Building a Bimodal Landscape with Varying Bed Thicknesses in Last Chance Canyon, New Mexico
Abstract. We explore how rock properties and channel morphology vary with rock type in Last Chance canyon, Guadalupe mountains, New Mexico, USA. The rocks here are composed of horizontally to near horizontally interbedded carbonate and sandstone. This study focuses on first and second order channel sections where the streams have a lower channel steepness index (ksn) upstream and transition to a higher ksn downstream. We hypothesize that differences in bed thickness and rock strength influence ksn values, both directly by influencing bulk bedrock strength but also indirectly through the production of coarse sediment. We collected discontinuity intensity data (the length of bedding planes and fractures per unit area), Schmidt hammer rebound measurements, and measured the largest boulder at every 40-foot elevation contour to test this hypothesis. Bedrock and boulder minerology was determined using a lab-based carbonate dissolution method. High resolution orthomosaics and DEMs were generated from drone photos. The orthomosaics were used to map channel sections with exposed bedrock. The high-resolution DEMs were used to measure channel slope and hillslope relief. We find that discontinuity intensity is negatively correlated with Schmidt hammer rebound values. Channel steepness is higher where reaches are primarily incising through more thickly bedded carbonate bedrock. Where there is more thinly bedded sandstone rock exposed, channel steepness tends to be lower. Furthermore, the effect that rock properties have on channel morphology is confounded by sediment input from hillslopes. Thickly bedded rock units on surrounding hillslopes contribute larger sized colluvial sediment to the channels, and these reaches have higher ksn. Larger and more competent carbonate sediment armors both the carbonate and the more erodible sandstone and dampens the negative effect sandstone bedrock has on channel steepness. We believe that in the relatively steep, high ksn downstream channel sections slope is primarily controlled by the coarse alluvial cover. We further posit that the upstream low ksn reaches have a baselevel that is essentially fixed by the steep downstream reaches, resulting in a stable configuration where channel slopes have adjusted to lithologic differences and/or sediment armor.
Samuel Anderson et al.
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2022-1285', Anonymous Referee #1, 24 Jan 2023
- RC2: 'Comment on egusphere-2022-1285', Anonymous Referee #2, 10 Feb 2023
- EC1: 'Comment on egusphere-2022-1285', Jens Turowski, 14 Feb 2023
Samuel Anderson et al.
Samuel Anderson et al.
Viewed (geographical distribution)
1. general comments
Overall, this manuscript deals with and disusses a relavant and well-set study topic fitting the current state of research on landscape development based on lithological setting. There is a good and clear hypothesis, though, there are several issues to be faced in the current version of the paper:
- the introduction needs a distinct backing-up and reasoning by more literature; the discussion needs to be straigthlined
- there is need of more discussion why the one outlier (L3.2) is valid as it is basic for some results/interpretations
- the outcomes developed in the discussion need to be more streamlined and several controdictions need to be cleared
- several figure panels should be combined to ease the interpretations
Thus, this study findings would generally be worth publishing after adressing the above mentioned tasks - below there are several detailed hints/notes/suggestions on how to address them - both scientificaly and technically.
2. specific comments
L1 use a more expressive statement
L23 abstract: we believe?
L27 (only) little debate?
L35 also Shobe++2021, GSA Bulletin
L37 needs definition what the ksn actually is (physically) or general description of channel profile descriptors (as they are more defined in the methods)
L45 needs info on the geochemical methods and data as background, there is no info yet
L47-L49 why inverse (physical explanation)?
L54 also Scott&Wohl2019, ESPL
L56 cf. Bursztyn++2015, EPSL
L57 to fluvial geomorphologists, too!
L58 intro of sediment availability, sediment size, btools and cover, and discharge variability is missing, also channel width vs. steepness is not mentioned - these topics are fundamental in this context!
L59 reason, why foirst-order channels (aslo, what are these)
L61 too colloquial text - e.g., "find rock mineralogy" ...
L65 landscape or river channels?
L68ff higher elevation in this scanario!
L70 is this for a ~steady state case the weaker erodibility may be deducted even? (cf. Mitchel&Yanites2021, ESURF)
L71 climate (so Kc) is assumably constant, i.e. can be ignored for this analysis?
L85 how about the sediment (size distribution, lithological partition) in the investigated reaches?
L105 Xi needs references
L109f which DEM; why 75m?
L112 are the San Gabriel Mountains reasonably comparable to your site (concerning chanel geometry, lithology, grainsizes, climate etc.)?
L117 a metric interval would be more tangible for the community
L118 why (only) the largest boulder - is this significant of anything (e.g., cover)? What is the relation to / meaning for smaller grainsizes?
L121 which unit
L133 define what plucking is, and why it is important here
L142 posting? you mean resolution?
fig4b end of caption is unclear; line colors in c and d are hard to differentiate - take a color-blind friendly range; indicate the Sitting Bull Falls in 4a (is this at L3.2?); also having notes on which channel holds which lithology (refer to fig.2) would be very helpful to get the point
fig5 how does a plot of discontinuity vs. Schmidt Hammer Rebound look like? What do the results tell you?
L187f the carbonate values are not much dfferent between steep and shallower sections
L201 how does the pattern look like if you clipp the actual in-channel boulders from the 500m window
L208ff I assume you refer to fig.7 - you state there "all boulders", but these are 'only' the largest boulders per reach, right? So, at least your result is not generally valid?
L221ff for the 5 points refer back to the figures, respectively!
L226 Shobe++2016, GRL
L229 didn't you measure larger Schmidt hammer values in the shallower sections above attesting them harder rock?
L231 that may be valid for you lithologies, but not generally
L236 I don't get this reasoning ...
L238 you mean there only is one data point for steep slopes that determines your whole interpretation above - correct; you say here you ignore it - so what about all the results?; why is this outlier there (is it an transient knickpoint? this would contradict L227ff)
L244ff several repetitions, reduce
L252 so why are there steep vs. shallow carbonate sections
L261 so erosion is focusing on the steep sections (until they are shallow enough to hold cover?) - both on carbonate and sandstone? Though, you say the opposite in L296ff
L267 not by fracture distance?
L272 so then - how is the correlation between bedding thickness with local rock dimensions
L272ff several repetitions, shorten; though you could repeat the bedding thickness values/orders for better evaluation of your discussions!
L283ff this section is missplaced and also repeats a lot; have this earlier in the interpretation - also fig9 partly repeats fig.4cd and should not show up here in the discussion; coul go to the supplement (or maybe show one example of the rock exposure as a panel in fig.4)
L296 contradicts L304f (and L261) - confusing and circular these two last paragraphs; solve for a reasonable, streamlined and consitent interpretation at one place in the text
L318 need to mention Carbonates here?
3. technical corrections
L39 not necessary
fig1/2 combine into panels
L146 why 40 foot and not [m]?
L189 for on
L209 combine fig.6 and the panels of fig.8 into 4 panels; fig.7 is wrong-placed
fig8 the caption indicates fig6 is added as a panel - do that
fig9 caption: left is right ...; what are the dots?; what are "high-order alluviated channels"; rock-coloring is hard to differentiate