how do wildfires affect water quality?
Comanche Reservoir in the Cache la Poudre watershed, after the 2020 Cameron Peak Fire
what happens to water after a wildfire?
Wildfires transform the landscape in a variety of ways, from burning vegetation, to altering soil chemistry, to changing the timing of snowmelt runoff. These changes can significantly impact processes in a watershed (the area of land where all the water drains into a common outlet, like a river) by increasing sediment and nutrients, leading to altered water chemistry, and potentially increasing the risk of algal growth.
Forests act as both a filter and a sponge for water. Forest vegetation catches water before it hits bare soil, protecting the ground from being overloaded by moisture. This helps prevent erosion and allows the water to slowly enter the ground rather than flowing over onto the land. As water flows through plants, soil, and rock, the land stores it during dry times when less water flows and naturally filters out sediment and nutrients. That’s why many cities in the Western U.S. rely on mountain forests for their drinking water—these watersheds help keep water clean before it even reaches the treatment plant.
But when the forest is altered by fire, the following things can happen:
Trees and plants are lost, so water hits the soil more quickly.
The heat from a fire can change the properties of soil so it repels water, known as hydrophobicity.
Soil becomes less porous and water moves over it instead of into it.
Reduced vegetation causes nutrients in the soil to runoff, rather than be absorbed by plants or filtered through groundwater.
Smoke and ash can be transported by wind into waterbodies and alter the water chemistry.
Ash causes land surfaces to darken—causing the snow to melt faster in winter and spring—which could lead to increased water temperature and evaporation in summer and fall.
In burned watersheds, when it rains or when snow melts, water moves rapidly over the soil rather than through it, which can cause ash, sediment, and debris, or even metals to be carried into otherwise clean streams, impairing key water quality indicators. Additionally, water high in sediment typically contains more nutrients, and can require additional treatment processes to meet safe drinking water standards.
Moreover, increased nutrients in post-fire environments could lead to more algae growth where there previously were constraints on algae—like heavy forests with historically low nutrients or snowmelt systems where colder water can curb algae growth. Algae is particularly concerning to municipal water treatment plants because it can clog water intakes and some forms of blue-green algae, known as cyanobacteria, can be harmful to humans and animals. High amounts of algae can also lead to water quality issues like oxygen depletion, which makes it difficult for fish to survive, and sunlight blockage, which can disrupt the entire food chain in a lake or reservoir.
Ash in the Cache la Poudre River
monitoring post-fire water quality - satellites, sensors, and sampling
The Cameron Peak fire was one of the largest wildfires in Colorado history, burning over 200,000 acres and approximately 44% of the Cache la Poudre River headwaters. To understand the fire’s impact on the Cache la Poudre Watershed, we use several methods:
We take regular physical water samples from reservoirs and the river, then analyze them in a lab using high-precision equipment that tells us what nutrients are present. These samples allow us to measure trace chemical components that aren’t easily seen using in-stream sensors or satellite imagery. We also use these samples to measure chlorophyll-a, which can tell us how much algae there is in a body of water.
A series of sensors placed in streams throughout the watershed continuously monitor water quality parameters like temperature, salinity (how salty the water is), and turbidity (how cloudy the water is). We can compare this data to our grab samples to get a fuller picture of water quality trends and conditions. Some of these sensors are part of the Poudre Water Quality Network, which measures water quality as the river passes through the city of Fort Collins.
To assess long-term trends of reservoir water quality in the watershed, we compare satellite imagery of reservoirs and historical temperature data. Satellites take both images and surface temperature data. Images can tell us a lot about the conditions of a body of water. Different properties of water can affect how the water absorbs and reflects light, resulting in different colors. Additionally, temperature is an important indicator of biological conditions in a reservoir or river and can affect how organisms living in water grow or respond to pollution or disease. We look at all the data we can for a given time period and use algorithms to determine long-term trends.
Kevin Horrell taking water quality grab samples from the Poudre River
post-fire water impacts: what we’re seeing
After researching water quality conditions following the Cameron Peak Fire, we have made a few key observations:
Firstly, fire impacts are complex and variable but we consistently observed higher amounts of nutrients in burned vs. unburned streams. For instance, we measured higher peak concentrations of a form of nitrogen—nitrate (NO3- )—in our burned watersheds and reservoirs. We also we measured an increase in other nutrients, like potassium, phosphorous, and dissolved carbon, in burned streams. You can read more about these findings in a paper published by our partners at Rocky Mountain Research Station.
Secondly, we measured higher levels of chlorophyll-a at burned reservoirs during the spring and early fall. However, these eutrophic events—periods where reservoirs had a lot of algal growth—didn’t last long. Most events subsided after 1-2 weeks.
Looking at satellite images and temperature data from 1984 to 2022, we found that water quality in the Cache la Poudre reservoirs changes a lot from place to place and year to year. Overall, we didn’t see a steady trend over time, and the fire didn’t cause a big sudden shift in water quality.
Lastly, we found that reservoirs, both burned and unburned, strongly influence water quality downstream, especially when it comes to algae growth (measured as chlorophyll-a). Sites closest to reservoirs tend to have more algae and that algae is often directly transported downstream. These increased levels can occur when water is released from a reservoir or from algae growing more during drier periods with lower flows. How much reservoirs affect downstream water quality is highly dependent on inputs from other streams and is something we still need to investigate further.
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