Résumé
Mechanical erosion and sediment transfers depend strongly on many natural parameters related to topography, climate and land cover [1]. Additionally, anthropogenic activities may affect sediment supplies to estuaries, deltas and the coastal zone [2]. The Red River (China/Vietnam, A=155 000 km²) is a typical tropical humid river originating from the mountainous area of the Yunnan Province in China. Based on daily discharges (Q) and suspended particulate matter (SPM) concentrations between 1960-2007 at SonTay gauging station (outlet of the river and entry to the Delta) provided by the national institute IMHE-MONRE, the mean annual SPM fluxes was estimated to 90 Mt/yr, corresponding to a sediment yield of 600 t/km²/yr, similar to the Ganges/Brahmaputra system [3], and probably due to same controlling factors. 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 did not account for high sediment fluxes, especially after 1989 when the HoaBinh dam was operated. Sediment rating curves (power law-type; SPM=aQb) were fitted for both periods (1960-1989; 1990-2007). The analysis of the pre- and post-1989 sediment rating parameters (a, b) suggests a downshift of a-parameter values after 1989, attributed to decreased sediment supply [4]. 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 <-5%). In contrast, applying the same rating curve to the 1986-2007 data resulted in systematic, important (up to 97%) overestimation. This suggests that the HoaBinh dam reduces annual SPM delivery to the delta by half, implying changes in nutrient and contaminant transport.