On The Study Of Drainage Basin, Paleoclimate And Anthropogenic Activities: Mechanism, Process, Index System And Methodology

This study examines the characteristics of sediment rating parameters recorded at various hydrological gauging stations in the Yangtze Basin, in relation to their controls. Our findings indicate that the distribution of parameters is closely associated with river channel morphology of the selected reaches. High b-value (>1.600) versus low log(a)-value (<-4.000) occur in the upper rock-confined valleys and characterize high unit-stream power, mostly controlled by steep gradients. Low b-value (<0.900, in general) and high log(a)-value (>-1.000) occur in the middle and lower Yangtze Basin associated with low-slope meandering reaches, and are taken to imply channel-bed aggradation of these low-stream power reaches. Of note are the higher b-value (0.900-1.600) versus lower log(a)-value (-4.000 - -1.000) that characterize the reaches between Yichang and Xinchang, immediately below the Three-Gorges valley. These values indicate channel erosion and bed instability resultant on changes in channel gradient from the upstream steep valley to downstream low slope flood-plain settings. Accompanying these changes are differences in channel morphology, i.e. upstream confined, V-shaped and downstream broad U-shaped channel patterns. There has been an apparent increase in channel bed erodibility of the middle and lower Yangtze in the past 40 years. This inference is based on sediment rating parameters from various hydrological gauging stations that record increasing b-value versus decreasing log(a)-value over that time. Analysis of the sediment load data also reveals a good correlation between the changes in sediment rating curve parameters and reduction of annual sediment budget (4.70×10~8 t/a to 3.50×10~8 t/a, from 1950s to 1990s), largely due to artificial damming of the Yangtze and subsequent sediment load depletion through siltation in the large-scale Dongting Lake located in the middle Yangtze reach. Short-term deviations from the general trends in the sediment rating parameters are related to hydroclimatic events. Extreme low b-values versus high log(a)-values signify the major flood years, while the opposite indicate drought events. When compared with riversfrom other climate settings, it is evident that wide range of values of the Yangtze rating parameters can reflect the huge discharge driven by the monsoon precipitation of the eastern China.