Wildfires and River Ecosystems Across the American West

Published: May 6, 2024 • By Rose Grenen

Introduction

This brief presents summaries of key impacts to aquatic ecosystems from increased frequency of wildfires across the American West. Findings from various studies reveal altered thermals and increased sediment loading across waterways in wildfire impacted regions. These direct inputs can lead to mortality among riparian vegetation, fish, macroinvertebrates and amphibians. However, moving further downstream in a watershed, delayed nutrient loading and sediment inputs may increase primary production. Understanding the broad impacts of wildfires involves analysis of the many factors that make a river or stream unique and the ripple effects for downstream environments.

Threat of Wildfires

Wildfire frequency and intensity has been rising across the Western United States. In the 2018 season alone, wildfires engulfed 76% more area of the Western United States than was observed 20 years ago. Furthermore, climate models project that the total area burned across the Rocky Mountains will increase by 175% by the year 2050 (Rust et al., 2019). Historically, the natural occurrence of wildfires has sculpted many ecosystems across the west, and as such, the increase in wildfire frequency is likely to drastically impact aquatic communities across the landscape (Jager et al., 2021).

Implications for Riparian Vegetation

Stream riparian ecosystems are directly impacted by burns from wildfires. The influence of fires on riparian ecosystems and aquatic fauna depends on fire severity and the amount of catchment area actually burned (Minshall et al., 2001). Extensive wildfire disturbance leads to burned riparian vegetation and can lead to enhanced hillslope erosion and sediment release to downstream areas (Robichaud et al., 2020). In some cases, riparian species can rebound post fire; however, if disturbances prevent recolonization of aquatic plants or there are significant modifications to the stream catchments, the riparian species may have a difficult time rebounding to pre-fire abundance (Jager et al., 2021).

a. Streams without recent stand-replacing wildfires in the watershed.

Dense forest scene with tall evergreen trees and lush green undergrowth. Fallen logs lie across a small stream, creating a tranquil, natural atmosphere.

 

 

 

 

 

 

b. Streams with moderate to high severity wildfires in watershed.

A serene, shallow stream meanders through lush green vegetation, under a bright blue sky, conveying a tranquil and natural wilderness setting.

 

 

 

 

 

 

c. Streams with moderate to high severity wildfires that resulted in channel reorganization. 

A barren landscape with charred tree trunks scattered across a dry creek bed. Sparse green grass and shrubs contrast with the desolate, post-fire scene.

Case Study

Muddy field with scattered dead fish surrounded by fallen branches and grass. The scene conveys a sense of desolation and environmental distress.

Photograph of the Trout Creek fish kill on August 3, 2014 courtesy of the Soward Ranch (Rust et al., 2018).

 

The West Fork Fire of 2013 consumed 44,360 hectares of forest across Colorado. Rain events following the fire delivered much sediment and turbid water to the Rio Grande River. These events caused widespread and extreme fish kills; for instance, it was reported that hundreds of trout were killed within a tributary in a single rain event. Turbidity reached 505 Nephelometric Turbidity Units (NTUs) during the fish kill event. Furthermore, elevated turbidity levels across the Rio Grande persisted for three years (Rust et al., 2018).

Implications for Aquatic Ecosystems

Wildfires alter the physical habitats in rivers through a variety of ways. Following a wildfire, many rivers and streams experience a change in water temperature, water chemistry, and deposition of ash and sediment (Minshal et al., 1989). Smoke during fires can alter temperatures through diminishment of short-wave solar radiation (David et al., 2018). On the other hand, the burning of riparian vegetation results in reduced shade, which can enhance stream temperatures post-fire (Dunham et al., 2007). Large quantities of sediment delivered to streams near wildfires can also degrade water quality and negatively impact local aquatic species (Sherson et al., 2015). Wildfires can cause a release of minerals from soils during the burn event (Perera & Buse 2014). Previous studies have even found elevated levels of dissolved metals in streams draining from burned watersheds across the American West (Rust et al., 2018). The cumulative impacts of temperature increase and sediment delivery from extreme fire events have been found to lead to short-term mortality of benthic macroinvertebrates, amphibians and fish (Dunham et al., 2007). For instance, an increase in large inputs of fine sediment from fires during fish spawning may fill and bury fish spawning locations, leading to decreased survival of fish populations (Greig et al., 2005).

Looking at a longer time scale, wildfires can increase delivery of debris, large wood, coarse sediment and nutrients to the downstream river networks (Miller et al., 2003). Increased input of allochthonous material, such as nutrients and carbon, can stimulate long-term increases in primary production. While invertebrate mortality has been seen in rivers that directly experienced wildfires, the long-term impacts of sediment and nutrient delivery to downstream river segments have actually seen an increase in abundance of some benthic macroinvertebrates post wildfire (Scrimgeour et al., 2001). Changes in fluxes from tributaries affected by wildfires may have elevated dissolved organic carbon and organic matter that can stimulate growth in microbes and invertebrates.

Flowchart illustrating wildfire impacts on ecosystems, showing connections between factors like vegetation, runoff, and fish habitats, with color-coded sections.

Diagram showing the direct and indirect effects of wildfire on aquatic populations (influenced by hydrologic, geomorphic, vegetative and biochemical pathways). © 2021 Oak Ridge National Laboratory, managed by UT‐Battelle, LLC. Ecology and Evolution published by John Wiley & Sons Ltd. This article has been contributed to by US Government employees and their work is in the public domain in the USA. Graphic adapted from Paul, M.J., S. LeDuc, M.G. Lassiter, L.C. Moorhead, P. D. Noyes, and S.G. Leibowitz

Furthermore, the increased delivery of nutrients downstream of fires may also cause algal blooms, which can lead to anoxic water conditions (Harris et al., 2015). Concentrations of dissolved and particulate nitrogen and phosphorus have been found to remain higher than natural levels in streams and rivers affected by fires for up to five years after a wildfire (Rust et al., 2018).

Wildfire impacts on aquatic species are dependent on fire severity, burn area, and distance downstream from the fire. While waterways directly downstream of a fire may see mass mortality, waterways much further downstream may actually see increase in growth and primary production. Understanding the broad impacts of fires involves analysis of the many externalities, both negative and positive, as well as the ripple effects of the various changes across the river ecosystem.

Connfounding Factors

River systems with a higher concentration of connected tributaries have been found to have aquatic species with higher resilience to disturbance, while species in more isolated systems have been found to have limited resilience (Jager et al., 2021). Isolated tributaries often result from human interventions, such as dams and reservoirs, or can naturally result from poor water quality or beaver dams (Terui et al. 2018). The preservation of branching complexity and connectivity in aquatic systems can lead to population stability (Terui et al., 2018). Conversely, when aquatic rivers systems are broken up, many species become vulnerable (Terui et al., 2018). As such, controlling human influences (such as habitat degradation and fragmentation) may be an even more effective method to manage fish and amphibians’ response to wildfires than managing the wildfires themselves (Dunman et al., 2007).

Comparative Study

Hedden et al. (2022) completed a study of two tributaries of the Gila River in New Mexico that had experienced large wildfires:

Black Canyon

  • The Silver Fire in 2013 burned 35% of upstream catchment of the Black Canyon Tributary.
  • The movement of sediment into the tributary reduced streamflow by 59% and increased quantity of substrates by 51%.
  • Fish densities dropped to zero following the wildfire and subsequent monsoonal flooding. It took approximately eight years for fish population recovery across Black Canyon.

Little Creek

  • The Miller Fire in 2011 burned 100% of upstream catchment of Little Creek Tributary.
  • The creek saw no notable change in streamflow or substrate content after the fire.
  • Fish density declined slightly immediately following wildfires and returned to pre-fire levels within one year.

Overall, the comparison of Little Creek to Black Canyon portrayed the varying responses different ecosystems can have to wildfires, even within a similar watershed and across similar topographies. Each wildfire response is dependent on various interacting factors, such as the extent of the wildfire, attributes of the watershed and post-fire rain events (Hedden et al., 2022).

As impacts of climate change are becoming more apparent across the American Southwest, the region is seeing longer fire seasons as well as increased fire frequency and intensity. The study along Black Canyon Tributary specifically revealed the extreme impact disturbances can have on fish communities, as they were nearly decimated after the wildfire. However, the eventual resurgence of native fish in Black Canyon following the fire revealed that the stream’s relative connectivity likely helped the fish recovery (Hedden et al., 2022).

Restoration and Management

Prescribed burns and forest thinning can be used to prevent forest fires and increase resilience (Jager et al., 2021). Spreading prescribed burns out in patches can lessen the subsequent sediment loads to watersheds (Hedden et al., 2022). Furthermore, the regrowth of riparian vegetation is a major factor in determining the speed at which aquatic populations will rebound (Jager et al., 2021). Directly after fires, management crews can implement slope stabilization and restore riparian vegetation (Hedden et al., 2022). More direct restoration may even include helping fish recovery through fish stocking or translocations (Hedden et al., 2022). There is a need to understand the relative importance of temperature and dispersal processes in different watersheds in order to identify which streams are most vulnerable to wildfires. Each water system will react differently to increased temperatures and channel disturbances from wildfires (Dunham et al., 2007). As such, a holistic approach is required when managing and restoring aquatic ecosystems after wildfires.

Policy Recommendations

Warming climates, overgrown forests and increased encroachment of housing developments into wildlife zones are contributing to a wildfire and forest health crisis (US Forest Service, n.d.). Public outreach can help raise awareness regarding the risks of wildfires, climate change, and improper land use.

According to the US Forest Service, some policies and practices that can help the issue include:

  • improving government and local agency communication related to forest fire management
  • recruit and maintain a workforce capable of providing the necessary riparian area and forest restoration
  • strengthen cross-boundary partnerships 
  • invest in open and easily accessible information and data sharing
  • share information regarding using prescribed fire as a tool for ecosystem resilience
  • help policy makers understand the benefits of managing forest resiliency

Looking Forward

The Bipartisan Infrastructure Law and the Inflation Reduction Act both address projects aimed at reducing wildfire risk. The plans will work on 21 landscapes across 134 firesheds in the Western United States. In many places, these projects have already gotten underway and are ready to expand (US Forest Service, n.d.).

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  • Dunham, J.B., Rosenberger, A.E., Luce, C.H. et al. Influences of Wildfire and Channel Reorganization on Spatial and Temporal Variation in Stream Temperature and the Distribution of Fish and Amphibians. Ecosystems 10, 335–346 (2007). https://doi.org/10.1007/s10021-007-9029-8
  • Fisher, J. T., & Wilkinson, L. (2005). The response of mammals to forest fire and timber harvest in the North American boreal forest. Mammal Review, 35, 51–81. 10.1111/j.1365-2907.2005.00053.x
  • Greig, S. M., Sear, D. A., & Carling, P. A. (2005). The impact of fine sediment accumulation on the survival of incubating salmon progeny: Implications for sediment management. Science of the Total Environment, 344, 241–258.10.1016/j.scitotenv.2005.02.01
  • Harris, H., Baxter, C. V., & Davis, J. M. (2015). Debris flows amplify effects of wildfire on magnitude and composition of tributary subsidies to mainstem habitats. Freshwater Science, 34(4), 1457–1467. https://doi.org/10.1086/684015
  • Hedden, C., Propst, D. L., Gido, K. B., Hedden, S. C., & Whitney, J. E. (2022). Differential responses of native fishes in two headwater tributaries of the gila river following severe wildfires. Western North American Naturalist, 82(1), 201-207.
  • Jager, H. I., Long, J., Malison, R. L., Murphy, B., Rust, A., Silva, L. F., Sollmann, R., Steel, Z., Bowen, M. D., Dunham, J. B., Ebersole, J. L., & Flitcroft, R. L. (2021). Resilience of terrestrial and aquatic fauna to historical and future wildfire regimes in western North America. Ecology and Evolution, 11(18), 12259–12284. https://doi.org/10.1002/ece3.8026
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  • Rust, Ashley J., et al. “Wildfire impacts on water quality, macroinvertebrate, and trout populations in the upper rio grande.” Forest Ecology and Management, vol. 453, Dec. 2019, p. 117636, https://doi.org/10.1016/j.foreco.2019.117636.
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  • Scrimgeour, G. J., Tonn, W. M., Paszkowski, C. A., & Goater, C. (2001). Benthic macroinvertebrate biomass and wildfires: Evidence for enrichment of boreal subarctic lakes. Freshwater Biology, 46, 367–378. 10.1046/j.1365-2427.2001.00682.x
  • Sherson, L. R., Van Horn, D., Gomez-Velez, J. D., Crossey, L. J., & Dahm, C. N. (2015). Nutrient dynamics in an alpine headwater stream: use of continuous water quality sensors to examine responses to wildfire and precipitation events. Hydrological Processes, 29(14), 3193–3207. https://doi.org/10.1002/hyp.10426
  • Terui, A., Ishiyama, N., Urabe, H., Ono, S., Finlay, J. C., & Nakamura, F. (2018). Metapopulation stability in branching river networks. Proceedings of the National Academy of Sciences of the United States of America, 115, E5963–E5969. 10.1073/pnas.1800060115Confronting the Wildfire Crisis | US Forest Service. (n.d.). US Forest Service. https://www.fs.usda.gov/managing-land/wildfire-crisis

Rose Grenen is a recent graduate from the Master of Science in Environmental Science program.

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