the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Feb 2024
The impact of bedrock meander cutoffs on 50 ka-year-scale incision rates, San Juan River, Utah
Abstract. Incision rates of major rivers may reflect the effects of drainage reorganization, hillslope processes, tectonic uplift, climate, the properties of rocks into which rivers incise, and other autogenic processes. On the Colorado Plateau, incision rates along the Colorado River have been interpreted as resulting from abrupt base-level changes produced by the integration of the Colorado River system. Specifically, the integration of the Colorado River in the location of Grand Canyon is thought to have created a knickpoint, enhanced by lithologic contrasts, which is retreating upstream. While evidence exists for a <1 Ma acceleration of incision on parts of the Colorado River, uncertainty about the processes reflected in shorter-term incision rates muddies comparison with longer-term averages. In this work, we combine a cosmogenic radionuclide depth profile exposure age and post-Infrared Infrared Stimulated Luminescence (p-IR IRSL) to date fluvial deposits adjacent to the San Juan River, a major tributary of the Colorado River, near Mexican Hat, Utah. The deposits, resting on a 32 m strath surface, are constrained to be ∼28–40 ka, suggesting an incision rate of 804–1151 m Myr-1, nearly an order of magnitude higher than the long-term rate of ∼140 m Myr-1 over the past ∼1.2 Ma. We observe fluvial deposits that were abandoned due to a bedrock meander cutoff, which partially explains our accelerated incision rate. We use a simple geometric model, informed by our field data, to demonstrate how planform river evolution may, in some circumstances, increase short-term incision rates, relative to long-term incision rates. These short-term rates may also reflect a combination of autocyclic and climatic processes, which limits their ability to resolve longer-term changes in incision rate that may be related to changes in base-level or tectonics.
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RC1: 'Comment on egusphere-2024-71', Anonymous Referee #1, 17 Mar 2024
This manuscript presented river incision rates of the Mexican Hat reach of San Juan river, inferred from dating of exposure and deposition age of the terrace fill. The inferred incision rate which averages over ~50 Ka is one order of magnitude higher than the incision rate inferred from a ~1.2 Ma aged terrace fill of the Bluff terrace upstream of the Mexican Hat reach. The incision rate difference between the two terrace sites is hypothesized to be caused by a meander-cutoff event at the Mexican Hat. A quantification model of cutoff-induced erosion intensification is proposed and applied to the Mexican Hat reach, revealing that 50% of the incision rate difference can be explained by the cutoff. The authors also discussed other alternative mechanisms, including base-level change erosion wave propagation of the bigger area, glaciation of the source area, and lithological effects. I think this manuscript is generally well-drafted, the presentation is in reasonable logic flow, and the meander cutoff model is interesting. Before this manuscript is advancing to the next phase, I think a few questions need to be addressed:
Specific comments :
- The causal relationship between the terrace Q3 and the meander cutoff is not evidenced clearly. Meandering cutoff age is constrained by the dated terrace at the quarry, Q3, in this study. I think it needs to be justified with more details. At the quarry, the meander course cuts through the Q3 according to the descriptions in section 4.1. That is saying the Q3 is formed before the cutoff, not formed from the cutoff event. The relative time between Q3 exposure and cutoff is unknown. Two patch clusters of Q3 are also found upstream of the quarry and the meander cutoff (Figure 5b, colored with orange and blue). The elevation difference between Q3 strath and the modern San Juan river bed is not 100% from the intensified erosion due to the cutoff. Should the relief difference (relative to the modern San Juan bed) between the Q3 patch clusters that were affected by the cutoff and those not affected by the cutoff be used to justify the intensified erosion due to the cutoff? I wonder what the authors think about this problem? In addition, Figure 4c doesn’t give enough topographic resolution to show the cut-cross relationship between the Q3 patch and the paleo- meandering river course. The paleo- meandering river course (black dash line) is drawn seemingly randomly on Figure 5b, instead of demonstrated by a high-resolution topographic map in the background, or by field evidences.
- Section 3.2 and 3.3
- In the presentation of section 3.2, sampling strategy, sample type, and sampling locations are missing in this section. Although the authors used published data of Wolkowinsky and Granger (2004), it is necessary to briefly introduce the sample strategies and samples that have been re-analyzed in this study. And in addition, locations of these re-analyzed samples from Wolkowinsky and Granger (2004) indicated on Figure 1 should be indicated distinctively to stand out from the background.
- I think section 3.2 and section 3.3 should be merged into one section. The methods of these two sections are the same, but apply two (same-type of samples I assume) different terrace sites. I would recommend the author to structure the new section 3.2 into theories and supporting equations, then present the Mexican Hat sampling strategies and samples, the Bluff re-analyzed samples come to the end.
- Section 4.1
Cutoffs and paleo-meandering course, in addition to the mapped terraces, are presented in this section to assist the demonstration of relative timings of multiple events (terraces formation and two cutoff events). This section is important to understand the purpose and logics of the study of this manuscript. I would recommend some restructure effort to be done for this section. I think the terraces, meandering cutoffs, cut-cross relationship between mapped terrace and paleo- meandering course of the studied San Juan reach and the nearby upstream reach should be presented in this section to frame the problem, and address the scientific problem above (Figure 5a). And it should be expanded to justify the relative timing of cutoff and terrace formation of Q3, and Q2.
Actually, this section reveals a bigger problem I see about the presenting style of this manuscript. I think 1) the derivation of the hypothesis needs to be sharpened in the introduction section; 2) backgrounds, methods, and results sections are not phrased to serve for evaluating the hypothesis. They were phrased in a way that is not solving problem-centered which have brought difficulties for reviewing. For the second point, I suggest replacing the technology-oriented subtitles of the results sections into subject-oriented subtitles. For example, the “4.2 Reassessing Bluff terrace dating” can be replaced with “4.2 Long-term incision rate of the San Juan River” to match the “long-term” and “short-term” incision rates that were described in the hypothesis.
Technical correction suggestions:
- Figure 2, the “T” symbols next to numbers are not explained in the caption or in the figure legend. I guess they are strike and dip of layered outcrops. I would also recommend the authors add explicit descriptions of rock types to the geological units in the legend.
- Line 111, the “Cutler Formation” is mentioned only here throughout the manuscript. Seems it is the formation above the Halgaito Tongue Formation. Could you clarify it in the legend of the geological unit of Figure 2?
- Line 153-155, not sure I understand the “long-term” and “our short-term” here. What does it mean? New community-accepted parameter values were used for recalculation, but the exposure or erosion rate with these new parameter values are still the same time scale as that calculated with “old” parameter values. I am confused.
- Line 170-171, why?
- Line 237-238 and Figure 4c, the IRSL samples are from Q3, but why the Q3 terrace extent in Figure 4c doesn’t cover the IRSL sampling location? Is the Q3 too disturbed in the quarry to map a confident boundary of Q3?
- Line 245-246: better to indicate the sample locations on Figure 5b again to clarify which two cutoffs because Figure 5a has more than two cutoffs there. Then Figure 5b should be referred to in this sentence, not the whole Figure 5.
- Line 246: “Both are now drained by tributary streams”, I am afraid I can’t find the tributaries in Figure 5.
- Line 248-249: I feel it is a confusing sentence. This long sentence seems to be describing Figure 5b. Could you separate this sentence into two so that one describes the relative spatial relationships, and the other describes the inferred sequential events? And refer to the respective figure in the description.
- Line 263: it will be better to explicitly point out the physical meanings in addition to the symbols only. For example, t, is the exposure age (?), and the mean erosion rate, eta, is the average between t to modern (?).
- Figure 6: could you indicate the two sites consistently of the a and b in the captions?
- Line 322, 323: S and Sc indicate two different physical quantities, better to use more distinctive symbols.
- Figure 7a: legend of dots and circles is missing.
- Line 293: I would add “of Q3” to this sentence since there were 6 terraces presented in the “Geomorphic Mapping” section.
- Line 296: “…to 84–140 m Myr-1 This incision rate …” “…to 84–140 m Myr-1. This incision rate…”
- Line 306-309: are you comparing similar-aged cutoffs in different rock types so that the relief difference is from rock-erodibility? Besides, where are they on figures (Figure 5a?)?
- Line 314: very interesting implications, is there any other literature to support it?
- Line 348: Figure 5BFigure 5b
- Line 351: “…rate to be ∼342 m Myr-1 Using the measured…”“…rate to be ∼342 m Myr-1. Using the measured…”. More places miss a period symbol (e.g. Line 374, 375). Please check that carefully.
- Line 376: “…depositional age for the same deposit of…”, after some literature digging, I realized the “same deposit” is actually the Bluff terrace fill. It does no harm to point it out clearly instead of saying “the same deposit”.
Citation: https://doi.org/10.5194/egusphere-2024-71-RC1 -
RC2: 'Comment on egusphere-2024-71', Joel Johnson, 26 May 2024
I think the manuscript by Steelquist et al. is clearly and well written, presents new age dates that will be useful for researchers understanding the evolution of the Colorado Plateau, and should be published after minor revisions. I only have a small number of suggested improvements.
Line 24: add “incision” after Grand Canyon?
33-35: I realize this is a broad summary of previous work, but Cook et al. 2009 find rates of 300-500 m/myr, higher than the estimates in the previous sentence.
Figure 3d: Can you specify which terrace levels these are, in relation to figure 4? Are these Q2 and Q1?
171: Formatting of Nishiizumi et al (2007).
190: I realize later you discuss the assumption of not terrace erosion; because fig3d sure makes me think there has been terrace erosion, I suggest you put a mention here that you’ll address this assumption later.
207: I realize you address the point of sand deposits on top of the gravel later as well, but I got stuck on it here. Suggest you also add a sentence or statement that you discuss the specific stratigraphic relations between underlying gravel and overlying sand deposits, and how that influences interpretations of dates and rates, below.
250: What is the “this may be the only location” statement based on? Seems pretty interpretive; try to work this point into the discussion instead of here.
266: Clarify what the 14+-4 rate is based on, since its lower than the long-term incision rate. Is this based on the inheritance, representing the upstream basin at that time? Not everyone is as well-versed in the depth-profile methods as the authors; I suggest guiding readers like me a little more through these numbers.
282: “while of the older” has an extra word or needs more editing.
285: Suggest not saying the measurements are “correctly ordered”. The underlying gravels dating to be younger than the overlying sand deposits isn’t correctly ordered. I like the interpretation of why, it just feels like saying they are correctly ordered is overselling the data.
Figure 7 caption: reword to not have “showing” twice.
Discussion, 301: remove “being”
303: In this landscape, in my experience steps are usually associated with strong and weak lithologic changes. The authors bring this up in the next paragraph, but are you sure the strath terraces don’t also reflect that?
348: Maybe change “sinuosity of around 7.6” to “local sinuosity of around 7.6”?
355-365, or 415-420, or somewhere: I appreciate the broader discussion of why erosion rates are variable, and I think the cutoff bedrock meander is a nice example of that. There are many other papers the authors could cite that I think are relevant to sediment supply and channel morphology influencing long-term calculated erosion rates; two that come to mind are Ouimet et al. (2009) and Gallen et al. (2015). It felt to me like the discussion could be a little broader.
Citations
Ouimet WB, KX Whipple, BT Crosby, JP Johnson and TF Schildgen (2008), Epigenetic Gorges in Fluvial Landscapes. Earth Surface Processes and Landforms 33, 1993-2009, doi: 10.1002/esp.1650.
Sean F. Gallen, Frank J. Pazzaglia, Karl W. Wegmann, Joel L. Pederson, Thomas W. Gardner; The dynamic reference frame of rivers and apparent transience in incision rates. Geology 2015;; 43 (7): 623–626. doi: https://doi.org/10.1130/G36692.1
Citation: https://doi.org/10.5194/egusphere-2024-71-RC2 -
AC1: 'Comment on egusphere-2024-71', Aaron Steelquist, 28 Jun 2024
We thank the editor and both reviewers for their careful and considerate assessment of our work. The revised manuscript is markedly improved, in our opinion, for having incorporated the variety of clarifications and revisions suggested. Full response to individual comments can be found in the attached file.
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-71', Anonymous Referee #1, 17 Mar 2024
This manuscript presented river incision rates of the Mexican Hat reach of San Juan river, inferred from dating of exposure and deposition age of the terrace fill. The inferred incision rate which averages over ~50 Ka is one order of magnitude higher than the incision rate inferred from a ~1.2 Ma aged terrace fill of the Bluff terrace upstream of the Mexican Hat reach. The incision rate difference between the two terrace sites is hypothesized to be caused by a meander-cutoff event at the Mexican Hat. A quantification model of cutoff-induced erosion intensification is proposed and applied to the Mexican Hat reach, revealing that 50% of the incision rate difference can be explained by the cutoff. The authors also discussed other alternative mechanisms, including base-level change erosion wave propagation of the bigger area, glaciation of the source area, and lithological effects. I think this manuscript is generally well-drafted, the presentation is in reasonable logic flow, and the meander cutoff model is interesting. Before this manuscript is advancing to the next phase, I think a few questions need to be addressed:
Specific comments :
- The causal relationship between the terrace Q3 and the meander cutoff is not evidenced clearly. Meandering cutoff age is constrained by the dated terrace at the quarry, Q3, in this study. I think it needs to be justified with more details. At the quarry, the meander course cuts through the Q3 according to the descriptions in section 4.1. That is saying the Q3 is formed before the cutoff, not formed from the cutoff event. The relative time between Q3 exposure and cutoff is unknown. Two patch clusters of Q3 are also found upstream of the quarry and the meander cutoff (Figure 5b, colored with orange and blue). The elevation difference between Q3 strath and the modern San Juan river bed is not 100% from the intensified erosion due to the cutoff. Should the relief difference (relative to the modern San Juan bed) between the Q3 patch clusters that were affected by the cutoff and those not affected by the cutoff be used to justify the intensified erosion due to the cutoff? I wonder what the authors think about this problem? In addition, Figure 4c doesn’t give enough topographic resolution to show the cut-cross relationship between the Q3 patch and the paleo- meandering river course. The paleo- meandering river course (black dash line) is drawn seemingly randomly on Figure 5b, instead of demonstrated by a high-resolution topographic map in the background, or by field evidences.
- Section 3.2 and 3.3
- In the presentation of section 3.2, sampling strategy, sample type, and sampling locations are missing in this section. Although the authors used published data of Wolkowinsky and Granger (2004), it is necessary to briefly introduce the sample strategies and samples that have been re-analyzed in this study. And in addition, locations of these re-analyzed samples from Wolkowinsky and Granger (2004) indicated on Figure 1 should be indicated distinctively to stand out from the background.
- I think section 3.2 and section 3.3 should be merged into one section. The methods of these two sections are the same, but apply two (same-type of samples I assume) different terrace sites. I would recommend the author to structure the new section 3.2 into theories and supporting equations, then present the Mexican Hat sampling strategies and samples, the Bluff re-analyzed samples come to the end.
- Section 4.1
Cutoffs and paleo-meandering course, in addition to the mapped terraces, are presented in this section to assist the demonstration of relative timings of multiple events (terraces formation and two cutoff events). This section is important to understand the purpose and logics of the study of this manuscript. I would recommend some restructure effort to be done for this section. I think the terraces, meandering cutoffs, cut-cross relationship between mapped terrace and paleo- meandering course of the studied San Juan reach and the nearby upstream reach should be presented in this section to frame the problem, and address the scientific problem above (Figure 5a). And it should be expanded to justify the relative timing of cutoff and terrace formation of Q3, and Q2.
Actually, this section reveals a bigger problem I see about the presenting style of this manuscript. I think 1) the derivation of the hypothesis needs to be sharpened in the introduction section; 2) backgrounds, methods, and results sections are not phrased to serve for evaluating the hypothesis. They were phrased in a way that is not solving problem-centered which have brought difficulties for reviewing. For the second point, I suggest replacing the technology-oriented subtitles of the results sections into subject-oriented subtitles. For example, the “4.2 Reassessing Bluff terrace dating” can be replaced with “4.2 Long-term incision rate of the San Juan River” to match the “long-term” and “short-term” incision rates that were described in the hypothesis.
Technical correction suggestions:
- Figure 2, the “T” symbols next to numbers are not explained in the caption or in the figure legend. I guess they are strike and dip of layered outcrops. I would also recommend the authors add explicit descriptions of rock types to the geological units in the legend.
- Line 111, the “Cutler Formation” is mentioned only here throughout the manuscript. Seems it is the formation above the Halgaito Tongue Formation. Could you clarify it in the legend of the geological unit of Figure 2?
- Line 153-155, not sure I understand the “long-term” and “our short-term” here. What does it mean? New community-accepted parameter values were used for recalculation, but the exposure or erosion rate with these new parameter values are still the same time scale as that calculated with “old” parameter values. I am confused.
- Line 170-171, why?
- Line 237-238 and Figure 4c, the IRSL samples are from Q3, but why the Q3 terrace extent in Figure 4c doesn’t cover the IRSL sampling location? Is the Q3 too disturbed in the quarry to map a confident boundary of Q3?
- Line 245-246: better to indicate the sample locations on Figure 5b again to clarify which two cutoffs because Figure 5a has more than two cutoffs there. Then Figure 5b should be referred to in this sentence, not the whole Figure 5.
- Line 246: “Both are now drained by tributary streams”, I am afraid I can’t find the tributaries in Figure 5.
- Line 248-249: I feel it is a confusing sentence. This long sentence seems to be describing Figure 5b. Could you separate this sentence into two so that one describes the relative spatial relationships, and the other describes the inferred sequential events? And refer to the respective figure in the description.
- Line 263: it will be better to explicitly point out the physical meanings in addition to the symbols only. For example, t, is the exposure age (?), and the mean erosion rate, eta, is the average between t to modern (?).
- Figure 6: could you indicate the two sites consistently of the a and b in the captions?
- Line 322, 323: S and Sc indicate two different physical quantities, better to use more distinctive symbols.
- Figure 7a: legend of dots and circles is missing.
- Line 293: I would add “of Q3” to this sentence since there were 6 terraces presented in the “Geomorphic Mapping” section.
- Line 296: “…to 84–140 m Myr-1 This incision rate …” “…to 84–140 m Myr-1. This incision rate…”
- Line 306-309: are you comparing similar-aged cutoffs in different rock types so that the relief difference is from rock-erodibility? Besides, where are they on figures (Figure 5a?)?
- Line 314: very interesting implications, is there any other literature to support it?
- Line 348: Figure 5BFigure 5b
- Line 351: “…rate to be ∼342 m Myr-1 Using the measured…”“…rate to be ∼342 m Myr-1. Using the measured…”. More places miss a period symbol (e.g. Line 374, 375). Please check that carefully.
- Line 376: “…depositional age for the same deposit of…”, after some literature digging, I realized the “same deposit” is actually the Bluff terrace fill. It does no harm to point it out clearly instead of saying “the same deposit”.
Citation: https://doi.org/10.5194/egusphere-2024-71-RC1 -
RC2: 'Comment on egusphere-2024-71', Joel Johnson, 26 May 2024
I think the manuscript by Steelquist et al. is clearly and well written, presents new age dates that will be useful for researchers understanding the evolution of the Colorado Plateau, and should be published after minor revisions. I only have a small number of suggested improvements.
Line 24: add “incision” after Grand Canyon?
33-35: I realize this is a broad summary of previous work, but Cook et al. 2009 find rates of 300-500 m/myr, higher than the estimates in the previous sentence.
Figure 3d: Can you specify which terrace levels these are, in relation to figure 4? Are these Q2 and Q1?
171: Formatting of Nishiizumi et al (2007).
190: I realize later you discuss the assumption of not terrace erosion; because fig3d sure makes me think there has been terrace erosion, I suggest you put a mention here that you’ll address this assumption later.
207: I realize you address the point of sand deposits on top of the gravel later as well, but I got stuck on it here. Suggest you also add a sentence or statement that you discuss the specific stratigraphic relations between underlying gravel and overlying sand deposits, and how that influences interpretations of dates and rates, below.
250: What is the “this may be the only location” statement based on? Seems pretty interpretive; try to work this point into the discussion instead of here.
266: Clarify what the 14+-4 rate is based on, since its lower than the long-term incision rate. Is this based on the inheritance, representing the upstream basin at that time? Not everyone is as well-versed in the depth-profile methods as the authors; I suggest guiding readers like me a little more through these numbers.
282: “while of the older” has an extra word or needs more editing.
285: Suggest not saying the measurements are “correctly ordered”. The underlying gravels dating to be younger than the overlying sand deposits isn’t correctly ordered. I like the interpretation of why, it just feels like saying they are correctly ordered is overselling the data.
Figure 7 caption: reword to not have “showing” twice.
Discussion, 301: remove “being”
303: In this landscape, in my experience steps are usually associated with strong and weak lithologic changes. The authors bring this up in the next paragraph, but are you sure the strath terraces don’t also reflect that?
348: Maybe change “sinuosity of around 7.6” to “local sinuosity of around 7.6”?
355-365, or 415-420, or somewhere: I appreciate the broader discussion of why erosion rates are variable, and I think the cutoff bedrock meander is a nice example of that. There are many other papers the authors could cite that I think are relevant to sediment supply and channel morphology influencing long-term calculated erosion rates; two that come to mind are Ouimet et al. (2009) and Gallen et al. (2015). It felt to me like the discussion could be a little broader.
Citations
Ouimet WB, KX Whipple, BT Crosby, JP Johnson and TF Schildgen (2008), Epigenetic Gorges in Fluvial Landscapes. Earth Surface Processes and Landforms 33, 1993-2009, doi: 10.1002/esp.1650.
Sean F. Gallen, Frank J. Pazzaglia, Karl W. Wegmann, Joel L. Pederson, Thomas W. Gardner; The dynamic reference frame of rivers and apparent transience in incision rates. Geology 2015;; 43 (7): 623–626. doi: https://doi.org/10.1130/G36692.1
Citation: https://doi.org/10.5194/egusphere-2024-71-RC2 -
AC1: 'Comment on egusphere-2024-71', Aaron Steelquist, 28 Jun 2024
We thank the editor and both reviewers for their careful and considerate assessment of our work. The revised manuscript is markedly improved, in our opinion, for having incorporated the variety of clarifications and revisions suggested. Full response to individual comments can be found in the attached file.
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Aaron T. Steelquist
Gustav B. Seixas
Mary L. Gillam
Sourav Saha
Seulgi Moon
George E. Hilley
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Supplement
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