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. These changes can significantly impact water 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, or creating environments for harmful algal blooms.
Forests act as both a filter and a sponge for water. The forest vegetation catches water before it hits the soil, protecting the ground from being overloaded by moisture. This helps prevent erosion and allows the water to slowly enter the ground so it can recharge groundwater aquifers and reduce the risk of flooding. The soil and rock help filter the water before it hits a 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.
Soil becomes less porous and water moves over it instead of into it.
When this happens, water moves rapidly over the soil rather than through it, which can cause ash, sediment, and debris, or even metals and pollution to be carried into otherwise clean streams. Increased sediment mobilization can impair key water quality indicators like dissolved oxygen (which fish and other aquatic wildlife depend on for survival). Moreover, increases in ash can cause more chlorophyl-a, leading to algal growth that can clog water treatment plants and even create toxic cyanobacteria harmful to humans and animals. Water with more nutrients and sediment is harder to filter before being delivered to people.
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. 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 special equipment that tells us what nutrients are present.
A series of sensors placed throughout the river continuously monitor water quality parameters like temperature and pH. 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 in the lower part of the watershed.
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 after the Cameron Peak Fire since 2021, we have made a few key observations.
Firstly, fire impacts are complex and variable but we consistently observed higher amounts of nutrients after the fire. For instance, we measured higher levels of a nutrient called chlorophyll-a, a pigment that plants use to photosynthesize or turn sunlight into food. Chlorophyll-a is a good measure of the amount of algae in a body of water. High amounts of algae or plant growth can 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.
Generally, the more a reservoir was burned, the more we measured an increase in other nutrients, like potassium, sodium, and chloride. While we did measure some eutrophic events or periods where reservoirs had a lot of algae/plant growth, these events weren’t sustained.
Secondly, our analysis of satellite imagery and satellite temperature data tell us that there is a lot of variation of water quality from 1984—onward. There was no clear water quality trend overtime, but there also wasn’t a big sudden shift in water quality after the fire.
Lastly, after collecting more than 1,000 water quality samples, we found large variation in water quality over space and time in the watershed, with high chlorophyll-a values observed in rivers and reservoirs primarily in early Spring and late Fall. While some reservoirs did experience eutrophic events (or high amounts of algal and plant growth) none of them were sustained for long.
This project is part of an ongoing study. Click the links below to explore!