the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Nov 2025
The multi-decadal hazard cascade of a tropical mountain wildfire
Abstract. Climate change is driving wildfires to higher elevations, yet the hazard cascades that follow the burning of pristine tropical mountain ecosystems remain largely unexplored. Here, we analyse the long-term cascade following a February 2012 wildfire that burned 31 km² of forest and wetland in Uganda's Rwenzori Mountains National Park. Combining remote sensing, humanitarian records, field surveys, and interviews, we document ten major floods since 2012, including two debris floods that required large-scale humanitarian responses. Post-fire increases in erosion and mass movement have widened the River Nyamwamba sevenfold since 2012, breaching copper-cobalt mine tailings and mobilising an estimated 744,000 tonnes of waste into the river. Slow vegetation recovery at high altitudes and positive feedbacks between hazards have prolonged this high-risk state, underscoring the susceptibility of tropical mountain ecosystems to long-term post-wildfire cascades. More monitoring and research are required to characterise key hazard interactions after tropical mountain fires, which can guide entry points for management seeking to mitigate and impede future cascades.
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Status: open (until 05 Jan 2026)
- RC1: 'Comment on egusphere-2025-5106', Anonymous Referee #1, 05 Dec 2025 reply
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RC2: 'Comment on egusphere-2025-5106', Anonymous Referee #2, 09 Dec 2025
reply
The manuscript investigates an important and understudied topic, namely tropical montane wildfire responses and management. The aim of synthesizing qualitative interviews with quantitative remoting sensing data is an interesting approach. Some general comments and specific suggestions follow. I look forward to seeing a revised version of the manuscript.
General Comments:
Terminology throughout the manuscript needs clarification and to be used more consistently. i.e. landslides are mentioned in the introduction as a common process in mountains following wildfire, yet hillslope landslides are not within the scope of this investigation. Instead, river bank erosion appears to be a dominant post-wildfire process measured in this study and therefore should be presented in the introduction. For example, the main quantitative data presents bank erosion measured as a change in channel width and/or area (however, the authors used the term “river cliff landslides” in the text instead of “bank erosion” which is confusing to the reader).
To understand the context for the post-wildfire processes and changes that are discussed, a brief description of the study area geology, topography, geomorphology, land uses, hydrology, and ecology (more specific than “tropical vegetation”) aligned with Figures 1, 3 , 4b is necessary (before the methods section).
Editing and clarification regarding Figure 2 is needed to best represent the actual data presented in this investigation. Human activities including storage mining debris adjacent to the river could be identified as a primary hazard in the conceptual model Figure 2, as it is the erosion of this stored material interacting with post-wildfire flooding and erosion that leads to the secondary runoff pollution hazard identified. The terminology in the caption does not match the figure: i.e. the caption says “(iii) catalysing/impeding” whereas the arrow symbol in the key is labeled “Positive feedback.” The terminology used in Figure 2 should match that used in the manuscript text. i.e. the secondary natural hazard: “River Channel Erosion” is referred to later in the manuscript as “river cliff landslides.”
I suggest that the authors consider adding a secondary Natural Hazard, namely “channel bed sediment deposition” as a distinct process from “channel bank erosion.”
The rationale for this is that the Results suggest that increases in channel width are an erosion hazard. The authors should consider if increased channel bed elevation from sediment deposition during post-wildfire floods led to the bank erosion quantified as widening. A more detailed process-oriented discussion of the feedback between flooding, channel bed sediment deposition and bank erosion, would be helpful.
Specific suggestions for clarification:
Abstract:
Abstract Line 25, and again in discussion: it would be more appropriate to suggest that information from monitoring could aid future management to “mitigate future hazards”, rather than to “impede future cascades” because climatic factors driving natural processes following wildfire will still occur. i.e. this has more to do with risk management than with controlling natural processes.
Introduction: Line 36 – this would be a good place to define “multi-hazard cascades” before the next two sentences in this paragraph. i.e. do the authors mean that numerous post-wildfire process occur, or the that one process leads to a subsequent process? Gil and Malamud, 2016 define a cascade as networks of interactions. Cite this work and explain what it means in the context of this investigation. Similarly, the italicized words “triggering, probability increasing, or catalysing/impeding” need definitions, not only citations.
Bank erosion (measured as changes in channel width and area) appears to be the main quantitative data presented in this investigation. Therefore, the topic of bank erosion, and it’s relation to post-fire changes in sediment transport processes should be presented in the introduction.
Convert units from English to metric units.
Clarify here if this is the topic of the research, or if this is a known fact (if the second, it needs a reference): " More than a decade later, the Nyamwamba catchment continues to experience flooding, debris flows, mass movements, erosion and water pollution at elevated intensity.” Also, do the authors mean “concentration” instead of “intensity”?
The introduction should clearly state the scope of the study, and the specific research questions the manuscript will address.
Methods: in the paragraphs reporting the data sets utilized, clarify what each data set was used for, the error/uncertainty, and limitations. i.e. for 2.1.3 River Erosion analysis, clarify if the remote sensing data sets are used to quantify channel bank erosion, or channel bed erosion, or both. Clarify the resolution of the datasets and results. Clarify If the data were used to quantify elevation changes, channel width changes, area or volume changes.
Results and Figure 2: It appears that the authors use of the term “landslides” refers to river bank erosion, not hillslope processes—this should be defied clearly the first time “landslides” are discussed. Otherwise, landslides would be considered a secondary rather than tertiary natural hazard, as they are the results of the same post-wildfire storms causing fluvial floods and debris flows.
Figure 2: wouldn’t sediment deposition also be an important hazard to consider—the authors should consider adding a separate box for “river bed deposition” rather than lumping both processes into one category called “river erosion”
Figure 2: wouldn’t River Channel Erosion and Runoff Pollution be considered tertiary hazards, as they are the result of fluvial floods.
Figure 3c and d and Figures in appendix F: Did bank erosion increase channel width because of post-wildfire sediment deposition in the channel? It appears to be a possibility.
Section 3.4: Erosion driven interactions: The increase in channel area reported in the previous section that is caused by river bank erosion would increase, not decrease channel capacity, thereby decreasing the probability of urban flooding, not increasing the probability of urban flooding. There is also the potential for sediment eroded from channel banks to be deposited on the channel bed, which would decrease capacity. The authors should correct or clarify this text.
Figures F1 and others, and throughout text: clarify if “River cliff erosion” is bank erosion – a more standard term for the processes being discussed. I suggest using the term “bank erosion” instead.
The photos in the appendix appear to show a relatively steep, coarse-grained river. A description of the gradient, grain size (if available) in a study area section would be helpful.
Citation: https://doi.org/10.5194/egusphere-2025-5106-RC2 -
RC3: 'Comment on egusphere-2025-5106', Anonymous Referee #3, 14 Dec 2025
reply
Review of “The multi-decadal hazard cascade of a tropical mountain wildfire”
This is an interesting manuscript that makes an important contribution to understanding global postfire hydrologic response, especially in places with limited fire history. It is valuable to document the events in the Nyamwamba basin to inform preparedness in similar tropical, mountainous areas that may experience wildfire. I appreciate the integration of social science. The introduction of quotes from the community brings a human aspect not often seen in work like this and increases its impact. I look forward to seeing a revised version of this manuscript published.
I have one major concern that is prevalent through the manuscript. The authors use the terms “debris flood” and “debris flow” interchangeably, yet they are different responses along the continuum from clearwater flood to debris flow (e.g., Hungr 2001; Pierson 2005 a,b; Oakley et al. 2025). Different responses along this spectrum are typically associated with different triggering rainfall intensities, different sediment concentrations and field evidence, and different magnitude of impact.
For example, at line 231, the section header says, “debris flooding”. The text in the section says, “two floods (2013 and 2020) included debris flows.” In line 352 these two events were then referred to as “debris floods”. Are they debris flows or debris floods? Furthermore, Table 2 indicates: “22 May 2024…debris flows, riverine flooding, and mudslides”. A “mudslide” is a term popularized in the media but has no scientific definition. What does mudslide refer to? Perhaps authors are also referring in some cases, especially multiple years after fire, to shallow landslides that mobilize into debris flows, but it is hard to tell.
My suggestion is that the authors review the information available and determine whether each response is best categorized as a debris flow or debris flood (or perhaps a debris flow in the headwaters that becomes a debris flood with increasing water concentration downstream). Then, apply consistent terminology when referring to each throughout the paper. This will provide clarification for the reader in the characteristics of the runoff response and will support accurate global cataloging of postfire debris flows and other runoff responses (e.g., McGuire et al. 2024).
Related, I wanted to question the “debris flows” mentioned in the 22 May 2024 event. The framing of the paper suggests that these are runoff generated postfire debris flows initiated within the burn area. However, this event is ~12 years postfire. Postfire debris flows tend to happen within the first few years after a fire (e.g. McGuire et al. 2024 and references therein for mid-latitude regions). Even with slow recovery, this seems excessively late for a runoff-generated debris flow to be related to postfire conditions. We do see a transition several years after wildfire to infiltration-driven shallow landsliding (e.g., Rengers et al. 2020; Thomas et al. 2025), is this debris flow in 2024 a shallow landslide that mobilized into a debris flow?
Line-by-line comments:
L47: Consider adding McGuire et al. (2024) to this list, which provides a comprehensive global catalog of postfire debris flow events that addresses your point well.
L56: thinner atmosphere is an odd phrasing, are you referring to the distance from the ground level to the top of the troposphere is less than at lower elevations, or are you referring to reduced partial pressure of oxygen at high elevations? Seems like the latter, consider rephrasing for clarity.
Fig 1: For the burn severity map, the applied classification may be an older approach. Currently in the US a 4-class Burned Area Reflectance Classification map is typically used, see Parsons et al. 2010 https://www.fs.usda.gov/rm/pubs/rmrs_gtr243.pdf
With classifications of unburned, low, moderate, high. Perhaps this is not used elsewhere. Wanted to make a note of it but not sure it is critical to change.
Fig 2— “Debris flows” mentioned many times in paper but not in figure.
Not sure how “debris floods” have a positive feedback. Key says they have high erosive power, but that does not explain how they have a positive feedback, which I interpret to mean if a debris flow occurs, it creates a scenario wherein more debris flows are likely to occur.
Why don’t fluvial floods trigger or increase probability of runoff pollution after wildfire?
After several years, wildfires could increase probability of landslides (see refs above), is that relevant here?
L159 consider “cause of ignition” preferred term over “trigger”
L163 perhaps need to specify “vegetation burn severity” since other parts of paper (e.g. Fig 1) relate to soil burn severity
Table 2: Are all the impacts listed after the fire meant to be tied to the fire? For example, the 2021 landslides in Kilembe town – if the landslides initiated in the terrain around the town, I do not think they can be related to the fire. Does this imply that landslides initiated in the burn area and sediment mobilized downstream?
L222: “5th May 2013 followed rainfall of only a 6.6-year estimated return period”. What duration does this refer to? Also L65 refers to “unprecedented debris flooding” in May 2013 but Table 2 does not indicate debris flows or debris floods, only riverine flooding. Postfire debris flows in mid-latitude regions may be triggered by <2 year return period rainfall at the <=1 hour duration. If there were debris flows documented in this event, it would be valuable to know the peak hourly-to-subhourly rainfall intensities and associated return periods.
Section 3.5—the impacts of flooding/debris flows are terrible but the pollution aspect was very sad. I hope this work brings light to this issue and helps the people impacted and mitigates this kind of outcome elsewhere.
L362: Consider pointing out some of the operational postfire hazard assessments that are used in other places (if a similar approach tailored to this region would be beneficial). For example, Sheridan et al. (2009) for Australia (or approach that has superseded this). In the US, federal Burned Area Emergency Response Teams (e.g., https://www.fs.usda.gov/naturalresources/watershed/burnedareas-background.shtml), The US Geological Survey emergency assessments of postfire debris flow hazards (https://landslides.usgs.gov/hazards/postfire_debrisflow/) or State of California Watershed Emergency Response Team assessments (https://www.conservation.ca.gov/cgs/bwg/recent)
References
Hungr, O., Evans, S. G., Bovis, M., & Hutchinson, J. (2001). Review of the Classification of landslides of the flow type: Environmental and Engineering Geoscience. Geological Society of America and the Association of Engineering Geologists, 7(3), 231-228.
McGuire, L.A., Ebel, B.A., Rengers, F.K. et al. Fire effects on geomorphic processes. Nat Rev Earth Environ 5, 486–503 (2024). https://doi.org/10.1038/s43017-024-00557-7
Oakley NS, Cheung DJ, Lindsay DN, Nash D. (2025) Insights from a 25-year database of post-fire debris flows in California. International Journal of Wildland Fire 34, WF25136. https://doi.org/10.1071/WF25136
Pierson, T. C., 2005, Distinguishing between debris flows and floods from field evidence in small watersheds: U.S. Geological Survey Fact Sheet 2004-3142, 4 p., (Available at: https://pubs.usgs.gov/fs/2004/3142/)
Pierson TC (2005) Hyperconcentrated flow – Transitional process between water flow and debris flow. In ‘Debris flows and related phenomena’. (Eds M Jakob, O Hungr) pp. 159–202. (Springer: Heidelberg, Germany)
Rengers, F.K., McGuire, L.A., Oakley, N.S. et al. Landslides after wildfire: initiation, magnitude, and mobility. Landslides 17, 2631–2641 (2020). https://doi.org/10.1007/s10346-020-01506-3
Sheridan, G.J, Lane, P.N., Smith, H. & Nyman, P. (2009) A rapid risk assessment procedure for post-fire hydrologic hazards; 2009/10 fire season. Technical report produced for the Department of Sustainability and Environment. The Department of Forest and Ecosystem Science, The University of Melbourne, Australia. 19 pages. https://rest.mars-prod.its.unimelb.edu.au/server/api/core/bitstreams/ec5e75e2-f25c-5e63-bd36-3311b811fb4a/content
Matthew A. Thomas, et al. Landsliding follows signatures of wildfire history and vegetation regrowth in a steep coastal shrubland. Geosphere 2025;; 21 (5): 823–840. doi: https://doi.org/10.1130/GES02856.1
Citation: https://doi.org/10.5194/egusphere-2025-5106-RC3
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- 1
The article "The multi-decadal hazard cascade of a tropical mountain wildfire" provides a perspective of post-wildfire flood impacts in a tropical montane system, a relatively under-published environment in the post-wildfire scientific circles. The paper is generally well written but could use some minor improvement.
General comments:
I recommend checking sentence structure, especially in the Introduction and Discussion. In particular many sentences appear to be fragmented or worded awkwardly. I also recommend checking for appropriate use of references, there are many statements in the Results section and Discussion section that include interpretation and do not include references to other published works. In some cases the interpretation is backed up by interview citations, which might be acceptable but only if there isn't further interpretation of natural processes by the authors.
Line comments:
Lines 34-35: Fragmented sentences.
Line 39: there should be a caveat with "they are tectonically active", this might be true for the study site but I doubt you mean to say that all tropical mountains are tectonically active.
Figure 2: I like the figure and I think it conveys a point but I worry that it is overly simplistic. For example Landslides can lead to Runoff Pollution as can Fluvial Floods. Avulsion Floods can have a feedback loop to Landslides, Runoff Pollution, and possibly Debris Flows. All of the bubbles in the figure are inter-connected with the exception of the triggering wildfire event. I recommend revising the figure or providing a better explanation to how this figure is correct.
Line 314: I would disagree that all fire-adapted systems have a rapid vegetation recovery. Catastrophic wildfires have created stand replacing fires in parts of Australia and Western North America (among other locations) that has created feedback loops that we have yet to have seen stabilize. I understand the purpose of this sentence but there should be some caution that while tropical mountain forest fires are unusually catastrophic (due to lack of historical fire) they are not completely unique in the long-term vegetation recovery.
Line 335: I think a little more discussion can be used to discuss mimicking natural process. There are a number of papers that talk about this, mostly from a fluvial erosion mitigation stand-point. There's an interesting article from Arizona (USA) on mimicking pre-wildfire alluvial fans to reduce downstream post-wildfire sediment migration. A full paragraph (or two) could be added to this Discussion section to provide a little more context to what is being worked on in other parts of the world to hasten the natural recovery.
In general I thought this was an interesting paper that marries a technical study with a socioeconomic and humanitarian citizen survey. I look forward to seeing the manuscript improved and published.