Dear Editor,

The manuscript by Maksymilian Fukś investigates the impact of dam construction on the ice regime of two rivers in central Europe. The author is able to demonstrate clear changes in the ice regimes before and after dam construction. Overall, the paper is well written, and I find little issue with the main result. My main concern is about the implications of what this result means, with concerns also about whether the available data have been used as effectively as possible to make the argument. I also have a concern about the methodology. On my pdf there are some annotations of minor edits that would be required to improve clarity in some places, but these are relatively simple. Overall, I do believe this paper is publishable, but major revision is needed in order to account for the issues presented below. I am very happy for the author to contact me if they have any questions about this feedback or if I have been unclear. If these issues are successfully revised, this paper will be a good contribution to the study topic.

Best wishes,

Andrew Newton – Queen’s University Belfast

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Major issues:

1) In the discussion I am not convinced it has explained what the dam has changed in the ice regime itself. Yes, the ice season has reduced, and it does appear that this might be associated with construction of the dams, but that in itself does not do enough in my view, particularly given the scale of impact that you state this paper might have. I think there needs to be a greater investigation into the potential causality of these changes. You outline some theoretical ideas, such as changes in discharge, but provide no evidence from the study site that could help to back this up. I strongly suspect you are correct in your hypothesis about what caused the impact, but it would make the paper significantly stronger if you could demonstrate some of this. Perhaps these data do not exist, but if they do, then I would encourage using these data to help make your point. If you have data such as Figure 9 for other time periods, then this would be a good place to start.

2) There is clearly a difference in the ice cover trend before and after the dam construction (Figure 2 shows that), but these trends have been derived from just two time periods – before and after. I would like to see you try some moving averages of that trend, perhaps a moving window of ~10-15 years within the before and after time periods. I suspect this would help to demonstrate that it was not a gradual change, but a rather blunt one. This would help to make your point of the dam influence, though it still does not explain the causal mechanism.

3) I would also like to see some greater statistical analyses of the climate change signal. This need not be complicated – e.g., some work can clearly be done on investigating the climatological trends in the weather station data and how these trends might relate to your ice regime changes. If there is no, or only a small, climate change signal in your weather stations, then this helps to make your point that it must have been the dam construction that impacted the ice regimes. This would be especially useful if data are not available to address point (1) above.

4) I would like to see a table that provides the geographical characteristics of the different water and weather stations, such as elevation. It would also be helpful to provide a comment on which weather stations are used to infer the air temperature at which water gauges – this is not currently as clear as it could be. This will provide a stronger link between the datasets.

5) I would like to see greater discussion in the text about how reflective the author feels the weather stations are of the temperature that is likely to have been observed at the water gauges. For example, the lack of geographical information about the different data sites – e.g., elevation – means that I cannot be sure what elevation difference there might be between them, and this will certainly impact how representative those observations are likely to be. Maybe there is little difference in elevation, but this needs to be explained in more detail, and if there are major differences in elevation between the weather stations and the associated water gauges, then I would think that this needs to be taken into account when drawing up the correlations between temperature and ice presence. The strong correlations do suggest this might not be a significant factor, but I still think this needs to be properly accounted for, or at least discussed.

6) The impact of the work needs to be developed further. The start of your concluding point (1) is very important but given the above issues I am not sure you have presented a strong enough case to robustly make this argument. If you can make the above revisions, then you certainly will have. There also needs to be some extra information on what this result would also mean for climate change studies – essentially that the trends of a declining river ice cover are not a de facto proof of climate change, and the wider hydrological setting needs to be taken into account in such settings. If you are able to make the revisions above you will be able to: 1) prove the dam influence is the case, and 2) prove that climate is not the main driver. This is a key selling point of the paper and if that case can be made more strongly, it will provide a good contribution to the literature.

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Minor issues:

1) In a large number of places in the text it is not always clear what is being referred to. The reader can often infer it, but it is better to be specific and unambiguous – e.g., instead of saying “their”, “this” etc., state what it is you are referring to.

2) I would encourage rewording of the locations from above/below the dam to upstream/downstream of the dam. This is much clearer language. I highlighted some instances, but not all of them.

This paper presents an interesting method for the quantitative assessment of the impact of reservoirs on river ice regimes. I consider that this paper is suitable for publication, but major revision is needed, some issues need to be explained in a more detail and the discussion should be improved.

Specific comments:

• Some details on the environment of the water gauges and climatological stations would improve the manuscript. Some examples: How far are the water gauges located from the dam? Are the water gauges located above the dam affected by the backwater of the reservoir? (Backwater has the opposite effect on river ice regime.) Has there been any other anthropogenic influence on the river bed conditions in the river section under investigation? At what altitude are the climatological stations are located?
• Why was the average air temperature of the 14 days prior to ice cover occurrence used for the modelling? Please explain it more detailed. Had other temperature averages or cumulative temperature sums been tested?
• How have the climatic conditions developed 1950–2020? What kind of temperature trend has been observed? The results shown in Fig.2, 4–5 suggest that there might have been changes in winter temperature conditions, especially since the 1990s.
• In case of the San River, the available data set for the pre-dam period is slightly short and incomplete for comparison.
• When comparing the results of the 2 dams, it should be taken into account, that the post-dam reference periods are different (Dunajec River: 1993–2020, San River: 1969–2020). Global trends show that winter warming has accelerated since the 1980-90s. If this is also the case here, it should also be taken into account when interpretating and discussing the results.
• Why was it necessary to use satellite data to study the spatial extent of dam effects on river ice regimes? Please justify this more detailed. This could be analysed on the basis of river ice observations at the water gauges. Has the result of the river ice classification been compared with the observational data for the winter of 2016/17? Satellite data could possibly be used to reduce the data gap in river ice observations after 2015.
• Fig.6 and Fig.7: Please use a thicker line to represent the rivers. Perhaps the inclusion of water gauges would help the navigation on the figure.