| Estuarine sediment flux is not only one of contents of Global Change Research, but also is a practical problem nearly related to estuarine engineering and estuarine environment. Aiming at sediment flux into ocean and taking Changjiang Estuary as background, several key problems are systematically studied with combined methods of GIS technique, sediment transport dynamics and hydrologic statistics. Mainly the problems are water and sediment fluxes of mid- to lower Changjiang River, nautical chart Depth Datum Levels of Changjiang Estuary, evolution process of Changjiang Estuary, sediment budget of Changjiang Estuary, sediment fluxes of Changjiang estuary, and so on.The runoff of Changjiang River increases from middle reach to lower reach. The yearly averaged runoff is 9.03 xlO'V in Datong Station (1951-1999), which isabout twice the amount in middle reach. Suspended sediment discharge decreases a little from middle reach to lower reach. The yearly averaged sediment discharge is 436 million tons in Datong (1953-1999). Comparing to suspended sediment, the amount of bedload is much small, which decreases form middle reach to lower reach. There aren't apparent trends of either decrease or increase for runoff flux hi Datong, but there is apparent decrease trend for sediment flux, especially since 1970s. Runoff changes monthly are closely related with the precipitation in the basin. Runoff in Datong Station mainly concentrates on the period from May to December and amounts to 71% of annual total runoff. The sediment discharge on flood seasons (from May to December) amounts to 85% in Datong Station.Calculations of theoretical depth datum (TDD) level are carried out with Matlab program language. The datum levels of 10 tidal stations in Changjiang Estuary are calculated with harmonic constants calculated with tidal level data in 1977 and the relationships among datum levels are discussed.Based on summarizing the definition, characters, application and development trend of GIS and DEM, the thought and method of calculating channel volumes and channel evolution analyses with GIS and DEM technique are brought forward. Withthis technique, silt-deposit amounts of several regions for over one hundred years are calculated, which separately are South Branch, South and North Channel, mouth bar area, offshore and Hangzhou Bay, etc. Moreover, the evolution regularities for different area under different hydrology conditions are discussed.There are so many frequently moving sandbars hi South Branch that local evolution often is very violent, but its total channel volume changes little. So the sediments from upper reach essentially pass through the region without detention.Channel volume of South Channel and North Channel usually change with reverse trends, which is closely related to the situation of bifurcation mouth. But the total volume of the two channels keeps equilibrium and changes little. Cumulative deposition trend is clearly showed in mouth bar area, and 6.8% of coming sediment is deposited here. Coming sediment mainly deposit in the submerged delta and Hangzhou Bay, and there is about 70% of sediment that go through Datong section into this region.Based on calculation of silt-deposit sediment, the mode of sediment budget of Changjiang Estuary is established for the first time. 10% of coming sediment is deposited in the channel from Datong to Xuliujing; 6.5% of it is deposited in North Branch; a little is eroded hi South Branch from Xuliujing to the mouth; 31% of it is deposited in the submerged delta; 40% of it is deposited hi Hangzhou Bay and its near sea; 4% of it is accreted to land; other sediment is mainly transported to the nearshore of Zhejiang and Fujian coast and thimbleful sediment is diffused into the blue water. Fluxes are nearly related with locations of sections. Sediment fluxes for some important sections hi Changjiang Estuary are calculated, thereinto that of Xuliujing is 406 million tons each year and that of the mouth is 370million tons each year.
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