Abstract
This study clarifies relationship between characteristics of channel geomorphology of lowland river and of riparian forest, toward riparian forest management. We classified channels into braided (B), free-meandering (F), irregular-meandering preceding bottleneck (Irre), and meandering that are deep on one side but not the other (D). At bottlenecks there are in-growing meandering (Ing) and intrenched meandering channels (Int). On plains, the bed gradient is gentler and water level fluctuation is larger in order of (B), (Irre), (D) and (F). Channel-bed degradation after river improvement work is generally small in bottlenecks, where the ratio of pre-work length divided by post-work length is small; it is large on plains, where that ratio is larger. Channel-bed degradation is particularly great in (B), whose decrease in sediment transport due to river improvement work are larger than in the other types of channels. The channels were analyzed in terms of micro-topography, sedimentation, and stand distribution and structure. We categorized channels into two: (F) whose range and strength of disturbance are small and where stable development of stand is seen from the area adjoining the channel; and (B), where the area of disturbance is large but the strength of disturbance is moderate. Normally, little vegetation or only young trees are seen on the floodplains after river improvement work and double-row sandbars formed in (B). However, a decrease in flood frequency due to bed degradation and dam construction, and a reduction in the magnitude of disturbance due to streamline adjustment resulting from construction of the low-water revetment resulted in moderate stability of stand. It was thus thought that forest in (B) decay and re-grow repeatedly, and those in (F) develop into stable silva. These findings provide us with a key for better riparian forest management.
