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Causes and Consequences of Meandering in Bedrock Rivers: how interactions between rock properties and environmental conditions shape landscapes

  • Author(s): Johnson, Kerri N.
  • Advisor(s): Finnegan, Noah J
  • et al.
Creative Commons Attribution-NonCommercial 4.0 International Public License
Abstract

The potential that sinuous bedrock river canyons archive information about past climatic and tectonic forcing of landscapes has intrigued geomorphologists for over 100 years, in fact they were one of the first landforms to inspire the use of landscapes to constrain geologic history. Yet because bedrock river erosion typically requires bedload impacts and abrasion, and because bedload transport is not tightly coupled with the small deviations in fluid shear stress that cause meandering in lowland alluvial rivers, it was never clear how or if the fluid dynamics understood to drive meandering in alluvial rivers could be applied to rivers confined to bedrock canyons. Without a theory for the process of active meandering in bedrock rivers, the assumption that bedrock sinuosity must be inherited from an antecedent alluvial state has remained pervasive despite evidence to the contrary.

In this thesis, exploration of well constrained field examples enables me to formulate two general requirements for a bedrock river to meander: 1. bank-rock must become susceptible to fluid erosion, and 2. mass wasting of steep bedrock cut banks cannot produce persistent talus to protect the bank from continued lateral erosion. With detailed field exploration, experimentation, and mapping, laboratory rock mechanics testing, weathering experiments, topographic analysis and literature review of mineralogy, we show that where meandering in bedrock occurs, weathering of bank rock satisfies these two conditions. In the mudstone lithology of Pescadero Creek, CA, we show that slaking (fracture due to wetting and drying) of exposed bedrock along cut banks both makes this rock transportable by fluid flow, and makes colluvium easily transportable. For channels in the basaltic lavas of the Kohala Peninsula, Hawai’i chemical weathering can satisfy these same two conditions. This chemical weathering has been shown to be closely controlled by precipitation, and I show a significant decrease in channel sinuosity across the peninsula’s strong orographic precipitation gradient from wet to arid regions. In both these examples, meandering is sensitive to climate. This understanding of how bedrock rivers meander provides a mechanistic foundation for climatic interpretations of sinuous bedrock river canyons (including their associated strath terraces).

In addition, I demonstrate that active meandering rearranges drainage networks and has far reaching consequences for landscape evolution and sediment routing. Specifically, I show that 1) growing meander bends can capture tributaries and cause large knickpoints, and 2) meander neck cutoff events cause terraces which isolate adjoining tributaries from the active mainstem, resulting in long lasting tributary convexities, and sediment storage. These autogenic consequences of active meandering mimic tectonic and climatic transience in landscapes over timescales similar to glacial/interglacial cycles (105-106 years), and also influence sediment transport and storage, which often plays a critical role for aquatic ecosystems.

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