Estimate of cumulative sediment trapping by multiple reservoirs in large river basins: An example of the Yangtze River basin

Most existing models are unable to model large spatial pattern of reservoir sedimentation due to the difficulty in accounting for trapping by upstream reservoirs in a multiple reservoir system. In this study we developed and applied a framework on 1358 of large and medium-sized reservoirs (≥ 107 maximum storage capacity) for calculating reservoir sedimentation rates in the multi-reservoir Yangtze River system while accounting for the effect of reduced sediment input due to upstream traps. We further used statistical inferences to assess the sedimentation rates of remaining 42,000 smaller reservoirs. Our results indicate that annual sediment accumulated in the Yangtze reservoirs is approximately 691 (± 93.7) million tons (Mt), 669 (± 89.1) Mt of which is trapped by 1358 large and medium-sized reservoirs and 22 (± 4.6) Mt is trapped by smaller reservoirs. Despite the large amount of sediment trapped by reservoirs, the reduction in sediment load at outlet (Datong station) was merely 305 Mt over the last 60 years. The difference may reflect uncertainties in estimates; but it also indicates the important discrepancy between the estimate of the current rate of sediment sequestration in reservoirs and the estimate of the reduction in the land–ocean sediment flux. We further estimated a mean annual rate of storage loss of 5.3 × 108 m3 yr− 1; but against the world trend, the Yangtze River is now losing reservoir capacity much lower than new capacity is being constructed.