Waterfall erosion and alteration of river form
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Authors
Rothman, Sophie
Issue Date
2024
Type
Dissertation
Language
en_US
Keywords
Geomorphology , Landscape Evolution , Waterfalls
Alternative Title
Abstract
Bedrock rivers are critical to landscape evolution because they transmit tectonic signals throughout the landscape, set the base level for hillslope erosion and remove eroded sediment. Because of their role at the center of landscape erosion, bedrock rivers can provide insight into the climatic and tectonic conditions within a drainage. In particular, bedrock river steepness can provide information about the relative erosion rate of a channel, and bedrock channel width can contain clues regarding sediment flux. However, it is possible that these signals may be altered due to waterfalls which are common features in steep bedrock rivers. While many waterfalls form as a part of a transient knickzone (an oversteepened river reach created through a change in external conditions), some waterfalls are known to self-form when Froude supercritical flows develop in steep bedrock channels. And although previous work has developed an understanding of knickzones retreat through a basin, it is not clear whether waterfalls which are not a part of a knickzones follow the same rules. Specifically, it is important to understand whether self-formed waterfalls are capable of altering long profile form and channel morphology independent from the influence of knickzone dynamics. This research addresses this knowledge gap by investigating waterfalls in many different settings, and through different methods. In Chapter 2, I use a finite difference model to examine how self-formed waterfalls alter longitudinal profiles. I find that self-formed waterfalls may create a reach with a constant slope in dynamic equilibrium, or alternatively, self-formed knickpoints, depending on the speed of waterfall erosion. Furthermore, I find these results consistent with some natural river profiles. In Chapter 3, I investigate how quickly waterfall-rich channels erode using 10-Be analysis. I find that waterfall-rich channels erode faster than the rest of the landscape on average and erode faster with increasing concentration of waterfalls. Through comparing my results with a physics-based model, I also find that waterfalls may erode slowly under conditions such as high sediment flux, large grain sizes, and low discharge. In Chapter 4, I extract channel widths and waterfall locations from lidar and use the data to show that waterfalls locally constrict channel width. I also demonstrate that the length scale of waterfall influence below a waterfall grows with increasing drainage areas, indicating an increase in waterfall erosion rates. Overall, this research provides a detailed look at the physical reality of waterfall erosion, which can be used to better understand knickzones dynamics, and channels in steady state.
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License
CC BY-NC-ND