Grain-size variability of river-bed sediments in mountain areas (the Gemenea and Slătioara rivers, Eastern Carpathians)

Abstract

Rivers in mountain areas have alluvial beds largely consisting of coarse alluvia (blocks, cobble, gravel) mobilized and transported downstream only during exceptional hydrological phenomena. Their distribution influences river-bed stability, sediment transport rates and high-water levels, because alluvial deposits define bed roughness. As a rule, especially in large rivers, the size of alluvia deposited on river-beds gradually decreases downstream (downstream fining), as the river transport capacity is reduced. This tendency can be interrupted by discontinuities, caused by the lateral input of alluvia or by anthropogenic interventions. The aim of this paper is to analyze the spatial distribution of the size of bed material along two small rivers, Gemenea and Slătioara, located in the northeastern part of the Eastern Carpathians, Romania. The phenomena of grain-size fining or coarsening of the material deposited along these river-beds are of low intensity. The high degree of connectivity between river-bed and hillslopes and scarps of meadow terraces has generally resulted in unimodal distributions centered on the coarse fraction (gravel, cobble and blocks, over 60% of the total), whereas the fine fraction (sands) is below 1%. A slight tendency of bimodality begins to emerge towards the lower course of the Gemenea river profile (sand increases to slightly over 5% of the total bed material). This occurs with meadow development and the reduction of the connectivity between river-bed and adjacent hillslopes. Hence the conclusion that lateral input and longitudinal sorting of particles are among the main causes of bed deposits bimodality in our study area.