Differentiated Evolution of Two Mid-Channel Bars in the Middle Yangtze River’s Urban Reach: Coupled Drivers and Terrestrial Habitat Assessment

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions in track changes in the re-submitted files.

2. Point-by-point response to Comments and Suggestions for Authors

Comments 1: The research methodology is well chosen and very well illustrated in tables. The principle of the presented method is somewhat similar to the River Habitat Survey (RHS) used in European Union to assess the hydromorphological condition of rivers. This European method also compares the degree of preservation of natural elements and the degree of transformation of the river by human activity. If the authors believe this to be the case, I suggest mentioning this method in the introduction where river habitat assessment methods are listed.

Response 1: Thank you for pointing this out. We agree with this comment. Therefore, we have added this method in the introduction where river habitat assessment methods are listed (line 83).

“Internationally, relatively complete river habitat assessment methods and systems have been developed, such as the rapid bioassessment protocols (RBPs) proposed by the U.S. Environmental Protection Agency [27], the qualitative habitat evaluation index (QHEI) developed at Ohio State University [28], the Riparian, Channel and Environmental Inventory index (RCE) proposed in Sweden [29], and River Habitat Survey (RHS) used in European Union [30].”

Comments 2: The structure of the article is clear and the aspects studied are described in detail, however, I suggest swapping the order of sections 3.2 and 3.3, as section 3.3 contains a detailed description of the Baishazhou bar in its introductory part which, in my opinion, should be known to the reader before knowing the study results. This note applies to analogous sections concerning the Tianxingzhou Sandbar.

Response 2: Thank you for pointing this out. We agree with this comment. Therefore, we have swapped the order of sections 3.2 and 3.3, sections 3.3 and 3.4.

Comments 3: Figure 1 shows the Thiebanzhou bar, which is not described in the paper. I suggest explaining this inconsistency in the text. I suggest also marking the direction of the river flow on this figure.

Response 3: Thank you for pointing this out. I/We agree with this comment. Tiebanzhou Bar lies at the edge of the Wuhan reach, far from Wuhan’s urban area, Baishazhou Bar and Tianxingzhou Bar, so it was not included in our study. We have explained it in the manuscript (line 104-106). The flow direction is also marked in the figure. (line 83).

“Baishazhou bar and Tianxingzhou bar in the Wuhan reach were selected as the study objects (Figure 1). Tiebanzhou Bar is located at the edge of the Wuhan reach and is geographically distant from the urban area of Wuhan, Baishazhou Bar and Tianxingzhou Bar. Therefore, it is not included in the scope of this study.”

Comments 4: Figure 2 shows that there was a sharp decline in sediment transport several years before the TGD was launched. I suggest attempting to explain this phenomenon, even if it is not supported by research.

Response 4: Thank you for pointing this out. We agree with this comment. Before the Three Gorges Dam (TGD) was completed, several dams (such as the Gezhouba Dam) had already been constructed upstream, trapping a large amount of sediment. They were simply not as large in scale or as strong in sediment trapping capacity as the TGD. Therefore, we have added the explanation in the manuscript (line 295-298).

“Before the operation, several dams had already been completed in the upstream reaches, resulting in a decline in sediment supply. TGD, in turn, drastically accelerated this downward trend.”

Comments 5: Line 443 contains information that: “Water quality was evaluated using the monitoring values of Baishazhou Water Treatment Plant in May 2025, which corresponds to Class II water quality” As far as I know, water purity classes vary in different regions of the world, so readers outside China may not understand the classification system used in this country. I suggest adding a brief note to give the reader an idea of whether class II water is highly polluted or, on the contrary, clean.

Response 5: Thank you for pointing this out. We agree with this comment. Therefore, we have added Table S9 about water quality into the Supplementary material.

Table S9. Chinese environmental quality standards for surface water.

Comments 6: Line 671 “largely natura.” Did you mean natural?

Response 6: Thank you for pointing this out. We agree with this comment. Therefore, we have modified the sentence (line 657).

“Baishazhou bar has no large-scale structures on the bar body and remains largely natural.”

Comments 7: The text suggests that Baishazhou bar may eventually be completely eroded, which would be a great loss considering the number of animal species that inhabit it. Are you able to calculate the estimated time remaining until the bar is completely eroded if external conditions remain unchanged?

Response 7: We sincerely appreciate the reviewer’s insightful and forward-looking question, which points out a valuable direction for our follow-up in-depth research.

Quantitatively estimating the time to complete erosion of the bar requires refined hydro-sedimentary numerical models, long-term continuous topographic monitoring data, and probabilistic simulation of extreme hydrological events, all of which are beyond the core research scope and available data support of this study.

The core focus of this work is to reveal the historical evolution mechanism and habitat health status of mid-channel bars based on remote sensing and hydrological data. We will carry out targeted research on this long-term prediction topic in our subsequent work, and respectfully ask for the reviewer’s understanding that we will not make relevant revisions to the current manuscript.

Comments 8: I appreciate the authors' explanation of the limitations they encountered during their research. In my opinion, examining a larger number of shoals, such as adding the first Tiebanzhou shoal visible in the figure to the analysis, would provide more data for drawing conclusions. The two described in the text differ in size, shape of the channel, and land use, which in itself affects the results relating to the C1, C2, and C3 characteristics mentioned by the authors in the methodology.

Response 8: We sincerely appreciate the reviewer’s constructive suggestion, which offers a valuable direction for our follow-up research.

This study focuses on mid-channel bars in Wuhan’s core urban reach with consistent hydrological and urban disturbance backgrounds. Tiebanzhou Bar, located at the edge of the reach, has markedly different environmental conditions, and its inclusion would introduce confounding variables that deviate from our core research focus.

The two selected representative bars fully support our conclusions. We will expand the sample size in our subsequent work, and respectfully ask for the reviewer’s understanding that we will not make this revision to the current manuscript.

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions in track changes in the re-submitted files.

2. Point-by-point response to Comments and Suggestions for Authors

Comments 1: It may be worthwhile to refer to Lane’s balance (1959) (see:
https://ponce.sdsu.edu/legacy_tales_the_lane_principle.html
https://www.mdpi.com/2073-4441/8/1/16 ).
I believe this concept should be explicitly addressed in studies of this type, as it represents a fundamental principle of fluvial hydraulics.

Response 1: We sincerely thank the reviewer for this valuable suggestion and for highlighting the fundamental significance of Lane’s balance in fluvial hydraulics. We fully agree that this principle is a cornerstone of fluvial process studies and holds essential guiding value for this type of research.

In the present study, as erosion type classification cannot be achieved solely via remote sensing approaches, we did not conduct a targeted analysis of Lane’s balance in the main body of the manuscript. However, we have clearly elaborated this limitation in the Limitations and Future Work section of the paper, and stated that we will explore the methodology of erosion type classification using remote sensing techniques in combination with Lane’s principle in our future research. (line 686-697).

“Several limitations should be acknowledged. First, planform changes were derived mainly from dry-season imagery; although this reduces stage-related uncertainty, residual water-level differences and image availability may still affect extracted bar areas. Second, the argumentation regarding local morphological changes (e.g., those associated with hydraulic structures such as bridges) is qualitative in nature, and the specific type of local scouring addressed in this study has not been explicitly defined and classified. This is largely because the remote sensing-based approach adopted cannot achieve refined classification of different scour types at the current stage. Third, this study does not explicitly incorporate Lane’s balance (Lane’s principle), a fundamental principle of fluvial hydraulics [1,2], to systematically interpret the intrinsic relation-ship between hydro-sediment regime shifts and channel morphological responses, which limits the in-depth analysis of the mechanisms driving channel erosion and deposition.”

Comments 2: In several places (e.g., when attributing local morphological changes to the presence of bridges), the argumentation is qualitative in nature and should be clearly identified as such. In my view, this also indicates a valuable opportunity for further research.

Response 2: Thank you for pointing this out. We fully acknowledge that the argumentation attributing local morphological changes to the presence of bridges is qualitative in nature.

As the reviewer rightly pointed out, this also represents a valuable opportunity for further research. In the present study, precise classification of erosion types cannot be achieved via remote sensing alone, which is why we did not conduct targeted quantitative analysis in the main body of the manuscript. We have clearly elaborated this limitation in the Limitations section of the paper, and stated that we will explore remote sensing-based erosion type classification methods to enable more robust and quantitative attribution of fluvial morphological changes in our future work. (line 686-697).

“Several limitations should be acknowledged. First, planform changes were derived mainly from dry-season imagery; although this reduces stage-related uncertainty, residual water-level differences and image availability may still affect extracted bar areas. Second, the argumentation regarding local morphological changes (e.g., those associated with hydraulic structures such as bridges) is qualitative in nature, and the specific type of local scouring addressed in this study has not been explicitly defined and classified. This is largely because the remote sensing-based approach adopted cannot achieve refined classification of different scour types at the current stage. Third, this study does not explicitly incorporate Lane’s balance (Lane’s principle), a fundamental principle of fluvial hydraulics [1,2], to systematically interpret the intrinsic relation-ship between hydro-sediment regime shifts and channel morphological responses, which limits the in-depth analysis of the mechanisms driving channel erosion and deposition.”

Comments 3: Both bars exhibit very low scores in the social attributes, yet they are still classified as “healthy”. From the perspective of environmental engineering and water resources management, such a conclusion is controversial and may be perceived as overly optimistic. I believe this issue should be explicitly discussed and critically addressed. One possible approach would be to follow one of the three options below:

1. Revise the final classification (e.g., “upper sub-healthy”);
2. Modify the threshold values used for the health classes;
3. Interpret the term “healthy” more critically in the context of long-term trends.

Response 3: We sincerely thank the reviewer for this critical and constructive professional comment. The core focus of the ecological health indicator system is the natural habitat of mid-channel bars. For this reason, we set a significantly higher weight for natural attributes (0.665) than social attributes (0.335). The final "healthy" classification mainly reflects the biological and ecological characteristics of the bars' terrestrial habitats, rather than bank slope stability and social service functions, which is also why we presented the morphological evolution analysis and habitat health assessment as separate modules in the Results section.

In response to your suggestion, we will follow your third proposal to add a critical interpretation of the "healthy" classification in the Discussion section, explicitly clarifying its natural habitat-oriented connotation and the potential long-term risks posed by the low social attribute scores, to fully address your concern. (line 662-669).

“Comprehensive health evaluation results show that both Baishazhou bar and Tianxingzhou bar fall into the healthy category, but their scores and the composition of health grades differ. It should be explicitly noted that the health evaluation system in this study is anchored in the natural habitat quality of mid-channel bars, with a substantially higher weight assigned to natural attributes than to social attributes. Accordingly, the final "healthy" classification primarily reflects the biological and ecological performance of the bars' terrestrial habitats, rather than their performance in social service functions, bank slope stability, and other social-attribute dimensions.”

Comments 4: Please also verify the manuscript for potential repetitions of content, which seem to occur particularly in the description of the Baishazhou bar.

Response 4: Thank you for pointing this out. We agree with this comment. Therefore, we have thoroughly checked the full manuscript, revised and streamlined the repeated content, especially the redundant descriptions of Baishazhou bar, to ensure the conciseness and logical coherence of the whole text. (line 363-429).

“To analyze the development, shrinkage, and scouring processes of Baishazhou bar over more than three decades (1989–2020), six dry-season remote-sensing images were selected to examine its morphological changes (Figure 4), and twelve dry-season re-mote-sensing images were used to measure the widths of the mainstream (left ana-branch) and branch channel (right anabranch) at three locations—the bar head, middle, and tail (Figure S2). Channel width change trends are presented in Figure 5(a), and annual change rates are illustrated in Figure 5(b). A positive annual change rate indicates channel widening (bar retreat/erosion), whereas a negative rate indicates channel narrowing (bar accretion).

During 1989–2001, Baishazhou bar was in a natural development stage, with the bar area increasing by 0.73 km². The bar head grew upstream, and the bar length in-creased by 39.70%. The 2001 image (Figure S1) shows that sediment deposition occurred at the bar head, forming a sand spit. By 2004, the bar area decreased from 2.02 km² to 1.42 km² (i.e., 70.44% of the 2001 area), with reductions in length (by approximately 0.9 km), width, and area compared to 2001. The sand spit migrated downstream during 2001–2004 and extended to the middle of the bar by 13 December 2004 (Figure 4 (c)).

In 2003, the mainstream width at the bar head decreased noticeably, a phenomenon explained by the sand spit reaching the measurement section (Figure S1) by that year. In 2004, the sediment load and runoff at Hankou Station decreased by 17.58% and 8.22%, respectively, compared with 2003. Under reduced sediment supply, Baishazhou bar experienced enhanced erosion and shrinkage, with widths at all three measurement locations increasing—most significantly at the bar head.

From 2001 to 2015, the bar continued to shrink: bar length and width decreased to 53.95% and 39.94% of their 2001 values, respectively, and the exposed area in 2015 (Figure 4 (e)) was only 30.90% of that in 2001. According to data from Hankou Hydrometric Station, the sediment load in 2015 (0.63×10⁸ t) decreased by 77.89% compared with 2001 (2.85×10⁸ t), which is a key driver of intensified erosion and shrinkage of mid-channel bars in the middle and lower Yangtze River. From 2004 to 2020, the annual change rates of mainstream width at the head and middle locations were mostly positive, consistent with continuous bar retreat driven by persistent sediment reduction. From 1989 to 2020, the mainstream width at the middle location increased by 22.26%, with local retreat exceeding 200 m.

During 2015–2020, the exposed area of Baishazhou bar increased slightly from 0.62 km² to 0.72 km² (Figure 4 (f)), showing a modest rebound with limited magnitude. This rebound is attributed to two factors: lower water levels in 2020 than in 2015, exposing more of the bar; and downstream bed incision induced by TGD operation, which lowered water levels under comparable discharges. Additionally, the bar-head shoal partitions upstream inflow into the left anabranch, the head shoal, and the right anabranch, functioning as a key water–sediment partitioning zone. Its adverse-slope and blocking effects can promote local deposition at the head, facilitating slight upstream extension under certain conditions. However, remote-sensing evidence confirms that post-TGD clear-water scour under low sediment concentrations has caused continuous erosion and retreat of the bar head, maintaining a long-term shrinking trend.

Local UAV images acquired in November 2020 and December 2021 (Figure 6) provide detailed insights into the bar head’s state. In the dry season, the shallow shoal at the bar head is exposed, and vortex-shaped sand pits of varying sizes form after flood recession. Comparative analysis of same-scale panoramas for the same region shows a slight decrease in the exposed area of the head shoal and a 35 m retreat of the vegetated zone. Severe bank collapse occurred along the right side, with exposed reed roots and steep slopes. The middle portion of the bar along the mainstream side suffered intense scouring; under the Baishazhou Bridge piers, substantial erosional retreat and obvious local collapse pits were observed. This confirms that post-TGD clear-water scour intensified mainstream-side erosion, while bridge piers induced local backwater and flow diversion, further enhancing scour and collapse.

Over the past 30 years, the bar tail has been covered by vegetation, resulting in minimal morphological changes and relative stability without downstream extension. In contrast, the bar head and mainstream-side middle section underwent significant scouring and retreat, with rapid shrinkage of the bar head driving overall area reduction. From 1989 to 2020, the exposed area of Baishazhou bar decreased by 0.57 km², corresponding to an overall shrinkage of approximately 43.83%. Without effective protection measures, the bar may continue to shrink, with acceleration under persistent clear-water scour.”

Comments 5: In the Introduction, the authors should more clearly emphasize the novelty of the study relative to previous research on the impacts of the Three Gorges Dam. The integration of morphodynamic analysis with terrestrial habitat assessment represents the main innovation of the study and should be explicitly highlighted, as it is a very promising research direction.

Response 5: Thank you for pointing this out. We agree with this comment. Therefore, we have revised the corresponding content in the Introduction section as suggested. We have clearly emphasized the novelty of this work relative to previous research on the impacts of TGD, and explicitly highlighted the core innovation of our study (line 85-98).

“Previous studies on the impacts of TGD on mid-channel bars in the Middle Yangtze River have mostly focused on individual morphodynamic or habitat-related analysis, with limited systematic integration of the two. In this study, time-series planform changes of mid-channel bars were extracted from Landsat imagery and high-resolution Google Earth images. By integrating hydrological data with field observations, we analyzed the morphological evolution of Baishazhou bar and Tianxingzhou bar, identified the processes driving their differentiated scouring, and explored underlying causes. Eight indicators were selected from two dimensions—natural attributes and social attributes—to construct an indicator system for assessing terrestrial habitat health on mid-channel bars, with the integration of morphodynamic analysis and terrestrial habitat assessment as the core novelty of this study. Indicator weights were determined using a combined Analytic Hierarchy Process (AHP) entropy weighting scheme, and terrestrial habitat health was evaluated for both bars. The study provides a reference for understanding mid-channel bar evolution in the Middle Yangtze River and supports conservation and restoration of bar wetlands.”

Comments 6: The authors should also clarify to what extent the two analyzed bars are representative of the river landscape in the studied region.

Response 6: Thank you for pointing this out. We agree with this comment. Therefore, we have added a clear statement in the manuscript to clarify that the two selected bars are representative of the main channel patterns and typical river landscape in the study reach. (line 102-110).

“The Middle Yangtze River lying between Yichang and Hukou has a length of 955 km. The reach has a typical meandering channel pattern, a multi-branched channel pattern, as well as a meandering and bar-braided channel pattern. The study area is the Wuhan reach of the middle Yangtze River. This reach extends from Shaomaoshan in Jiangxia District, through Wuchang, to Yangluo Town in Xinzhou District, Wuhan City. Baishazhou bar and Tianxingzhou bar in the Wuhan reach were selected as the study objects (Figure 1). These two bars are fully representative of the mid-channel bar river landscape in the study region, as they correspond to the two dominant channel patterns of the Wuhan reach, and cover the primary morphological characteristics and anthropogenic disturbance modes of bars in the core urban section.”

Comments 7: Furthermore, it is necessary to specify which type of scour is being addressed (see:
https://link.springer.com/chapter/10.1007/978-3-319-70914-7_15
https://ace.il.pw.edu.pl/Keyword-local+scouring/174343 ).

Response 7: We sincerely thank the reviewer for this valuable and constructive suggestion, as well as for providing the key references to guide the explicit specification of scour types. We fully acknowledge that clearly defining the type of scour addressed is essential for the rigor of fluvial process studies.

In the present study, precise classification and targeted analysis of specific scour/erosion types cannot be achieved solely via remote sensing approaches, which is why we did not conduct relevant in-depth analysis in the main body of the manuscript. We have clearly elaborated this limitation in Limitations and Future Work section of the paper, and stated that we will focus on exploring remote sensing-based classification methods for scour and erosion types, to enable precise identification and targeted analysis of different scour processes in our future research. (line 686-697).

“Several limitations should be acknowledged. First, planform changes were derived mainly from dry-season imagery; although this reduces stage-related uncertainty, residual water-level differences and image availability may still affect extracted bar areas. Second, the argumentation regarding local morphological changes (e.g., those associated with hydraulic structures such as bridges) is qualitative in nature, and the specific type of local scouring addressed in this study has not been explicitly defined and classified. This is largely because the remote sensing-based approach adopted cannot achieve refined classification of different scour types at the current stage. Third, this study does not explicitly incorporate Lane’s balance (Lane’s principle), a fundamental principle of fluvial hydraulics [1,2], to systematically interpret the intrinsic relation-ship between hydro-sediment regime shifts and channel morphological responses, which limits the in-depth analysis of the mechanisms driving channel erosion and deposition.”

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions in track changes in the re-submitted files.

2. Point-by-point response to Comments and Suggestions for Authors

Comments 1: The description of remote sensing–based bar morphology extraction remains insufficient and lacks evaluation uncertainty. In Section 2.2, the manuscript states that dry-season imagery was selected and that water-level differences were considered limited; however, no detailed workflow regarding shoreline delineation (e.g., classification approach, threshold selection, manual digitization protocol, or object-based segmentation) is provided. Please provide brief supporting documentation (e.g., Excel file) of how the paper was written in the supplemental materials.

Response 1: Thank you for pointing this out. We fully recognize the importance of detailing the remote sensing-based bar morphology extraction workflow. As recommended, we have already included the complete detailed workflow of bar extraction (covering the full shoreline delineation procedure, including the classification approach, threshold selection criteria, and processing protocols) in the supplemental materials of this manuscript.

Comments 2: The construction of the habitat health indicator system requires improved transparency. Although the manuscript presents the hierarchical framework and indicator definitions, the rationale for selecting the eight indicators is not sufficiently explained. Please briefly re-present the basis for selecting the eight indicators and their sources (references).

Response 2: Thank you for pointing this out. We fully agree that a clear rationale for indicator selection is essential to ensure the transparency of the habitat health evaluation framework. In the revised manuscript, we have clearly elaborated the detailed selection basis, theoretical rationale, ecological applicability, and corresponding supporting references for each of the eight indicators in the relevant section. (line 171-183).

“An ecological health indicator system for terrestrial habitats on mid-channel bars was established [32]. The model comprises three hierarchical levels: the goal layer, the criterion layer, and the indicator layer. The terrestrial habitat health index of a mid-channel bar was chosen as the goal layer. Two dimensions—natural attributes and social attributes—were used as the criterion layer to comprehensively reflect both ecosystem conditions and social service functions. The eight indicators were selected based on the study area’s core characteristics and mature frameworks from existing river and wetland health assessment studies [32,33], with C1–C4 targeting core natural habitat quality and C5–C8 reflecting anthropogenic impacts and social service functions, fully corresponding to the two criterion dimensions. The indicator layer includes eight indicators (Table 2): exposure days of the water–land ecotone (C1), diversity of terrestrial plant species (C2), diversity of terrestrial animal species (C3), vegetation cover (C4), rate of area change (C5), stability of protection works (C6), permeability of protection works (C7), and water quality (C8).”

Comments 3: This seems to overlap slightly with the request I made in #1. The workflow for extracting shorelines or sandbar boundaries is also not sufficiently described, making it impossible to assess extraction reproducibility. Given that land-water boundaries extracted from satellite imagery are affected by spatial resolution, spectral variability, and residual water-level differences, please provide a clear boundary-extraction methodology once again.

Response 3: Thank you for pointing this out. We fully recognize that a detailed boundary extraction workflow is critical to ensure the reproducibility of our results. As requested, we have provided the complete, detailed methodology for shoreline and sandbar boundary extraction in the supplemental materials, which fully addresses the impacts of imagery spatial resolution, spectral variability, and residual water-level differences.

Comments 4: The basis for classifying habitat health levels (e.g., “very healthy,” “healthy,” “unhealthy”) requires further justification. Although score intervals are defined in Section 2.3, the manuscript does not explain how the threshold values (e.g., 80 or 60) were determined, nor whether they are supported by ecological benchmarks, empirical calibration, or prior literature. Please add references to the basis for evaluating habitat health levels, comparing the basis for “very healthy,” “healthy,” and “unhealthy” with comparable studies conducted around the world.

Response 4: Thank you for pointing this out. We fully acknowledge that explicit justification for habitat health level classification and threshold determination is critical to ensure the scientific rigor and ecological validity of our evaluation system. As requested, we have detailed the complete rationale for the threshold values in the supplemental materials, including their underlying ecological benchmarks, empirical calibration process. The scoring criteria for each indicator are also clearly elaborated therein.