Effects of dams on water and sediment delivery to the sea by the Huanghe (Yellow River): The special role of Water-Sediment Modulation

Large dams on the Chinese Huanghe (Yellow River) have altered its water and sediment fluxes, suspended sediment concentration, grain sizes, and inter-annual patterns of water and sediment delivery to the sea. Sediment entrapment by the Sanmenxia and Xiaolangdi reserovirs along with increasing water consumption associated with dam regulation are mainly responsible for curtailed water and sediment discharges. After Xiaolangdi Reservoir was constructed in 1999, peak flows have decreased, with low flow (<2000 m ³/s) now dominating the Huanghe discharge most of the year. Since 2002, a managed water release system through Xiaolangdi Dam, known as Water-Sediment Modulation (WSM), has played a vital role in regulating the delivery of material from Huanghe to the sea. The WSM produces ∼50% of the annual sediment to the sea, of which 60% derives from scoured coarse sediment from the riverbed of the lower reaches. The suspended sediment concentration of Huanghe during operation of the WSM is 17.3 kg/m³, compared with just 6.9 kg/m³ in other times of the year. The WSM also leads to intense riverbed scouring in the lower reaches, which increases transport capacity and reduces flood risk. Sediment infilling in the Xiaolangdi Reservoir remains high, however, and riverbed scouring during the WSM has weakened since 2006. The Huanghe provides an example of management issues when large dams eventually lose their impoundment.