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Legacy of severe wildfires on stream water quality

COLORADO – A new study severe seeks to find out the impact of wildfires on rivers and reservoirs years after they burn. This study found that, 15 years after the Hayman Fire, watersheds with extensive high-severity wildfire still had elevated levels of streamwater nitrogen. While elevated nitrogen and carbon in burned watersheds are not a threat to drinking water quality, they do exceed expected levels for healthy streams in this area.

The recently published study, “The legacy of a severe wildfire on stream nitrogen and carbon in headwater catchments,” evaluated the site of the 2002 Hayman Fire, located 50 km southwest of Denver, Colorado. The Hayman Fire, the largest in recorded Colorado history, burned watersheds with streams that flow into the South Platte River, affecting the drinking water supply for residents of the Denver area as well as agricultural and industrial users.

Fortuitously, Rocky Mountain Research Station scientists and USDA Forest Service managers began to analyze stream water quality in tributaries of the South Platte River prior to the fire. This provided before and after information allowing researchers to evaluate the short-term effects of the fire on nutrients, sediment and stream temperatures and return to examine current conditions and the long-term effects of the fire. 

“This study shows that severe wildfires that burn large portions of some watersheds affect stream nutrients for longer than we had previously thought,” said Chuck Rhoades, lead author of the study from RMRS. “If the systems are becoming less efficient at retaining nutrients, then you may have water quality problems downstream that can lead to increased algae, changes in fish habitat, and challenges for water treatment plants.”

Recovery of ponderosa pine forests in the burned areas has been slow. The watersheds that impact water quality the most are the least revegetated.  In this area, ponderosa pine is only regenerating near the edge of the unburned forest, so without help it will take a long time for the forest to regrow. Scientists believe that the higher stream nutrient levels originate in the soil, where plant demand for nitrogen has remained low since the fire. The researchers also measured higher levels of dissolved carbon in streams draining from watersheds that experienced moderate-intensity burns. Extensively burned watersheds no longer act as strong nitrogen “sinks” that retain more than 90 percent of atmospheric nitrogen inputs, as they did before the Hayman Fire.

“Studies like this, that point to the long-term water quality effects of severe wildfire can help flag locations in need of post-fire restoration, like tree planting,” said Rhoades.

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