Depositional system of an anastomosing river with low organic content: an example from the Oława Valley, SW Poland

Abstract

Large amounts of organic matter (OM) deposited in a cohesive floodplain are considered a typical feature of a low-energy anastomosing river in a humid climate. Studies in the Oława Valley (Silesian Lowland) were designed to determine the forms of occurrence and the conditions of deposition of OM in relation to the main sedimentary sub-environments. The results show that the proportion of OM in the sediments of the Oława floodplain is low and usually assumes values typical of mineral soils (up to several percent). In particular, the deposits of floodbasins and natural levees have a low content of OM. The rapidly decaying OM there mainly accumulates in epipedons and is dominated by leaf and wood debris. A somewhat higher content of OM was found in the channel alluvia, especially in the rhythmically stratified sediments of the upper part of the river bars. Plant detritus in the form of species-rich wood, bark, remains of leaves, fruits and seeds was deposited directly from the tree canopy or together with mud in the last phase of the flood. The remains of diatoms, porifera and bryozoans are typical of sandier strata. Organic sediments from frequently flooded bars may contain more rhizodermis and epidermis of amphiphytes. The greatest potential for accumulation and long-term preservation of OM is found in abandoned channels. The palaeochannel fills mainly contain the remains of autochthonous wood, rhizodermis and epidermis. The remains of mosses, Cladocera and Chironomidae are characteristic of this sedimentary sub-environment. In general, however, the area occupied by peat swamps in the study area is disproportionately low (less than 1%) compared to the examples of anastomosing-river, organic-rich floodplains cited in the literature. In the studied sections, the Oława River thus represents a type of anastomosing system in temperate, humid areas that has been overlooked by classical classifications and is characterised by a low organic sediment content.