Bedrock channel response to tetonic, climatic and eustatic forcings
Author(s)Snyder, Noah P
Massachusetts Institute of Technology. Dept. of Earth, Atmospheric, and Planetary Sciences.
Kelin X. Whipple.
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The response of bedrock channels to external forcings is investigated in this thesis. The approach is to test and constrain a theoretical model for bedrock-channel incision based on shear stress using field data. The primary study area is a series of 21 small, coastal drainage basins in northern California, USA with known, varying rates and history of rock uplift. The initial application of a simple form of the model to the stream profiles suggests that (1) the channels are eroding at rates approximately equal to uplift rates (i.e. steady-state fluvial incision), and (2) erosion processes are proportionally more effective in the high-uplift-rate zone, with factors in addition to channel gradient responding to tectonic forcing. These results lead to the rest of the study, in which some of the assumptions of the simple model are rejected in order to explain the second observation. A more sophisticated model that includes both a stochastic distribution of floods and a threshold shear stress to initiate bedrock erosion predicts that a greater part of the distribution of flood events will exceed the threshold in steeper channels. Therefore, higher-gradient channels have proportionally higher erosion rates, as is observed in the high-uplift-rate streams of the California field site. The shear-stress model is tested and constrained through a detailed, field-based analysis of topography, lithology, stream morphology and regional hydrology to isolate those factors that respond to tectonics. The stochastic model is able to incorporate the observed variation in stream discharge due to orographic enhancement of precipitation by high topography associated with high uplift rates.(cont.) This increase in discharge appears to play a second-order role in setting the erosional effectiveness of the high-uplift zone. Other factors, including channel width, lithologic resistance and sediment flux, do not appear to vary in an important way with uplift rate, although this conclusion is based on analyses that have some limitations. The importance of thresholds is underscored by a direct calculation of critical shear stress during a rare bedrock-incision event in a low-erosion-rate creek in New York state (= 100-200 Pa). This event, the only one that caused significant bedrock plucking at the site in an -40-year period, is consistent with a low erosion rate, with few events that exceed the threshold. In contrast, similar 'z values are exceeded during high-frequency flood events in the steep, rapidly eroding California streams. Inclusion of an erosion threshold accounts for the observed relationship between channel gradient and rock-uplift rate in the California site. In summary, by using field examples, the shear-stress bedrock-incision model with a stochastic distribution of flood events and an erosion threshold is demonstrated to be an effective and powerful tool for exploring relationships amongst climatic, tectonic and surficial processes. In the final section of this thesis, a numerical modeling study couples the shear-stress model for onshore fluvial incision with a simple rule for offshore wave-based erosion of bedrock to explore the response of uplifting streams to eustatic fluctuations ...
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2001.Includes bibliographical references.
DepartmentMassachusetts Institute of Technology. Dept. of Earth, Atmospheric, and Planetary Sciences.
Massachusetts Institute of Technology
Earth, Atmospheric, and Planetary Sciences.