HR: 1340h
AN: H13H-1411    [Abstracts]
TI: Distribution of Fluvial Knickzones Along Mountain Bedrock Rivers in Japan
AU: * Hayakawa, Y S
EM: hayakawa@csis.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Tokyo, 113-0033 Japan
AU: Oguchi, T
EM: oguchi@csis.u-tokyo.ac.jp
AF: Center for Spatial Information Science, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, 277-8568 Japan
AB: Knickpoints are significant features in bedrock river longitudinal profiles, but their broad distribution has not been fully investigated. Here we propose a quantitative method to extract knickzones from 50-m DEMs, to enable broad-scale analyses on knickzone distributions. Japanese mountainous regions, having various lithologic, tectonic and climatic settings, provide many bedrock rivers to be analyzed. The rivers generally have steep gradients and large flood discharge, resulting in active erosional conditions. Analyses were carried out on all the major Japanese mountain rivers whose total length is approximately 70,000 km. Stream gradients were obtained with various measurement scales, which behave as a function of the measurement length. The stream gradients were classified into local and trend types. The local gradient is given by measurement scales of several hundred meters, and the trend gradient is obtained by measurement scales of several kilometers. The local gradient higher than the trend gradient at a point indicates that the location is relatively steeper than the neighbors. A set of such relatively steep points can be regarded as a knickzone. The difference between the local and trend gradients for each measurement location was evaluated based on the slope of a regression line fitted to the plot of calculated gradients against measurement lengths. Segments along a river with the gradient decrease rates larger than their total standard deviation value were then extracted as knickzones. The extracted knickzones are abundantly present in the upper steep reaches of the rivers. The stream trend gradient correlates positively with the knickzone abundance, indicating that those upstream knickzones can be the result of hydraulic actions like step-pool morphology but in a much larger spatial scale and a longer time scale. While the effects of lithologic substrate on knickzone distribution are unclear, tectonic conditions, which can also affect stream gradient, show strong spatial correlations with the knickzone existence, confirming that the knickzones are a result of stream erosive conditions rather than local lithology.
UR: http://home.csis.u-tokyo.ac.jp/~hayakawa/
DE: 1815 Erosion
DE: 1819 Geographic Information Systems (GIS)
DE: 1825 Geomorphology: fluvial (1625)
SC: Hydrology [H]
MN: Fall Meeting 2005