Abstract

Spatial distribution of bedload sediment grain size in rivers largely depends on geological composition of their drainage basins. The presented map of channel-forming bedload sediment of the Eastern European rivers shows the two main patterns. Firstly, different size of bedload sediment is associated with rivers draining different geomorphic and orographic provinces. Secondly, among lowland rivers there is a prominent trend of decreasing bedload sediment grain size from north to south. First tendency emphasizes strong control on river channel bedload size exerted by drainage basin geological and geomorphic structure, mainly bedrock lithology and dominant surface gradients. In mountainous areas characterized by erosion-resistant bedrocks and high surface gradients, all river channels are formed by coarse bedload sediments from gravels to boulders. Rivers draining upland plains composed of horizontally stratified solid sedimentary rocks are usually characterized by sandy-gravel or gravel-pebble bedload. In contrast, rivers flowing through lowlands composed of sands or sandy loams transport mainly sandy bedload sediment, with silt, silt-organic or sandy-silt bed material dominant in rivers of the most prominent depressions. The second trend can be explained by influence of periodic glaciations during the Pleistocene history of the Eastern Europe. Rivers with gravel or sand-gravel bedload sediment are widespread at western and northern part of the Eastern Europe where valleys are cut into the Middle and Late Pleistocene glacial deposits rich in coarse particles up to boulders. Widespread occurrence of silty and sandy-silty bedload in rivers of the southern part of the Eastern Europe is associated with fluvial reworking of loessy loams deposited under periglacial conditions along the southern margins of ice sheets and clayey marine deposits left by transgressions of the Black and Caspian seas. Rivers of central and eastern parts of the Russian plain commonly have sandy bedload sediment. Main sources of sandy particles are glaciofluvial deposits of the Pleistocene zandur plains and older alluvial sands left from the past river networks of the Russian plain.

Keywords: Bedload sediment, river channels, river valleys, bedrock types, history of geomorphic development, Eastern Europe