I apologize with authors and editors for the delay in submitting this review.

I must disclose that Prof. Parker was my research advisor during the doctorate, and I worked with him as post-doc. In short, he was my advisor/supervisor/mentor for 6 years (2005-2011). Prof. Parker and I co-authored two papers in the past four years.

I really enjoyed reading the manuscript. It clearly discusses how pea gravel can be transported by flowing water. This is a largely overlooked topic in the sediment transport literature. The lack of studies specifically designed to characterize transport of sediment with characteristic grain size between 1 mm and 10 mm resulted, in my opinion, in confusing arguments on streamwise and vertical sorting of mix-size sediments. This work fills the knowledge gap for uniform sediment and illustrates how models for the entrainment of mix-size sediment can be modified to account for the non-uniformity of the bed material.

In particular, the re-analysis of the Garcia-Parker data and the modification of the Garcia-Parker entrainment relation for uniform sediment provides a solid ground to determine if and when pea gravel is transported in suspension. The modified Wright and Parker formulation for mix-size sediment provides a place to start for size specific suspended load calculations. The discussions on the mode of pea gravel transport (section 5) and on streamwise sorting (section 7), along with the comment on how effects of particle shape on grain mobility can be incorporated in the present formulation (section 7) clearly show that this work is relevant and readily applicable. Example of application is the study of gravel-sand transitions indicated in the conclusions. The comment on how to account for particle shape indicates a path on how to extend entrainment relations to particles with non-spherical shapes, such as plastic fibers that also have a smaller density that many natural sediments.

I only have very minor comments and suggestions that are indicated in the edited manuscript. Working on an edited pdf file may not be the most efficient way to prepare a revised manuscript, I thus report two comments that are not related to text clarity or figure formatting below.

Lines 255-256: is the presence of a massive, basal unit (see Figure 17 in Carling, 2013) also indicative of the thick grain flow?

Carling, P.A. (2013). Freshwater megaflood sedimentation: What can we learn about generic processes? Earth-Science Reviews 125, 87-113.

Lines 354-356: is the formation of a diffuse (not abrupt) gravel-sand transition a possible consequence of the longitudinal sorting described for the mixture of 1/3 gravel, 1/3 pea gravel and 1/3 sand?

Enrica Viparelli

The authors set out to provide a physical basis for the distinct behavior of sediment in the grain size range 1-5 mm. The manuscript summarizes the state of knowledge and practice in the field of sediment transport as it relates to sediment finer/coarser than this grain size range. Then by revisiting existing sediment transport relations using a revised approach to the role of viscosity, authors argue that there is indeed a physical basis for “special”-ness of pea gravel.

Overall I found the framing, which challenges the paradigm of the gravel-sand distinction, to be very engaging. The manuscript generally makes a strong case that the gravel-sand distinction has been an organizing force in this field over several decades and I think the review of how and why this came to be is a great launching point for the analysis that follows. The quantitative reasoning is clear and convincing overall.

The comments below note several places where the clarity can be improved, both in describing the key mechanisms and in making the manuscript more accessible to those outside the trenches of sediment transport. While these are there are a number of suggestions below they should be quick to consider. I have no major criticisms – I think this work is timely, interesting, and impactful.

L34-38. Adding another reference or two would strengthen the case that the 2-mm grain size is a standard dividing line for classifying clast vs. matrix support.

L70-71: “repetition, convergence and rearticulation” – the basic idea comes across, but I am curious about how each of these social phenomena are distinct from one another. As this phrase also appears in the abstract, and the Butler reference is not well known in this field, a bit more explanation of the social science would be helpful.

Figure 2a: The reason for flagging “granite” particles is clear in the text, but at first glance it is mildly confusing why “granite” is included as a category as the other two are grain sizes (gravel or sand). Recommend Tweaking the legend entry to indicate a grain size (e.g., “granitic pea gravel”) or briefly explaining in the caption.

Figure 2a: missing units for D60.

L75-96. The discussion of Figure 2a and 2b is in reverse order compared to the figure itself. Suggest swapping the order of the subfigures and adding specific references to Figure 2a and 2b.

L98. Missing word around “may”

L99. Didn’t follow how “patchiness” is related to the abruptness/non-abruptness of the gravel-sand transition.

L101: Anthropogenic effects such as?

L101: “show this” – show what? The sentence is drawing a contrast for a pristine vs. a human-modified river described in the previous sentence, so it’s hard to follow what is consistent between the two rivers.

L109-110: Brief rationale for the role of viscosity would be helpful.

L117: What is this parameter with dimensions?

L140-142: If it’s the exposure of grains that makes them easier to move, why not refer to this as the exposure effect (rather than the hiding effect)? That would seem to draw a clearer contrast between the two effects: one effect that makes particles easier to move vs. one effect that makes them harder to move.

L158-160. Very helpful explanation.

L167. “little difference” still looks like a factor of a few. Perhaps clarify that the difference is smaller than one would expect given the ratio of the two grain sizes.

L204. It’s a bit ambiguous what “just as well” means here, as what’s being compared are two theoretical predictions (rather than predictions vs. observations). Suggest condensing this with the following sentence to cut straight to the finding that the predicted behavior is unchanged for D = 0.25 mm but drastically different for D = 4 mm.

L212-213. “predicts the data as well as the original form.” I’m confused, doesn’t Figure 4 compare two theoretical predictions? What are the data?

L215-216. Concordance issue. Should be “relations…encompass”

L279-281. I don’t know if anyone who isn’t familiar with the Shields number will be reading, but just in case, it may be helpful to go back to basics here and state that the particles are easier to move relative to their weight. Also, while the “lubrication” metaphor is evocative I left this paragraph confused as the physical mechanism by which viscosity makes smaller grains easier to move. I get that it comes out of the equations but can you help the reader build some physical intuition?

L299: This looks like a key point but seems like you’d need to dig into Novak and Nalluri (1975) to fully understand how the change in near-bed flow affects the Shields number. Slightly more explanation here would be helpful.

L330-331. “now…applicable” – good place to restate what this study has added in order to make this application possible.

L348. “pea gravel tends to become diluted over a long reach.” Reasoning is not fully clear until the thought experiment in the next paragraph – a phrase like “as described in the following though experiment” would help reader stay in the flow.

L356. I like the thought experiment, but the allusion to preferential mobility is somewhat unsatisfying because the thought experiment has only described a static condition (distributions of grain size in upstream and downstream reach). Perhaps this would be more effective if it considered two time steps: an initial condition before transport, and then final condition after transport. Might consider adding a sketch figure also.

L378-379: I find the latter description of pea gravel not being able to concentrate in a location where it can “dominate the deposit” to be more clear and informative than the “dilution” metaphor that is used elsewhere in the manuscript. Whereas dilution implies that the pea gravel is getting lost by addition of another grain size, the mechanisms that are the focus have more to do with the mobility of pea gravel relative to other grain sizes.

Figure 3b. In legend, indicate that blue line is the Dietrich relation.

L385. This table is very helpful. It did take a moment to figure out that the “[1]” indicates dimensionless variables. Perhaps this is a standard notation but I haven’t seen it before. A brief note to indicate that [1] indicates dimensionless would clarify.