HR: 17:00h
AN: H34A-05 INVITED     [Abstracts]
TI: Knickpoint Generation and Persistence Following Base-Level Fall: An Examination of Erosional Thresholds in Sediment Flux Dependent Erosion Models
AU: * Crosby, B T
EM: mountain@mit.edu
AF: Dept. Earth, Atmospheric, and Planetary Science, MIT, Cambridge, MA 02139 United States
AU: Whipple, K X
EM: kxw@mit.edu
AF: Dept. Earth, Atmospheric, and Planetary Science, MIT, Cambridge, MA 02139 United States
AU: Gasparini, N M
EM: nicoleg@alum.mit.edu
AF: Dept. of Geology and Geophysics, Yale University, New Haven, CT 06511 United States
AU: Wobus, C W
EM: cwobus@MIT.EDU
AF: Dept. Earth, Atmospheric, and Planetary Science, MIT, Cambridge, MA 02139 United States
AB: Non-lithologic knickpoints, or discrete convexities in longitudinal river profiles, are commonly considered to be the mobile, upstream extent of a transient incisional signal. Downstream of the knickpoint, the landscape is responding to a recent change in base level, uplift rate or climatic condition, while upstream of the knickpoint, the landscape retains its relict form, relatively ignorant the transient signal. Though this model of knickpoint mobility and their capacity to communicate incisional signals throughout basins works well with standard formulations of the stream power erosion model, the recent development of sediment flux dependent erosion models contain explicit thresholds that limit the upstream extent of knickpoint-mediated fluvial adjustment. Sediment flux dependent erosion models fail to communicate incisional signals at small drainage areas as sediment and water discharges are insufficient to effectively erode the bed. As well, if knickpoint slopes increase beyond a threshold value, sediment impacts against the bed become too infrequent and too oblique to continue knickpoint propagation by fluvial mechanisms. This threshold in fluvial erosion could lead to the stagnation of incisional signals and the generation of hanging valleys. This theoretical expectation aligns with our observation that in numerous actively incising landscapes around the world, relict low drainage area basins are often found elevated high above and disconnected from the mainstem by extremely over-steepened channel reaches often composed of one or more near-vertical steps. In order to better understand how river networks respond during transient pulses of incision, we employ a numerical landscape evolution model (CHILD) to test the sensitivity of three different sediment flux dependent erosion models to different base-level fall scenarios. This technique allows us to observe the propagation of the signal throughout a fluvial network composed of tributaries of variable drainage area. We are also able to track sediment flux during the transient adjustment and monitor its influence on the propagation of the incisional signal. We find that depending on which sediment flux dependent erosion model is used, hanging valleys can be generated at tributary junctions where sediment and water discharge decrease dramatically. We also find that as the incision signal propagates up the mainstem, the magnitude and rate of incision diminishes and results in differing responses between upstream and downstream tributaries. The hanging valleys we observe forming within the model have variable longevities and styles of decay. At present the model does not adequately represent the processes responsible for this decay, but demonstrates need for further study. If hanging valleys are generated due to thresholds in sediment flux dependent erosion models, we may be currently underestimating the response times of river basins during the transient response to incision.
UR: http://www.mit.edu/~mountain/work/meetings
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1826 Geomorphology: hillslope (1625)
DE: 1847 Modeling
DE: 1856 River channels (0483, 0744)
SC: Hydrology [H]
MN: Fall Meeting 2005