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July 15th, 2002

Wildfire erosion effects will show in reservoirs, ecosystems

At least three reservoirs likely will be contaminated by erosion in areas burned by this season's record wildfires, according to a University of Colorado at Boulder researcher.

The South Platte watershed and Cheesman Reservoir, one of Denver's important water sources, will be affected by erosion from the Hayman fire, while the Missionary Ridge fire could affect the Vallecito and Lemon reservoirs, according to John Gartner, a research assistant at CU-Boulder's Institute for Arctic and Alpine Research.

"The magnitude of erosion depends on how much heavy rain we receive before the grasses and shrubs reestablish," he said. "Native plants are especially important for re-vegetation, because they are adapted to flourish after wildfires. My research area is like a flower garden right now."

Gartner is studying areas burned by the 2000 Hi Meadow Fire and the 1996 Buffalo Creek Fire to learn more about erosion, flooding and mitigation techniques. As much as 100 times more erosion can occur after fires, causing damaging floods and choking freshwater ecosystems and hydroelectric power generators with sediment and debris, he said.

After the Buffalo Creek fire, hydroelectric water intakes in the Strontia Springs reservoir were clogged with sediment, debris and burned logs, and the lifespan of the reservoir was shortened, Gartner said. "In the first year after the fire, the reservoir filled with an amount of sediment that wasn't expected for 15 to 30 years" under normal conditions.

The reservoir had to be drawn down to flush out charcoal, but residents still reported that the water had a smoky smell and taste. Gartner said that manganese, which is stored in pine needles and released in abundance after wildfires, created another nuisance in the reservoir water because it stained clothing and required the use of more chlorine to treat the water.

"Some could see this as evidence endorsing the construction of more reservoirs. This does not support creating more reservoirs, but the ones we have may need to be dredged sooner than expected," Gartner said.

An increase in sediments like clay and silt in streams and reservoirs can significantly alter natural ecosystems. "It can kill fish and riparian vegetation, and the flooding can drastically alter the physical habitat," Gartner said. "For example, a flood after the Buffalo Creek fire destroyed beaver dams and the riparian habitat found there."

Gartner said some species benefit from excess sediment. Salmonid fish species need gravel for spawning grounds, and some types of vegetation thrive after a disturbance. "Some species may initially be decimated by a sediment-laden flood, only to thrive later when they re-colonize the area," he said. "This complexity of responses is essential for healthy ecosystems, but also difficult to understand and manage."

Flooding and debris slides on steep mountain slopes are other dangerous consequences of wildfire. In burned areas, the chance for flooding increases with erosion. "This is because there is no vegetation to protect the soil and slow the overland flow of water from hill slopes to streams," he said. "Some think that the fire also changes the chemistry of the soil, making it more water repellant."

Many techniques are used to reduce the sediment transfer and flooding after fires, Gartner said, but most are ineffective. "The latest research shows that spreading mulch on burned areas is most effective. But there may be ecological concerns with this, such as disrupting nutrient balances, slowing native plant re-growth, and introducing non-native species."

In addition to his research at INSTAAR, Gartner is completing a master's degree in geography at CU-Boulder. He hopes to conduct research on the areas burned by the wildfires of 2002 before completing his degree.