Initial stages and evolution of recently abandoned meanders revealed by multi-proxy methods in the Odra River (Czech Republic)

Oxbow lakes represent important geomorphic and depositional units of meandering river systems, whose rates of sediment accumulation may significantly exceed those of surrounding floodplains. Natural and artificial oxbow lakes may differ considerably, although both can offer unique sedimentary records of recent past events in river catchments. We provide an insight into the initial stages of oxbow lake evolution in terms of morphology, dynamics, sediment accumulation rates, sediment architecture and switching between sediment source areas. The study is based on a combination of geomorphic observations, multi-proxy stratigraphic analysis and geochemistry of sediment cores in four abandoned meanders of the Odra River, in the northeastern part of the Czech Republic. Two of the meanders were formed in non-regulated parts of the floodplain following a flood in 2010. In addition, two abandoned meanders formed by the artificial shortening of an upstream section of the Odra River were studied in order to compare processes in both sections. Several site-specific factors controlled the sedimentation patterns. In the naturally abandoned meanders, the initial stages of oxbow lake succession were characterised by extremely high sediment accumulation rates (up to 32 cm/yr), which caused the Stará Bělá abandoned meander to completely fill up over the course of several years. Relative to natural oxbow lakes, artificial lake successions revealed much slower sedimentation rates and higher levels of organic sedimentation and eutrophic conditions. The accumulation rates were influenced by both local (oxbow lake size, position and geometry) and catchment-wide factors (sediment yield). The annual occurrence of low-magnitude floods was an important factor in the natural reaches of the Odra River. The geochemical composition of sediments in naturally and anthropogenically formed abandoned meanders also differed due to the fact that the river tributaries drain two geologically contrasting areas, the Variscan Bohemian Massif and the Cretaceous-Cenozoic Outer Western Carpathians, respectively.