PATTERNS OF BEDROCK CHANNEL EROSION ON THE BOSO PENINSULA, JAPAN

Three channels on the Bose Peninsula, Japan, were examined to assess h ow cross-sectional to reach-scale channel morphology relates to substr ate variables, and how cross-sectional to reach-scale erosional patter ns relate to basin-scale longitudinal profile. Two of the channels, Sh ichiri-gawa and Torii-zawa are deeply incised into interbedded sandsto ne and mudstone, whereas the third channel, Futama-gawa, is incised in to mudstone. Reach-scale channel morphologies include knickpoints, chu tes and pools, plane beds, and both transverse and longitudinal bed er osional 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 wit h bends and with coarse-clast deposition. Magnitude of erosional bed f eatures correlates with hydraulics and flow energy expenditure along F utama-gawa, and with variability in substrate resistance along Shichir i-gawa and Torii-zawa. Thickness and orientation of the bedrock strata strongly control erosional patterns along Shichiri-gawa and Torii-zaw a. Reaches dominated by sandstone have shallower pools or a plane bed. Reaches with a higher proportion of resistant mudstone and with strat a that strike across the channel have more knickpoints and a stepped m orphology. Comparison of main stem and tributary drainage areas and gr adients indicates that a simple stream power-dependent erosion law doe s not fully account for the evolution of longitudinal profiles on the Bose Peninsula. Ability to incise does not necessarily increase with s tream power or basin area for channels on the Bose Peninsula because o f substrate influences on bed erosional forms and on coarse clast resi stance.