Summary

The manuscript presents findings from laboratory studies on confluences in mountain streams. It builds upon a recently published work by St. Pierre Ostrander et al. (2023). In this paper, the experimental program has been extended by 30 additional tests to focus on the effect of the tributary gradient and confluence angle. The experiments were conducted for different total discharges, sediment concentration, tributary gradient, and confluence angle. The model values were based on information of more than 100 confluences in Italy and Austria. The results demonstrate that the discharge and sediment concentration affect the morphology of the confluence. In contrast, tributary gradient and confluence angle had only a minor effect. For all tests, similar morphological features have been observed, including deposition cone, separation zone bar, or scour.

Main comments / Conclusion

The paper is well written and the differences or extensions compare to St. Pierre Ostrander et al. (2023) are clear. The experiments have been conducted thoroughly and the findings are presented in a comprehensive way. I have the following main comments:

1. Main outcome of the paper is that the confluence angle does not affect confluence morphology, which is different compared to low-land rivers. Please add to Introduction and Conclusions what the findings of the low-land rivers literature were with respect to the confluence angle and describe the physics behind it. Why is this different for mountain streams? 
2. The focus of this paper was on confluence angle and tributary gradient: why did the paper include only 2 different angles and 2 different gradients, while 5 total discharges and 3 different sediment concentrations (per discharge) were tested? In addition, why did you decide to not vary the discharge ratio, as this was also mentioned in the outlook of St. Pierre Ostrander et al. (2023)?
3. How was “equilibrium” defined? Was it achieved after the 20 minutes of test run and did it not take longer? See for example Ancey (2020a,b): https://doi.org/10.1080/00221686.2019.1702594 and https://www.tandfonline.com/doi/full/10.1080/00221686.2019.1702595

I provide additional comments per line below. Based on my review, I recommend minor revision in form and content.

Comments per section/line

1. L116: Please add why these hypotheses have been formulated; what potential processes or governing parameters lead to these hypotheses?
2. L151: see also main comment 2, but why 5 discharges and why steady-state?
3. L155ff: Please add the accuracy of the measurement devices.
4. L162: Why 20 minutes? Which scaling factor did you choose and what was the reference value to derive this duration?
5. Table 1: Not all parameters have been introduced in the text or in the table (e.g., Qm, Qt). Please check entire manuscript. Please add Froude number and stream power to the table.
6. Equation 1: please introduce right after you first mention it and also introduce the parameter.
7. L244: “Discharges and related unit stream power above 45 l s-1” … please add unit stream power or write 45 l/s after discharge, otherwise confusing.
8. L249: Reference to subcritical flows – would be interesting to know Froude numbers of this study (see recommendation for Table 1)
9. Section 3.2+3.3.: Consider stating the value of the tributary gradient in addition to the test number; difficult to remember the test number that refers to a certain gradient. This would be especially helpful in the Figure captions and I recommend making them consistent.
10. L278: Here, I recommend to state the gradient value instead of “from the decrease in gradient”. For example: “A smaller tributary gradient of 5% led to reduced velocity and … compared to the depositional forms with a tributary gradient of 10%” or similar.
11. Figure 4: figure caption “supporting a qualitative representation of morphological differences” is different to L279 that the gradient did not have an effect. I recommend deleting the sentence on the qualitative representation in the figure caption.
12. L295: add gradients and add test numbers in brackets + refer to Table 1
13. Figure 5: please add test number.
14. Figure 6: please add tributary gradient
15. L329: I recommend deleting the sentence on the qualitative representation in the figure caption.
16. Figure 8: please add test number.
17. L349: Where are the results of the depositional volume plotted? Please add.
18. L356: Please revise this sentence; the statement is not clear to me. Consider adding the values of the confluence angle for clarification instead of “greater” – to what?
19. Figure 9: add angle to the caption
20. Table 5+6: add sigma also to the last 3 columns
21. Figure 10: add reference to Table 4
22. L434: Add more details on why turbulence increasing with increasing confluence angle.
23. Table 7: Why was a different statistical test used compared to the other factors? T-Test for angle compared to ANOVA for discharge and sediment concentration?
24. L452: See main comment 1. Please add references and briefly summarize physics behind it so the reader understands the differences between lowland and mountain streams.
25. L453: I recommend to remind the reader of hypothesis 1 and explain why it was confirmed. References to main plots in paper would be helpful.
26. L456: sympathetically? Not clear
27. L472: I recommend to remind the reader of hypothesis 2 and again explain why the same geomorphic units occurred.
28. Conclusions: see main comment 1 and consider rewriting the conclusion to add an explanation why the angle does not have an effect in mountain streams compared to lowland rivers. Last sentence comes a bit as a surprise and not so clear what is meant by sediment buffer zones.

The authors perform a series of experiments in a scaled physical model, to better understand the geomorphological dynamics at confluences of steep, high-sediment-load tributaries, into larger rivers, during a flash flood event (in the tributary). I agree that this is an area needing more research, and opine that this work is a good contribution.

My comments are separated into major issues, minor issues, and writing suggestions/corrections.

1. Major comments:

A.1. The manuscript needs a more extended and improved discussion of a series of scaling issues, including:

(i) Experiment run times vs. the expected range of actual flood durations in the prototype tributaries

(ii) Sediment density and size, and a better explanation of how these were chosen

(iii) Whether it was ensured that flow remains fully rough turbulent throughout

(iv) More importantly, as D50 is about 2 mm, and D16 around 0.7 mm, there needs to be a discussion about scale effects on the threshold for entrainment (critical Shields number), given that it will not scale geometrically for smaller sediments. This effect might not be that important, because D84 may be large enough (6.0 mm) but still, this needs to be explained/discussed.

A.2. In general, many of the figures, plots and tables are not at the adequate scale, or seem unnecessarily complicated, and/or do not have complete descriptions (captions).

(i) The “depositional geomorphic units” discussed in Section 3.1 deserve a specific figure, describing each one such units in more detail, at a better scale, so readers can understand exactly what is meant, visually. Figure should contain typical cross-sections and longitudinal profiles of the identified features or units.

(ii) Figures 3, 4, 7, A1 to A9 are all too small to allow for a clear visualization of what is happening at the confluence. Trying to show the total extent of both flumes makes it very difficult to clearly see what happens where it really matters. The focus should be on the confluence, showing the detail at a better scale (i.e., zoom into the confluence). This will probably make it impossible to keep all the figures in the paper, but I would rather see fewer of them, as long as they have the right combination of size and quality that allows me to visualize what is happening. Can’t a “supplemental material file” include each one of the figures you are now giving, at that smaller scale? I am sure there must be a better way; as it is now, even viewing the pdf file at 200% zoom does not allow me to resolve the details at the confluence in Figures 3 and 4, for example.

(iii) Related to the previous comment, on L.239-41 you state that “Deposition cones formed for all configurations and sediment concentrations when Q was 15 L/s and 1.5 L/s in the main channel and tributary, respectively.” But this is not clear at all from the panels and images mentioned in L.232-33! Visually, it would seem that deposition cones were formed in all cases except A1 (d) & (e), A4 (c) to (e). Thus, I do not understand the statement.

(iv) In Table 3, both text and table header need better explanations of what is being given here. Please mention that  0.5% and 5% and 10% are the longitudinal slopes.

(v) In Figs. 5, 6, 8, and 9, the combined use of two different types of charts (bars and scatterplots) to show the same type of differences is very confusing, visually.

2. Minor comments:

B.1. Why do the authors repeatedly refer to “sediment concentrations.” Is there an expectation behind this language that only suspended sediment matters?

B.2. The wordage "introduced factors" in 113, 118, 183, etc. is too general, and thus inadequate.

B.3. On L.214-15 you state “Volume samples were taken after an experiment with sample locations corresponding to both morphologic and hydraulic zones occurring in the channel.” Shouldn't you state "after each experiment?" Also, please explain with more detail the reference to “both morphologic and hydraulic zones”

B.4. On L.225-26, please indicate the values of Dmax and rho-s for your sediment

B.5. At different locations in the ms, but mostly on L.231 and thereafter, you mention “unit stream power.” I assume you refer to some kind of "length-wise averaged unit stream power," computed with the slope of the main flume and the sum of the discharges, right? And not to some local measure of Ω, the unit stream power at the location where each type of feature or unit occurs, computed with the local value of Sf, right?

More importantly, what does it mean to associate a single value of Omega to each type of unit? I ask because I do not understand the point of giving single values of average Ω corresponding to each experiment, in Table 3. What the table really does is not "associate units to unit stream power," but give the value of omega (as well as the geomorphic units) observed for each experiment. I also note that this is a “reach-scale averaged Ω” as discussed above, so what is the point of doing this? This should be explained better. You could actually add a column to Table 1 to give the values of Ω for each run…

B.6. What is exactly meant by "gradients of the deposited sediment" in L.306? This gradient is a local characteristic that will change continuously along and across the flume. This needs to be better explained.

B7. In the discussion on L.306-315 about bed aggradation/degradation, it seems to me that in the second case, that of aggradation in the tributary, the whole issue is a bit "artificial" and/or unrelated to the topic of study,  as (i) it relates to the impact of procedural choices (feed rate), (ii) that happen outside the confluence. I am saying this because the aggradation in the tributary starts at its upstream end only because too much sediment is being fed there, which would not happen in an actual river system (unless there were another steep tributary contributing too much sediment to our “main tributary, ” or else the tributary had a sharp decrease in slope or increase in cross-section,  just before its confluence with the main river).

B8. Because you have the previous discussion about deposition/erosion (aggradation/degradation) in the tributary, as discussed in Comment B.7, maybe it would be a good idea to remind the reader at the beginning of Section 3.4, in the Overview (Subsection 3.4.1), that all response variables here refer to the main channel, not the tributary.

B9. Discussion on lines 433-36: More than "the degree of turbulence increasing," I would think that this is caused by the increased lateral or transversal momentum with respect to the main, longitudinal direction (which in turn should result in increased turbulence). I don't understand the following comment about "improved mixing." IMO, it is the other way around: when the confluence angle is decreased, the flows align more closely (the transversal momentum is decreased) which should result in less mixing between the two flows, not more.

B10. “When more channel was available,” on L.457. Can you explain this better, please?

B11. Sentence on L.515-17: (i) Writing issue in that you can’t say “Tending towards sth was characterized…;”  you need a noun (e.g., “The tendency towards” or “The evolution ..”). (ii) More importantly, the meaning is not clear when you state that this “Tendency towards … was characterized by the geomorphic units… .” Please re-write giving more details, indicating what exactly varies with the geomorphic units (I mean: you stated before that you obtained the same units across all experiments so now that you say that these units “are indicators of the flood magnitude” – which varies – you must indicate what is specifically changing with the units). Also, maybe “reflected” is a better word than “characterized?”

B12. Your single sentence about future work, L.523-24, does not contain sufficient information to understand how “ecologically valuable measures” such as “sediment buffer zones” relate to the work at hand (beyond the fact that they are related to sediment).

3. Writing suggestions/changes:

L.111: laser scans?

L.128: … but considers an additional case for the tributary gradient as well as for the confluence angle

L.132: …while the tributary channel had a fixed bed

L.157:  … a more natural river bed, while the post-run ….

L.177:  Experiment 30 could not be completed…

L.186:  “subtle variations” in what, exactly?

L.222: grain size analysis

L.288-89: … of morphological differences at confluences caused by different tributary gradients"

L.361: “Statistical evidence of…” or else “Statistical analysis of …?”

L.375: Please replace “provoked” by “resulted in” or “caused” or sth else

L.379-80: Please rephrase this sentence, as the three main ideas (clauses) are not well linked logically

L.380: concentration instead of concertation; same thing on L.519

L.382: semi-colon instead of comma

L.411-12: tributary-transported sediment

L.416: Add semi-colon to “… on the response variables; (sigma) is the standard …”

L.417: Add semi-colon immediately after D

L.438: “two” instead of 2? And a semi-colon before sigma

L.441: Delete comma

L.443 and 445: The reference to Fig. 9 seems wrong

L.451: Maybe add some nuance to the sentence, like “… as one of the main drivers of confluence morphology, thus affecting the spatial distribution of hydraulic zones, in the case of lowland confluences”?

L.462-63: Please flip the sentence to get the grammar right. Maybe “It has been previously observed in mountain rivers (citations) that the tributary channel gradient responds to ...”?

L.474-75: Maybe “Adjustments to sediment concentration were reflected in varying ranges for deposition and erosion depths and volumes, as well as varying extents for these geomorphic units.” (“these” refers to the mention in the previous sentence that the same units occur across experiments)

L.484: Replace “the transport capacity” by “its transport capacity?”

L.508: I would just say “… the findings of the literature dealing with …”

L.514-15: Replace the second “as” with a “while?” Can’t have “… as …. as …”

L.559: Should say “the geomorphic units” (plural); also, “than” instead of “then”

Many locations: I don’t know whether this is an EGU thing, but I see that you separate numbers from %, e.g., “5 % tributary gradient,” but at the same time keep numbers together to units, e.g., “above or below 0.01m.” The rules I am used to are the exact opposite: “the channel has a 5% slope and is 12 m wide.”