sternberg, 1875; Knighton, 1980; Seal and Paola, 1995; Morris and Williams, 1999; Gomez et al , 2001; Frings, 2008). As a result of abrasion and sorting, rivers with gravel beds show strong downstream fining processes with their grain sizes decreasing exponentially with distance if there are no lateral inputs of coarse sediment from tributaries and valley sides (Mackin, 1948; Dawson, 1988; Dietrich et al , 1989; Wilcock and Southard, 1989; Paola et al , 1992; Hoey and Ferguson, 1994; Kodama, 1994; Paola and Seal, 1995; Ferguson et al , 1996; Rice and Church, 1998; Hoey and Bluck, 1999; Gomez et al , 2001; Radoane et al , 2008). Field data furthermore show that this downstream fining is always punctuated by significant tributaries and other lateral sediment sources, and thereby shows a sequence of sedimentary links within which fluvial fining processes operate and fining trends develop (Rice and Church, 1998; Rice, 1999).…”

Downstream patterns of bed material grain size in a large, lowland alluvial river subject to low sediment supply

“…These lateral sediment sources, where significant, bound sedimentary links along the main stem of a river such that median grain size increases and each link has its own exponential function of grain size decline with distance downstream [ Constantine et al , 2003; Knighton , 1980; Rice and Church , 1998; Rice , 1999], although there is clearly a hierarchy of influential tributaries [ Rice , 1998]. Others have identified a marked downstream transition in grain size from gravel to sand, which generally occurs over very short length scales [ Cui and Parker , 1998; Ferguson et al , 1996; Ferguson , 2003; Gomez et al , 2001; Ichim and Radoane , 1990; Knighton , 1999; Parker and Cui , 1998; Radoane et al , 2008; Sambrook Smith and Ferguson , 1995].…”