Erosional microtopography of the Nuporomaporo river [Hokkaido, Japan] floodplain.
1994
Takahashi G. | Sasa K.
A precise survey was carried out to clarify the geomorphological processes of the Nuporomaporo River floodplain located in the northern part of Hokkaido, Japan. Two types of channel course changes were recognized on the floodplain which is characterized by remarkable meandering, few riparian forests and few woody debris. The first type is the continual shifting of the channel course to the outer side and the neck cutoff. The second one is the abrupt change caused by the blockage of the main channel section with the accumulation of sediment in channel bends. Woody debris generally induces bedload deposition intermixing with itself, and possibly causes a channel course to change abruptly. Because of the small amount of woody debris in the Nuporomaporo floodplain, the major process of meander development is not the abrupt channel course change but the continuous shifting, which could develop channel bends to a high degree. Thus the small amount of woody debris is thought to be a reason for the remarkable meandering. Specific erosional features were observed adjacent to the current channel. They were "Semi-globular concavity" and "Trench-like groove". The concavity was considered to have been formed by both pipe flow which spouted through the stream bank and whirlpool formed over the concavity. The grooves were suggested to have a relationship with the dynamics of groundwater or throughflow. Water movement is one of the agents which form a floodplain. Further detailed investigations including the dynamics of underground water are needed to clarify the geomorphological formative process of a floodplain.
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