The impact of large to extreme flood events on floodplain evolution of the middle and lower reaches of the Yangtze River, China

Abstract

Fluvial floodplain evolution is poorly understood despite its importance in sediment deposition, the carbon cycle, and the unique ecosystems it represents. Here we present how different magnitude and frequency of river floods contribute to floodplain evolution by analyzing three profiles of the middle and lower reaches of the Yangtze River. To investigate the formation of the Yangtze flood deposits we determine the percentage of flood events with different magnitude that can be identified from sediment cores and analyze their contribution to floodplain deposition. Our study demonstrates that magnitude of flood events strongly controls the identifiability of flood event deposition, and therefore the evolution of floodplains. Analyses show that detectability of flood layers resulted from large floods is remarkably influenced by sedimentation rates within the floodplain. High occurrence frequency of large to extreme flood events tend to correspond to high sedimentation rates, regardless of decreasing suspended sediment loads over the wet season. This implies that frequent large to extreme floods can counterbalance the decrease in suspended sediment load to maintain a high sedimentation rate in the floodplain. High flood stage is a principal factor that accounts for acceleration of floodplain sedimentation induced by large to extreme floods in the middle and lower Yangtze River. However, acceleration of sedimentation is limited by the quantity of sediment supply.