Patterns of bedrock channel erosion on the Boso Peninsula, Japan

Three channels on the Bose Peninsula, Japan, were examined to assess how cross-sectional to reach-scale channel morphology relates to substrate variables, and how cross-sectional to reach-scale erosional patterns relate to basin-scale longitudinal profile. Two of the channels, Shichiri-gawa and Torii-zawa are deeply incised into interbedded sandstone and mudstone, whereas the third channel, Futama-gawa, is incised into mudstone. Reach-scale channel morphologies include knickpoints, chutes and pools, plane beds, and both transverse and longitudinal bed erosional features. Step-backwater modeling of a September 1996 typhoon-related flood flow along each channel indicated that unit stream power is highly variable downstream. Stream power minima are associated with bends and with coarse-clast deposition. Magnitude of erosional bed features correlates with hydraulics and flow energy expenditure along Futama-gawa, and with variability in substrate resistance along Shichiri-gawa and Torii-zawa. Thickness and orientation of the bedrock strata strongly control erosional patterns along Shichiri-gawa and Torii-zawa. Reaches dominated by sandstone have shallower pools or a plane bed. Reaches with a higher proportion of resistant mudstone and with strata that strike across the channel have more knickpoints and a stepped morphology. Comparison of main stem and tributary drainage areas and gradients indicates that a simple stream power-dependent erosion law does not fully account for the evolution of longitudinal profiles on the Bose Peninsula. Ability to incise does not necessarily increase with stream power or basin area for channels on the Bose Peninsula because of substrate influences on bed erosional forms and on coarse clast resistance.