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Abstract: In 2000 and 2003 before the closure of 3-Gorges Dam, numerous sediment samples were taken from the middle-lower Yangtze River channel to examine sediment transport processes and associated hydromorphological nature of the river. Analytical results show that the riverbeds consist mostly of medium to coarse sands and gravelly sands, and fine sand occurs locally, especially near the river coast. The results further indicate a downstream fining trend in riverbed sediment from Yichang to the river mouth, totaling about 1900 km long with 12 sediment zones (I-XII). These were identified as alternate coarse- and fine-grained sediment on the riverbed, although the zonation of I–III below Three-Gorges Dam site is weaker. The mode of sediment transport in the river is dominated by saltation (20–80%), followed by bed-load transport with 3–15%; transport by suspension is quite low. Grain-sizes associated with hydrological parameters have greater values in the Jingjiang Reaches (from Yichang to Chenglingji; unit stream power: 5–18 N m−1s−1, boundary shear stress: 14 Nm−2 and mean flow velocity: 2–3.2 ms−1), whereas the values obtained from Chenglingji downstream are considerably low (<5 N m−1s−1, 1–4 Nm−2 and <0.7–1.5 ms−1). These values, when compared with on-site measured velocity of the ADP flow column, revealed the erosive riverbed sediment transport in the Jingjiang Reaches, and the accumulative riverbed transport downstream, from Wuhan to the river coast. Hydrological parameters together with distribution of grain-sizes indicate a coarsening riverbed in the Jingjiang river, largely because damming peaked since the last half-century. This corroborates the weakening sediment zonation in the Jingjiang Reaches, which is expected to extend further downstream towards the river coast in response to the potential impact of 3-Gorges Dam in the coming decades. [Copyright &y& Elsevier]

Abstract: This paper examines the characteristics of historical floods and associated monsoon precipitation in the Yangtze Catchment, with specific attention given to the middle and lower Yangtze basin. Based on in situ field observations and historical discharge, flood duration, and land-use information, etc, we propose a number of flood characteristics of the basin: 1) floods have occurred more frequently after 1950s, however, changes in discharge can only partially explain the occurrence of the floods; 2) spectral analysis on the annual mean water discharge during 1860–1985 in Hankou hydrological gauging station of the middle Yangtze River reveals 16.4-year and 2.5-year flood recurrences, which become shortened from what has been recorded in the Chinese historical literature on millennial time scale; 3) floods after 1950s tend to have higher water levels (both mean and peak) and longer duration than before, but with remarkably limited inundation area; and 4) a series of flood waves generated by monsoon precipitation in 1998, initiated from the middle Yangtze, highlights the basin-wide climate hazards with a recurrence of around 60 years. Intensifying anthropogenic activity in the last century, including deforestation, dyking, and lake–coast reclamation etc. that results directly in riverbeds aggradation and shrinkage of lake water area in the middle Yangtze basin, are the key causes for recently human-induced floods in the basin. Threatened land property in the basin reminds us to harmonize with the river and provide more space for flooding. [Copyright &y& Elsevier]