Sedimentary record of 2010 and 2011 Warta River seasonal floods in the region of Poznań, Poland

The Warta River near Poznań (West Poland) represents a meandering lowland river modified by hydro-engineering projects. Recently, two large floods occurred during the summer of 2010 and the winter of 2011. Rapid response surveys were conducted to document erosion and sedimentary records of the floods (spatial extent, grain size, composition, grain microtextures). Follow-up studies, which were conducted during the two years after the floods, assessed postdepositional changes in the flood deposits. A thick sand layer formed that ranged from an average of 10–15 cm (summer) to 30–35 cm (winter), building natural levees, side bars and crevasse splays. The sand layers consisted of fine- and medium-grained sands that were well-sorted, fine skewed and sourced from the river channel. The longer but smaller winter flood produced thicker and coarser deposits at different sites compared with the summer flood. An organic-rich mud layer and algal mats, which were short-existing, were also detected on the floodplain. The study proved that the flood record on engineered rivers may be only in the sand bodies near the river channel and their preservation is mainly controlled by their thickness. A single site may not record all floods, as particular events tend to leave deposits in various places.