Sedimentation Processes in a Typical Riparian Zone Along the Yangtze Mainstream of the Three Gorges Reservoir

A typical transect of the riparian zone along the Yangtze mainstream of the middle Three Gorges Reservoir is selected todetermine the lateral distribution of sedimentation rates against elevation, and to explore the effect of the reservoir water levelregulation and the seasonal variation of suspended sediment concentrations of the Upper Yangtze River. It is found that the netsedimentation mainly takes place within the portions between 145-168 m. The cumulative net sediment depth ranges from 1.1-39.9cm during the 2010 survey and it decreases with the increase of the elevation. The average sediment depth is 14.9 cm between theelevations of 145-155 m, and 2.6 cm in the portions above. The sediment depth ranges from 3-80 cm during the 2013 field survey,indicating the severity of the sediment accretion during the initial period of dam operation. The riparian sediments become coarserwith the increase of the elevation, which indicates that the sandy fraction of the bulk sediment increases at a higher elevation. Thesediment grain-size composition at lower elevations is close to that of the suspended sediment, while that at a higher elevation iscoarser than the suspended sediment. The sedimentation in the riparian zone is closely related to the water level regulation induced bythe regular dam operation. During a wet season when the reservoir water level is low, a high fluvial suspended sediment yield leads toa significant sedimentation in the lower portions of the riparian zone. During the dry season when the fluvial suspended sedimentsupply is limited, much fewer sedimentation occurs at the upper portions. Relatively longer inundation duration also contributes to ahigher sediment depth in the lower portions of the riparian zone. It can thus be concluded that the sediment in the lower portions ofthe riparian zone is due to the fluvial suspended sediment yield during the wet season, while the sediment in the upper portions of theriparian zone originates from the bank erosion.