Fires & floodplains: the Holiday Fire vs. restored and unrestored areas of the South Fork Mckenzie

Year: 2023
Presenter/s: Colin Thorne
Symposium Session: 2023 - 08 - Restoration as Risk Reduction: Wildfire
Topics covered: adaptive management and monitoring, climate change, fish-salmon, floodplain, riparian, risk and resilience, wildfire, and wood


This presentation is directly relevant to river restoration science, as well as being of wider interest to a range of environmental disciplines. It identifies a novel co-benefit of fully-restoring river-floodplain connectivity in anthropogenically-incised streams (a goal which is colloquially termed “restoration to Stage Zero”). Rehydrating the floodplain at base flow is known to alter the behaviour of wildfires that sweep through river valley floors, through replacing a uniformly-severe burn with what is termed a ‘fire mosaic’. Interactions between the habitat mosaics on the floodplain generated by the impacts of (a) flood inundation and (b) fire may generate benefits for biodiversity resilience and recovery, wildfire management, and people who live, work or rely on continued functioning of the river and its floodplain. Broadly, we use the emerging results of an ongoing study of the impacts of Holiday Farm Fire (September 2020) and subsequent post-fire recovery to investigate the hypothesis that pyro-diversity increases biodiversity. The study site is at, and adjacent to, the reach of the South Fork Mckenzie River restored to ‘Stage Zero’ by the US Forest Service, starting in 2018. This is an interdisciplinary effort, drawing on elements of ecology, geomorphology and river science, with strong practical applications. Scientific rigour in our case study is assured by the breadth and depth of the large team of environmental specialists performing the work, which includes both academic and government researchers, as well as the river management and restoration practitioners who are most familiar with the river and its floodplain. Our approach is based on sound principles, best field study practices and appropriate analytical treatments, coupled with cautious interpretation of our results that avoids over-stating our initial conclusions. As the frequency, intensity and extent of both floods and wildfires are expected to increase due to climate change, it is essential that we develop integrated management approaches that leverage the positive environmental and ecological impacts of floods and fires, while minimising their adverse impacts on people, property, and infrastructure.

This presentation contributes to advancement of the science and practice of river restoration because at present, the academic literature elucidating interactions between riverine ecosystems and wildfire is limited. Its originality stems from the fact that, to date, no published study has presented a first-hand account comparing and contrasting fire impacts and recovery trajectories in an area of floodplain restored to ‘Stage Zero’ with an otherwise equivalent, unrestored area of the same floodplain. Our study is in its early stages, but its initial findings suggest that flood and fire interactions differ between unrestored and restored floodplains, with the former being more adversely impacted and slower to recover, and the latter less adversely impacted and trending towards increases in biodiversity and resilience. Clearly, our findings must be classed as preliminary and we plan to present them in that spirit, aiming to inform and stimulate a lively discussion at the presentation’s conclusion.