Unraveling the Mysteries of Asymmetric Topography at Gabilan Mesa, California. Issue 7 (23rd July 2020)

• Record Type: Journal Article
• Authors: Richardson, P. W.; Perron, J. T.; Miller, S. R.; Kirchner, J. W.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 7(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-07-23
• Subjects: topographic asymmetry -- geomorphology -- microclimates -- landscape evolution model -- hillslopes -- channel migration; Geomorphology -- Periodicals; 551.3
• DOI: 10.1029/2019JF005378
• Languages: English
• ISSNs: 2169-9003

Abstract

Abstract: We investigated the potential causes of topographic asymmetry at Gabilan Mesa, CA, a site that exhibits large aspect‐dependent differences in hillslope gradients and microclimates. Competing hypotheses have been proposed to explain the asymmetry observed at Gabilan Mesa. One hypothesis states that different microclimates on opposing slopes are responsible for differences in runoff or soil strength, which generates asymmetric topography. A second hypothesis states that differences in sediment flux from opposing slopes causes southward lateral channel migration and oversteepening of north facing slopes. To test these hypotheses, we carried out numerical modeling experiments, terrain analysis, and field measurements. We also considered the role of initial tilting in causing the asymmetry. We found that saturated hydraulic conductivity is considerably lower on south facing slopes in one highly asymmetric basin. This is consistent with the hypothesis that aspect‐dependent runoff is responsible for the asymmetry. We also used cosmogenic radionuclide‐derived erosion rates and topographic characteristics at Gabilan Mesa to test predictions from numerical landscape evolution models that incorporate asymmetry‐forming mechanisms. The aspect‐dependent models reproduce the erosional and topographic characteristics of Gabilan Mesa better than the lateral channel migration model. We conclude that aspect‐dependent runoff is the most likely explanation for most of the topographic asymmetry at Gabilan Mesa. Our results do not rule out tilting as a possible influence on the initial development of asymmetry nor do they rule out the possibility that lateral channel migration has contributed to the asymmetry, but we suggest that tilting and lateral channel migration are not primarily responsible for it. Key Points: Cosmogenic nuclide erosion rates for asymmetric valleys imply that lateral channel migration is not widespread at Gabilan Mesa, CA Increased runoff due to lower infiltration rates on sunnier slopes is likely the main contributor to valley asymmetry at Gabilan Mesa Initial tilting of the mesa may have contributed to the high degree of asymmetry witnessed in some valleys at Gabilan Mesa