Characterizing the transient geomorphic response to base level fall in the northeastern Tibetan Plateau

Abstract:

Analyses of hillslope gradient, landscape relief, and channel steepness in the Daxiahe drainage basin along the northeastern margin of the Tibetan Plateau provides evidence of a transient geomorphic response to base level fall along the main stem Yellow River. The upper portions of the watershed are characterized by low-gradient channels and gentle hillslopes and are separated from a steeper, high relief landscape by a series of convex knickzones along channel profiles. Downstream projection of the upper channel profiles implies ~500-600 m of incision, consistent with terrace records of post ~1.7 Ma incision in the Linxia basin. We characterize erosion rates across this transient landscape using both optically-stimulated dating of fluvial terraces and catchment-averaged ¹⁰Be concentrations in modern sediment. Both data sets are consistent and suggest erosion/incision rates of ~300 m/Myr below knickpoints and ~50-100 m/Myr above. Field measurements of channel width (n=48) and bankfull discharge (n=9) allow us to determine local scaling relations among channel hydraulic geometry, discharge, and contributing area that we employ to estimate basal shear stress, unit stream power and bedload transport along the main stem of the Daxiahe River. We find a clear downstream increase of incision potential across this transient landscape, consistent with topographic observations and erosion rates. In contrast to recent studies, we find no evidence for adjustment of channel width across the transition from slowly eroding to rapidly eroding portions of the watershed. We hypothesize that this behavior is consistent with detachment-limited models of fluvial incision, despite the presence of significant sediment in channel bed and banks. Our results imply that the controls on hydraulic geometry along actively incising rivers remain incompletely understood.