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Severe wildfire may alter steep mountain streams by increasing peak discharges, elevating sediment and wood inputs into channels, and increasing susceptibility to landslides and debris flows. In the Pacific Northwest, where mean annual precipitation is high and mean fire‐return intervals range from decades to centuries, understanding of steep stream response to fire is limited. We evaluate the hydrologic and geomorphic response of ~100‐m‐long steep stream reaches to the large‐scale and severe 2020 fires in the Western Cascade Range, Oregon. In the two runoff seasons after the fires, peak flows in burned reaches were below the 2‐year recurrence interval flood, a level sufficient to mobilize the median grain size of bed material, but not large enough to mobilize coarser material and reorganize channel morphology. Sediment inputs to study streams consisted of two road‐fill failure landslides, slumps, sheetwash, and minor bank erosion; precipitation thresholds to trigger debris flows were not exceeded in our sites. There was a 50% increase in the number of large wood pieces in burned reaches after the fires. Changes in fluxes of water, sediment, and wood induced shifts in the balance of sediment supply to transport capacity, initiating a sequence of sediment aggradation and bed‐material fining followed by erosion and bed‐material coarsening. Gross channel form showed resilience to change, and an unburned reference reach exhibited little morphologic change. Post‐fire recruitment of large wood will likely have long‐term implications for channel morphology and habitat heterogeneity. Below‐average precipitation during the study period, combined with an absence of extreme precipitation events, was an important control on channel responses. Climate change may have a complex effect on stream response to wildfire by increasing the propensity for both drought and extreme rain events and by altering vegetation recovery patterns.
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Seventy years of watershed response to floods and changing forestry practices in western Oregon, USA
Abstract This study examined the 70‐year history of clearcutting of old‐growth forest and associated road construction, floods, landslides, large wood in rivers, and channel change in the 64 km2Lookout Creek watershed in western Oregon, where forestry practices began in 1950 and largely ceased by the 1980s. Responses differed among three zones with distinctive geomorphic processes within the watershed: a glacially sculpted zone, an earthflow‐dominated zone, and a debris slide and debris flow‐dominated zone. Watershed response to floods was more related to the timing of road construction and clearcuts, past geomorphic events, and forest dynamics than to flood magnitude. Even small (1–3 year) floods generated geomorphic responses in the period of initial road construction and logging (1950–1964) and during ongoing logging in the early part of a 30‐year period between large flood events (1966–1995). The floods of 1964/65, 15 years after the onset of logging, produced much larger geomorphic responses than the flood of record (1996), more than a decade after logging ceased. Geomorphic response was negligible for the third largest event on record (2011) during the last period (1997–2020), when former clearcuts were 20 to 70‐year‐old forest plantations. Watershed response in each of five distinct time periods depended on conditions created during prior periods in the three zones. Understanding of watershed response to forestry requires integrated observation of forestry practices, floods, landslide susceptibility, wood delivery and movement, and channel change on time scales that capture responses to past and ongoing management practices and geophysical and biological factors and events.
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- Award ID(s):
- 2025755
- PAR ID:
- 10474282
- Publisher / Repository:
- Earth Surface Processes and Landforms
- Date Published:
- Journal Name:
- Earth Surface Processes and Landforms
- Volume:
- 48
- Issue:
- 6
- ISSN:
- 0197-9337
- Page Range / eLocation ID:
- 1103 to 1118
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract ContextWildland-urban interface (WUI) areas are facing increased forest fire risks and extreme precipitation events due to climate change, which can lead to post-fire flood events. The city of Flagstaff in northern Arizona, USA experienced WUI forest thinning, fire, and record rainfall events, which collectively contributed to large floods and damages to the urban neighborhoods and city infrastructure. ObjectivesWe demonstrate multi-temporal, high resolution image applications from an unoccupied aerial vehicle (UAV) and terrestrial lidar in estimating landscape disturbance impacts within the WUI. Changes in forest vegetation and bare ground cover in WUIs are particularly challenging to estimate with coarse-resolution satellite images due to fine-scale landscape processes and changes that often result in mixed pixels. MethodsUsing Sentinel-2 satellite images, we document forest fire impacts and burn severity. Using 2016 and 2021 UAV multispectral images and Structure-from-Motion data, we estimate post-thinning changes in forest canopy cover, patch sizes, canopy height distribution, and bare ground cover. Using repeat lidar data within a smaller area of the watershed, we quantify geomorphic effects in the WUI associated with the fire and subsequent flooding. ResultsWe document that thinning significantly reduced forest canopy cover, patch size, tree density, and mean canopy height resulting in substantially reduced active crown fire risks in the future. However, the thinning equipment ignited a forest fire, which burned the WUI at varying severity at the top of the watershed that drains into the city. Moderate-high severity burns occurred within 3 km of downtown Flagstaff threatening the WUI neighborhoods and the city. The upstream burned area then experienced 100-year and 200–500-year rainfall events, which resulted in large runoff-driven floods and sedimentation in the city. ConclusionWe demonstrate that UAV high resolution images and photogrammetry combined with terrestrial lidar data provide detailed and accurate estimates of forest thinning and post-fire flood impacts, which could not be estimated from coarser-resolution satellite images. Communities around the world may need to prepare their WUIs for catastrophic fires and increase capacity to manage sediment-laden stormwater since both fires and extreme weather events are projected to increase.more » « less
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Garcia-Ayllon_Veintimilla, Salvador (Ed.)Historical information about floods is not commonly used in the US to inform land use planning decisions. Rather, the current approach to managing floods is based on static maps derived from computer simulations of the area inundated by floods of specified return intervals. These maps provide some information about flood hazard, but they do not reflect the underlying processes involved in creating a flood disaster, which typically include increased exposure due to building on flood-prone land, nor do they account for the greater hazard resulting from wildfire. We developed and applied an approach to analyze how exposure has evolved in flood hazard zones in Montecito, California, an area devastated by post-fire debris flows in January 2018. By combining historical flood records of the past 200 years, human development records of the past 100 years, and geomorphological understanding of debris flow generation processes, this approach allows us to look at risk as a dynamic process influenced by physical and human factors, instead of a static map. Results show that floods after fires, in particular debris flows and debris laden floods, are very common in Montecito (15 events in the last 200 years), and that despite policies discouraging developments in hazard areas, developments in hazard zones have increased substantially since Montecito joined the National Flood Insurance Program in 1979.We also highlight the limitation of using conventional Flood Insurance Rate Maps (FIRMs) to manage land use in alluvial fan areas such as Montecito. The knowledge produced in this project can help Montecito residents better understand how they came to be vulnerable to floods and identify action they are taking now that might increase or reduce their vulnerability to the next big flood. This science-history-centric approach to understand hazard and exposure evolution using geographic information systems (GIS) and historical records, is generalizable to other communities seeking to better understand the nature of the hazard they are exposed to and some of the root causes of their vulnerabilities, in other words, both the natural and social processes producing disasters.more » « less
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García-Ayllón Veintimilla, Salvador (Ed.)Historical information about floods is not commonly used in the US to inform land use planning decisions. Rather, the current approach to managing floods is based on static maps derived from computer simulations of the area inundated by floods of specified return intervals. These maps provide some information about flood hazard, but they do not reflect the underlying processes involved in creating a flood disaster, which typically include increased exposure due to building on flood-prone land, nor do they account for the greater hazard resulting from wildfire. We developed and applied an approach to analyze how exposure has evolved in flood hazard zones in Montecito, California, an area devastated by post-fire debris flows in January 2018. By combining historical flood records of the past 200 years, human development records of the past 100 years, and geomorphological understanding of debris flow generation processes, this approach allows us to look at risk as a dynamic process influenced by physical and human factors, instead of a static map. Results show that floods after fires, in particular debris flows and debris laden floods, are very common in Montecito (15 events in the last 200 years), and that despite policies discouraging developments in hazard areas, developments in hazard zones have increased substantially since Montecito joined the National Flood Insurance Program in 1979.We also highlight the limitation of using conventional Flood Insurance Rate Maps (FIRMs) to manage land use in alluvial fan areas such as Montecito. The knowledge produced in this project can help Montecito residents better understand how they came to be vulnerable to floods and identify action they are taking now that might increase or reduce their vulnerability to the next big flood. This science-history-centric approach to understand hazard and exposure evolution using geographic information systems (GIS) and historical records, is generalizable to other communities seeking to better understand the nature of the hazard they are exposed to and some of the root causes of their vulnerabilities, in other words, both the natural and social processes producing disasters.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1002/esp.5537
