Sediment cores from river dams as flood archives

Studies of sediment cores originating from a dam of the River Mulde near Bitterfeld (Saxony-Anhalt, Germany) confirmed that river dams act as excellent archives of flood events. The records were performed by significant changes of the composition and quality of suspended matter and sediment. To combine the investigated sediment core with a reliable time scale, suitable reference data (e.g. radionuclides, geochemical markers) were included into the study. Subsequently, flow data of the Mulde (from 1975 up to now) were used to assign specific floods (>300 m3/s) to the event layers detected in the sediment cores. In addition to this, further time markers were inspected towards a more reliable adjustment of the time scale. In detail we made use of the 137Cs activity maximum (caused by Chernobyl fallout), as well as of the drop in organic pollution (following the collapse of East Germany's industrial sector 1989/90), and finally of the decrease in the concentration levels of elements and isotopes (a consequence of the reduced activities of mines and metallurgical plants in the River Mulde catchment). Exemplarily we present the results for a short sediment core, which originates from the Friedersdorf basin of the Mulde river dam (sampled in September 2002). The most recent such layer occurring in this sediment core was caused by the flood in August 2002. This event proved to be an outstanding flood time marker with regard to its thickness and geochemical properties.