Climate warming is lighting up our forests. Warmer temperatures are leading to dry conditions, meaning that fires are igniting sooner, and are more intense, far-reaching, and harder to control (1). The effects of fire are dramatic and immediate; communities are evacuated, and air quality plummets.
Last year I moved from Ontario to Alberta and the summer season was a shock to me – I had never dealt with such consistently poor air quality in Canada. However, the effects of wildfire don’t stop there. Wildfires can have dramatic, negative impacts on water quality. As our climate continues to warm and fires become more frequent and severe, understanding these effects becomes increasingly important.
How do forests manage water quality?
Forested watersheds provide many services, some of them include providing clean drinking water, hydropower, and food , and managing flood risk. The ability to provide high-quality drinking water from pristine forest areas saves the U.S. $4.1 trillion in water treatment and infrastructure costs (2). Forests manage water quality in different ways. As water passes through forests, soil filters out pollutants, such as heavy metals . Erosion, or the loss of soil, rock, and dissolved material, is also prevented by forest cover. Roots stabilize the soil, and leaf litter protects the soil from the erosive force of rain and wind. Without forest cover, soils are left vulnerable and are broken down; these break-down products, called sediment, and are transported downstream as runoff enters rivers and streams.
Wildfires and water quality
Wildfires disrupt a forest’s ability to manage water quality in many ways:
- As forest cover is lost, erosion can greatly increase as the soil is no longer protected.
- Wildfires create a temporary water repellent layer over the soil which further increases soil erosion as more water flows over land rather than infiltrating into the soil (2).
- Erosion leads to larger loads of sediment being transported downstream; sediment affects the colour and clearness of water and may also contain contaminants like heavy metals (3).
- Heavy metals attach to sediment particles – they’re safe when stored in the soil but can be mobilized through erosion as particles enter rivers and streams (3). Winds can also disperse metals over great distances (2).
- Sediments can promote bacterial growth and make detecting bacteria/viruses more difficult during water treatment (3).
Forest fires are becoming more severe with climate warming. Image from pexels.com.
Predicting water quality changes
How can we predict the effects of wildfires on water quality? Well, it’s complicated.
Not all fires result in increased erosion, runoff, and sediment transport. The degree to which this happens depends on multiple factors: fire severity, rainfall, slope of the land, and distance to rivers/streams, among others (2). Specifically, low intensity fires are less likely to significantly increase erosion(2). Among this, intense rainfall, especially shortly after a fire, will have a greater effect on water quality as rainfall is what erodes the soil, and transports materials downstream. (2). Materials are also transported more easily in sloped areas vs. flat areas – an example being landslides occurring in mountainous areas after heavy rainfall. How far away water sources are from forested areas will affect whether materials are deposited back onto the landscape or into rivers/streams.
More research is needed
Emergency rehabilitation techniques have been used to minimize the movement of materials post-fire. These include planting grass or clover to stabilize soil, applying mulch to protect soil, installing dams to prevent flooding and allow sediment to settle out, and many others (4). However, more research is needed to accurately predict the impact of wildfires on water quality in different regions and how far downstream these impacts reach (5). Knowing this will help alert downstream communities to changes in water quality and allow municipalities to adjust water treatment appropriately to deal with fire.
1. Robinne F, Hallema DW, Bladon KD, Flannigan MD, Boisramé G, Bréthaut CM, et al. Scientists’ warning on extreme wildfire risks to water supply. Hydrological Processes. 2021 May 16;35(5).
2. Abraham J, Dowling K, Florentine S. Risk of post-fire metal mobilization into surface water resources: A review. Science of The Total Environment. 2017 Dec;599–600:1740–55.
3. Smith HG, Sheridan GJ, Lane PNJ, Nyman P, Haydon S. Wildfire effects on water quality in forest catchments: A review with implications for water supply. Journal of Hydrology. 2011 Jan;396(1–2):170–92.
4. Robichaud PR, Beyers JL, Neary DG. Evaluating the effectiveness of postfire rehabilitation treatments. 2000.
5. Ball G, Regier P, González-Pinzón R, Reale J, van Horn D. Wildfires increasingly impact western US fluvial networks. Nature Communications. 2021 Dec 30;12(1):2484.