the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Jul 2025
Fluvial fans in the Himalayan Terai: A gradual shift of the Karnali River from double to single branch
Abstract. Fluvial fans in the Himalayan Terai are essential for water resources and provide crucial habitats for endangered species, including tigers. Switching of the dominant channel in these fans influences such habitats by changing the distribution of water and sediment. This study addresses such a transition in the Karnali River, one of the least human-altered large rivers in Nepal and India. For over two centuries, the Karnali maintained a double-branch system, but in recent years it has gradually consolidated into a single branch. Our primary objective is to describe this shift and to assess its trigger. By analyzing flow duration curves, fluvial fan topography, and channel properties, we suggest the cause is an extreme monsoon season in 2009, when two major peak discharges seem to have initiated the subsequent gradual deposition of coarse sediment at the upstream end of the eastern branch (Geruwa), effectively gradually plugging its flow. To better understand the balance between natural and anthropogenic influences, we compare the Karnali with the more heavily altered Koshi River. While embankments and infrastructure developments have significantly shaped the Koshi’s morphology, the Karnali’s shift appears to be driven primarily by natural sediment dynamics. Human interventions (such as embankments and existing hydropower dams) appear to have played little to no role in the transition. With rapid hydropower expansion and ongoing modifications to the river system, we anticipate that Karnali’s single-channel configuration will persist, with profound implications for water distribution and habitat conservation in Bardiya National Park.
- Preprint
(25594 KB) - Metadata XML
- BibTeX
- EndNote
Status: open (extended)
-
CC1: 'Comment on egusphere-2025-2926', James Pizzuto, 26 Aug 2025
reply
-
AC1: 'Reply on CC1', Kshitiz Gautam, 19 Nov 2025
reply
We thank the reviewer for the constructive feedback on our manuscript. Our detailed responses are provided in the attached document.
-
AC1: 'Reply on CC1', Kshitiz Gautam, 19 Nov 2025
reply
-
RC1: 'Comment on egusphere-2025-2926', Anonymous Referee #1, 31 Dec 2025
reply
This manuscript investigates the gradual transition of the Karnali River fluvial fan system from a long-lived double-branch configuration to a single dominant channel. The study addresses an important and timely topic in fluvial geomorphology and river–fan dynamics, particularly in large Himalayan rivers where natural processes still dominate over engineering control. Overall, the manuscript is well written, and presents a possible explanation for Karnali River based on multiple datasets. However, I still have two major questions that need to be addressed.
- The paragraphs are very short and lack clear logical flow, making the manuscript feel like a compilation of disconnected information. I suggest restructuring the paper around a coherent framework, such as scientific hypothesis → evidence → discussion → conclusion. Meanwhile, the Discussion section should be strengthened with more mechanistic explanations, as the current presentation of information is somewhat confusing.
- The authors suggest that the river shift may be related to the 2009 moonsoon season. Such as heavy 2009 monsoon season formed a sediment ‘plug’ in the upstream reach of the Geruwa branch. I think that this evidence appears relatively weak. Could the authors provide additional evidence to support this hypothesis?"
In addition to the above, there are some minor issues that I also suggest the authors consider.
Line 16-25. Could more recent literature be added here to highlight the significance of your study?
Line 40-51. At interannual timescales, human activities and climate change are the dominant drivers of river change; however, in certain regions or during extreme events, abrupt tectonic activity (such as earthquakes) and short-term sea-level fluctuations can also significantly influence river systems.
Line 61. Which natural factors are included here? These have not been specified earlier in the manuscript.
Line 143-144. This sentence should appear in the Discussion section.
Line 156-159 This is just a simple description; I would rather know more about the underlying mechanisms.
Line 187 “Sinuosity and Braiding Indices and Sinuosity Dynamics for”. Should the first letter be capitalized?
Line 206-208 This is a good hypothesis, but how can it be tested? Is there any new evidence to support it?
Line 210-249 the Discussion section is overall too simplistic, merely ruling out some possible factors. I hope you can build on your results to provide more mechanistic insights.
Line 250-260. Could this be written as one or two paragraphs? There’s no need for so many separate paragraphs.
Citation: https://doi.org/10.5194/egusphere-2025-2926-RC1 -
CC2: 'Comment on egusphere-2025-2926', Ivo Pink, 09 Jan 2026
reply
The article investigates the evolution of the Karnali fan over the past decades using data from historical surveys and remote sensing imagery. The authors discuss the implications of the fan evolution on ecosystems, which is a novel and important aspect. Furthermore, the authors present the first article (that I am aware of), which uses historical maps of the Karnali fan dating back to the 18th century, and hence adds to our understanding of the fan.
Main criticism
I have one major concern regarding the claim that the Karnali River transitioned from a multi-branch river to a single-branch river following the 2009 monsoon season. This statement is repeated from the title to the conclusion, but I cannot see sufficient evidence to support this transition. I am not convinced that the Karnali is a single-branch river for several reasons:
- Satellite images after 2009 show that the Eastern branch still carries water and is, thus, still an active channel. Please see the attached images of Oct. 2014-2021 as an example.
- Figure 6a shows that the Western branch had a lower water surface area between 2000-2009 than the Eastern branch between 2010-2020. It is not logical to argue that it transitioned to a single-branch river after 2009, when the low-flow partitioning is more balanced after 2009 than before.
- The analysis is based on imagery during low-flow conditions. Dingle (https://doi.org/10.1130/G46909.1 supplements) shows that normal to high flows overflow the plug, and that the Eastern branch still receives water during low flows despite the plug. Hence, the Eastern branch is active, and low flow conditions are insufficient to determine the activity of a branch.
Minor aspects
- Branch characteristics: The Eastern and Western branches have different channel characteristics, whereas the Eastern branch is composed of more but narrower braid channels. These narrower channels are more difficult to classify from 30m satellite imagery, which likely leads to an underestimation of the water surface area in the Eastern branch. It may be worth addressing this limitation in the discussion.
- Potential future evolution: The authors argue that with human interventions, the likelihood of future channel switches decreases (lines 245ff). I believe that this section needs more context. Dingle, 2017 (doi:10.1038/nature22039) shows that the gravel supply to the fan originates from the Siwaliks. The fan will not be blocked from its gravel supply unless a hydropower station is constructed at the mountain gauge. Upstream stations may reduce the flow rate (and hence transport capacity), which may lead to a decrease in gravel supply. However, increasing frequency and magnitude of intense rainfall may counteract and lead to increasing gravel supply (e.g. https://doi.org/10.3126/jalawaayu.v1i1.36448)
Additional thoughts:
- 2009 monsoon season: There is a book chapter on bifurcation switches in the Karnali fan by C. Cload called monsoon-driven changes to river bifurcations in Nepal (DOI: 10.1201/9781003475378-6), which may be of interest.
- It may be worth assessing the variation of water surface area with flow (e.g. comparing multiple images during a year). This may be beyond the scope of the study, but I believe it would add depth to the article because it provides evidence about how sensitive medium to high flows are to bifurcation changes. Without any consideration of medium to high flows in the analysis, it should be clearly stated that the findings relate to low-flow conditions only.
I disagree with the authors that the Karnali transitioned to a single-branch channel and feel that the narrative of the article needs to be adjusted. Nonetheless, I believe that it is a valuable article because it improves our understanding of the channel evolution during low flow conditions, which prevails for most of the year and is important for ecosystems and communities. I would further like to highlight that the figure presentation is excellent.
Ivo Pink
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 1,106 | 231 | 32 | 1,369 | 48 | 61 |
- HTML: 1,106
- PDF: 231
- XML: 32
- Total: 1,369
- BibTeX: 48
- EndNote: 61
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
The authors present a well-crafted, well-written investigation of recent changes to two fans of the Himalayan Terai region, with specific focus on the evolution of the Karnali River from a two-branch to a single-branch system. The topic is of considerable interest, and the manuscript is in pretty good shape – I found only a very few minor issues to comment on throughout.
Nonetheless, I think that the authors need to do a better job on two aspects of the study. First, their study would be clearer and make more sense to the reader if it was presented as a hypothesis-driven investigation. Second, the available data to test their leading hypothesis (which they present as a “finding” in the discussion) is quite limited, and I think they should explicitly acknowledge that additional work would be desirable to provide a more comprehensive test.
On the first point: the introduction reads as if the authors are going to just look at some data and see if anything interesting emerges. But I am sure that they had some specific ideas about why a fan system could evolve from a 2-channel to a single-channel system, and that these ideas guided their data collection. The paper would be much improved if specific hypotheses were offered at the outset, and the data collection effort was presented as an effort to test these hypotheses. As written, the author’s main conclusion is presented first at the end of the Results section, which is really too late to introduce this. Better to see specific hypotheses at the beginning of the paper, and then explain which of these hypotheses are supported by the author’s data.
On the second point: the only real support available that I could discern for the idea that one of the channels was “plugged” by coarse bed material in 2009 is the large grain size at the outside of a bend near the head of the fan. This seems like minimal support to me: ideally some evidence of the deposit itself might be available, or some other supporting evidence. I would like the authors to either more clearly summarize the existing evidence (in case I have missed something), and then discuss additional evidence would could (should?) be collected before accepting this working hypothesis (e.g., before and after topographic surveys, field mapping of the coarse deposit from 2009, and so on). This discussion would strengthen the scientific impact of the author’s work.
These suggestions are important, and perhaps constitute a significant revision of the presentation, but should be easily accomplished and really only a minor effort for the authors. I have indicated major revisions, but I think they will be easily accomplished and the present manuscript greatly improved as a result.
Jim Pizzuto
Dept. of Earth Sciences (retired)
University of Delaware, USA
Some detailed comments, keyed to the text:
1. Line 20. Should probably cite Figure 1 somewhere in this paragraph.
2. Line 51. In the paragraph above on anthropogenic stresses, land-use is not mentioned. Perhaps it should be included in the paragraph above, or, if it is not important here, not mentioned at all...???
3. Line 67. Include: Section 6, which presumably interprets information from the previous sections and leads to the conclusions of Section 7.
4. Introduction, general comment. I think it would be helpful to present some specific hypotheses to test. This would provide context for the methods. Otherwise, why investigate flow characteristics, dynamics, channel properties, and so on? Presumably these investigations are designed to answer specific questions derived from the authors' hypotheses....and the reader should know what these questions and hypotheses are.
5. Line 208. The hypothesis to be tested should be presented in the introduction, and specific methods available to test it should be outlined in the methods. Evidence from grain size only is somewhat weak, it seems to me. Is the hypothesis supported by topography analyses, or are these data insufficient or unavailable?
6. Line 215. But little direct evidence besides grain size has been presented for this. I think it remains a "working hypothesis", rather than a "finding". This may be semantics, but nonetheless important.