Recent trends in the suspended sediment loads of the world's rivers

Abstract

The transport of sediment by rivers to the oceans represents an important pathway in the global geochemical cycle, a key component of the global denudation system, and an important measure of land degradation and the associated reduction in the global soil resource. With the growth of interest in global environmental change, it is important to consider the extent to which this important index of the functioning of the earth system is changing. Evidence from longer-term sediment load records indicates that river sediment fluxes are sensitive to many influences, including reservoir construction, land clearance and land use change, other forms of land disturbance, including mining activity, soil and water conservation measures and sediment control programmes, and climate change. Some of these influences cause sediment loads to increase, whilst others, namely, soil and water conservation and sediment control programmes, and reservoir construction cause decreased sediment fluxes. In many cases, it is difficult to disentangle the influence of climate change from that of other changes in catchment condition. Although there is clear evidence that the sediment loads of some rivers are changing, others show little evidence of any significant temporal trend. This could reflect either lack of change in the controlling factors or the buffering of any change by the river basin. To provide a preliminary assessment of current trends in the sediment loads of the world's rivers, longer-term records of annual sediment load and runoff were assembled for 145 major rivers. Simple trend analysis of these data indicated that ca. 50% of the sediment load records showed evidence of statistically significant upward or downward trends, with the majority evidencing declining loads. In the case of the annual runoff series, far fewer rivers (i.e. ca. 30%) showed evidence of statistically significant trends. The evidence afforded by the sample of the world's rivers indicates that reservoir construction is probably the most important influence on land–ocean sediment fluxes, but the influence of other controls resulting in increasing sediment loads could also be detected. A larger database, however, is required to provide a more definitive assessment of current trends in land–ocean sediment transfer by the world's rivers.