Channel initiation by surface and subsurface flows in a steep catchment of the Akaishi Mountains, Japan

Channel initiation, which is a key factor in the evolution of mountain landforms, is caused by a combination of various hydrogeomorphic processes. We modeled the channel initiation in steep mountains on the basis of the physical mechanism for sediment transport by surface and subsurface flows. Field investigations and Geographic Information Systems (GIS) analysis in the Higashi-gouchi catchment of central Japan showed that our model can well explain the area–slope relationship in steep and highly incised subcatchments, in which surface flow and shallow underground water would be the dominant flow components. In contrast, the area–slope relationship is not clear in gentler subcatchments, in which the contribution of deeper flow components (i.e., deep underground water) on the entire runoff is not negligible. Thus, the contribution of each runoff component to the total runoff is an important factor affecting the location of the channel head. Most channel heads in the deeply incised subcatchments in the Higashi-gouchi catchment have been formed by surface and subsurface flows, although many landslides have also occurred around the channel heads. Compared with the dominant flow components, activity of sediment supply from hillslopes might be a minor factor in determining the area–slope relationship for locating the channel head.