Why the Yellow River is less yellow
Background:The erosion, transport and re-deposition of sediments are key processes that modify the geomorphology of the Earth’s surface, and consequently impact the structure and function of ecosystems and human society. The Yellow River is the world’s largest carrier of fluvial sediment, but over the last 40 years this load has substantially decreased.
Question: The reduction in sediment is equivalent to 5% to 7% of the global total land-ocean sediment ﬂux; yet it is not clear why this has occurred.
Method: Here, we use the past 60 years of runoff and sediment load observations, with a Kaya identity approach, to investigate the possible drivers behind the observed trend in sediment. We study the middle reach of the Yellow River, i.e., the Loess Plateau, the source of nearly 90% of the sediment.
Results: We find that human activities have played a more important role than climate change in driving the reduction in sediment from the Loess Plateau during this period. Landscape engineering including terracing, and the construction of check dams and reservoirs was the main driver during the 1950s to 1990s, reducing sediment flux into the Yellow River from the Loess Plateau by about 3.37 Gt yr-1 (54% of the total reduction). Since 2000s, large-scale vegetation restoration projects emerged as the main contributor, reducing both runoff coefficient and suspended sediment load (a further 0.23 Gt yr-1, to give 57% of the total reduction). As check dams and reservoirs gradually silt up they lose their ability to trap sediment, and soil and water conservation in the Loess Plateau will increasingly have to rely on reducing soil erosion at source by making further improvements to land and vegetation management. Thus, maintaining a healthy and sustainable vegetated ecosystem is the key to effective soil and water conservation in the fragile Loess Plateau.
Conclutions: Our results should sound a warning bell for other areas of the world where deforestation and other land-use changes are exposing vulnerable soils to erosion. Without active soil conservation measures the sediment load across many of the world's major rivers will increase. This will result in not only the loss of a valuable natural resource, but also the premature demise of major civil engineering structures designed for irrigation and hydroelectric power generation.