The effects of river regime changes in the Changjiang Estuary on hydrodynamics and salinity intrusion in the past 60 years I. River regime changes

River regime is an essential factor of hydrodynamics and salinity intrusion in estuaries. In this paper, the digitized sea chart of the Changjiang Estuary in 1950s and 1970s, and measured data in 2012 were used to analyze the river regime changes of the Changjiang Estuary since 1950s. The Changjiang Estuary is a bifurcated estuary, which had two-order bifurcations in 1950s and the situation of the three-order bifurcations and four outlets into the sea was formed since 1970s. Comparing the Changjiang Estuary in 1970s and in 1950s, the volumes of upper reaches、middle reaches and lower reaches of the North Branch had changed by -64.13×10⁶,-306.60×10⁶ and -639.27×10⁶ m³, the rates were -16.30%,-22.74% and -25.69%, the volume of the North Branch decreased significantly because of the serious siltation; the volumes of upper reaches、middle reaches and lower reaches of the South Branch had changed by -28.61×10⁶,-35.69×10⁶ and 126.43×10⁶ m³, the rates were -1.30%,-2.12% and 4.36%; the volumes of upper reaches and lower reaches of the North Channel had changed by 109.21×10⁶ and -797.14×10⁶ m³, the rates were 5.01% and -15.25%, the volume of lower reaches decreased significantly because of the siltation of Chongming Shoal and Hengsha Shoal; the volumes of upper reaches、north lower reaches and south lower reaches of the South Channel had changed by -238.95×10⁶, 203.58×10⁶ and 153.34×10⁶ m³, the rates were -8.96%, 6.85% and 3.26%, the volume of upper reaches decreased because of the siltation, the volume of northern lower reaches had increased because the cutting off of Tongsha Shoal increased the depth of water. Comparing the Changjiang Estuary in 2012 and in 1970s, the volumes of upper reaches、middle reaches and lower reaches of the North Branch had changed by -199.06×10⁶,-504.61×10⁶ and -654.12×10⁶ m³, the rates were -60.45%,-48.44% and -35.38%, the volume of the North Branch decreased significantly because of the serious siltation and reclamation; the volumes of upper reaches、middle reaches and lower reaches of the South Branch had changed by 92.34×10⁶, 193.01×10⁶ and -163.62×10⁶ m³, the rates were 4.24%, 11.73% and -5.40%; the volumes of upper reaches and lower reaches of the North Channel had changed -154.64×10⁶ and -511.79×10⁶ m³, the rates were -6.75% and -11.55%, the volumes had decreased because of the reclamation of Qingcaosha Reservoir and Hengsha eastern tidal flat; the volumes of upper reaches、north lower reaches and south lower reaches of the South Channel had changed by 136.39×10⁶,-658.28×10⁶ and -1 266.11×10⁶ m³, the rates were 5.62%,-20.73% and -26.06%, the volume of upper reaches had increased because the channel in the upper reaches became deeper and the volume of lower reaches had decreased because of the siltation and reclamation of Jiuduansha and Nanhui tidal flat.