Comparative analysis of sedimentation rates in meanders and natural levees of the Tisza River floodplain (Bereg Plain, Hungary) utilizing Cesium-137 (¹³⁷Cs) as a tracer.

The active floodplain of the Gyüre in the upper Tisza River on the Bereg Plain in Hungary has a lot of different landforms, like meanders and natural levees. To understand how the sedimentation deposits in the Tisza River changed over time, we need to know how sediment builds up in this area. The aim of this study is to assess the sedimentation rate and analyze the distribution of ¹³⁷Cs in two vertically oriented paleochannels (the second and third meanders of the active Gyüre floodplain). We also intended to assess the influence of diverse geomorphological sites on sediment deposition. Satellite images and a LiDAR-based digital terrain model of the study area were used to choose the sampling points in order to ensure that the description of floodplain morphology to depth is accurate

We excavated four profiles for this research to a depth of 100-130 cm. Two in meanders (GYM3, GYM2) and two on natural levees (GYFH3, GYF2). Soil samples were collected at every 2 cm interval for high resolution. We characterized sediment samples by laboratory measurements, including particle-size distribution (Köhn pipette method), pH, EC, CaCO₃, and humus content (Tyurin method). Sequential framework created by measuring ¹³⁷Cs by gamma spectrometry at the HUN-REN Institute for Nuclear Research.

The profiles' depth indicates different sedimentation processes between meanders and natural levees. The ¹³⁷Cs distribution in GYM3 and GYM2 (two meanders) indicates continuous vertical accumulation over the last approximately 70 years. We estimated the sedimentation rate for the third meander (GYM3) to be about 0.4 cm/year, and it is about 0.6 cm for the second meander (GYM2) annually. Conversely, the natural levee samples demonstrated minimal ¹³⁷Cs downward movement, and the peak concentration of ¹³⁷Cs was restricted to topsoil layers. There is no ¹³⁷Cs accumulation in the natural levee profile, and the maximum quantity in natural levee samples is probably driven by plant root activity and soil particles.

In conclusion, the results indicate that the processes of sedimentation in the floodplain vary greatly from one place to another. Meanders are active sediment reservoirs that hold sediment at a rate of 0.4 to 0.6 cm per year. Natural levees stay pretty stable even though they don't get a lot of new sediment. The different levels of ¹³⁷Cs in meanders and the high levels of ¹³⁷Cs on the surface of levees show how important it is to do high-resolution radio-tracer studies of the different ways sediment builds up in the upper Tisza River floodplain.

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Azin Rooien is funded by the Stipendium Hungaricum Scholarship under the joint executive program between Hungary and Iran.