Deltaic responses to dam regulation on river system: Example of the Yellow River, China

Abstract:

Construction and operation of large dams in river basins have trapped large volumes of freshwater and sediment, which not only alters the natural seasonal rhythm of river hydrological cycles, but also creates a disconnection between rivers and their deltas. As a result, the water and sediment discharged to the coastal ocean have been greatly reduced, which triggers profound responses in coastal region including delta destruction, accelerated rise of relative sea level and changes in coastal primary production. The Yellow River has been a well documented system with significant impacts of dam regulation. Recent sediment load and freshwater (2002-2012) delivered to the sea have been reduced to 0.16 billion tones per year and 17.9 km³/yr, approximately 13% and 34% of those in 1950s-1960s, a period without significant dam impacts. Dam interception and dam-facilitated water regulation play a dominated role in reducing the flux of water and sediment to the sea, as well as in changing the grain-size composition of sediment. Consequently, the process of estuarine sediment dynamics has changed and the delta has recently been converted into a destructive phase with strong coastal erosion due to insufficient rive sediment supply although the accretion of the active delta lobe was evident because of rapid local deposition of coarsening river sediment around the river mouth. The delta coast erosion has thus become a major source for sediment transport in the Bohai Sea and even to those in the Yellow Sea given the critical role of monsoonal climate on coastal resuspension and coastal currents. Delta erosion and subsidence have therefore accelerated the rate of relative sea-level rise, considerably higher than the global mean, which has put the mega-delta to be at risk. In addition, recent works have identified two peaks of chlorophyll-a within annual cycle in the delta coastal region, one of which is closely associated with the river delivery of nutrients transferred with freshwater and sediment. This implied a significant decrease in coastal primary production resulting from the reduction of river water and sediment. Therefore, the coastal ecosystem in the delta region is expected to be greatly altered given the combined effects of decreasing primary production and destruction of coastal wetland due to the enhanced coastal erosion.