24 Feb 2021
24 Feb 2021
Bedrock River Erosion through Dipping Layered Rocks: Quantifying Erodibility through Kinematic Wave Speed
- Indiana University, Bloomington, Indiana, USA
- Indiana University, Bloomington, Indiana, USA
Abstract. Landscape morphology reflects drivers such as tectonics and climate but is also modulated by underlying rock properties. While geomorphologists may attempt to quantify the influence of rock strength through direct comparisons of landscape morphology and rock strength metrics, recent work has shown that the contact migration resulting from the presence of mixed lithologies may hinder such an approach. Indeed, this work counterintuitively suggests channel slopes within weaker units can sometimes be higher than channel slopes within stronger units. Here, we expand upon previous work with 1-D stream power numerical models in which we have created a system for quantifying contact migration over time. Although previous studies have developed theory for bedrock rivers incising through layered stratigraphy, we can now scrutinize this theory with contact migration rates measured in our models. Our results show that previously developed theory is generally robust and that contact migration rates reflect the pattern of kinematic wave speed across the profile. Furthermore, we have developed and tested a new approach for estimating kinematic wave speeds. This approach utilizes stream steepness, a known base level fall rate, and contact dips. Importantly, we demonstrate how this new approach can be combined with previous work to estimate erodibility values. We demonstrate this approach by accurately estimating the erodibility values used in our numerical models. After this demonstration, we use our approach to estimate erodibility values for a stream near Hanksville, UT. Because we show in our numerical models that one can estimate the erodibility of the unit with lower steepness, the erodibilities we estimate for this stream in Utah are likely representative of mudstone and/or siltstone. The methods we have developed can be applied to streams with temporally constant base level fall, opening new avenues of research within the field of geomorphology.
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Nate A. Mitchell and Brian J. Yanites
Status: open (until 01 May 2021)
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RC1: 'Comment on esurf-2021-3', Sarah Boulton, 24 Mar 2021
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This paper sets out to test, using computer modelling, two recent theories (proposed by Perne et al. (2017) and Darling et al. (2020)) for the development of river profiles and rates of bedrock erosion with strong contrasts in bedrock strength with either horizontal or slightly dipping lithological contrasts. This broad research area has also been the focus of a number of other modelling and field-based investigations and the influence of lithology has often been an overlooked aspect of river evolution. As such this is a timely investigation into the role of bedrock strength that will be of use for the fluvial geomorphic community. However, there are issues that need addressing to improve the paper prior to final publication. I hope that these comments are useful to you in this regard.
Clarity and readability.
Unfortunately, I find that the paper is rather long and quite wordy and in places repetitive, so much so that often the key points that the you are trying to are lost on me. My main recommendation would be for you to cut the length of the methods and results sections to make these parts shorter and clearer. The methods section is 11 pages long. Could some of this information go in the supplements for the interested reader but for non-modellers only the key parameters and assumptions are described?
In addition, in the results section figures are often described in the text using virtually the same words as the accompanying figure captions but the key data, trend or observation that the reader should take away from these plots is not clear. This occurs for example on lines 552- 560, lines 682 – 686; line 704 – 709 etc. This means that often I am confused as to the key point being made and I would prefer the result to be stated not a figure description (e.g., What is high/low r2 values? Lines674/675; what the different spatial patterns? line 600)
Rock strength
There have been a number of recent papers based on field measurements of rock strength to determine K (i.e., Kent et al., 2020; Zondervan et al., 2020a; b in addition to those studies that you already cite) but you don’t refer to these when discussing how you chose the K values for the strong and weak rocks. This is important as several of these studies indicate the in the real world erobilities are many orders of magnitude less than values used in models including these used on here. Given that the stated erodibilites are also stated to 3 significant figures – how did you come to these numbers and what are the implications for your models if all the rocks are ‘weak’ in comparison to the limited field data available?
Tank Wash
I would like to see greater justification for the use of the Tank Wash site as your 'field' area. It appears that there is no published geological map or other field constraints for the area – so is this really the best location? Although you recognise that this is ‘far from ideal’ there must be reasons you chose this site over somewhere else nearby on the map? But I'm not clear as to what these are? You do mention the stepped river profile but surely there are other similar locations that have better constrained bedrock data?
Additionally, what is the justification of using K values across such a large range? Is this using Stock and Montgomery (1999)?
On line 845 you state that dating of terraces is used to constrain uplift, but incision recorded by terraces often does not equal uplift owing to the processes of aggradation as well as incision that occur during terrace formation. Do you have any other constraints on uplift? What is the error on this parameter?
Minor comments
Line 56 – Reference needed for metrics of rock strength and channel steepness, maybe Zondervan et al. (2020b) or Bernard et al. (2019) would be suitable here.
Line 71 – is the word possible in this sentence appropriate – common would be my experience in most regions of the world.
Line 208 – what observations are you referring to here?
Lines 570 – 585 I’m not sure if this section is describing results or background information, maybe consider the location of this information.
Lines 434/437 - Why are dips expressed as negatives?
References not in pre-print
Bernard T, Sinclair HD, Gailleton B, Mudd SM, and Ford M., 2019. Lithological control on the post-orogenic topography and erosion history of the Pyrenees. Earth Planet Science Letters, v. 518, p. 53–66.
Kent, E., Whittaker, A.C., Boulton, S.J. and Alçiçek, M.C., 2020. Quantifying the competing influences of lithology and throw rate on bedrock river incision. GSA Bulletin.
Zondervan,, J.R., Whittaker, A.C., Bell,. R.E., Watkins, S.E., Brooke,. S.A.S. and Hann, M.G., 2020a. New constraints on bedrock erodibility and landscape response times upstream of an active fault, Geomorphology, v. 351, p. 106937-106937
Zondervan, J.R., Stokes, M., Boulton, S.J., Telfer, M.W. and Mather, A.E., 2020b. Rock strength and structural controls on fluvial erodibility: Implications for drainage divide mobility in a collisional mountain belt. Earth and Planetary Science Letters, 538, p.116221.
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RC2: 'Review of “Bedrock River Erosion through Dipping Layered Rocks: Quantifying Erodibility through Kinematic Wave Speed', Boris Gailleton, 21 Apr 2021
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The comment was uploaded in the form of a supplement: https://esurf.copernicus.org/preprints/esurf-2021-3/esurf-2021-3-RC2-supplement.pdf
Nate A. Mitchell and Brian J. Yanites
Nate A. Mitchell and Brian J. Yanites
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