HYDROLOGIC AND EROSION RESPONSES OF BURNED WATERSHEDS


 

Scientific Questions

The enhanced probability of catastrophic wildfires in the western United States and elsewhere in the world has increased the need to understand the flooding risk and the erosion and depositional responses of burned watersheds. In addition, surface water flowing from burned areas may carry increased levels of sediment, organic debris, and chemicals that may contribute to significant degradation of municipal water supplies and aquatic habitats. Our project has three main thrusts: (1) we are investigating the relation between rainfall intensity and peak water discharge from burned watersheds, a relation that depends on the size of the rainstorm, the size of the burned area and burn severity, and the changes in infiltration capacity of the soil; (2) we are investigating the hillslope and channel erosion and deposition processes after wildfire with a focus on predicting these processes on a watershed or landscape scale rather than on a single hillslope plot or channel cross-section scale; and (3) we are examining the water quality impacts of wildfire and are synthesizing post-fire water-quality sampling protocols.

 

Mission or Goal

An extensive body of literature exists on the effects of wildfire on watersheds. Wildfires have burned across the landscape of the western United States for centuries, but the magnitude of the geomorphic effect on the landscape is unknown. By understanding the magnitude of the runoff response and the erosion and deposition responses of recent wildfires, we can provide data for landscape evolution models in areas prone to wildfire. In addition, an understanding of the runoff response will contribute to better methods of predicting post-fire flooding to minimize the loss of life and property. Watershed-scale predictions of erosion and deposition from these natural disasters can be used by land managers to prioritize forest treatments based on erosion potential before and after wildfires. Moreover, we hope to contribute to an understanding of wildfire as an element of an ecosystem’s disturbance regime.

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Alluvial Fan

The Buffalo Creek Fire in May 1996 burned 4,690 hectares in the mountains southwest of Denver, Colorado. This wildfire lowered the erosion threshold of the watershed.  As a consequence of this wildfire, a 100-year rainstorm in July 1996 caused erosion upstream and deposition of this alluvial fan at the mouth of a tributary to Buffalo Creek. Buffalo Creek is flowing to the right at the bottom of the photograph. Photo by R. H. Meade

Flash Floods

A consequence of wildfire is the increased probability of flash floods. This flash flood occurred in Spring Creek on 29 July 1997 within the area burned by the Buffalo Creek Fire. The view  is upstream and the discharge is about 5.0 m3/s from a maximum 30-minute rainfall  intensity of about 19 mm/h. Rainfall-runoff relations suggest a rainfall threshold at about 10 mm/h above which much larger flash floods occur. Photo by John A. Moody 

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Erosion

Another consequence of wildfires and subsequent rainfall is erosion.  This erosion of a drainage created an incised channel after the Cerro Grande Fire near Los Alamos, NM.  The view is upstream and the blue backpack is about 1 meter tall.  The maximum 30-minute rainfall intensity was about 20 mm/h.  The incision seen in this photo was after the wildfire and rain storm; prior to the storm this drainage had no definite banks.  Photo by John A. Moody

Deposition

Channels draining burned areas have zones of erosion and zones of deposition. This deposition was downstream from an erosion zone shown in the previous photo. The peeled bark indicates the highest level of water and debris during a flash flood. Sediment is coarse sand and gravel. The view is downstream and the blue backpack is about 1 meter tall.  Photo by John A. Moody

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Debris and Sediment

Organic debris and sediment were deposited in Strontia Springs Reservoir, which supplies drinking water to the cities of Denver and Aurora. This debris came from two watersheds (Buffalo Creek and Spring Creek) burned by the 1996 Buffalo Creek Fire. Associated with this debris was an increase in manganese, which increased the chlorine demand of water treated for municipal usage. Photo by John A. Moody

Rill Erosion

Rill erosion on a burned hillslope after the Buffalo Creek Fire. Photo by John A. Moody

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