PUBLISHED: 14 July 2016

How wildfires impact a watershed

About 1,200 wildfires on average are reported in Alberta each year. Half of the wildfires are caused by humans and close to half are caused by lightning [1].
As we have seen dramatically over the last two decades, wildfires impact communities, people, wildlife and habitat as undergrowth burns, trees come down, and increased sediment enters streams. The specific impacts of a wildfire on a watershed can be unpredictable and depend on variables including existing (pre-fire) river or lake chemistry, topography and bedrock, and vegetation.
From a water quality perspective wildfires can cause changes in a number of parameters of interest or concern including nutrients, sulfate, pH, total dissolved solids, turbidity, organic carbon, chloride, iron, color, taste, and odor [2].
Water quantity impacts are generally observed following intense rainfall or snowmelt in a watershed impacted by fire. Increased flooding and peak flows as well as debris flows are examples of wildfire impacts related to water quantity [3].
Click on an impact arrow in the diagram to learn more about it.

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List of all wildfire impacts

Clouds and weather

If the heat of a fire is lifted high enough it can create a water cycle of its own through pyrocumulus clouds. These clouds may rise above the smoke from a major wildfire [4] and rain can fall from these clouds, which may help put out the fire or, in a worse scenario, cause lightning that creates another fire in the same or neighbouring watershed.

Ash and PAHs

Polycyclic Aromatic Hydrocarbons (PAHs) are a group of chemical compounds that show up after any sort of fire that involves organic matter. They appear after a wildfire and they can also appear within car exhaust fumes or cooking a burger on a BBQ [5]. After a wildfire, if there is environmental monitoring nearby, the PAHs produced from the wildfire may interfere with those recordings [6].
Along with all the other ash and materials PAHs travel by air until they settle on the land and in water.

Fire retardant

Although fire retardant does have an environmental impact, in particular to fish and aquatic life, its impacts are dwarfed by the impacts of the ash and high temperatures from the fire [7]. A study published in 2006 [8] compiled data from post-fire surface water monitoring programs where fire retardant constituted ammonia, phosphorus, and cyanide was measured (data was available in the public domain). This study found these chemicals were also found in similar concentrations to streams in burned areas where retardant was not used.

Reduction in trees and plants

Following a wildfire the number of trees and plants in the impacted area can be dramatically reduced. The absence of trees and plants and a decreased canopy can contribute to mudslides and floods. However, over time a reduction in trees and plants can allow new growth to take root—fire can be an important contributor to natural regrowth and habitat change which contributes to overall diversity in a watershed.

Tree seed release

Although wildfire may kill off and remove some plants and trees there are some types of trees, such as the Lodgepole Pine (an evergreen conifer which is also the provincial tree of Alberta) whose pinecone scales are held closed by resin and only open from the heat of a wildfire or direct sunlight [9].

Decreased canopy

A decrease in trees and plants mean there is less, or in some cases no, interception of snowfall to the ground. This results in an increase in the amount of snow that reaches and stays on the ground [10], creating bigger snow pillows. Bigger snow pillows may result in higher peak flows as the snow melts or contribute to flooding or mudslides through the greater volume of snowmelt in specific areas.

Occurrence of runoff

Decreased trees and plants means there are fewer opportunities for precipitation to be trapped and soaked into the ground. This causes higher surface runoff and increased erosion, which increases water quantity and decreases water quality [11]. Further, runoff during the first year after a wildfire can increase by as much as 30% [12].

Flooding and mudslides

Immediately after a wildfire occurs many of the services normally provided by trees and plants go missing. Intense rainfall and/or snowmelt (increasing water quantity) combined with the decreased tree canopy can contribute to flooding and mudslides. Flooding impacts after a wildfire can be exacerbated by debris flows with large amounts of soil, rocks, and trees from a burned area. The risk of flooding and debris flow in a watershed can be determined using a combination of slope (or ruggedness), road density, and other data. The more rugged a watershed is, the more susceptible it is to debris flows after a wildfire [13] [14].

Land stability and hydrophobicity

Less trees and plants mean fewer roots holding together the soil. As a result soil and dirt in the burned area is less stable. Without the protective role of vegetation on soil there is potential for mudslides. Interestingly, in severe, slow-moving fires the combustion of vegetative materials creates a gas that penetrates the soil profile. As the soil cools, this gas condenses and forms a waxy coating. This causes the soil to repel water – a phenomena called hydrophobicity [15]. Hydrophobicity can exacerbate runoff impacts.

Sediment and erosion

If rainfall occurs after a wildfire, the ash and soot that fell during the fire will be flushed through the watershed [16]. Long term impacts of a fire and sediment depend on the characteristics of the watershed (lakes, rivers, or both), the severity, and the reoccurrence of rain events following the fire. Sediment can affect stream structure and function; headwater reaches will undergo erosion and can become unstable, while flatter downstream reaches will receive sediment and may become clogged with fine material [17].

High sediment and aquatic life

The large amount of sediment after the fire can overwhelm fish and aquatic habits [18]. Since the 2003 Lost Creek fire in Alberta some of the aquatic ecology still has not recovered [19].

Water treatment plants and reservoirs

The proximity of the water treatment plant to the surface water source will affect how strongly the plant is affected by wildfire-related water quality changes [20].
Another consideration for water treatment plants after a wildfire is turbidity—which refers to the cloudiness of the water; clear water is not very turbid while muddy water is very turbid. A typical water treatment plant is prepared to deal with normal levels of turbidity and spikes in turbidity. However increased turbidity after a wildfire may require more treatment chemicals or cause additional wear and tear to water treatment filters. For example, membranes (ultrafiltration and microfiltration) can handle occasional turbidity spikes however over time productivity of the plant is impacted due to more frequent backwashes, which uses more water and increases ‘downtime’.
Excess sediment and debris flows may fill or otherwise disrupt reservoirs, infiltration basins, or treatment works [21]. In particular, mobilization of sediment can result in reservoir sedimentation, curtailing the useful life of a reservoir [22].

Recreation areas

The scars of a wildfire including singed vegetation, less trees and plants, flooding, or mudslides change public areas and may have a positive or negative impact on recreation and tourism. Areas may be closed for some time after a wildfire as burned trees can suddenly fall or lose limbs. Kootenay National Park, which boundaries Alberta’s Banff National Park, has positioned the impacts of fire on landscapes as a visitor attraction, “The ghostly spindles of once-burned trees carpet many parts of Kootenay. The fresh green trees and plants among them is awe-inspiring evidence of the destructive and regenerative power of fire – vital to forest renewal and health.” [23]

Support for new wildlife and animals

During a wildfire many of the animals will find ways to escape, either by travel, or by burrowing underground. However the strategy of burrowing underground fails when the intensity of the fire is too great. Once a fire is over its remnants offer new scavenging opportunities for animals [24]. Over time, as the forest returns, so do the wildlife, adding to the overall biodiversity of a watershed and its ecosystems.

Cold water streams

Fire can destroy vegetation that shades cold-water streams, which helps keep them cool. This impact on the watershed is not beneficial to favored angling species such as trout, which require a steady supply of clean, cold and silt-free water [25].

Sediment, water quality and new aquatic life

As the years pass after the wildfire, sediment is flushed downstream. This happens faster when a stream is undammed [26]. Over time as the stream recovers the aquatic life will return.


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[3] J. Coen and H. Friedli, “Wildfire, weather & climate,” UCAR, June 2012. [Online]. Available: [Accessed 2016].
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[14] S. Ryan, K. Dwire and M. Dixon, “Impacts of wildfire on runoff and sediment loads at Little Granite Creek, western Wyoming. Geomorphology. 129: 113-130.,” 2011. [Online]. Available: [Accessed 2016].
[15] R. Waskom, J. Kallenberger, B. Grotz and T. Bauder, “Addressing the Impacts of Wildfire on Water Resources,” August 2013. [Online]. Available: [Accessed July 2016].
[16] W. Snowdon, “Wildfire contaminants could sully Fort McMurray water supply,” 19 May 2016. [Online]. Available: [Accessed July 2016].
[17] D. Gill, he Impacts Of Forest Fires On Drinking Water Quality. Thesis Presented in Partial Fulfillment of the Requirement for the Degree Master of Science, Arizona State University, 2004.
[18] T. Meixner and P. Wohlgemuth., Wildfire Impacts on Water Quality. Southwest Hydrology., 2004.
[19] Parks Canada, “Kootenay National Park,” March 2016. [Online]. Available: [Accessed July 2016].
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[21] D. INKLEY, “5 WAYS WILDFIRES THREATEN WESTERN WILDLIFE,” National Wildlife Federation, 19 June 2012. [Online]. Available: [Accessed July 2016].
[22] eWater CRC, “Background Ecological Aspects of Stream Recovery after Bushfire,” bushfires and catchments, [Online]. Available: [Accessed July 2016].