Quantification of internal system dynamics and external forcing in bedrock river networks
AuthorBeeson, Helen Willemien
AdvisorMcCoy, Scott W
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Landscapes evolve in response to tectonic and climatic boundary conditions towards an equilibrium state in which rock uplift is balanced by erosion. In unglaciated mountainous terrain, bedrock rivers transmit tectonic and climatic conditions through the landscape, carving it into ridges and valleys and controlling the relief structure by setting the boundary condition for hillslopes. Despite the importance of bedrock rivers, the manner by which bedrock river networks respond to changes in tectonic and climatic conditions and the associated timescales for these responses remain poorly constrained. As a result, difficulties exist in reconstructing tectonic or climatic histories of landscapes and fundamental questions remain regarding the relative importance of external forcing versus internal system dynamics or complex system response in shaping landscape morphology. I use a combination of quantitative terrain and network analysis, geochronology, and numerical landscape evolution modeling applied to two postorogenic landscapes in North America – the Ozark dome and the Sierra Nevada – to investigate the complex system response to tectonic forcing in bedrock river networks. Specifically, I seek to contribute to the following outstanding questions in geomorphology: (1) Do the erosional dynamics between contiguous river basins (“river basin dynamics”) prevent landscapes from reaching steady-state conditions in which erosion everywhere balances uplift? (2) To what extent do internal system dynamics, particularly river basin dynamics, shape landscape morphology in bedrock landscapes? (3) To what degree can internal system dynamics obscure geomorphic signatures of external forcing, and what methods can be used to see through the noise of internal system dynamics to robustly recover histories of external forcing from bedrock landscape form? I show that planform geometric disequilibrium of river basins may hold landscapes in a persistent transient state for potentially hundreds of millions of years in which erosion gradients generate asymmetric divides, the formation of elevated, low-relief surfaces, and discrete events such as stream capture. Although the majority of river longitudinal profiles may approach a near-equilibrium state, if the river network is in geometric disequilibrium, landforms will not be time-invariant and the broad-scale morphology can reflect the dynamics between contiguous river basins. I demonstrate that the transient fluvial response to tectonic and climatic perturbations leaves signatures in bedrock river channels and networks that are distinct from signatures of topologic change. I use these signatures to support a robust reconstruction of the tectonic history of a rejuvenated postorogenic landscape, the Sierra Nevada, despite the presence of heterogeneous lithology, Pleistocene glaciation, active river reorganization, and major beheading of the majority of mainstem rivers. I show that while drainage area exchange in the form of both steady divide migration and discrete stream capture modifies bedrock longitudinal channel profiles in the Sierra, the signature of a major late Cenozoic tectonic perturbation can be recovered provided a systematic approach to reconstructing tectonic histories is taken in which river basin dynamics is taken into account. Further, I demonstrate through analysis of the Ozark dome and Sierra Nevada that while tectonic and climatic boundary conditions generally control the relief structure of landscapes, river basin dynamics can drive substantial deviations in relief.