Severely Declining Suspended Sediment Concentration in the Heavily Dammed Changjiang Fluvial System

As a key component of global change, dam‐induced sediment reduction occurs in large rivers worldwide, which has profound implications on the fluvial systems. However, the systematic change of suspended sediment concentration (SSC) and its dynamic processes are not well known. We summarize typical SSC changes and propose a new sediment modeling framework for heavily dammed fluvial systems with the Changjiang (Yangtze River) as a background. We find that the fluvial SSC has declined by an order of magnitude, i.e., from ∼1.0 to ∼0.1 kg/m3, and even to ∼0.01 kg/m3 locally. The SSC distribution pattern along the mainstream has changed remarkably, with the sediment source/sink being partially reversed. Downstream of the Three Gorges Dam, the SSC recovery capacity gradually decreases with the sediment erosion quantity accumulated over time, and the SSC contribution rate of a linked large lake (Dongting) will change from negative (ca. −39%) to positive (ca. 17%), in the coming decades. Plain Language Summary Suspended sediment plays a key role in fluvial nutrient transport and aquatic ecological processes. Globally river damming has led to sediment discharge reduction to the sea, yet the systematic riverine concentration (SSC) changes and their mechanisms remain poorly understood. Herein, we propose a new modeling framework for reproducing and forecasting suspended sediment processes for dammed fluvial systems with the Changjiang as a reference. Results show a sediment source/sink reverse, downstream recovery capacity decrease and a change in river‐lake relationship under severe SSC decline. These findings offer essential guidelines for river management subject to super reservoirs, which have been built (e.g, the Three Gorges Reservoir) or planned in large rivers, such as the Congo and Amazon River. Key Points Suspended sediment concentration of the Changjiang River has decreased by an order of magnitude in recent 3 decades from ∼1.0 to ∼0.1 kg/m3 Sediment source/sink reverse partially and downstream recovery capacity decrease exponentially under the reservoir operation Predicted by a new sediment modeling framework, the river‐lake relationship in the fluvial system will change considerably in 2030s