Planform characteristics and development of inter- channel wetlands in a gravel-bed anastomosing river, Maqu Reach of the Upper Yellow River

摘要

Both interchannel wetlands and multi-channels are crucial geomorphologic units in an anastomosing river system. Planform characteristics and development of interchannel wetlands and multi-channels control the characteristics of anastomosing rivers. To understand the role that interchannel wetlands play in the development of anastomosing rivers, a study was conducted on the Maqu Reach of the Upper Yellow River(MRUYR), a gravel-bed anastomosing river characterized by highly developed interchannel wetlands and anabranches. Geomorphologic units in the studied reach were extracted from high resolution satellite imagery in Google Earth. The size distributions of interchannel wetlands and interchannel wetland clusters(IWCs), a special combination of interchannel wetlands and anabranches, were investigated. Geomorphologic parameters, including the ratio of interchannel wetland area to IWC area(P), shoreline density(D_l), and node density(D_n) were used to analyze planform characteristics of IWCs and the development of multi-channels in the studied reach. The results suggest that small or middle sized interchannel wetlands and large or mega sized IWCs are more common at the study site. The area of IWC(S_u) is highly correlated with other geomorphologic parameters. P increases with increasing S_u, and the upper limit is about 80%, which indicates that the development of interchannel wetlands and anabranches in the IWC is in the equilibrium stage. In contrast, D_l and D_n show a tendency to decrease with increasing S u due to diverse evolution processes in IWCs with different sizes. There are three main reasons leading to the formation of IWCs: varying stream power due to the meandering principal channel; development of the river corridor due to the weakening of geologic structure control; and high stability of interchannel wetlands due to conservation by shoreline vegetation.