Disequilibrium river networks dissecting the western slope of the Sierra Nevada, California, USA, record significant late Cenozoic tilting and associated surface uplift: Reply

Abstract

Abstract In his comment, Gabet summarized his critiques regarding our identification of migrating knickpoints in the Sierra and our estimates of both timing and magnitude of surface uplift made using a wide variety of independent measures of fluvial geomorphology. Here, we review the evidence we used to interpret particular features as migrating knickpoints in a transient landscape and demonstrate that the evidence presented by Gabet in no way shows these are stationary features that can be attributed to heterogeneous lithology alone. The challenges we faced and assumptions we made to determine timing from these river profile forms were summarized in the original paper and again in Gabet’s comment. We chose a range-wide approach using systematic topographic analysis and an ensemble of methods precisely because we recognize the modern heterogeneity and dynamic late Cenozoic geologic history. With this approach, we identified trends in timing and magnitude of surface uplift that were consistent along much of the length of the range and in both tributary and mainstem rivers. We were able to identify signatures of late Cenozoic tilting amidst the influence of heterogeneous lithology, latitudinal climate gradients, drainage area exchange, and variable depositional histories. We hope that readers recognize that rather than ignoring local complexity, the tectonic mechanism of late Cenozoic westward tilt and the ensuing transient response can explain the diversity in observed geomorphology. Particularly, geomorphic features such as the along- and between-river variability in incision as well as the distribution and form of both mainstem and tributary knickpoints and knickzones can be reconciled when such a transient response is filtered through the heterogeneous lithology and basin area exchange characteristic of the west side Sierra Nevada. Thus, while challenges preclude pinpointing timing of the most recent tectonic event within the late Cenozoic using river profile forms alone, we maintain that a significant tectonic event perturbed Sierra river profiles in the past 11 million years. In many landscapes, an estimate with such a degree of uncertainty would not move the needle on our understanding but for the Sierra, where some contend no major tectonic event has occurred since the late Cretaceous, this uncertain estimate is valuable.