Sharp change of the channel bar pattern within the river delta: Insights from modern Ganjiang River delta in Jiangxi province, China

Abstract

Channel bars (midchannel bars or point bars) in river deltas present challenges to water transportation; meanwhile, they provide a source of building materials, habitats, and farmlands, and they could become targets in hydrocarbon reservoirs. Researchers focus on the channel bar pattern in rivers, while ignoring the channel bar pattern in distributary channels. We have analyzed a sharp change of the channel bar pattern from the river to the delta. The integration of remote map analysis, fieldwork, and a flume experiment has resulted in a quantified sharp change in the channel bar pattern in the Ganjiang River delta and revealed a hydrodynamic mechanism. The result finds that distributary channels develop much finer-grained, smaller-scale, less channel bars with the lower width-length ratio, compared to the upstream river. Distributary channels develop more point bars and fewer midchannel bars than the upstream braided river, and they develop fewer point bars than the upstream meandering river. Sharply weakening bank erosions in distributary channels lead to the formation of the sharp change in the channel bar pattern from the upstream river, due to backwater effect-induced sharp gentling of landform gradient. The backwater effect impedes bank erosions in the distributary channels, but it promotes bank erosions in upstream rivers, which, in turn, enhances the sharp change of the channel bar pattern. We provide insights into channel bars’ growth in modern river deltas and provide new facies models for the river-delta system.