Water Quality Concerns

Photo by city of Boulder

Return to Drought, Fire & Flood Homepage

By: Donna Scott, City of Boulder, Water Quality and Environmental Services

The following is an outline of some of the water quality concerns the city of Boulder has related to the recent Walker Ranch fire.

Fires and Fish

First and foremost, we must remember that wild land fires are critical for forest health. However, fires create a number of management challenges, including erosion, runoff and water quality.

Forest fires alter the aquatic ecosystem by changing the deposition of woody debris, sediment suspension, nutrient cycling, leaf litter input and the types of aquatic organisms present. The effects of forest fires on streams can be divided into three categories. Immediate impacts are those directly attributed to the fire itself (e.g. increase temperature, water chemistry, and food quality) Delayed impacts are associated with runoff and include physical disturbances such as sediment deposition and turbidity. The impact of fires varies proportionally with the intensity and extent of the burned area in the stream basin. The impact of fire is also regulated by factors influencing runoff and erosion. Slope, gradient, geology and soil depth play a role in erosion.

The Walker Ranch Fire

The potential impact of a fire on water quality is dependent on the erosion potential of the burned area and the proximity of streams. The U.S. Department of Agriculture/ Natural Resource Conservation Service is currently evaluating the soils and runoff potential of the Water Ranch Area to determine erosion control strategies.

Potential water quality impacts involve a major tributary to Boulder Creek and a drinking water supply reservoir. It is estimated that 500,000 people receive their drinking water from water resources affected by this fire. These include South Boulder Creek, which is a drinking water source for the cities of Louisville, Lafayette and the town of Superior and Gross Reservoir, a water supply for Denver Water as well as serving Arvada. In addition, several small drainage ways cross the area. Tom Davis Gulch is an intermittent stream which runs west to east right through the most heavily burned areas and is a tributary to South Boulder Creek, just upstream of the city of Lafayette's and the town of Louisville's diversion structures. Gross Reservoir's northern corner is within a few hundred feet of the fire area, and South Boulder Creek forms most of the eastern boundary of the fire.

Short-term Concerns

The Walker Ranch Fire presents a number of short-term and long-term concerns related to the water quality - for human health and for aquatic life. The immediate concern is the impact of the fire retardant used. A September 20 article in the Daily Camera indicated that the fire slurry used on the Walker Ranch fire might have contained sodium ferro cyanide, which "when exposed to sunlight, releases pure, deadly cyanide." However, it is unclear if the fire retardant used in the Walker Ranch fire contained the cyanide additive. Water samples taken on September 20, during the first storm event have been analyzed for ammonia and cyanide, as well as other water quality constituents. Laboratory results are not yet available.

The Colorado Department of Health and Environment indicated that if the cyanide additive were present, it would not be a drinking water concern or human health risk.

Long-term Concerns and Erosion Control and Forest Rehab

The long-term water quality concern is the impacts of runoff from the burned area. The sediment load has a potential severe detrimental effect on water quality and aquatic habitat, particularly over the next two years.

The difference in the burned areas which had forest management / prescribed burns and those areas which did not, is remarkable. The areas that had prescribed burns not only were instrumental in stopping the fire but in the long term provide the best erosion control measure. In these areas, the fire did not burn as intensely, therefore grass roots and patches of forest remain intact.

In contrast, in the areas with no forest management, the fire burned the hottest and left a stark landscape, void of vegetation. This will be the area with the most erosion potential and will have to have the most costly erosion control measures applied. Of particular concern in this area is channel erosion in Tom Davis Gulch, which may be responsible for producing and delivering the largest sediment load into South Boulder Creek.

Fire fighting efforts were in full swing at the Walker Ranch fire as the State Forest Service started working on revegetation and erosion control activities. After the fire, Boulder County Open Space coordinated an erosion control and forest rehabilitation plan with other agencies including City of Boulder, Colorado Department of Health, State of Colorado Forest Service, Eldorado State Park, Denver Water, U.S. Department of Agriculture/ Natural Resource Conservation Service. A citizen group, the Boulder Creek Watershed Initiative volunteered to assist as well. Over the next year, Boulder County will be working with these agencies to implement the plan they develop to minimize the impact of the Walker Ranch Fire on water quality. The city of Boulder's Water Quality and Environmental Services will continue to monitor water quality of South Boulder Creek to help evaluate fire mitigation measures and assess drinking water concerns.


Minshall, G. Wayne, J.T. Brock, J.D. Varley. 1989. Wildfires and Yellowstone’s Stream Ecosystems. BioScience 39:10. 707-715.

Berry, Vern P.. December, 1990. An Evaluation of Stabilization and Rehabilitation Procedures at Black Tiger Gulch Following the 1989 Fire.

Return to Drought, Fire & Flood Homepage

INVITATION BASIN is a community project actively seeking public participation. We appreciate all feedback and welcome comments, suggestions and contributions. To find out more about how you can be involved, click here.

Home | Site Map | Glossary | Bibliography | Contributors
About BASIN | Attribution | Feedback | Search