| 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.
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.