Numerical simulations of inherited sinuosity and valley entrenchment in mixed alluvial- and bedrock-banked meandering rivers
Abstract
Highly sinuous rivers inset in bedrock canyons have long piqued the interest of geomorphologists because this sinuous morphology evokes comparisons to more actively migrating rivers with relatively weak sediment banks. An extensive literature debates the degree to which meandering rivers in bedrock valleys have inherited their sinuosity from previous alluvial states, or instead actively maintain sinuosity through bank migration at a rate sensitive to environmental characteristics (e.g., climate and lithology). This distinction is essential to evaluating whether bedrock channel sinuosity and valley morphology can be used to infer regional base-level and climate history, or instead reflect initial conditions. Previous work has considered sinuosity inheritance to occur by river vertical incision directly translating the form of a meandering channel from its alluvial state to a bedrock-bound state, yet the rate of vertical incision required to cause this state transition is uncertain. Here we explore the hypothesis that feedbacks between channel lateral migration and vertical incision, communicated through the composition of the channel banks, may influence channel planform geometry during an alluvial-to-bedrock transition. We use a numerical model to evaluate the transient morphologic and kinematic response of an alluvial river to different vertical incision histories and initial valley configurations. The model uses a vector-based framework for bank-material tracking to precisely model feedbacks between channel lateral migration and bank strength. Model results suggest that the tendency for a channel to remain highly laterally mobile depends on the vertical incision rate, but also on the width of the valley formed by lateral channel migration: bedrock valley floors are more easily bevelled by channel sweeping within narrow valleys than for wide valleys. For cases in which the channel cannot bevel across the initial valley width, lateral variations in bank strength cause more frequent cutoffs, which reduces channel sinuosity, stalls channel migration, and confines the channel to bedrock banks. Resultant channel morphology is less sinuous under cases of steady vertical incision than for cases with a rapid pulse of vertical incision (e.g., due to climate change). These results suggest that a channel's planform morphology may hold clues to its vertical incision history, and may assist in discriminating hypotheses for the evolution of bedrock river valleys. We use these insights to reevaluate potential dynamical histories of several bedrock rivers, including the San Juan River, Utah.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMEP22B..03L
- Keywords:
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- 1825 HYDROLOGY Geomorphology: fluvial;
- 1847 HYDROLOGY Modeling;
- 1872 HYDROLOGY Time series analysis;
- 1856 HYDROLOGY River channels