Środowisko sedymentacji osadów przykorytowych rzek uregulowanych na przykładzie górnej Odry i górnej Wisły

Abstract

Presented study focuses on vertical accretion sediment deposited during the last 250 years in the upper reaches of the rivers Odra and Vistula (Southern Poland). In this study rates of vertical accretion of floodplain sediments were determined using chronological markers provided by the first occurrence of coal dust, plastic artefacts and 137Cs. The rivers Odra and Vistula are meandering sand-bed rivers with average discharges of 41 m3s–1 and 62 m3s–1 respectively. Both of the investigated river reaches are located within submountainous basins of southern Poland: the Odra River in the Raciborz Basin and the Vistula River in the Oswiecim Basin. The aerial extent of the catchment up to the investigated area averages 4666 km2 for the Odra and 5301 km2 for the Vistula River. The valley floor slope reaches respectively 0.4 and 0.3 m/km and the sediment transportation 322 000 t/year for the Odra and 312 000 t/year for the Vistula. Maximum discharges usually occur in July in association with summer rainfalls in mountainous headwater areas. The natural processes of erosion and sedimentation on both rivers were disturbed in the 19th century when the authorities started the river regulation program for navigation purposes. The Upper Odra River and Vistula land use is characterised by agriculture and some industrial centres. The valleys itself are used for agricultural purposes. Hard coal mining started in the middle of 18th century in Katowice in 1750 AD and in Ostrava in 1763 AD. Hence these dates provides the maximum age for coal dust supply to the tributaries. Floodplain sediments were examined at sites on the Odra and on the Vistula River. Pits were excavated adjacent to the channel up to the groundwater level (approximately 3 m below surface). Sediment samples were collected from each layer of alluvia. Grain size distribution was determined by sieving at 0.25 phi intervals. The presence of 137Cs was determined for the grain size finer than 0.4 phi in gamma ray spectrometer. The coal dust layers and artefacts were found by direct visual inspection in the field. The investigated floodplain deposits of the Vistula and Odra Rivers, deposited in the zone adjacent to the river channel, are dominated by vertically accreted sets of sand and silty sand bed represent a significant flood event. Study sites on both rivers provide evidence of increasing rates of vertical accretion during the last 50 years. The very high recent sedimentation rate in Vistula and Odra sites (1,7 to 6 cm/year) is comparable with observed at other European rivers.