PWAT-D-25-00045

Developing environmental flows and metrics to quantify river ecosystem needs for regional water planning in Georgia, USA

PLOS Water

Dear Dr. Rack,

Thank you for submitting your manuscript to PLOS Water. After careful consideration, we feel that it has merit but does not fully meet PLOS Water's publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Please submit your revised manuscript by Jun 19 2025 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at water@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pwat/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

• A rebuttal letter that responds to each point raised by the editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.
• A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.
• An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

We look forward to receiving your revised manuscript.

Kind regards,

Vasanthavigar Murugesan, M.Sc., Ph.D.,https://orcid.org/my-orcid?orcid=0000

Academic Editor

PLOS Water

Journal Requirements:

1. Please note that PLOS Water has specific guidelines on code sharing for submissions in which author-generated code underpins the findings in the manuscript. In these cases, we expect all author-generated code to be made available without restrictions upon publication of the work. Please review our guidelines at https://journals.plos.org/water/s/materials-and-software-sharing#loc-sharing-code and ensure that your code is shared in a way that follows best practice and facilitates reproducibility and reuse.

2. We have noticed that you have uploaded Figure files, but you have not included a list of legends. Please add a full list of legends for your Figure files after the references list.

3. Figure 1: please (a) provide a direct link to the base layer of the map (i.e., the country or region border shape) and ensure this is also included in the figure legend; and (b) provide a link to the terms of use / license information for the base layer image or shapefile. We cannot publish proprietary or copyrighted maps (e.g. Google Maps, Mapquest) and the terms of use for your map base layer must be compatible with our CC-BY 4.0 license.

Note: if you created the map in a software program like R or ArcGIS, please locate and indicate the source of the basemap shapefile onto which data has been plotted.

If your map was obtained from a copyrighted source please amend the figure so that the base map used is from an openly available source. Alternatively, please provide explicit written permission from the copyright holder granting you the right to publish the material under our CC-BY 4.0 license.

Please note that the following CC BY licenses are compatible with PLOS license: CC BY 4.0, CC BY 2.0 and CC BY 3.0, meanwhile such licenses as CC BY-ND 3.0 and others are not compatible due to additional restrictions.

If you are unsure whether you can use a map or not, please do reach out and we will be able to help you. The following websites are good examples of where you can source open access or public domain maps:

* U.S. Geological Survey (USGS) - All maps are in the public domain. (http://www.usgs.gov)

* PlaniGlobe - All maps are published under a Creative Commons license so please cite “PlaniGlobe, http://www.planiglobe.com, CC BY 2.0” in the image credit after the caption. (http://www.planiglobe.com/?lang=enl)

* Natural Earth - All maps are public domain. (http://www.naturalearthdata.com/about/terms-of-use/)

Additional Editor Comments (if provided):

As per reviewer's suggestion manuscript need major revision

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Does this manuscript meet PLOS Water’s publication criteria?>

Reviewer #1: Yes

Reviewer #2: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?-->?>

Reviewer #1: N/A

Reviewer #2: N/A

**********

3. Have the authors made all data underlying the findings in their manuscript fully available (please refer to the Data Availability Statement at the start of the manuscript PDF file)??>

The PLOS Data policy

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English??>

Reviewer #1: Yes

Reviewer #2: Yes

**********

Reviewer #1: I have reviewed the manuscript entitled, “Developing environmental flows and metrics to quantify river ecosystem needs for regional water planning in Georgia, USA” (PWAT-D-25-00045) by Rack et al.

The manuscript provides an excellent case study of the application environmental flows and discusses lessons learned/recommendations for future applications by water resource managers. The manuscript is well written and should be of broad interest to readers of PLOS Water. I recommend it be accepted for publication with the following minor suggestions:

1. Please discuss how managers can reconcile the application of functional flows, which are intended to be broadly protective of ecological processes and habitat in general, with metrics that relate to specific species life history needs. Will the species-specific requirements nest within the more general functional flow recommendations? With they be distinct or complementary?

2. Many of the metrics and recommendations relate to annual magnitudes and durations (as discussed on Pg. 15). However, couldn’t the recommendations also be seasonal or tied to specific types of years (e.g., critically dry years)? Please consider this option as well.

3. In the Discussion section, there is a discussion of the application of thresholds; I suggest the authors include language relating to the uncertainty (or confidence) behind those thresholds which would facilitate decisions that consider this uncertainty. Alternatively, probability-based targets could be used (e.g. the probability of having 10days above 425 m3/s).

4. I really like Figure 4 as a depiction of the overall application of functional flows approach. Is there a way that the recommendations could be converted (or depicted as) an annual hydrograph that a manager could use to set seasonal flow requirements.

Thank you. I enjoyed reading this manuscript.

Reviewer #2: Dear authors and editor,

I find this manuscript very interesting. The main focus is on the transformation of ecological information into practical thresholds, in a small number, in order to facilitate the integration of the e-flows in the decision-making and in water resource management system at the same level as the so-called “human uses”. The manuscript includes the general situation of the study area, a large river basin, and the general frame of water management in that region, a relevant part of the state of Georgia. The authors decided to use an approach based on functional flows (sensu Yarnell et al). In my opinion, the main goal of proposing functional flow is well developed. Other “themes” (L343-348) are explained with less detail. While the elaboration of a functional flows proposal may not be highly novel, it represents a meaningful and valuable contribution to the field of environmental flows.

I have tried to explain the suggestions in reading order, I hope they are clear. I think they can help improve the manuscript in several aspects and for a wider audience. I only have one concern related to the natural flow regime as the reference for the e-flows regime; my other comments are suggestions of smaller relevance in general.

- It seems there is limited explanation provided regarding key riparian processes—such as dissemination, establishment, and riparian water recharge for riparian vegetation—in the "foundation/method" section of the manuscript. Could you kindly consider elaborating on these aspects?

- It may be valuable to discuss how the prescription of flushing flows could potentially interact with modified (typically incised) channels. These rivers could experience intensified channel incision due to inappropriate flushing flow practices. I know there are experiences in this concern, I hope authors can find some documents about this topic. Additionally, specifying which dams are located in the Piedmont region and addressing whether these dams have contributed to channel incision could enhance clarity.

- concerning the “functional flows”, the first references where I found that concept were the following; please would you make clear if you have used this conceptual framework, and be more specific about the process of the method? This seems to be different from Yarnell’s,

- Escobar‐Arias, M. I., & Pasternack, G. B. (2010). A hydrogeomorphic dynamics approach to assess in‐stream ecological functionality using the functional flows model, part 1—model characteristics. River research and applications, 26(9), 1103-1128.

- Escobar‐Arias, M. I., & Pasternack, G. B. (2011). Differences in river ecological functions due to rapid channel alteration processes in two California rivers using the functional flows model, part 2—model applications. River Research and Applications, 27(1), 1-22.

- The elaboration of thresholds from scientific studies is the central content. Such studies supporting the functional flow components are explained (L199 and on), but I miss a clearer and more organised structured text, by e-flows component, to explain the studies better. To be technically sound, the explanations and justifications in this part are essential. Same comment for the flow recommendations below the Fall Line. Better use sub-sections, if possible. In both “sub-regions”, I suggest including some of the key species involved in some thresholds (if that makes the text too long, it could be in Annexes), I think that is helpful for many readers. Please balance the citations, the second sub-region should also include references. The readers should be able to obtain all the documents cited.

- Background. The method to extract thresholds seems to be the reading of ecological studies; however, previous e-flows methods that integrate e-flows into water management can provide interesting ideas, methods, and indicators, which are not mentioned anywhere, at least to show some solid background. In general, in a manuscript, the authors discuss their proposal against previous ones. E.g., there can be interesting indicators in the BBM, ELOHA, EEDS (eco-engineering decision scaling), etc. At the end of this document, I have added some references (apart from the fundamental papers of well-known methods that you know for sure).

- L215. Maybe the third one is just "moderate depth and velocity"?, or "intermediate" instead of deep?

- For channel maintenance flow, in some regions or countries, the bankfull discharge is identified as the channel maintenance flow (in some river basin mgmt. plans, and e-flows studies) to be released from dams, because it is equivalent to the effective discharge (maximum work of sediment transport, calculated upon a flow time series), is it not valid in your context ? I assume you have studies on the effective discharge and bankfull discharge, as there are some "regional" studies and regional curves to determine the bankfull stage.

- Concerning the components with no ecological studies, have you considered using the intervals of the EFC of the RVA approach? For me, it sounds logical to propose - at least- an interval of each indicator based on natural/unregulated (or naturalised) flow records. And the estimation of the EFC in the IHA software is quite easy.

- L262. Regarding "historical flow records or current conditions," there appears to be some confusion between the natural flow regime and the present (regulated) flow regime. It seems the present flow regime is being treated as a "natural reference." Could you kindly ensure a specific mention of the natural flow time series available at each of the points evaluated in the river basin? I assume these points would correspond to several of the red dots in the map—gauging sites—and large dams, rather than just one. Additionally, as you are aware, clearly distinguishing between natural and regulated flows across text, figures, and tables is essential for clarity and accuracy.

- It is interesting to note that, despite various studies, no indices have been identified relating to the duration of low-flow or high-flow events. In some rivers, the number of consecutive days below a habitat threshold is considered a crucial indicator for fish populations. For example, such events can severely impact fish populations, as shown by Capra et al., 2003. Additional references mentioned at the end of these notes provide other examples of indicators, such as those related to environmental flows (e-flows) and water management. In my view, for certain species, the continuous duration of a low-habitat event during a biologically significant period could be more critical than the total number of sparse days it occurs throughout the year. It would be greatly appreciated if the authors could clearly distinguish between “frequency” and the duration of events throughout the text. Although the frequency and duration of low flows are briefly mentioned (Lines 269–270), they are not utilized. Could you kindly elaborate on the reasoning behind this approach?

- The comparisons in Figures 2 and 3 present separate periods of similar length. However, it would be valuable to distinguish which of these periods feature flows that are dominantly regulated versus natural (if applicable). A common practice when evaluating indices is to observe their variations under these two distinct conditions. One reason for this distinction is that dam operation can prevent certain indicators from being maintained within the natural limits previously observed. Since the values of the indices under natural conditions are not presented, readers might struggle to assess or infer whether the thresholds of these indicators are achievable under the current dam operation rules.

- In some approaches of e-flows it is common good practice that the indicators should be discussed with the stakeholders, and also evaluated under different evaluation criteria. In the manuscript, the focus is only on the ecological criteria as the base for the thresholds. However, what authors call the theme 1 (L343-44) is also important. I would appreciate more explanation about the process where the partnerships around the e-flow metrics were developed (mentioned in L366-67), that is, the frame for stakeholders participation. There is a section about the general situation of the river basin, water uses, etc., but the reality about stakeholders participation, their rights, negotiation, etc., are relevant aspects to understand the water management framework.

- To further clarify the previous point: Regarding the assessment of the feasibility of the e-flows proposal, I’d like to emphasize that the IFIM methodology, along with other approaches, suggests that each e-flow regime should be evaluated against multiple criteria, including efficiency, feasibility, and risk analysis. Another example can be found in the EEDS method, where multiple layers of information are integrated to identify a feasible multivariate space. This approach aims to provide a mutually acceptable solution for the various stakeholders involved. While the thresholds appear to be grounded in ecological studies, it seems that additional criteria to assess the proposal might be absent. Could the authors briefly expand on this aspect, focusing on what they consider essential for readers to better understand the decision-making process?

- back to the question of the reference (natural) period, fig. 4 has the same limitation; the manuscript suggests that the year 2004 is an example of the "ecological baseline", a good natural reference with a natural flow regime that would respect all the riparian processes. But I suppose the river was regulated then. Please make clear what 2004 means for the authors, and why there is no natural flow regime as a reference.

- if possible, could you improve Figure 4 to clearly display more of the 5 components? The text and figure caption should clearly indicate if 2004 is an average year under flow regulation (altered flow regime, to a degree we cannot determine with the presented information).

- In my view, the manuscript should address an essential point: clearly stating that the natural flow regime is accepted as the reference for maintaining a healthy ecosystem and ensuring that the data is presented consistently with this principle. If the authors disagree with this perspective, it would be good to explain their reasoning and discuss it further. It is evident that functional flows can be applied or integrated with other methodologies (Lines 394–395), which reinforces the assumption that the natural flow regime serves as the fundamental reference, as is the case with most widely recognised e-flows methods.

- This manuscript primarily focuses on ecohydrology, analysing and formalising the relationships between flow indicators and biota, while also incorporating specific habitat studies that offer valuable quantitative assessments (ecohydraulics). These two approaches are complementary; however, as the authors have noted, direct flow-ecology relationships can be challenging to establish, and the associated uncertainty can be considerable. Rather than focusing solely on limitations, it might be beneficial to include examples that highlight positive perspectives on the methods and some reasonable outcomes. To illustrate, cases such as those presented by Webb et al., 2017, and Fornaroli et al., 2020, could serve as valuable references from different continents.

- A similar observation applies to the discussion on population dynamics and positive cases. Some models of fish populations have demonstrated that habitat indicators, such as the weighted usable area (derived from physical habitat simulations), are highly relevant (e.g., Sabaton et al., 1997; Gouraud et al., 2001; Capra et al., 2003). As the authors may be aware, the manuscript by Lancaster and Downes (2010) spurred a significant discussion in various subsequent publications. Based on my careful analysis of that and related papers, my conclusion is that the manuscript cited by Lancaster and Downes included examples that were either incorrect or inappropriate. It appears to lack serious consideration of robust scientific studies conducted by multiple research teams across several countries, many of which have demonstrated highly positive results. Consequently, I recommend replacing this citation with others that are more credible and reflective of the broader scientific consensus.

- The authors mention that flow-ecology relationships can be "complex for rivers like those in the southeastern U.S. that harbor high species diversity." While I agree with this statement, it could be elaborated further by incorporating examples from the extensive body of research developed over the years. Methods such as the BBM, ELOHA, and the Savanna process—applied in regions like the USA, South Africa, and Australia—have significantly advanced our understanding of e-flows. Additionally, numerous studies on e-flows in diverse rivers across the USA could provide valuable insights. Are there no compelling examples from the USA that demonstrate robust e-flows studies in rivers with high biodiversity? Including such examples would strengthen the discussion and provide a more global perspective.

- Regarding strategic monitoring and model-updating, I recommend the authors consider one paper, in my opinion, one of the very good papers published in this matter: Souchon et al. (2008). Detecting biological responses to flow management: missed opportunities; future directions. River Research and Applications, 24(5), 506-518.

- Concerning non-prescriptive methods of e-flows, I would like to remark that other well-known methods are neither prescriptive. For instance, the IFIM and the DRIFT are not prescriptive; they evaluate alternative scenarios of water management, and then they establish thresholds (minimum flow in different biological significant periods, high flows for fine sediment flushing, etc.) to be evaluated, modified and selected by the stakeholders.

- Additional manuscripts concerning e-flows, thresholds and water resource management that you could consider for discussion (comparing shifts in the annual frequency or duration of flows above/under thresholds, climate change scenarios, dam operation,...):

- Muñoz-Mas, R., Macian-Sorribes, H., Oliva-Paterna, F. J., Sangelantoni, L., Peano, D., Pulido-Velazquez, M., & Martínez-Capel, F. (2024). Adaptation measures to global change in the Serpis River Basin (Spain): An evaluation considering agricultural benefits, environmental flows, and invasive fishes. Ecological Indicators, 161, 111979.

- Solans, M. A., Macian-Sorribes, H., Martínez-Capel, F., & Pulido-Velazquez, M. (2024). Vulnerability assessment for climate adaptation planning in a Mediterranean basin. Hydrological Sciences Journal, 69(1), 21-45.

Cited References

Fornaroli et al., 2020. https://doi.org/10.1016/j.scitotenv.2020.138052

Sabaton et al., 1997. https://doi.org/10.1046/j.1365-2400.1997.00084.x

Gouraud et al., 2001. https://doi.org/10.1002/rrr.655

Capra et al., 2003. https://doi.org/10.1002/rra.729

**********

what does this mean? ). If published, this will include your full peer review and any attached files.

Do you want your identity to be public for this peer review? If you choose “no”, your identity will remain anonymous but your review may still be made public.

For information about this choice, including consent withdrawal, please see our Privacy Policy

Reviewer #1: No

Reviewer #2: No

**********

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

PWAT-D-25-00045R1

Developing environmental flows and metrics to quantify river ecosystem needs for regional water planning in Georgia, USA

PLOS Water

Dear Dr. Rack,

Thank you for submitting your revised manuscript to PLOS Water.

We apologize for the delay in the reviewing process for your resubmitted manuscript. The two reviewers of your original submission were not available to consider your revisions, and your first Handling Editor also became unavailable.

Nevertheless, after careful consideration and internal editorial review, we feel that it has merit but does not yet meet PLOS Water's publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

In general we are satisfied that you have responded appropriately to both thorough and relevant reviews. There are two exceptions to this, one to do with referencing, and one to do with stakeholder engagement. These two points, along with a third comment about tense, are outlined in "Additional Comments" below. Accordingly, my recommendations here are that:

1. the two matters (exceptions) are attended to;

2. the entire manuscript should be edited carefully for tense; and 

3. the referencing is carefully checked throughout the manuscript.

Please submit your revised manuscript by Dec 17 2025 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at water@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pwat/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

• A rebuttal letter that responds to each point raised by the editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.
• A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.
• An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

We look forward to receiving your revised manuscript.

Kind regards,

Pierre Horwitz

Editor-In-Chief

PLOS Water

Journal Requirements:

If the reviewer comments include a recommendation to cite specific previously published works, please review and evaluate these publications to determine whether they are relevant and should be cited. There is no requirement to cite these works unless the editor has indicated otherwise. 

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article's retracted status in the References list and also include a citation and full reference for the retraction notice.

Additional Editor Comments:

First, the authors have responded to a comment from Reviewer 2 thus: “We have added text throughout the manuscript to better describe the interactions with the water council members, specifically - see page 17 lines 629 [sic]-549 and paragraph 2 in the discussion.” Assuming that the authors are referring to the whole of paragraph 2 in the Discussion (lines 431-450 in the cleaned document), I accept this explanation, but I am confused by what was done as part of this research and reported, and what could or should next be done with respect to stakeholder engagement.

Furthermore, the need for clarification around what was done is exemplified in the section on “Communication and Contextualization of Environmental Flow Thresholds” which precedes the Discussion. Starting in Line 378 the authors explain what was communicated to water planners/water council – no text is provided to show how the water planners responded to this communication. To compound matters, this section changes tense several times: starting in lines 378 the past tense to convey what was communicated (presumably). The next paragraph shifts to what 'can be' and 'what is' (lines 386-400), then shifts back to past tense (lines 400-403) again presumably for what was communicated, and finally back to the present tense (lines 403-405) for what is presumably a generalization.

Second, Reviewer 2 made the comment “The readers should be able to obtain all the documents cited.” I understand (and completely agree with) the authors’ comments about the importance of contextualization and that this necessarily requires reference to unpublished works and the grey literature in general. However, quite a few of the references are referred to loosely, without sufficient detail for them to be located by an interested reader. Worse, some internet citations are unavailable (for example those cited in: lines 304-5 = “Sorry! That page doesn't seem to exist.”; lines 645-7 = “Forbidden”; lines 648-9 = “problem loading page”). More referencing issues are listed below.

Minor matters requiring editing:

- authors use U.S.. US, U.S.A., and USA – please standardize for an international audience)

- line 98 close bracket

- Line 110-1 - and set in an adaptive management framework - this point could also be made in the Discussion

- Line 276 FERC not FREC

- Line 310 Table 2 caption – add the word “number” after “gage”. Also, does USGS need to be spelled out for an international audience?

- Line 330-1 (and elsewhere) USGS 2024. Is a reference? If so, shouldn’t it be in the references and formatted accordingly?

- Line 358 what does “(table)” mean?

- Line 369 Sinclair

- Line 370 I think “1970s” is correct.

- Line 400-1 “two of the three functional”

- Line 465 – to whom was it acceptable?

- Standardise journal references either abbreviate journals or write them in expanded form, not both.

- Line 694 I think it is Pine III WE

- Line 729-731 This should be 2014 and give page numbers.

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

Developing environmental flows and metrics to quantify river ecosystem needs for regional water planning in Georgia, USA

PWAT-D-25-00045R2

Dear Ms Rack,

We are pleased to inform you that your manuscript 'Developing environmental flows and metrics to quantify river ecosystem needs for regional water planning in Georgia, USA' has been provisionally accepted for publication in PLOS Water.

Before your manuscript can be formally accepted you will need to complete some formatting changes, which you will receive in a follow-up email from a member of our team. 

Please note that your manuscript will not be scheduled for publication until you have made the required changes, so a swift response is appreciated.

IMPORTANT: The editorial review process is now complete. PLOS will only permit corrections to spelling, formatting or significant scientific errors from this point onwards. Requests for major changes, or any which affect the scientific understanding of your work, will cause delays to the publication date of your manuscript.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they'll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact water@plos.org.

Thank you again for supporting Open Access publishing; we are looking forward to publishing your work in PLOS Water.

Best regards,

Pierre Horwitz

Editor-In-Chief

PLOS Water

***********************************************************

Additional Editor Comments (if provided):

Reviewer and additional editorial comments have been addressed satisfactorily.

Reviewer Comments (if any, and for reference):