Lessmann et al. presented a very interesting study that reveals the importance of CH4 emissions due to reservoir flushing. To the best of my knowledge, this is one of the first studies that estimate CH4 emissions due to reservoir flushing. Their results indicated that this CH4 emission pathway could be an important missing piece for the reservoir CH4 cycle. Besides, Lessmann et al. also answered several questions that I had in my mind before reading the work, including 1) how this CH4 emission pathway compares with other pathways in this reservoir; 2) how this CH4 emission pathway interacts with other CH4 emission pathways; 3) how important this CH4 emission pathway might be for other reservoirs in the globe. In addition, the work also provides reasonable operation advice to reduce CH4 emissions from this pathway. Although the estimates still have many gaps and the conclusion may not be applied to other reservoirs of different environments, as a pioneer study it will help encourage more following studies to bridge these gaps and address the transferability issue. Overall, I think that it is well-written and all results are clearly explained. I recommend its publication in this journal.

Some comments for the authors to consider.1) The manuscript does not provide the information of reservoir age. Previous studies showed that the transition of carbon dynamics with reservoir aging is significant (Maavara et al., 2020). It is thus valuable that the authors can put their estimates and discussion in this context and warn the audience that the importance of this pathway can change significantly in time. Maavara, T., Chen, Q., Van Meter, K., Brown, L. E., Zhang, J., Ni, J., & Zarfl, C. (2020). River dam impacts on biogeochemical cycling. Nature Reviews Earth & Environment, 1(2), 103-116.

2) The method to estimate diffusive CH4 flux from sediment to the water column is only accurate when there aren't any large CH4 production or oxidation in the surface sediment layers. But Figure 3d shows that the gradient of CH4 concentration changes between 0 and 2.5 cm depth, implying that CH4 oxidation occurred during April 2019 sampling and CH4 production occurred during both June and September 2020 sampling. Do the authors have a sense of the related estimate uncertainty?

3) Is the y-axis of Figure 2b really sediment depth? I suspect it is still water depth, consistent with Figure 2a. Anyway, I cannot tell that DO in September 2020 were oversaturated near the water surface and above 2 mg throughout the entire water column from the figure, as described in Line 180.

This study measures vertical profiles of methane in reservoir sediment porewater to estimate the potential for enhanced methane emission during sediment flushing events (a management strategy employed to reduce sediment accumulation in reservoirs).  The authors then compare the magnitude of these potential emissions to existing estimates of methane emission from the reservoir via more commonly recognized pathways.  The study is the first that I know of to consider the potential methane emission impacts of a management practice that is likely quite common in certain types of reservoir ecosystems (I was previously aware of this practice particularly for reservoirs in tropical/monsoon driven climates).  While the study is limited in it’s reach (e.g. it doesn’t document emissions during a flushing event; it just estimates potential), it points out an important gap in our understanding and the findings suggest that this emission pathway should be studied in more detail. 

I have two major comments: 1.) I think the authors should dial back their conclusions a bit and place more emphasis on the need for empirical study of these flushing events.  I suggest adding a bit more text discussing the potential for methanotrophy once sediment is mobilized and how this may depend on travel time to the dam outlet.  2.)  To the extent possible, the authors should spend a bit more time familiarizing the readership with flushing events as a management practice (for example: do we have any idea how common a practice is this? how well constrained are estimates of sediment erosion during these events (both total volume and spatial/depth relationships?).  I think a table describing some of this literature would be helpful for putting this in context and helping the readership to understand the potential importance.  Particularly, the information about flushing frequency and all the papers cited on lines 63-65.  How different are the reservoirs where flushing is conducted (e.g. do they generally have to be small enough for the sediment plume to have a relatively short residence time?)

Overall I think this is a creative and timely paper that points to important areas for future work with potentially important management implications.

Line by Line:

Line 6:  consider rephrasing: “Reservoirs are globally significant sources of the greenhouse gas methane”

Line 16: add “seasonal” between “depends on the” and “timing”

Lines 16-17:  Another potentially more straightforward way to say this would be: “Larger flushing events that mobilize deeper sediments lead to non-linear increases in CH4 mobilization"

Lines 21-22: I think I’d use this sentence to describe important next research steps—we need to know a lot more before we can start including this pathway in large scale estimates.

Line 24: I’m not a fan of stating the number of global reservoirs this precisely… it makes it sound like there isn’t much uncertainty in this number (even thought there is).  Consider “Worldwide millions of reservoirs have been constructed”—and possibly add a reference to Couto and Olden 2018 (https://doi.org/10.1002/fee.1746).

Line 28: omit “the required”

Line 30:  Check units on this reported range (your high number looks too high).  See a new review by Lauerwald et al. 2023 (https://doi.org/10.1029/2022GB007657) for a range of 10-52 Tg CH4 yr-1 from existing global reservoir estimates (which includes the lower estimate by Johnson et al. 2021).  I also suggest you include yr-1 in your units (rather than just saying “annual” budget)

Lines 33, 42 and throughout:  I suggest avoiding the term “drawdown” flux as synonymous with degassing flux.  When I hear the term “drawdown” flux I think of either: 1.) the ebullition flux that occurs when hydrostatic pressure drops during reservoir drawdown or 2.) the flux from drying littoral sediments that occurs when water levels are drawn down

Line 41:  I suggest referencing Denfeld et al 2018 (https://doi.org/10.1002/lol2.10079) here and in the discussion.  I think it would be helpful to discuss the importance of constraining how much of the flushed sediment methane might get oxidized before passing downstream.  You could draw from the turnover literature for this, but also discuss the importance of the sediment plume residence time.   

Line 45:  I suggest citing Harrison et al. 2021 (https://doi.org/10.1029/2020GB006888) who estimated that over half of the global methane emissions from reservoirs may occur via degassing.

Line 49:  Or load following/hydropeaking operations & ship lock induced changes in water level (Maeck et al 2014 https://doi.org/10.5194/bg-11-2925-2014; Harrison et al. 2017)

Line 115: This is the first paper where I’ve seen NaCl used to reduce the solubility of methane in the water sample (and force it into the headspace). I see there is another published paper using this approach, but it isn’t common to my knowledge. According to my bunsen’s solubility calculations, the concentration of NaCl you used would reduce solubility by about an order of magnitude (from 0.03 to 0.0015 liters per liter at STP).  I might just add a line explaining whether you calculated the methane still dissolved in solution (or whether this was just assumed to be nominal?)

Line 154:  Looking at the rest of the paper, I’m unclear what the C CH4sed estimates deeper than 15 cm were used for?  The estimates provided were all using the 1-15cm layer (unless I missed something)?  If you do use these deeper concentration estimates, then I suggest bounding them (since the April profile suggests that methane concentration likely continues to increase with depth past 15 cm).  

Line 174:  Cite the papers that you are using for CH4 emissions via other pathways at Schwarzenbach here.

Figure 2:  I’m confused by your y-axes.  A standard approach in limnology is to plot depth on the y axis (with values in reverse order where 0m is at the top and 40m is at the bottom).  Also, I think you are plotting water column dissolved oxygen concentrations in panel B, but the y axis suggests these may be porewater concentrations?

Line 210-211:  Is there much literature on different erosion depths?  If so, then explain your assumption.  If not, then argue for more study of this in the future. 

Figure 3d and Figure 4b seem very similar to me. Do you need 4b?

Line 239: Deemer and Harrison 2019 (https://doi.org/10.1007/s10021-019-00362-0) is another study that contains sediment porewater methane concentrations for a small reservoir—maybe not necessary to add, but it does also discuss how timing of drawdowns (relative to seasonal methane accumulation) can be important for determining water column flux from sediment. 

Line 246:  add “relative to methane oxidation rates” after “are enhanced at higher temperatures”.  Also, the Shelley paper you reference showed that oxidation may keep up with methane production as temperatures warm in some systems, so you could do a “but see” reference there.

Line 254:  Might spatial variability in bottom water dissolved oxygen concentration also explain this?

Line 261 (and elsewhere):  How rapidly might this transport happen?

Line 265-267:  I think you should mention this scoping study way earlier (when you introduce the study site).  As I read the paper I was wondering if flushing was actually ever conducted at Schwarzenbach until I got to this line in the paper.

Line 268-270:  Cool insight?  Could you work this factor of 2 estimate into the abstract?

Lines 285-291:  Make sure to tell the reader that the reservoirs are similar in size--  I had to look back in the study site description to see that the volumes were similar.  Similarly on line 298 you could provide an areal rate in addition to a total mass.

Line 302:  change “provide” to “replenish”

Line 310:  you could point out that theory would suggest replenishment rates would be higher in more eutrophic reservoirs

Lines 314-315:  suggest rephrasing for clarity—“In addition to methane emissions driven by sediment erosion, the water level drawdowns that accompany reservoir flushing events may also lead to enhanced methane emissions.”

Line 321-324:  This sentence is hard to digest.  I suggest rephrasing to something that emphasizes uncertainty a bit more… maybe something like “The relative role of erosive forces versus hydrostatic pressure drops in driving methane emissions during reservoir flushing events is an area for future work.”

Lines 324-325:  Enhanced methane emissions from drying sediments is not a ubiquitous pattern & some studies show dried sediments can act as a methane sink (Yang et al. 2012; https:// doi.org/10.1029/2011JD017362). I suggest rephrasing that this is an important area for more research.

Line 332- change “causes” to “will likely cause”.  Your study is estimating emission potential, but you don’t know how methanotrophy will play into ultimate emission dynamics.  I think you should be sure to emphasize that emissions should be studied during an actual reservoir flushing event to learn more.

Line 336:  delete “might be severely”—I wouldn’t call the values you present “severe”, but you can state that ignoring the pathway leads to underestimation.