Long-term monitoring (1960-2008) of the river-sediment transport in the Red River Watershed (Vietnam): temporal variability and dam-reservoir impact.

The Red River (China/Vietnam, A=155,000 km(2)) is a typical humid tropics river originating from the mountainous area of Yunnan Province in China. Based on information on daily discharge (Q) and suspended particulate matter (SPM) concentration between 1960-2008 for the SonTay gauging station (outlet of the River and entry to the Delta) provided by the National Institute IMHE-MONRE, the mean annual SPM flux was estimated at 90 Mt/yr, corresponding to a sediment yield of 600 t/km(2)/yr. The temporal variability of annual SPM fluxes (ranging from 24 to 200 Mt/yr) is strongly related to the interannual hydrological conditions. However, some years of high water flow were not associated with high sediment fluxes, especially after 1989 when the HoaBinh dam came into operation. Therefore, the median discharge pre- (3389 m(3)/s) and post 1989 (3495 m(3)/s) are similar indicating there was little or no change between both periods. Sediment rating curves (power law-type; SPM=aQ(b)) were fitted for both periods (1960-1989; 1990-2008). The analysis of the pre- and post-1989 sediment rating parameters (a, b) suggests a downshift of b-parameter values after 1989, attributed to a decrease of the sediment supply due to the commissioning of the HoaBinh dam. A single sediment rating curve derived from 1960-1989 data was used to simulate the annual variability of former sediment delivery, generating excellent cumulative flux estimates (error ~1%). In contrast, applying the same rating curve to the 1990-2008 data resulted in systematic and substantial (up to 109%) overestimation. This suggests that the HoaBinh dam reduces annual SPM delivery to the delta by half, implying important metal/metalloid storage behind the HoaBinh dam.