the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jul 2025
Discriminating fluvial fans and deltas: Channel network morphometrics reflect distinct formative processes
Abstract. Recent recognition of a new type of fluvial system – fluvial fans – introduces a fan-shaped channel network that appears similar to that of river-dominated deltas. Deltas form where rivers enter lakes and oceans, while fluvial fans are terrestrial landforms. However, fluvial fans can reach the shorelines of oceans or lakes, and in such cases the distinction between fluvial fan and river-dominated delta channel networks become ambiguous. We currently lack fundamental of these two landforms’ morphometric differences, despite their high socioeconomic significance, vulnerability to natural hazards, and key differences in how these landforms respond to global climate change and urbanization. Here we review the relevant conceptual differences in delta and fluvial fan network morphodynamics, propose a set of quantitative morphometric criteria to distinguish fluvial fan and delta channel networks, and test these criteria on 40 deltas and 40 fluvial fans from across the world. This initial attempt to distinguish deltas and fluvial fans demonstrates that quantifying channel network angles, and trends in normalized channel widths and lengths provides efficient criteria, but some ambiguities remain that need to be resolved in future work. This research advances our mechanistic understanding of fluvial fan and delta channel networks and the recognition of modern and ancient landforms on Earth and other planetary bodies, such as Mars and Saturn’s moon Titan.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-2497', John Shaw, 11 Aug 2025
- AC1: 'Reply on RC1', Luke Gezovich, 16 Sep 2025
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RC2: 'Comment on egusphere-2025-2497', Ellen Chamberlin, 20 Aug 2025
- AC2: 'Reply on RC2', Luke Gezovich, 16 Sep 2025
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RC3: 'Comment on egusphere-2025-2497', Anonymous Referee #3, 03 Sep 2025
This paper uses two morphometrics, bifurcation angle and the relationship between channel width, channel length, and channel order, derived from satellite imagery, to distinguish between fluvial fans and deltas. The topic is within the scope of Earth Surface Dynamics and has the potential to contribute to ongoing conversations about how we classify and interpret depositional systems. At present, however, the analysis and framing feel limited. With significant revision, especially in the choice of metrics, the engagement with existing literature, and the way the classification is presented, I think the paper could be suitable for publication.
Major considerations
The choice of metrics is my main concern. Restricting the analysis to bifurcation angle and downstream channel width/order feels too narrow. Other metrics such as lateral channel mobility or avulsion frequency would provide a richer basis for comparison and may lead to more meaningful separation. When a fan or delta terminates in standing water, downstream boundary effects become critical, and prior work (e.g., Carlson et al., 2018; Wang et al., 2019) shows that boundary conditions strongly influence channel number, lateral migration rate, and sediment bypass. These findings should inform the interpretation here. Even if channel depth cannot be measured from imagery, alternatives such as migration rates or wetted frequency maps (e.g., Piliouras et al., 2017) could help test whether channel dynamics differ between deltas and fluvial fans. This would require using data from more than a single snapshot in time. The authors are working on scales where existing channel metric tools could be applied, and doing so would provide a clearer picture of system dynamics across time and discharge conditions.In general, the paper would be stronger if the classification were less prescriptive. Rather than setting a framework in advance, I would like to see populations emerge from the data, and then understand when a fluvial fan behaves like a delta and when it does not. That approach would make for a more compelling contribution.
Minor considerations
- The selection of case studies is unclear. How is the threshold for fluvially dominated deltas quantified?
- Should confinement be considered as a control?
- Figure 2 needs clearer labeling (e.g., upstream/downstream).
- Several figures would benefit from improved color schemes and alternative presentation. Scatter plots, for example, could more clearly show whether two populations emerge.
- The GitHub link provided does not work.
References for consideration
- Wang, J., Muto, T., Urata, K., Sato, T., & Naruse, H. (2019). Morphodynamics of River Deltas in Response to Different Basin Water Depths: An Experimental Examination of the Grade Index Model. Geophysical Research Letters, 46(10), 5265–5273. https://doi.org/10.1029/2019GL082483
- Carlson, B., Piliouras, A., Muto, T., & Kim, W. (2018). Control of Basin Water Depth on Channel Morphology and Autogenic Timescales in Deltaic Systems. Journal of Sedimentary Research, 88(9), 1026–1039. https://doi.org/10.2110/jsr.2018.52
- Piliouras, A., Kim, W., & Carlson, B. (2017). Balancing Aggradation and Progradation on a Vegetated Delta: The Importance of Fluctuating Discharge in Depositional Systems. Journal of Geophysical Research: Earth Surface, 122(10). https://doi.org/10.1002/2017JF004378
Citation: https://doi.org/10.5194/egusphere-2025-2497-RC3 - AC3: 'Reply on RC3', Luke Gezovich, 16 Sep 2025
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- 1
This manuscript describes an empirical analysis of channel networks on modern river deltas and fluvial fans. Bifurcation angles, widths, and lengths were measured on 80 systems. Bifurcation angles were found to be similar to a theoretical angle for deltas and significantly smaller on fans. Lengths and widths decreased with channel order. These findings are new, and could significantly improve the interpretation of ancient and planetary landscapes. I have several comments that I think will improve the manuscript, but I think that at its core this is significant work that is worthy of ESURF.
Major comments
The treatment of river and tide dominated deltas is confusing. On L186 you say that we neglect W- and T- dominated, but then you do treat them in Fig. 7a. You state that you looked at quantitative metrics for process dominance (L300), but don’t give details of any cutoff (e.g. 50% river dominated). I don’t mind if you didn’t go this specific: one of my key takeaways is that the process regime doesn’t particularly influence the angle, which is nice and a worthy paper conclusion. Then in L371 it sounds like wave dominated deltas were omitted from the statistics.
L253, Several aspects of network mapping need to be described better. Particularly, is there a lower limit of channel width that you stop measuring at? Do short side branches or tie channels count as channels? I don’t mean for you to be endlessly bogged down in these choices, but it would be good to include your choices and shapefiles as a supplement so that your work can be introduced.
L393 I don’t believe that Normalized Channel Length is defined anywhere. Is it a channel length divided by apex channel width? Average channel width of the reach? It might be interesting to cite Jerolmack (2009), which does the same normalization.
In section 5.1 you discuss how small angles are more likely to be from a fan than a delta. Are you arguing that 60 or 64 degree average angle is a good cutoff for fans vs deltas? A number like this would be really helpful, but I think it requires some statistical modeling beyond what is here. For example, if you had just 2 measurements on an ancient system and their average was 66 degrees, you wouldn’t have much confidence. I’m not requiring more analysis, but this is an important passage and as yet we don’t have good guidelines for applying this work.
Minor comments
L22, fluvial fans and deltas have distinguished before (Van Dijk et al., 2009). Perhaps specify that you are looking at quantitative differences in channel network morphometrics.
L144 “Deltas always form where the mouth of a river enters a body of water.” This is not an essential point to the paper, but it is easy to find counterexamples to this, such as the Amazon. Perhaps change always to generally.
L179-182 While E&S did propose this reason for channel length reduction, I do not think that it is broadly confirmed. I find that your empirical results are interesting, but that normalized channel lengths of 100s and 1000s are far too long to be explained by momentum. My point here is to just state that the proposed mechanisms have not been firmly established yet, and your certainty overstates the case.
L245 remove existing
L262 By “not considering channels” I think you mean that you measure them but don’t have them influence the channel ordering, right?
L287-288 branching into three channels is sometimes called a “furcation” (Shaw et al., 2018) or a polyfurcation (Chamberlain et al., 2018).
L308, confusing sentence structure.
L321, Python- and Pandas- readable
L451-452 this observation is consistent with Coffey and Shaw
L459, I think “diffusion in unchannelized flow” could be improved. Perhaps, “flow patterns at channel tips well-explained by diffusive processes.
Our recent paper (Shaw et al., 2025) shows that many large systems have both proximal fan and distal delta platform components. Could this possibly explain why angles tend to be larger at the higher order channels at the distal end of fans?
Chamberlain, E. L., Törnqvist, T. E., Shen, Z., Mauz, B., & Wallinga, J. (2018). Anatomy of Mississippi Delta growth and its implications for coastal restoration. Science Advances, 4(4), eaar4740. https://doi.org/10.1126/sciadv.aar4740
Jerolmack, D. J. (2009). Conceptual framework for assessing the response of delta channel networks to Holocene sea level rise. Quaternary Science Reviews, 28(17–18), 1786–1800.
Shaw, J. B., Miller, K., & McElroy, B. (2018). Island Formation Resulting from Radially Symmetric Flow Expansion. Journal of Geophysical Research: Earth Surface, 2017JF004464. https://doi.org/10.1002/2017JF004464
Shaw, J. B., Sanks, K. M., & Piliouras, A. (2025). Basin confinement influences river delta elevation profiles. GSA Bulletin. https://doi.org/10.1130/B38386.1
Van Dijk, M., Postma, G., & Kleinhans, M. G. (2009). Autocyclic behaviour of fan deltas: an analogue experimental study. Sedimentology, 56(5), 1569–1589. https://doi.org/10.1111/j.1365-3091.2008.01047.x