Difference in riverbed micromorphology of two world large lowland rivers: Implication of natural and human effects

Dunes of the lowermost Mississippi River (LmMR) in USA and the lowermost Yangtze River (LmYR) in China have been recently investigated in some reaches. Yet, our understanding of dune formation in large lowland alluvial rivers is still limited. This study compared characteristics of dunes in an 8-km long reach in the LmMR and a 35-km long reach of the LmYR to discern similarity and difference in dune development. The primary goal of the study was to assess morphological responses of dunes to natural factors and human interventions in these two world's large rivers. Multi-beam bathymetric measurements in the river reaches were used to quantify dune length, height, stoss and lee angles. The results showed that both the LmMR and LmYR had large (10–100 m in length), medium (5–10 m in length), and small dunes (0.6–5 m in length), but very large dunes (dune length >100 m) only developed in the LmYR. Large dunes were predominant, amounting to 88.4% in the LmMR and 54% in the LmYR. There was a close correlation between dune height and dune wavelength in both the LmMR and LmYR. Significant differences (p < 0.0001) in dune size, geometry (height/wavelength), and symmetrical characteristic (stoss slope/lee slope) were found between the LmMR and the LmYR. On average, dunes in the LmYR were longer and taller (20.08 m and 0.91m) than those in the LmMR (17.62 m and 0.87 m), which was likely owing to the human activities in the LmYR and the backwater effects in the LmMR, as well as the influence of river discharge and flow strength. Little difference was found in dune roughness between the LmMR and the LmYR, indicating that the flow resistance is similar in the two regions. The findings suggest that human activities are the dominant factor affecting dune morphology in large lowland rivers.