Water Quality Impacts of Forest Fires on Watersheds

by Ben Alexander

Ben Alexander is Senior Process Design Engineer for the City of Fort Collins water treatment program. He chairs the Watershed Assessment Committee for BTWF. He also holds a seat on the board of directors for the Colorado Lake and Reservoir Management Association (CLRMA) and serves as co-chair on the statewide post-fire water quality monitoring committee of the Colorado Water Quality Monitoring Council.

Forest fires can help maintain healthy forests. They are often viewed as beneficial if they don’t become too hot or cover too large of area. But what happens if too much forested land burns all at once? Burning of forested areas increases the rate and distance of the transfer of the elements contained in the forest’s vegetative matter.

All vegetation, including trees, takes up carbon, water, and smaller amounts of other elements while they are growing. When forests burn, these compounds are released either back into the atmosphere, where they are readily lifted aloft by the intense heat generated by the fire, or onto the soil in the form of ash and charred embers. The process, especially during intense fires, is much like a dam breaking, with elements released over a very short period of time. In addition to the sudden release of chemical compounds, the soil that was once held in place by the vegetative cover becomes exposed and susceptible to erosion until vegetative cover reestablishes. This can take years, and in some cases, even decades. Under post-fire conditions, only the lowest rates of precipitation can be absorbed by the soil. Any precipitation that results in surface runoff causes the transport of sediment. With each rainfall, the soil and ash deposits begin working their way down the mountain towards streams, lakes, and reservoirs.

For people and the environment, the impact of sediment alone can be enormous. Lakes and reservoirs can become filled with sediment. Denver Water Board lost $20 million worth of storage capacity in Strontia Springs Reservoir as a result of the 1996 Buffalo Creek Fire. The mudflows that often occur when runoff flows from burned-over areas can drastically alter in-stream habitat. Stream benthic organisms and fish populations are often buried and suffocated by the sediment. Mountain homes are sometimes damaged by post-fire mudslides and highways are frequently blocked or washed out. In extreme cases such as the Yellowstone Park fire, hydrologic features of some geyser systems were permanently obliterated by the fire-caused erosion and sedimentation. Endangered species, already on the edge of survival, could be pushed to extinction as a result of a forest fire.

The aftermath of the Bobcat Fire, in the Big Thompson watershed. Photo by Ben Alexander.

In addition to the sediment, some of the chemical compounds that are mobilized by fire are detrimental to humans and the environment in general. Nutrients such as nitrogen and phosphorous are abundant in the ash. Until they become incorporated into the soil or taken up by emerging vegetation they are easily transported downstream to destinations where they can be problematic. The transport can take them far away from burned areas where they are needed to fertilize and thereby help reestablish vegetative cover. Excess nutrients in the water promote excessive algal growth, which can trigger a chain reaction that frequently results in anoxic conditions and fish kills as well as causing adverse impacts on drinking water and recreational use. Fire caused fish kills recently occurred in Cheeseman Reservoir as a result of the 2002 Hayman Fire. The potential for fish kills resulting from the Missionary Ridge Fire of 2002 is also an on-going concern at Vallecito Reservoir in La Plata County.

Several metals are known to accumulate in the trees, pine needles and other forms of vegetation in the forest. Some of them, such as manganese, cause serious aesthetic problems for drinking water providers and their customers when suddenly released into the water supply by forest fires. Others such as mercury are highly toxic. Mercury has always been present in the air from man-made and natural causes such as burning of fossil fuels and vegetation and from volcanic action. Recent studies show that nearly all forms of vegetation contain mercury that has been absorbed out of either the soil or the atmosphere. In a laboratory experiment conducted in part by the National Center for Atmospheric Research, about 95% of the mercury contained in all types of forest vegetation was released to the atmosphere upon burning and 5% remained in the ash. BTWF’s own monitoring discovered that samples of runoff from the Bobcat Fire in the Big Thompson watershed contained mercury levels in excess of 100 times the stream standard. These levels would have been from the fraction contained in the ash. The other 95% likely traveled downwind to destinations in the eastern United States that may well be having problems with mercury accumulation in fish and the higher (human) portion of the food chain. Preliminary monitoring results from samples of runoff from one of the Colorado fires of 2002 demonstrates that lead and arsenic are being transported from the burned areas into the runoff receiving streams and reservoirs. For the entire period of analytical record neither of these substances was measurably present in the water prior to the fires.

Issues of forest fires and forest management practices will be a topic of debate that will likely run longer and may be every bit as heated as the fires of 2002. We must insist that sound science prevail over special interests and that the human population’s total dependency upon forested watersheds of the West be fully taken into account by those responsible for managing the forests.

Related websites:

Front Range Forest Fire and its Impacts on the Big Thompson River in our November 2000 issue.

Prof. Lee MacDonald, Colorado State University, conducts research on the effects of fire on runoff and erosion in the Colorado Front Range.