LONGITUDINAL PROFILE DEVELOPMENT INTO BEDROCK - AN ANALYSIS OF HAWAIIAN CHANNELS

Analysis of topographic maps of rivers incised into dated Hawaiian lav a flows shows that the long term average bedrock erosion rate along ce rtain reaches is linearly related to stream power. Field observations suggest that two processes may control Hawaiian channel downcutting: ( 1) stream power-dependent erosion, including abrasion of the channel b ed by transported particles, and (2) step-wise lowering caused by knic kpoint propagation. Modeling results indicate that a simple stream pow er-dependent erosion law predicts the straight to weakly convex longit udinal profiles characteristic of Kauai channels but is insufficient t o predict two other characteristic features: the upslope propagation o f knickpoints and the straight 5-8-degrees channel slopes below the kn ickpoints; thus more than this single transport law is apparently requ ired to model bedrock channel incision. Field surveys also indicate th at significant portions of the channel lengths below the knickpoints a re mantled with large boulders. We propose that the boulder mantling o f long channel reaches inhibits channel incision, reducing downcutting to a rate set by boulder weathering, breakdown and transport of the m aterial, and perhaps by knickpoint propagation sweeping under the boul der armor. Partial boulder mantling of bedrock-dominated channels is c ommon in mountainous regions, and a theory which takes boulder armorin g into account will have broader applications than one which ignores t hese limiting effects.