The paper discusses the extremely pertinent topic of over-harvesting of rivers through sand mining. The region of focus chosen by the research team is particularly important as it is one of the highest impact global regions with respect to sand mining. Given the relatively limited academic literature around quantification of sand mining from different river ecosystems across the world, this paper makes an extremely important contribution to both literature and data on sand mining. I was very excited to read this paper and the figures that emerge from it. This paper provides new, and higher numbers which are highly likely to much closer to the reality of sand extraction in the LMK than previous estimates. I also hope to see further work from the authors as this is important, urgent and necessary data.

However, I am not an expert in GIS mapping and the other technical aspects of this research paper, and hence cannot comment on the details of the scientific assessment process. I hope other relevant reviewers can address technical issues and preform the quantitative check.

The abstract needs some grammatical improvements.

P1, L10: ..in part due to (followed by only one reason)… seems incomplete //either more “reasons” could be added here or there could be a minor edit to this statement. Next line: grammatically incorrect sentence: Can it be replaced with…?: Current estimates, based on 2013 figures, indicate that basin wide sand extraction in the Mekong River stood at least 50 Mt.

“Year on year” could be changed to: on a yearly basis? Or just “yearly”?

“We use…..respectively” is a very long statement and can easily be broken down into two or even three to enhance clarity for the reader.

P1, L29: Although it is correct that massive growth has been observed in the last three decades (around 1990s with neoliberal reforms being pushed through post Washington Consensus, trends show the increase since 1970s already). It might be interesting to include this paper… See Miatto et. al., 2016 paper quantifying global non-metallic mineral consumption.

P1, L30: Although correct, it might be better to specify: Estimates indicate that currently/today, at minimum, between 32-50 billion tons of aggregates…

P2, L32: …; with impacts manifesting as… statement could end with …, among others. Given the many more serious ecological and environmental impacts of the over-extraction of sand. You could see/quote Bisht, 2021 paper on global sand extractivism for this.

P2, L35: is reaches correct? Or is it “regions”?

P2, L39: …pervasive and diffused…

P2, L56: “changed”

P2, L54-56. The statement is too long and seems a little unclear to me (The last part is referring to the recent bans in extraction and export of sand to Singapore?). Can this be split at “…(Hackney, 2020). However, tighter contractions on locations and rates of extraction, …., are currently lacking…

P2, L62: could “sand transport” here be replaced with another phrase? Maybe, sediment transport. I was a bit confused about whether this meant the amount of sand being transported out of the LMK, until I read the details of the methodology below. 

This paper quantifies sand extraction from the lower Mekong River using remote sensing. The authors document the number and dimensions of sand mining vessels from PlanetScope and Google Earth images respectively. Based on sand mining vessel dimension and number, the authors estimate the volume of sand extracted each month from 2016–2020. The extraction rate is compared to the total sand transport estimated from previous studies, and it is determined that the extraction far exceeds transport. As a consequence, the LMR bed is scouring. The authors demonstrate that the deepest scours of the channel bed correspond to the most active areas of mining, and therefore conclude that the primary cause of these bathymetric lows is driven by sand mining. This paper offers a simple method to estimate sand mining in the world’s large rivers. The content is timely, as the industrial demand for sand continuously increases.

Comments:

The authors nicely demonstrate a correlation between intense sand mining and localized channel bed scour (e.g., Figure 7). It would be helpful if the authors made an explicit connection between these highly localized observations and the broader impacts of sand mining outlined in the introduction (L48–51). Do the scour locations here correspond to enhanced bank erosion or other morphological responses?

It would be helpful for the authors to explicitly state why Planet data are used to estimate the number of mining vessels and aerial images from Google Earth are used to get dimension. Why not use the daily images on Planet to estimate dimension? Presumably this is due to resolution, but the resolution of the Google Earth images is not stated.

Are the scours filled at all during low mining times?

Line Edits:

L12: remove “and”. “…sand extraction are 50 Mt based on estimates from 2013.”

L21: specify if this incision is localized or widespread.

L108: Please provide more information about the aerial images. How many images were used? What dates do the images span? Resolution?

L120: Is it assumed that mining vessels are active 7 days a week?  

L127: Be consistent when reporting ranges (e.g., L127 “2016  –  2020” vs. L59 “2016–2020”)

155: Be consistent reporting negative values. Sometimes a space is included, sometimes not (e.g., L157 “- 0.6 m” vs. L21 “-0.26 m”

L193: “negate the negative” – Improve word choice

L187–188: Please clarify this sentence. What is meant by “basin sand supply”? Sand supply to the basin?

L216: Should read “saw mining operations begin in 2017” rather than “saw mining operations began in 2017”

L228: “Attendant”? Word choice

283: Label the apex on Figure 1

Figure 1 caption: It’s not clear what the author means by “basin country.”

Figure 3 caption: State why water level is important here. (High water levels correspond to low vessel traffic).

Figure 4: Please state the source of data for the natural sediment supply at Kratie. Also, please choose a color-blind friendly palette.

Figure 7 L453: The word “change” is missing. The median rate of bed elevation…(insert change)