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Abstract: Suspended sediment dynamics are still imperfectly understood, especially in the upper Yellow River where few studies on this issue have been conducted. This study analyzed the variability in suspended sediment concentration at different temporal scales (within-flood, monthly-seasonal and annual). Using a dataset up to 49years long (1954–2002) at Toudaoguai station, this paper provided insight into the characteristics of suspended sediment transport in the Ningxia-Inner Mongolia reaches of the upper Yellow River. The results show that, in a single flood event, changes in SSC are not simultaneous with changes in discharge, resulting in counterclockwise hysteretic loops, clockwise loops or figure-eight loops. The C–Q loops and sediment rating curves for different seasons had different characters. The diversiform C–Q loops in the Inner Mongolia reaches of the upper Yellow River may be mainly caused by the sediment inputs from tributaries, the different propagation velocity of sediment and water discharge, spatial and temporal storage–depletion processes of available sediment. At annual scale, the variations of annual mean discharge and SSC displayed a similar gradually deceasing trend, and the relation between water discharge and SSC during 1954–2002 could be described by a cubic polynomial function. Since the joint operation of Liujiaxia and Longyangxia reservoirs after 1986, the transport capacity of the Upper Yellow River has been much reduced. [Copyright &y& Elsevier]

Abstract: The suspended sediment deposition (SSD) in the alluvial reach between the Qingtongxia station and the Toudaoguai station of the upper Yellow River has shown a dramatic variation since 1952. However, its spatial and temporal changing pattern and driving factors are still not clear. This paper examines the detailed spatial and temporal variations of the SSD based on the annual suspended sediment load at five gaging stations located in this alluvial reach in 1952–2007. The mean annual SSDs in four sub-reaches (Qingtongxia–Shizuishan, Shizuishan–Bayangaole, Bayangaole–Sanhuhekou, and Sanhuhekou–Toudaoguai) were calculated and analyzed for different periods. The mean annual kilometric SSDs in 1952–2007 were −0.228∗105, 0.97∗105, 0.165∗105 and 0.006∗105 ta−1 km−1 in the four sub-reaches, respectively, with the highest SSD occurred in the Shizuishan–Bayangaole sub-reach. Results also suggested that SSD was mainly accumulated in 1952–1959 before major reservoirs were constructed and in 1986–2007 after three major reservoirs (Qingtongxia, Liujiaxia and Longyangxia) were constructed in the main stream. During 1960–1985, all sub-reaches except the Shizuishan–Bayangaole sub-reach experienced channel erosion. Although climate change may play some roles, the changes in SSD were mainly influenced by human activities. In particular, the water and soil conservation actions, such as the construction of check dams from 1958 in upper stream tributaries, intercepted significant amount of suspended sediment and cause the net erosion of the main channel in the 1960s. The operation of the Qingtongxia reservoir reduced the SSD in this downstream reach in the 1970s and 1980s because upper stream suspended sediment was trapped by the reservoir and reservoir-released relative clear water scoured the downstream channel. Although the Liujiaxia and Longyangxia reservoirs trapped relatively small amount of suspended sediment, they stored a large amount of water during the flood season, reducing the discharge and the erosion capability of the downstream flow. Therefore, this operation mode may increase the SSD in this river reach. In addition, with the gradual loss of the Qingtongxia reservoir''s capacity in sediment storage, more suspended sediment was released to the downstream channel, causing strong SSD in this reach after 1990. [Copyright &y& Elsevier]