The ms is extremely well written and easy to read. The approach is very well throight throuigh and the findings and recomendations clear. There are some minor typological corrections that can be picked uo by a copy editor. I would emphasise that is it extremely rate for me to come to such a conclusion on the fost draft. Indeed I only remember such a positive reaction to one other ms in the hundreds of reviews that I have done.

General comments:

Overall the paper has strong scientific significance and quality as well as strong presentation quality. The authors show how Colombian dams drive changes in temperature, dissolved oxygen, and suspended sediment in rivers using monitoring data. They also show if and how stratification is observed in reservoirs, and how that relates to downstream impacts. They connect their findings with the drivers such as the upstream catchment properties and dam design, and thus make a strong practical connection. The impacts on aquatic ecosystems of the tropical Andes are also presented. The authors end with clear and concise recommendations to regulators and planners, which makes it potentially highly impactful and socially-relevant. The authors connected all of these themes seamlessly and made it an enjoyable and insightful paper to read.

The authors were working with a limited dataset due to the data scarcity in the tropical Andes, which is a common challenge in other parts of the world where dams are booming. Thus, this paper has implications beyond the tropical Andes. The figures communicate clearly communicate important information and are helpful models for other studies to follow.

I have made minor comments which I hope will help make the manuscript even stronger, as outlined below. While the literature review is overall good, I believe the paper would benefit integrating information from other literature in other places, as I mention in the specific comments. I also believe that the authors have strong communication to regulators and planners, but could make some slight improvements for an even stronger impact. Lastly, I think that there could be a few more sentences towards the end that connect to the regional and global implications of the study (I didn’t have any specific comments on this). I think it’s pretty clear from the introduction, however would be interested to see more directly what the broader implications are for reservoir impacts on water quality from the perspective of the authors. Thank you for this important work!

Specific comments:

Lines 38-39: I question the use of “unintended”… it seems that consequences of dam construction now are well-known globally so I’m not sure that it can be generalized that consequences are unintended.

Line 55: A recent and relevant study that you may consider citing here is the following:

Flecker, Alexander S., Qinru Shi, Rafael M. Almeida, Héctor Angarita, Jonathan M. Gomes-Selman, Roosevelt García-Villacorta, Suresh A. Sethi, et al. “Reducing Adverse Impacts of Amazon Hydropower Expansion.” Science 375, no. 6582 (February 18, 2022): 753–60. https://doi.org/10.1126/science.abj4017.

Lines 55-58: It seems that some of these are specific to the Amazon, not global studies as the sentence implies. Consider using all global studies. Some examples to be considered are below:

Fragmentation:

Grill, G., B. Lehner, M. Thieme, B. Geenen, D. Tickner, F. Antonelli, S. Babu, et al. “Mapping the World’s Free-Flowing Rivers.” Nature 569, no. 7755 (May 2019): 215–21. https://doi.org/10.1038/s41586-019-1111-9.

Sediment:

Vörösmarty, C. J., Meybeck, M., Fekete, B., Sharma, K., Green, P., & Syvitski, J. P. M. (2003). Anthropogenic sediment retention: Major global impact from registered river impoundments. Global and Planetary Change, 39(1–2), 169–190. https://doi.org/10.1016/S0921-8181(03)00023-7

Syvitski, J. P. M., Vorosmarty, C. J., Kettner, A. J., & Green, P. (2005). Impact of Humans on the Flux of Terrestrial Sediment to the Global Coastal Ocean. Science, 308(April), 376–381.

Line 61: Flecker et al. (2022) (cited above) could also be a good example of a regional study. There are also several examples of strong regional studies from the Mekong river basin, such as the following:

Kummu, M., X. X. Lu, J. J. Wang, and O. Varis. 2010. “Basin-wide sediment trapping efficiency of emerging reservoirs along the Mekong.” Geomorphology 119 (3–4): 181–197. https://doi.org/10.1016/j .geomorph.2010.03.018.

Line 70: These parameters “are fundamental to the condition of aquatic ecosystems” seems vague. Perhaps it should be “are fundamental to understanding the condition”…. Or discussion about how the parameters being in specific ranges is fundamental to maintaining healthy aquatic ecosystems.

Line 99: Consider providing the link to the data source

Lines 101-102: Were you only able to access/analyze 2017-2018 data, and not other years? Or did you only select those years? Perhaps it would help to clarify so that readers better understand accessibility.

Line 124: I think “We feel that” should be removed as it dilutes the recommendation and makes it seem like an opinion that can more easily be ignored by regulators

Lines 212-215: This is an important point- perhaps it can be stated earlier in the paragraph as a topic sentence.

Line 264: See Dunn et al 2019 as another relevant  (and more recent) study

Dunn, Frances E, Stephen E Darby, Robert J Nicholls, Sagy Cohen, Christiane Zarfl, and Balázs M Fekete. “Projections of Declining Fluvial Sediment Delivery to Major Deltas Worldwide in Response to Climate Change and Anthropogenic Stress.” Environmental Research Letters 14, no. 8 (August 1, 2019): 084034. https://doi.org/10.1088/1748-9326/ab304e.

Lines 278-311: This section is strong, however mainly focuses on the need to increase monitoring frequency. There were other regulatory implications discussed throughout the text, such as the need to change the temperature regulation from 5 deg C to 2 deg C. I think it would help to briefly reiterate those various items in this section. It’s possible a regulator would skim the rest of the paper and look closely at this section—what would be the most important things to reiterate?

Line 302: Would satellite remote sensing be another viable option for monitoring some of the parameters, for at least a first-order approximation? Since in situ monitoring in the Andes is challenging, it seems to be a practical option to consider. There would be several limitations to consider, of course. I am aware of studies in the Mekong River basin where satellite data is used to monitor impacts of dams on sediment and temperature—see citations below (you don’t necessarily need to cite them in your paper, but perhaps they could help your investigation)

Bonnema, Matthew, Faisal Hossain, Bart Nijssen, and Gordon Holtgrieve. “Hydropower’s Hidden Transformation of Rivers in the Mekong.” Environmental Research Letters 15, no. 4 (April 1, 2020): 044017. https://doi.org/10.1088/1748-9326/ab763d.

Beveridge, Claire, Faisal Hossain, and Matthew Bonnema. “Estimating Impacts of Dam Development and Landscape Changes on Suspended Sediment Concentrations in the Mekong River Basin’s 3S Tributaries.” Journal of Hydrologic Engineering 25, no. 7 (July 2020): 05020014. https://doi.org/10.1061/(ASCE)HE.1943-5584.0001949.

Figure 1: Consider labeling the hydropower reservoirs on the map, since you refer to them by name in the text/figures, and the relative location seems to be important. Or, at least label the eight reservoirs in Table S3. I understand that labeling the reservoirs might made the figure too busy, but you could also use numbers to label in the map and add a table. Could also consider adding major cities, such as Medellin and Bogota (since urban effects are discussed in the text)

Figure 3: Consider adding horizontal lines for + 2 degrees and – 2 degrees (they could be in another color like gray?), since that’s an important threshold that you mention. Could also indicate the regulatory limit (5 deg) to emphasize the difference (which could help with messaging to regulators)

Figure 4: Consider adding line for -2 mg/L since this is the threshold that makes the downstream waters below the regulatory limit.

Technical corrections:

Line 128: “we are unlikely to [be] capturing”—need to add “be”

Line 180: Grammar is awkward— perhaps say “while other reservoirs” or “and other reservoirs”

Line 247: Here you say “the authors” but in other parts you say “we” – edit for consistency

Line 258-259: Need comma after “delivery” for consistency. Also, would be good to clarify that “downstream” refers to reaches downstream of the dam but upstream of the delta. Consider breaking this into two sentences to clarify these things as they might be confusing for people not familiar with the concept.

Line 299: should be “justifies” and no comma is needed before that

Figure 1: In legend for hydropower reservoirs, it should be >70 km2 (not <70 km2).

Figure 4: Y-axis on left side should be “DO” not “OD”

Table S2:  “Reserervoir” should be “Reservoir”

The paper “Patterns and drivers of water quality changes associated with dams in the Tropical Andes” by Winton et al. presents an assessment of the effect of Colombian dams on downstream water quality, specifically focusing on temperature, oxygen availability and sediment loss. Even though I find the paper clearly written, I think that is has some methodological drawbacks that need to be clarified. Given the relevance of the paper for the management of water resources in the tropical Andes, I consider it could be suitable for publication in HESS after some points described below are implemented in the manuscript. 

General comments

Data quality assessment: Even though the authors mention that the presented data has been curated by the ANLA (L.309), there is no information on how this procedure was carried out, so that the presented data can be reliable. Also, the authors must have carried out a quality control of the available data before using it in their analyses. This procedure is remarkably important considering that the data in the ANLA “repository” comes from different sources. The methodology should clearly specify how such a procedure was conducted so that the use of all presented data and the results from their analysis are justified.

Measurement methods: The paper should include a table summarizing the methods used for obtaining the presented data (e.g., measurement method, accuracy) at each sampling site as supplementary material.  

Description of study sites: Section 4.1 nicely describes some specific features of the monitoring sites which are important for interpretation. However, the paper would benefit is such features are described earlier in the manuscript (e.g., a new section 2. Description of study sites). The study areas should be described in the paper and summarized in a table including relevant information such as coordinates, elevation, state/province, depth, length and volume of reservoir, management. Part of this information is currently presented in tables and figures in the supplement, but is it important that the reader has direct access to it in the main body of the paper. Such information could be added to Table S1 and embedded directly in the manuscript.     

Temporal component: Considering the heterogeneity of the data (1 to 12 sampling dates in 2018), it would interesting to specify when the monitoring was carried out at each of the study sites (e.g., the different symbols used in Fig. 6 for Urra Reservoir). However, using too many symbols might complicate the plots visually. I suggest that the year is divided in 4 season (e.g., January-March, April-June, July-September, October-December) and that different colors per each season are used. In that way, differences among seasons could be observed and perhaps some additional insights into the role of temporal variability on the effects of dams on water quality parameters. This could even be done in current figure 6 in which the use of 12 different symbols makes it complicated to detect monthly temporal variations.

Ground water inflows: I would appreciate if the authors could comment on the potential influence of groundwater inputs on thermal and oxygen stratification in section 4.1.

Specific comments:

Titile: I think the title is a bit too broad both in terms of the topic and the region. I suggest the following title that I consider more appropriate based on the presented data and geographic scope or something along those lines: “Changes in thermal regimes, oxygen availability and sediment loss associated with dams in the Colombia”

P1.L20: specify the number of sites included in the analysis

P1.L22: specify percentage of all sites or number of sites  

P1.L25: specify percentage of all sites or number of sites

P3.L75: not sure what this means “with hypoxia is intolerable”

P3.L92: there is only one Andean Cordillera, but with different branches. Please correct accordingly

P3.L95: specify if the companies are private, public, and/or both types.

P4.L103: I suggest including Table S1 directly in the manuscript instead of as a supplement.

P4.L116: specify what “deep waters” mean in this context

P4.L117: explain how the sites are assigned to these categories

P4.L117-118: specify the data, equation and assumptions used to estimate Froude number

P4.L124: please rephrase without using the verb “feel”

P5.L143: here it says that data are from 2018, but in figures 2-5 it is reported (either 2017 or 2018). Please revise and correct where needed

P5.L149: Table S2 is very relevant. I suggest including it in the main body of the paper instead of as supplementary material

P5.L153: I suggest including Figure S1 in the manuscript and keeping S2 and S3 as supplementary material.

P6.L171: even though the text mentions 12, the figure actually shows 15. Please revise and correct accordingly

Section 4.1. The description of the sites should be included earlier in the manuscript, and here only the relevant aspects should be emphasized.

P5.L190: how was the residence time of deep water estimated? Please specify

P5.L191: Again, I think it is worth it including Fig. S1 in the paper

P10.L300-304: This paper (Pesántez et al., 2021) presents an automated sensor which allows measuring chemical water quality parameters in tropical Andean stream at high-temporal frequency. I think it could be useful in this part of the discussion.

Technical corrections:

P1.L22-25: Very long sentence, please split into 2 or more shorther ones.

P1.L30: “These findings HIGHLIGHT the importance of IMPLEMENTING environmental monitoring…..”

P2.L36: “… hydropower GENERATION, which is…”

P2.L39: replace trade-off by effects

P2.L42: replace too often by frequently

P2.L56-58: use comma instead of semicolon when listing

P3.L79: replace though by through.  “… largely GOVERNS …”

P3.L82: Does stratification lead to … (i.e., delete “the” from the question)

P4.L113: qualitatively ASSESSED

P8.L230: BOCACHICO

FIGURES

Figure 1: I suggest replacing “GENERAL LOCATION” by SOUTH AMERICA in the inset map. Also, the legend of the largest circle should be >70km2, not “<”

Figure 3: add dashed horizontal lines at +2C and -2C for reference of the thresholds described in the paper

Figure 4: add a dashed horizontal line at -2 for reference of the threshold described in the paper

Figure 5: add dashed horizontal lines at -50 and -99% for reference of the thresholds described in the paper

SUPPLEMENTARY MATERIAL

Table S1: missing important information such as coordinates, elevation, period of sampling. Include this in the main body of the text.

Table S3: Specify what the colors mean

Figures S1, S2, and S3: the text of these figures are not in in English, please revise and correct.

REFERENCES I USED

Pesántez, J., Birkel, C., Mosquera, G. M., Peña, P., Arizaga-Idrovo, V., Mora, E., McDowell, W. H. and Crespo, P.: High‐frequency multi‐solute calibration using an in situ UV ‐visible sensor, Hydrol. Process., (August), 1–15, doi:10.1002/hyp.14357, 2021.