Effects of flood abrasion of the Carpathian alluvial gravels

Floods occurred in the drainage basins of two large mountain tributaries of the Vistula—the Soła and the Skawa in 1996, 1997 and 2001. This paper shows changes in petrographical composition of gravel bars and characteristics of clasts in both rivers after the episodes of flood transport. The drainage basins of both rivers lie within the Carpathian mountains in their southern, headwater parts, and the in the Oświęcim Depression, belonging to the foreland, in the northern part. The Soła and Skawa are gravel-bed rivers which transport mostly sandstone and conglomerate gravels derived from erosion of flysch rocks. Both drainage basins share many characteristic features, the main difference being their hydrotechnical infrastructure. The Soła is regulated by three dams and the discharge below the dams is artificially controlled. There is no dam in the Skawa and its discharges are natural. Petrographical composition of gravels, roundness, shape and size of clasts were studied in 12 sites in the Soła valley and in 11 sites in the Skawa valley after each flood. More than 5000 clasts were studied in total. ometrical characteristics of the sediments changed markedly after the floods. Both increase and decrease in mean size of largest clasts in gravel bars were observed. The results point to the important role played by the tributaries, which control the characteristics of clasts in the main rivers. In the foreland, lateral supply is not so important, because the bedrock consists of argillaceous rocks which do not produce gravels. The processes observed there include selection of material and changes in its composition caused by the presence of dams. Conglomerates are the least resistant to flood transport among gravel components. The most resistant are quartz clasts, but they are present only in the finest size classes. The most persistent characteristic of the clasts is their shape. The influence of the dams on clasts characteristics, contrary to expectations, is only slight. It is visible only in the greater proportion of quartz clasts in the alluvium of the Soła where the dams are present.