Influence of stream channel characteristics on bed morphology and step-pool structure in a rhyolitic headstream watershed

The headstream area is an important channel segment for investigating the self-organization mechanism of a stream channel or the occurrence characteristics of stream bed morphology owing to its sediment source and low anthropogenic impact such as that from check dams or channel modifications. However, less work has been conducted on the transition of stream bed morphology in channels with very high gradients, unlike in step-pool channels. This study describes the field observation results of the stream bed morphological characteristics and the relationship between the step-pool structure and stream channel characteristics in the headstream areas of rhyolitic mountains. The transition of the stream bed morphology associated with different bed slopes was confirmed through field observations. The dominant stream bed morphology changed from a fall to a cascade to a step, from upstream to downstream. Furthermore, the relationship was different between the step formation processes (hydraulic control and keystone types) and the jamming state, which was caused either by the supply process of the step constituent boulder or the external force of the step. Mountainous streams are strongly affected by geology, and this tendency is likely to be more pronounced in headstream areas. Further, the factors influencing stream bed morphology or stream channel characteristics include the formation age of mountains and the climate, geology, and the absence of fault zones. Therefore, knowledge of the stream bed morphology of headstream areas under various geological and meteorological conditions is required.