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This paper discusses successive changes in the evolution of the fluvial system observed throughout the Bladzikowo Formation at the Mrzezino key site in northern Poland. The study focuses on the development of fluvial sedimentation during Marine Isotope Stage 7. The sedimentary record of the Pleistocene at the study site shows a well-developed sequence that is over 30 m thick. The Bladzikowo Formation shows a quick evolution of the fluvial system from a high-energy sandy braided river, to a meandering sandy river and, finally, to a low-energy meandering/anastomosing river with significant suspension settling. This transition is explained as a decrease in flow energy, increase in aggradation ratio and expansion of accommodation space during deposition time. A change in the fluvial system has not been manifested in the variations of the plant cover. Palynological analysis indicates that climate conditions were generally stable, cool and humid (boreal). Sediment transport occurred towards the N-NW, through a valley with a NW-SE axis. Infrared optically stimulated luminescence (IR-OSL) dating shows the sequence formed from 225.6 +/- 17.2 to 210.5 +/- 16.6 ka. Well preserved fluvial cycles and 35 m of thickness suggest the intensive accumulation of the Bladzikowo Formation with decreasing ratio of lateral erosion. It suggests an increasing influence of base level rise on evolution of fluvial system, mostly during MIS 7. (C) 2017 Elsevier Ltd and INQUA. All rights reserved.