Insights from a 25-year database of post-fire debris flows in California

December 10, 2025

Figure 1. Examples of PFDFs. (a) Deposits from a small PFDF on the 2020 CZU fire that demonstrates lateral levees characteristic of PFDFs. Photo: Matthew Thomas, US Geological Survey (USGS). (b) Multiple large PFDFs on the 2017 Thomas Fire caused extensive damage in Montecito and nearby areas. Image shows a location where a home was knocked off its foundation and large boulders of various sizes were deposited by the flow. Photo: Don Lindsay, California Geological Survey.

A new paper, “Insights from a 25-year database of post-fire debris flows in California” by Nina Oakley, Derek Cheung, Donald Lindsay (California Geological Survey, CGS) and Deanna Nash (CW3E), was recently published in the International Journal of Wildland Fire. The study presents a comprehensive 2000–2024 database of California postfire debris flows (PFDFs) compiled from scientific, media, and agency sources, and analyzes their spatial distribution, seasonal timing, atmospheric drivers, and interannual variability. PFDFs are fast-moving slurries of soil, ash, rocks, burned vegetation, and water, typically initiated by short-duration (<1 h), high-intensity rainfall within the first few years after a wildfire.

This work directly supports a key priority in CW3E’s Strategic Plan (Atmospheric Rivers and Extreme Precipitation Research, Prediction, and Applications) and strengthens Research and Operations partnerships through close collaboration with the California Geological Survey. It advances understanding of where, when, and under what atmospheric conditions PFDFs occur across the state.

The new database documents 595 individual PFDFs from 97 storm events, with the highest concentrations in the Transverse Ranges and Sierra Nevada. Roughly one-third of PFDFs occur during the warm season (Jun–Sep), typically linked to the North American Monsoon, tropical cyclones, or thunderstorms. The remaining two-thirds occur in the cool season (Oct–May).

Atmospheric rivers (ARs) were associated with 55% of the PFDF events. Most PFDFs tied to ARs occurred during the more frequently occurring weak- and moderate-strength events (AR Scale 1–3; Ralph et al., 2019), highlighting that the strongest ARs are not necessary to produce PFDF triggering rainfall. This challenges the common assumption that the most intense ARs dominate PFDF activity. Fine-scale (mesoscale) processes within the mid-latitude cyclone/AR system and their interaction with topography often drive the high-intensity rainfall capable of triggering PFDFs (e.g., Collins et al. 2020; Cordeira et al. 2025; de Orla-Barile et al. 2021; Oakley et al. 2017; Oakley et al. 2018; Schwartz et al. 2021; Zou et al. 2023).

The study also finds substantial interannual variability: PFDF frequency is highest following years with well above-average area burned, while wetter-than-average water years do not necessarily correspond to more PFDFs, and vice versa.

By establishing a long-term, statewide record, this database supports hazard planning and mitigation and provides a foundation for tracking changes as California’s climate continues to warm. This work underscores the importance of representing the processes within the AR/cyclone system that produce short-duration, high-intensity rainfall in weather forecast models. Building on these findings, future work will focus on verifying short-duration, high-intensity precipitation in West-WRF prior to PFDF events.

Figure 2. Figure 2. Panel (a) presents the storm category or proportion of storms in each category associated with PFDF events at each wildfire location. Each wildfire that experienced PFDF events in this study is represented by a pie graph at or near the centroid of the fire perimeter; some are off-centroid for visibility. The size of the pie is proportional to the number of PFDF events that occurred on that fire. Panel (b) provides the count of PFDF events in each storm category. Panel (c) shows the count of PFDF events by month, grouped by storm category.

Paper Citation

Oakley, N. S., Cheung, D. J., Lindsay, D. N., & Nash, D. (2025). Insights from a 25-year database of post-fire debris flows in California. International Journal of Wildland Fire, 34(12), WF25136. https://doi.org/10.1071/WF25136

References

Collins, B. D., Oakley, N. S., Perkins, J. P., East, A. E., Corbett, S. C., & Hatchett, B. J. (2020). Linking Mesoscale Meteorology With Extreme Landscape Response: Effects of NCFR. Journal of Geophysical Research: Earth Surface, 125, e2020JF005675. https://doi.org/10.1029/2020JF005675

Cordeira, J. M., Kawzenuk, B. K., Bartlett, S. M., Hecht, C., Castellano, C., Roj, S., & Ralph, F. M. (2025). A Case Study of Forecast Uncertainty Prior to a High-Impact Landfalling Atmospheric River in California in January 2021. Weather and Forecasting, 40(8), 1543-1561. https://doi.org/10.1175/WAF-D-24-0088.1

de Orla-Barile, M., Cannon, F., Oakley, N. S., & Ralph, F. M. (2021). A Climatology of Narrow Cold-Frontal Rainbands in Southern California. Geophysical Research Letters, 48, e2021GL095362. https://doi.org/10.1029/2021GL095362

Oakley, N. S., Lancaster, J. T., Kaplan, M. L., & Ralph, F. M. (2017). Synoptic conditions associated with cool season post-fire debris flows in the Transverse Ranges of southern California. Natural Hazards, 88, 327–354. https://doi.org/10.1007/s11069-017-2867-6