This study describes the sediment transportation and its relating phenomena typical in mountain streams. In a stream with the step-pool system, the sediment transportation depends greatly on sediment storage in the pools. The destruction of armor coat results in the destroy of step-pool system, which causes a Change of stream morphology. In addition, a meandering channel, the characteristics of which accords with the degree of the flood, is formed.
Stream channels do not change so much in the conditions the armor coat can not be destructed even if a flood comes. In such cases, the sediment transportation depend greatly on the sediment storage in the step-pool system. In the case that the destruction of armor coat occurs, stream channels change very much.
Debris flow brings a great change to the stream channel in the areas of its running-out; deep erosion in the occurrence areas and rapid aggradation in the ceasing area. In the deposition area, a new stream channel is formed due to the eroding action of water flow after the event.