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.

中文翻译：

---

揭开加利福尼亚加比兰梅萨的非对称地形之谜

我们研究了加利福尼亚加比兰梅萨（Gabilan Mesa）地形不对称的潜在原因，该地点在坡度和微气候方面表现出很大的坡度依赖性。有人提出了竞争性的假设来解释在Gabilan Mesa观察到的不对称性。一种假设指出，相对斜坡上不同的微气候是造成径流或土壤强度差异的原因，这会导致地形不对称。第二个假设指出，相对坡度的泥沙通量差异会导致向南的横向河道迁移和朝北的坡度过度陡峭。为了检验这些假设，我们进行了数值建模实验，地形分析和现场测量。我们还考虑了初始倾斜在引起不对称性中的作用。我们发现，在一个高度不对称的盆地中，朝南的斜坡上的饱和水力传导率要低得多。这与假说有关，径流取决于非对称性。我们还使用了Gabilan Mesa的宇宙成因放射性核素引起的侵蚀速率和地形特征，以测试来自包含不对称形成机制的数字景观演化模型的预测。依赖于方面的模型比横向通道迁移模型更好地再现了Gabilan Mesa的侵蚀和地形特征。我们得出结论，与坡向有关的径流是加比兰梅萨大部分地形不对称的最可能解释。