Abstract: Sediment source is an important information that connects particles from hill slopes and small watersheds to river channels. In order to reveal the basic law of sediment transport and deposition in the riparian zone of the Three Gorges Reservoir, we investigated the horizontal and vertical variation of particle size distribution and ¹³⁷Cs activity among sediment samples collected in the study area, and analyzed the implication of such variations on sediment sources. Results showed that median particle size (D₅₀) of sediments gradually became finer along the flow direction and stabilized in general in Zhongxian County and its lower reach; while the ¹³⁷Cs activity of sediment samples rose along the flow direction but was not as stable as particle size in the downstream of Zhongxian County. Sediments collected around H=148 m had the largest particle size and the lowest ¹³⁷Cs activity compared to samples from other altitudes. Except for this river-adjacent area, particle size grew larger with the increase of altitude when the soil could be eroded easily in the upper hill slope; on the contrary, particle size became smaller with the increase of altitude when the upper hill slope was used for forest, indicating slight soil erosion. Therefore, vertical variation of particle size was greatly influenced by the land use of the hill slope above the riparian zone. In contrast, vertical variation of ¹³⁷Cs activity in three sampling transects were all falling at first and rising later with the increase of altitude despite the different land use of upper hill slope. On the basis of these variations, it could be judged that sediment deposited at the riparian zone in the fluctuant backwater area was sourced mostly from the Jinsha River and the Jialing River. In the permanent backwater area, coarse sediment with a grain diameter ≥62 μm in the riparian zone had distinct sources compared to fine sediment whose grain diameter was <62 μm. Coarse sediments from the Wujiang River deposited at the reach from Fuling to Renjia. In the reach from Renjia to the Three Gorges Dam, coarse deposited sediment sourced dominantly from soil erosion in the tributaries within the Three Gorges Reservoir basin. All potential sources including the Jinsha River, the Jialing River, the Wujiang River and soil erosion within the basin all showed significant contributions to the fine sediments in the permanent backwater area. In addition, wave erosion redistributed deposited sediments. Surface particles could be eroded into the water and deposit on the locations with lower altitudes.

Key words: sediment sources, tracing analysis, particle size, cesium-137, riparian zone, the Three Gorges Reservoir