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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]

Fluvial suspended sediment has multi-fold environmental implications and the study of the variation in suspended sediment load (SSL) of rivers is important both in environmental earth sciences and in river environmental management. Based on data collected for the upper Yellow River of China in the past 50-60 years, the purpose of this study is to elucidate the impact of human activity and climate change on SSL, thereby to provide some knowledge for the improvement of the drainage basin management. The results show that the SSL of the upper Yellow River exhibited a remarkable decreasing trend. A number of reservoirs trapped a considerable amount of sediment, resulting in a reduction in SSL at Toudaoguai station, the most downstream station of the upper Yellow River. The analyses based on Mann-Kendall'U and double-mass plot indicate some turning points, which were caused by the Liujiaxia and Longyangxia Reservoirs, two major reservoirs on the upper Yellow River. The implementation of soil and water conservation measures reduced the runoff coefficient, and therefore, the intensity of soil erosion. The climate change also played a role in reducing sediment yield. The increase in air temperature enhanced the evapo-transpiration and reduced the runoff, by which the SSL decreased. The decreased frequency of sand-dust storms reduced the amount of wind-blown, sand and dust to the river reaches located in desert, also reducing the SSL. Seven influencing variables are selected to describe the changing human activity and climate. As some of the influencing variables are strongly inter-correlated, the principle component regression was used to establish the relationship between SSL and the influencing variables. The squared multiple correlation coefficient is R = 0.823. Some further research is suggested with the minerals and pollutants related with the SSL. [ABSTRACT FROM AUTHOR]