How wildfires affect streams and lakes as well as forest landscape

With more than 50 fires starting in the past week, acres burned and percentages contained litter headlines across the state. Montanans just need to glance at front-page photos or smoky skies to see that fire season is in full swing. If they’re patient, maybe they’ll be able to see it in their streams and lakes too.

National Forest Service Hydrologist Craig Kendall said the biggest concern with fire and water bodies is erosion. When fires sweep an area, they leave a rich layer of ash while taking the plant roots that hold soil in place. Any precipitation can take those loose layers into water bodies.

While that means more sediment overall, Kendall said the real focus was nutrients. Phosphorus and nitrogen are the most closely monitored because algae use them for food.

“Nitrogen compounds are really mobile in the soil so you typically see a spike,” Kendall added.

“A pulse of nutrients can jump-start the aquatic system, provide more nutrients for plants and insects and also the growth of fish,” Kendall said.

Fires also leave larger things in the water. Kendall noted fallen trees and other burnt debris provide new habitat and woody materials that make high quality fish habitat for species like bull trout.

Flathead Lake Biological Station Research Scientist Tom Bansak said streams experience a blaze differently from lakes.

Bansak referenced a study from Montana’s 1988 fire season. Large fires near Polebridge and Yellowstone National Park provided a chance to sample water bodies during major wildfires. Station researchers sampled streams during the fires and every 10  years since.

Bansak said although the study showed fire can warm stream waters, temperatures don’t have a big impact. Temperature increases were short term and animals like fish are evacuated.

“You do get some stream temperature increase but they don’t get high enough that critters will die,” he said.

Smoke and ash left a bigger impression. Bansak said in 1988 researchers found chunks from ash and smoke in stream waters. They also recorded higher nutrient levels that were later connected to dissolved smoke.

More permanent changes came after the blazes were put out.

“The bigger effects of fire on a stream ecosystem tend to be in the first few years,” Bansak said.

Without vegetation to provide shade, researchers recorded slightly increased temperatures. Erosion deposited loose soils into streams and increased nutrients.

Bansak said those factors combined to change the look of streams. More sun and more nutrients promote algae. More algae attract more plant-eating insects. Some streams return to pre-fire conditions in 10 to 15 years.

“In an ecological sense, 10 to 15 years is not a long time,” he said.

Bansak said that the impacts disappear with the water flow and the long-term changes are found wherever streams empty.

“Streams are less impacted during the fire than lakes can be,” he said.

Glacier National Park has roughly 200 lakes. Right now it also has two fires that have burned more than 15,000 acres.

Shawn Devlin, a postdoctoral researcher at Flathead Lake Biological Station, said the key difference between fire impacts on lakes and streams is how long the water sits. While streams flow, lakes cycle through water.

The amount of time it takes for a lake to refill is called residence time. According to the biological station, Flathead Lake’s residence time is a short three years. For comparison, they list the residence time of Lake Tahoe, another large freshwater lake, at 650 years.

Devlin said the size of a lake negates any potential temperature rise. Its surface area compared to its shoreline makes the amount of new habitat from debris small. For lakes, nutrients are the focus.

Devlin said because lake water sits, smoke plays a larger role. Smoke-deposited nutrients stay in the water longer. Weather and geography could concentrate the effects by creating inversions that trap the smoke at a lake’s surface.

Lakes are more vulnerable to contamination by air. Devlin used Flathead Lake as an example, saying studies showed 98 percent of its water comes from rivers but 50 percent of its ammonium, a major source of nitrogen, is deposited by air.

More nutrients by air, stuck in the water long periods, create a visible change: a greener and less clear lake. The drop in water quality comes from a boom in algae populations feeding on more nutrients.

Devlin said the effect is not immediate: not every smoky day will lead to an algae bloom.

While fire impacts aren’t as quickly obvious in water as on land, scientist and researchers like hydrologist Craig Kendall know they’re just as significant.

“From an ecological perspective fire plays a really important role in shaping the landscape,” he said.