HR: 0800h
AN: H51C-1136    [Abstracts]
TI: Hanging Valleys in Fluvial Systems: A Failure of Stream Power and Implications for Landscape Evolution
AU: * Wobus, C W
EM: cwobus@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AU: Crosby, B T
EM: mountain@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AU: Whipple, K X
EM: kxw@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AB: We document the existence of hanging valleys in an unglaciated landscape in the eastern Central Range of Taiwan. Analyzing fluvial long profiles from over 140 channels, we establish a set of morphologic criteria defining a hanging valley, including: 1) channel gradient at the tributary mouth; 2) position of the oversteepened reach in relation to the tributary junction; and 3) height of the perched tributary lip above the trunk stream. Using these criteria, we suggest that 25 tributary basins in this fluvial system are effectively hanging valleys. Our conceptual model for the initiation of hanging valleys is consistent with a recently published mechanistic model of bedrock incision, which suggests that the highest transport stage flows are actually less efficient at eroding bedrock than moderate stages. As tributary mouths steepen and transport stage increases in response to an incisional pulse in the mainstem, channel gradients may pass a threshold value beyond which erosional efficiency is hindered, giving rise to a mismatch between trunk and tributary erosion rates. This mismatch is naturally expected at tributary junctions, where a step-function decrease in drainage area also leads to sharp contrasts in water and sediment discharge between trunk and tributary basins. The presence of hanging valleys in fluvial landscapes suggests that our most simplified parameterizations of bedrock erosion -- which typically assume a monotonic positive correlation between channel gradient and incision rate -- may be applicable only to a range of moderate channel gradients. More importantly, the presence of these features may have important implications for landscape evolution, since the catchments above these oversteepened reaches may be insulated for some time from incisional signals propagating through the channel network, increasing landscape response time beyond that expected based on the stream power incision model. The results of this study underscore the need for a more complete understanding of bedrock erosion processes and their incorporation into landscape evolution models. Finally, the presence of hanging valleys in Taiwan indicates significant perturbation away from steady-state conditions in the eastern Central Range.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting