Spatial and temporal variation of in-reach suspended sediment dynamics along the mainstem of Changjiang (Yangtze River), China

摘要

Four major reaches of Chiangjang (Yangtze River) were analyzed for the period 1956-2007 in terms of long-term trends in suspended sediment gain and loss in each reach, the relation of input and output to within-reach sediment fluxes, and the impacts of dam construction on reach sediment dynamics. The temporal and spatial patterns of within-reach sediment dynamics correspond closely with river morphology and also reflect basin conditions over the past few decades; thus they are conditioned by coeval trends in climate, hydrology, and land use. The study period encompasses both unusually wet years (e.g., 1998) and unusually dry periods (e.g., 2006). There are suggestive coincidences between changes in the suspended sediment regime and innovations in land use, sediment mining, and dams. The relatively strong statistical relation between sediment input and output for each of the four study reaches, and weak relations between storage and output, imply that most sediment supplied to each reach is delivered from upstream and that relatively small amounts of sediment are recruited within each reach. Cumulative departure plots reveal epicycles of sediment exchange varying from 15 to 40 years, implying a characteristic fine sediment storage time on the order of decades. Shorter epicycles (5-8 years) are commonly related to sediment influx associated with major floods.