Recent adjustments of pool-riffle distribution along the channels in the Pearl River Delta, China

Abstract

The adjustments of pool-riffle distribution along the deltaic channels in the Pearl River Delta (PRD) under the combined effects of human interventions and natural evolution are examined in the three periods, namely 1952–1977 period, 1977–1999 period, and 1999–2014 period, corresponding to the different dominant factors. The residual depths along the channel thalweg and their spatial autocorrelation are introduced to quantify the pool-riffle distribution. The results show that the average pool depths of the deltaic channels increased significantly and the percentages of longitudinal length occupied by riffles decreased significantly by large-scale sand excavation during the 1977–1999 period and by sediment input decrease during the 1999–2014 period. It is highlighted sand excavation has a greater impact on the pool-riffle distribution of the North River channel and sediment input decrease has a greater impact on the pool-riffle distribution of the West River channel. The changes in spatial autocorrelation of pool-riffle sequence reveal a dramatic decrease of the average pool spacing after sand excavation, reflecting a rearrangement in the pattern of pool-riffle sequence. Moreover, the rearrangement process of the North River channel is ahead of that of the West River channel. In addition, the inhomogeneity indexes of deltaic channels are decreasing during the study period, indicating a deterioration of aquatic habitats diversity. It is anticipated that these results are useful for river management and utilization in the PRD and other river deltas worldwide that are subject to large-scale human interventions.