Abstract
The tropical river basins of India are important because of the coastal ecosystem that they sustain and the densely populated economic zones that they serve. This study examines the recent trends in sediment load and also explores the influence of the climatic and human forcing mechanisms on the land–ocean fluvial systems. A large dataset comprised of the sediment time series of different timescale during the period 1986–87 to 2005–06 from 133 gauging stations spreading across tropical river basins of India was analyzed. Results indicate dramatic reductions in sediment load in the tropical river basins, which is beyond the fold of assignable natural variability. Around 88% (62%) of the total 133 gauging stations showed decline in sediment loads in the monsoon (non-monsoon) season. The significant downward trends outnumbered the corresponding upward trends in high proportions for both the seasons. Striking spatial coherence was observed among the significant trends, suggesting the presence of the cross-correlation among the sediment records. The regional trends, which account the spatial correlation, also indicated the widespread nature of the sediment declines. The rainfall, which is characterized by the non-significant decreasing trends and also frequent drought years, is the primary controller of the sediment loads for most of the river basins. It may be inferred that a little change in rainfall towards the deficit side leads to a significant reduction in sediment load. This is due to the diversion and storage of runoff to meet the manifold increases in water requirements for the agriculture and industry. Among the tropical rivers, the maximum reduction in sediment flux has taken place for the Normada River (−2.07 × 10 6 t/yr) due to the construction of dam. Although the sea level is rising, we speculate that the significant reduction in sediment loads may also have influenced the coastal erosion in recent years. The results of this study can be utilized for the sustainable management of the tropical river basins in the backdrop of a predicted erratic monsoon rainfall and the growing anthropogenic stresses.
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