Unprecedented sedimentation in response to emerging cascade reservoirs in the upper Yangtze River Basin

Abstract

Anthropogenic interferences to large river basins are triggering unprecedented changes to earth surface processes. The upper reach of the Yangtze River Basin (UYRB) is a typical example, where a large number of mega dams and reservoirs (storage capacity >108 m3) have been, being, or will be constructed in clusters especially in the 21st Century. Limited studies have quantitatively demonstrated the impact of these reservoirs in trapping sediment; the spatial and temporal patterns of the responses of sediment flux and sedimentation have yet to be elucidated. We develop formulas to estimate basin-wide reservoir trapping efficiency typically for mega cascade reservoirs which better reflect the reality than traditional formulas. Results show that the sediment load at Yichang (downstream of the Three Gorges Reservoir or TGR) dropped below 1×108 t, smaller than the amount trapped in reservoirs upstream (1.6×108 t) for the first time in 2003, and it further decreased to 2% of the historical value since 2006. The overturn of sedimentation-flux relation also occurred at Pingshan (outlet of the Jinsha River Basin) at 2014, after the mega dams Xiluodu and Xiangjiaba were put into operation. The total annual sedimentation in UYRB increased from 3×108 t in 2001–2010 to 4.7×108 t in 2011–2015, meaning that 92% of the sediment was trapped; no considerable increase will be made in the future (after 2021) despite new reservoirs still being constructed. After 2016, the Jinsha River Basin became the most important trapper while the most unimportant contributor to the incoming sediment to the TGR where the annual sedimentation was apparently relieved (from 1.4×108 t before 2015 to 0.7×108 t after 2016). Serious channel erosion is expected to happen due to the unprecedented sediment shortage downstream of the cascade reservoirs, especially at Panzhihua, Pinsghan and Yichang.