The Quarterly Newsletter of the

 

Front Range Forest Fire and its Impacts on the Big Thompson River

by Bob Lange and John Stednick

Bob Lange is a graduate student in Watershed Science, Department of Earth Resources at Colorado State University. He is studying the effects of fire on water resources and especially impacts on water quality.

John Stednick is a Professor in Earth Resources at Colorado State University and teaches courses in land use, water quality, and watershed problem analysis.

The Bobcat fire started on June 6, 2000 and resulted in over ten thousand acres of burned forest around Cedar Park, a small community in the Big Thompson Canyon near Drake. A thunderstorm on August 16 caused flooding and transported significant quantities of ash and sediment into the Big Thompson River. The Bobcat burn area is a small portion of a 2,500 square mile zone of mostly ponderosa pine forests along the Front Range of Colorado, which is primed for severe wildfires.

Large fires in Larimer County are typically in the neighborhood of 3,000 acres, according to Sarah Gallup of the Forest Service, who spoke at Big Thompson Watershed Forum’s General Assembly in July. The Bobcat fire was three times that size and was the largest wildfire in Larimer County’s fire fighting history. The size of the Bobcat fire may be an indication of a trend toward larger and more frequent fires due to increased fuel loads in Front Range forests.

Changes in water quality downstream of burned forests can be dramatic, especially in increased streamflow after rainstorms. These conditions have rarely been measured, since samples must be taken during or shortly after a rainstorm. A warning system consisting of two rain gauges with radio transmitters and a base station in Fort Collins was set up shortly after the Bobcat fire. This system sent out pager messages to samplers when rainfall indicated a potential increase in streamflow. The trigger for the system is based on a threshold value established to determine conditions suitable for water quality sampling (i.e. 0.5 inch/half-hour or 1 inch/hour).

Through a research project sponsored by the USDA Forest Service and conducted by the Department of Earth Resources at Colorado State University, water quality samples were collected during and shortly after rainstorms in the Bobcat fire area. Sampling sites were selected in Bobcat Gulch, the North Fork of the Big Thompson, the mainstem of the Big Thompson, Jug Gulch and along Buckhorn Creek. Where possible, sites were selected at streamflow gaging sites monitored by Big Thompson Watershed Forum participants, and Forum participants conducted the majority of the water quality analysis.

Storm Samples after August 16,2000 Storm in Bobcat Gulch

The August 16, 2000 rainstorm resulted in flooding near Drake and dramatic changes in water quality. Water quality was the worst at the time of the flood and then improved as streamflow decreased. For example, total suspended and total dissolved solids indicate changes in water quality in response to the storm. The figure above illustrates the peak of the suspended and dissolved solid concentrations during the peak of the flood (i.e. August 16, 2000, 2:30 p.m.), and their corresponding decrease with time. The pattern shown in the water quality data for solids mirrors the analysis of the flows at the North Fork of the Big Thompson during and after the storm.

It is hoped the combination of these efforts along with other ongoing research projects will give water resources managers a good idea of what kinds of water quality problems they can expect from fires in the Big Thompson Watershed. With the recent fire behavior and increased fuel loads along the Front Range, the timing could not be better.

Related websites:

Water Quality Impacts of Forest Fires on Watersheds by Ben Alexander in our Fall 2002 issue.

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