Can Reservoir Regulation Along the Yellow River Be a Sustainable Way to Save a Sinking Delta?

Today's deltas are impacted negatively by (1) accelerated subsidence (e.g., from ground fluid extraction), (2) global eustatic sea level rise, and (3) decreased sediment supply, which increasingly starves these landforms of sediment necessary to sustain their footprint. This growing vulnerability threatens many megacities that have developed due to the rich resources offered by deltas and therefore urgently calls for efforts to maintain sustainability. The Yellow River of China is classic example of such a landform under threat and which requires human intervention to maintain its resilience. Since 2002, the Yellow River Conservancy Commission has enacted an annual water and sediment regulation scheme (WSRS) by coordinated operation of three large reservoirs in the mainstream. Here we evaluate the efficiency and sustainability of this man-made experiment on delta evolution. The impulsive delivery of muds and sands, within similar to 20 day intervals (averaged duration of the WSRS), did indeed move the present Yellow River delta from a destructive phase to an accretion phase. With continuous scouring, however, the downstream riverbed erosion efficiency has decreased, due to coarsening of surface bed material sediment. Concomitantly, sediment delivery has decreased, resulting in the present delta once again entering an erosive (destructive) phase, since 2014. From a perspective of delta restoration, the WSRS on the Yellow River is effective but potentially unsustainable. Restoring delta resilience necessitates an enhanced, coordinated effort, relying upon new sciences advances, rather than simply assuming channel scour will address the sediment deficit of the delta. Plain Language Summary Deltas are highly dynamic systems which lie on the interface between rivers and oceans. Due to accelerated sea level rise and terrestrial sediment decrease, over half of global deltas are presently under a growing risk of being submerged. How best to protect these important terrestrial surface has become an urgent problem to be resolved. With our manuscript, we describe the successes and unintended consequences learned from one of the largest man-made experiments in the world. Since 2002, the Yellow River Conservancy Committee started to release flushing water from the Xiaolangdi Reservoir annually to remove accumulated sediment from the reservoir and to erode the lower riverbed of the Yellow River. This man-made experiment did increase the quantity and particle size of sediment reaching the sea, and thereby nourishing the Yellow River delta successfully. With gradually eroding, however, the enhanced particle size has armored the lower riverbed, making it progressively difficult to be eroded by reservoir-released flushing water. Consequently, the sediment delivery from the Yellow River to the sea declined, resulting in that the Yellow River delta, has once again retreated landward since 2014.