Is drainage reorganization a plausible explanation for late Cenozoic incision of Yosemite Valley and within the Kings and Kaweah watersheds (Sierra Nevada, California)? Open Access

Increases in the rates of river incision can be driven by a variety of causes, such as tectonic uplift, changes in sediment supply, changes in climate, and drainage reorganization. Here, I evaluate whether drainage reorganization may have driven late Cenozoic episodes of accelerated river incision in the Sierra Nevada (California) that have previously been attributed to uplift. I propose several lines of evidence useful for distinguishing between these two potential drivers of incision and apply them at three sites with well-documented histories of late Cenozoic incision: Yosemite Valley and Tenaya Canyon, along the South Fork Kings River, and in the Kaweah River watershed. These lines of evidence include the spatial patterns of knickpoints and relief, the presence of paleochannels, the morphologies of alluvial fans, and results from numerical modeling. The analyses suggest that a river draining a Miocene–Pliocene volcanic edifice along the crest of the northern Sierra Nevada temporarily extended into Tenaya Creek, greatly increasing the latter’s erosive power and incising Tenaya Canyon and Yosemite Valley. This hypothesis explains previously unsolved issues, such as the presence of voluminous andesitic volcaniclastic deposits in the Merced River paleofan, despite their absence in the Merced watershed, and the discrepancy in relief between Tenaya Canyon and neighboring Little Yosemite Valley. Results for the Kings River drainage show that deep incision since the mid-Pliocene has been confined to the upper portion of its watershed and that it occurred in a rapid pulse, consistent with an episode of stream piracy. Albeit limited, the geomorphic evidence for drainage reorganization in the Kaweah watershed includes a 1800-m-high convex knickpoint on a trunk stream and an anomalously large alluvial fan. This study demonstrates the importance of considering drainage reorganization as an alternative to uplift when investigating abrupt and short-lived increases in river incision rates.