Zróżnicowanie wypełnień holoceńskich starorzeczy Warty w Uniejowie
Diversity of the infillings of Holocene oxbow lakes of the Warta River in Uniejów

Summary/Abstract: The study area is located an extensive alluvial plain called the Uniejów Basin (Klatkowa, Załoba 1991). The oxbow lakes studied are located within the Warta River valley floor, in this region located at almost the same latitude as the level of the low terrace. Together they form a common morphological horizon (Forysiak, Kamiński 2011). It is diversified by numerous cut-off river channels. Based on cartographic materials and aerial photographs, an inventory of the cut-off channels was carried out. Preliminary corings (70) of organic sediments constituting the deposits of several oxbow lakes were then performed. Four of them were selected as a subject of further detailed analyses. The objects selected are distinguished by differing morphological and lithological features of their deposits. Oxbow lake No. 1 is entirely filled with sediments (up to the surface of the flood plain). A straight-line section (with the lowest total tortuosity) of approximately 55 m, with a width from approximately 6 m to 12 m was studied in detail. The highest recorded thickness of sediments amounts to 2.5 m. The contribution of organic matter in the sediments varies from 0.6 % to 33.6 %, and CaCO3 content in the sediments from 0 % to 4.36 %. Sedge peat from a depth of 2.31–2.36 m was dated for 1440 ± 60 years BP. Oxbow lake No. 2 is also filled with overbank deposits, but in contrast to the previous one, it is clearly visible in the land relief as a crescent-shaped depression in the surface of the Warta River flood terrace. The total tortuosity of the channel is 1.34. The entire oxbow lake with a length of 215 m and width from 7 to 20 m was subject to corings. The highest recorded thickness of sediments amounts to 1.69 m. The contribution of organic matter in the sediments varies from 0.5 % to 32.7 %. Content of CaCO3 in the sediments has values from 0 % to 0.52 %. Sandy peat deposited at a depth of 0.88–1.05 m was dated for 4140 ± 80 years BP. Oxbow lake No. 3 is partially filled with sediments. It is visible in the land relief as a quite deep depression in the surface of the flood plain. A straight-line section with a length of 56 m and width from 4 to 11 m was analysed in detail. The highest recorded thickness of sediments amounts to 1.1 m. The contribution of organic matter varies from 1.2 % to 26.3 %. Content of CaCO3 ranges from 0 % to 2.6 %. Oxbow lake No. 4 is partially filled with water, and is clearly visible in the land relief in the form of a crescent-shaped depression in the surface of the Warta River flood plain. The total tortuosity of the channel is 1.27. The oxbow lake has a length of approximately 360 m and width from 10 m to 32 m. The lowest recorded thickness of sediments filling the channel is 1.5 m. The contribution of organic matter in the sediments varies from 1 % to 12.5 %. Content of CaCO3 in the sediments ranges from 0 % to 4.16 %.

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