China's Yangtze river is being dammed at Three Gorges, and this structure will alter the fluvial regime and cause
Present sediment provenance and dispersal regime will be altered, and before Dam closure it is urgent to acquire a sediment database by sampling along the entire Yangtze drainage basin. The current study (first year) is to focus on the lower Yangtze drainage basin, where one hundred fluvial transects were particularly surveyed by sampling with an interval of 10 km. Together with the sampling, various fluvial morphologies were examined to indicate:
- river channel modification below the Dam,
- reduced freshwater to the lower Yangtze,
- saltwater incursion into estuarine lowland, and
- accelerated coastal erosion along shoreline.
- lower Yangtze fluvial regime, as presented by braided river channel in 2-4 wide and 10-20 m deep, and with gentle riverbed slop ranging from 0.5-1.0 x 10-5;
- fluvial topography, including longitudinal bar, gravel shoal and stepped flood plains;
- rock exposures as mostly marked by pre-Quaternary carbonates and vermiculated red earth of early to middle Pleistocene, meaning
- prevailed neo-tectonic rise relative to river channel incision during that time;
- elevated water stage of flood level from 3-5 m above mean sea level in estuary to ~8 m in upstream, as recorded by stepped fluvial banks and marks on rock exposures; and
- natural hazard, including slide and slumping and mud flow.
In addition, our one-hundred samples indicate that the mean grain size of the lower Yangtze river channel ranges generally from 0.03 to 0.30 mm, and tends to be finning in size downstream towards the estuary. In addition, the mean grain size distribution is closely associated with river channel patterns, indicating that medium to fine sands occur in relative straight river channel and fine sand to clayey silt in curvy river channels, where longitudinal sand bars usually prevail. The above sedimentological database will certainly become the primary "pre-Dam" sediment gauge available for "post-Dam" studies of the Yangtze system.
Acknowledgement: This project is funded by US National Geographical Society (Grant no. #6693-00)